113 results on '"Nemšák, S."'
Search Results
2. Emergent phenomena at oxide interfaces studied with standing-wave photoelectron spectroscopy
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Kuo, C. -T., Conti, G., Rault, J. E., Schneider, C. M., Nemšák, S., and Gray, A. X.
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Condensed Matter - Materials Science - Abstract
Emergent phenomena at complex-oxide interfaces have become a vibrant field of study in the past two decades due to the rich physics and a wide range of possibilities for creating new states of matter and novel functionalities for potential devices. Electronic-structural characterization of such phenomena presents a unique challenge due to the lack of direct yet non-destructive techniques for probing buried layers and interfaces with the required Angstrom-level resolution, as well as element and orbital specificity. In this review article, we survey several recent studies wherein soft x-ray standing-wave photoelectron spectroscopy, a relatively newly developed technique, is used to investigate buried oxide interfaces exhibiting emergent phenomena such as metal-insulator transition, interfacial ferromagnetism, and two-dimensional electron gas. Advantages, challenges, and future applications of this methodology are also discussed.
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- 2021
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3. Simultaneous ambient pressure X-ray photoelectron spectroscopy and grazing incidence X-ray scattering in gas environments
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Kersell, H., Chen, P., Martins, H., Lu, Q., Brausse, F., Liu, B. -H., Blum, M., Roy, S., Rude, B., Kilcoyne, A., Bluhm, H., and Nemšák, S.
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Physics - Instrumentation and Detectors ,Physics - Chemical Physics - Abstract
We have developed an experimental system to simultaneously observe surface structure, morphology, composition, chemical state, and chemical activity for samples in gas phase environments. This is accomplished by simultaneously measuring X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) in gas pressures as high as the multi-Torr regime, while also recording mass spectrometry. Scattering patterns reflect near-surface sample structures from the nano- to the meso-scale. The grazing incidence geometry provides tunable depth sensitivity while scattered X-rays are detected across a broad range of angles using a newly designed pivoting-UHV-manipulator for detector positioning. At the same time, XPS and mass spectrometry can be measured, all from the same sample spot and in ambient conditions. To demonstrate the capabilities of this system, we measured the chemical state, composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in O$_2$ atmosphere at various temperatures. These simultaneous structural, chemical, and gas phase product probes enable detailed insights into the interplay between structure and chemical state for samples in gas phase environments. The compact size of our pivoting-UHV-manipulator makes it possible to retrofit this technique into existing spectroscopic instruments installed at synchrotron beamlines. Because many synchrotron facilities are planning or undergoing upgrades to diffraction limited storage rings with transversely coherent beams, a newly emerging set of coherent X-ray scattering experiments can greatly benefit from the concepts we present here., Comment: 21 pages, 4 figures
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- 2021
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4. Hard x-ray standing-wave photoemission study of the interfaces in a BiFeO$_3$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ superlattice
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Martins, H. P., Khan, S. A., Conti, G., Greer, A. A., Saw, A. Y., Palsson, G. K., Huijben, M., Kobayashi, K., Ueda, S., Schneider, C. M., Vishik, I. M., Minár, J., Gray, A. X., Fadley, C. S., and Nemšák, S.
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Condensed Matter - Materials Science - Abstract
Hybrid multiferroics such as BiFeO$_3$ (BFO) and La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) heterostructures are highly interesting functional systems due to their complex electronic and magnetic properties. One of the key parameters influencing the emergent properties is the quality of interfaces, where varying interdiffusion lengths can give rise to different chemistry and distinctive electronic states. Here we report high-resolution depth resolved chemical and electronic investigation of BFO/LSMO superlattice using standing-wave hard X-ray photoemission spectroscopy in the first-order Bragg as well as near-total-reflection geometry. Our results show that the interfaces of BFO on top of LSMO are atomically abrupt, while the LSMO on top of BFO interfaces show an interdiffusion length of around 1.2 unit cells. The two interfaces also exhibit different chemical gradients, with the BFO/LSMO interface being Sr-terminated by a spectroscopically distinctive high binding energy component in Sr 2p core-level spectra, which is spatially contained within 1 unit cell from the interface. From the electronic point of view, unique valence band features were observed for bulk-BFO, bulk-LSMO and their interfaces. Our X-ray optical analysis revealed a unique electronic signature at the BFO/LSMO interface, which we attribute to the coupling between those respective layers. Valence band decomposition based on the Bragg-reflection standing-wave measurement also revealed the band alignment between BFO and LSMO layers. Our work demonstrates that standing-wave hard x-ray photoemission is a reliable non-destructive technique for probing depth-resolved electronic structure of buried layers and interfaces with sub-unit-cell resolution. Equivalent investigations can be successfully applied to a broad class of material such as perovskite complex oxides with emergent interfacial phenomena.
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- 2020
5. Bulk Electronic Structure of Lanthanum Hexaboride (LaB6) by Hard X-ray Angle-Resolved Photoelectron Spectroscopy
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Rattanachata, A., Nicolaï, L., Martins, H. P., Conti, G., Verstraete, M. J., Gehlmann, M., Ueda, S., Kobayashi, K., Vishik, I., Schneider, C. M., Fadley, C. S., Gray, A. X., Minár, J., and Nemšák, S.
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Physics - Applied Physics ,Condensed Matter - Materials Science - Abstract
In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials., Comment: Total 26 pages, Total 11 figures
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- 2020
- Full Text
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6. Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy
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Schaab, J., Shapovalov, K., Schoenherr, P., Hackl, J., Khan, M. I., Hentschel, M., Yan, Z., Bourret, E., Schneider, C. M., Nemsák, S., Stengel, M., Cano, A., and Meier, D.
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Condensed Matter - Materials Science - Abstract
Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er0.99Ca0.01MnO3. By combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Our study introduces an X-PEEM based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times in the order of 0.1-1 sec., Comment: 15 pages, 6 figures
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- 2020
- Full Text
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7. Bulk electronic structure of lanthanum hexaboride (La B6) by hard x-ray angle-resolved photoelectron spectroscopy
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Rattanachata, A, Nicolaï, LC, Martins, HP, Conti, G, Verstraete, MJ, Gehlmann, M, Ueda, S, Kobayashi, K, Vishik, I, Schneider, CM, Fadley, CS, Gray, AX, Minár, J, and Nemšák, S
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physics.app-ph ,cond-mat.mtrl-sci - Abstract
In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials.
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- 2021
8. Hard x-ray standing-wave photoemission study of the interfaces in a BiFeO$_3$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ superlattice
- Author
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Martins, HP, Khan, SA, Conti, G, Greer, AA, Saw, AY, Palsson, GK, Huijben, M, Kobayashi, K, Ueda, S, Schneider, CM, Vishik, IM, Minár, J, Gray, AX, Fadley, CS, and Nemšák, S
- Subjects
cond-mat.mtrl-sci - Abstract
Hybrid multiferroics such as BiFeO$_3$ (BFO) and La$_{0.7}$Sr$_{0.3}$MnO$_3$(LSMO) heterostructures are highly interesting functional systems due to theircomplex electronic and magnetic properties. One of the key parametersinfluencing the emergent properties is the quality of interfaces, where varyinginterdiffusion lengths can give rise to different chemistry and distinctiveelectronic states. Here we report high-resolution depth resolved chemical andelectronic investigation of BFO/LSMO superlattice using standing-wave hardX-ray photoemission spectroscopy in the first-order Bragg as well asnear-total-reflection geometry. Our results show that the interfaces of BFO ontop of LSMO are atomically abrupt, while the LSMO on top of BFO interfaces showan interdiffusion length of around 1.2 unit cells. The two interfaces alsoexhibit different chemical gradients, with the BFO/LSMO interface beingSr-terminated by a spectroscopically distinctive high binding energy componentin Sr 2p core-level spectra, which is spatially contained within 1 unit cellfrom the interface. From the electronic point of view, unique valence bandfeatures were observed for bulk-BFO, bulk-LSMO and their interfaces. Our X-rayoptical analysis revealed a unique electronic signature at the BFO/LSMOinterface, which we attribute to the coupling between those respective layers.Valence band decomposition based on the Bragg-reflection standing-wavemeasurement also revealed the band alignment between BFO and LSMO layers. Ourwork demonstrates that standing-wave hard x-ray photoemission is a reliablenon-destructive technique for probing depth-resolved electronic structure ofburied layers and interfaces with sub-unit-cell resolution. Equivalentinvestigations can be successfully applied to a broad class of material such asperovskite complex oxides with emergent interfacial phenomena.
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- 2020
9. Hard x-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction
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Conlon, C. S., Conti, G., Nemšák, S., Palsson, G., Moubah, R., Kuo, C. -T., Gehlmann, M., Ciston, J., Rault, J., Rueff, J. -P., Salmassi, F., Stolte, W., Rattanachata, A., Lin, S. -C., Keqi, A., Saw, A., Hjörvarsson, B., and Fadley, C. S.
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Condensed Matter - Materials Science ,Condensed Matter - Mesoscale and Nanoscale Physics - Abstract
The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically, and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, non-destructive, in-situ measurement ideal for this system. We have thus applied hard x-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few keV energy range to probe the structure of an epitaxially-grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO (001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 $\pm$ 4 angstrom, with minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and x-ray reflectivity measurements. Through the STEM measurements we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 angstrom. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few angstrom precision., Comment: 37 pages, 13 figures
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- 2019
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10. Establishing structure-sensitivity of ceria reducibility: Real-Time observations of surface-hydrogen interactions
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Duchoň, T, Hackl, J, Mueller, DN, Kullgren, J, Du, D, Senanayake, SD, Mouls, C, Gottlob, DM, Khan, MI, Cramm, S, Veltruská, K, Matolín, V, Nemšák, S, and Schneider, CM
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Macromolecular and Materials Chemistry ,Materials Engineering ,Interdisciplinary Engineering - Abstract
The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure-reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts.
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- 2020
11. Characterization of free standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy
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Conti, G., Nemšák, S., Kuo, C. -T., Gehlmann, M., Conlon, C., Keqi, A., Rattanachata, A., Karslıoğlu, O., Mueller, J., Sethian, J., Bluhm, H., Rault, J. E., Rueff, J. P., Fang, H., Javey, A., and Fadley, C. S.
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Condensed Matter - Materials Science - Abstract
Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with angstrom precision. We determined that: i) the exposure to air induced the formation of an InAsO$_4$ layer on top of the stoichiometric InAs(QM); ii) the top interface between the air-side InAsO$_4$ and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; iii) the bottom interface between the InAs(QM) and the native oxide SiO$_2$ on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO$_2$/(Si/Mo) substrate was determined by HXPS. The value of $VBO = 0.2 \pm 0.04$ eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO$_2$ heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials., Comment: three figures
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- 2018
12. Probing single-unit-cell resolved electronic structure modulations in oxide superlattices with standing-wave photoemission
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Yang, W, Chandrasena, RU, Gu, M, Dos Reis, RMS, Moon, EJ, Arab, A, Husanu, MA, Nemšák, S, Gullikson, EM, Ciston, J, Strocov, VN, Rondinelli, JM, May, SJ, and Gray, AX
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cond-mat.mtrl-sci ,Bioengineering - Abstract
Control of structural coupling at complex-oxide interfaces is a powerful platform for creating ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy, in conjunction with scanning transmission electron microscopy, to probe the depth-dependent and single-unit-cell resolved electronic structure of an isovalent manganite superlattice [Eu0.7Sr0.3MnO3/La0.7Sr0.3MnO3]×15 wherein the electronic-structural properties are intentionally modulated with depth via engineered oxygen octahedra rotations/tilts and A-site displacements. Our unit-cell resolved measurements reveal significant transformations in the local chemical and electronic valence-band states, which are consistent with the layer-resolved first-principles theoretical calculations, thus opening the door for future depth-resolved studies of a wide variety of heteroengineered material systems.
- Published
- 2019
13. Hard x-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction
- Author
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Conlon, CS, Conti, G, Nemšák, S, Palsson, G, Moubah, R, Kuo, C-T, Gehlmann, M, Ciston, J, Rault, J, Rueff, J-P, Salmassi, F, Stolte, W, Rattanachata, A, Lin, S-C, Keqi, A, Saw, A, Hjörvarsson, B, and Fadley, CS
- Subjects
cond-mat.mtrl-sci ,cond-mat.mes-hall ,Mathematical Sciences ,Physical Sciences ,Engineering ,Applied Physics - Abstract
The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, nondestructive, in-situ measurement ideal for this system. We have thus applied hard X-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 ± 4 Å, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and X-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision.
- Published
- 2019
14. An Efficient Algorithm for Automatic Structure Optimization in X-ray Standing-Wave Experiments
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Karslıoğlu, O, Gehlmann, M, Müller, J, Nemšák, S, Sethian, JA, Kaduwela, A, Bluhm, H, and Fadley, C
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SWOPT ,YXRO ,SO-I ,Data analysis ,Optimization ,physics.comp-ph ,cond-mat.mtrl-sci ,Chemical Physics ,Atomic ,Molecular ,Nuclear ,Particle and Plasma Physics ,Condensed Matter Physics ,Physical Chemistry (incl. Structural) - Abstract
X-ray standing-wave photoemission experiments involving multilayered samples are emerging as unique probes of the buried interfaces that are ubiquitous in current device and materials research. Such data require for their analysis a structure optimization process comparing experiment to theory that is not straightforward. In this work, we present a new computer program for optimizing the analysis of standing-wave data, called SWOPT, that automates this trial-and-error optimization process. The program includes an algorithm that has been developed for computationally expensive problems: so-called black-box simulation optimizations. It also includes a more efficient version of the Yang X-ray Optics Program (YXRO) [Yang, S.-H., Gray, A.X., Kaiser, A.M., Mun, B.S., Sell, B.C., Kortright, J.B., Fadley, C.S., J. Appl. Phys. 113, 1 (2013)] which is about an order of magnitude faster than the original version. Human interaction is not required during optimization. We tested our optimization algorithm on real and hypothetical problems and show that it finds better solutions significantly faster than a random search approach. The total optimization time ranges, depending on the sample structure, from minutes to a few hours on a modern computer, and can be up to 100x faster than a corresponding manual optimization. These speeds make the SWOPT program a valuable tool for real-time analyses of data during synchrotron experiments.
- Published
- 2019
15. Atomic-layer-resolved composition and electronic structure of the cuprate B i2 S r2CaC u2 O8+δ from soft x-ray standing-wave photoemission
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Kuo, CT, Lin, SC, Conti, G, Pi, ST, Moreschini, L, Bostwick, A, Meyer-Ilse, J, Gullikson, E, Kortright, JB, Nemšák, S, Rault, JE, Le Fèvre, P, Bertran, F, Santander-Syro, AF, Vartanyants, IA, Pickett, WE, Saint-Martin, R, Taleb-Ibrahimi, A, and Fadley, CS
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cond-mat.supr-con ,cond-mat.mtrl-sci ,cond-mat.str-el - Abstract
A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO2 layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to Bi2Sr2CaCu2O8+δ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large c represent a promising future direction.
- Published
- 2018
16. Interface properties and built-in potential profile of a LaCr O3/SrTi O3 superlattice determined by standing-wave excited photoemission spectroscopy
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Lin, SC, Kuo, CT, Comes, RB, Rault, JE, Rueff, JP, Nemšák, S, Taleb, A, Kortright, JB, Meyer-Ilse, J, Gullikson, E, Sushko, PV, Spurgeon, SR, Gehlmann, M, Bowden, ME, Plucinski, L, Chambers, SA, and Fadley, CS
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cond-mat.mtrl-sci - Abstract
LaCrO3(LCO)/SrTiO3(STO) heterojunctions are intriguing due to a polar discontinuity along [001], exhibiting two distinct and controllable charged interface structures [(LaO)+/(TiO2)0 and (SrO)0/(CrO2)-] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged (LaO)+/(TiO2)0 intermediate layer at the LCOtop/STObottom interface and a negatively charged (SrO)0/(CrO2)- intermediate layer at the STOtop/LCObottom interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies.
- Published
- 2018
17. Interface properties and built-in potential profile of a LaCrO3/SrTiO3 superlattice determined by standing-wave excited photoemission spectroscopy
- Author
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Lin, S-C, Kuo, C-T, Comes, RB, Rault, JE, Rueff, J-P, Nemšák, S, Taleb, A, Kortright, JB, Meyer-Ilse, J, Gullikson, E, Sushko, PV, Spurgeon, SR, Gehlmann, M, Bowden, ME, Plucinski, L, Chambers, SA, and Fadley, CS
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Physical Sciences ,Chemical Sciences ,Physical Chemistry ,cond-mat.mtrl-sci ,Chemical sciences ,Engineering ,Physical sciences - Abstract
LaCrO3(LCO)/SrTiO3(STO) heterojunctions are intriguing due to a polar discontinuity along [001], exhibiting two distinct and controllable charged interface structures [(LaO)+/(TiO2)0 and (SrO)0/(CrO2)-] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged (LaO)+/(TiO2)0 intermediate layer at the LCOtop/STObottom interface and a negatively charged (SrO)0/(CrO2)- intermediate layer at the STOtop/LCObottom interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies.
- Published
- 2018
18. Depth-resolved charge reconstruction at the LaNiO3/CaMnO3 interface
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Chandrasena, RU, Flint, CL, Yang, W, Arab, Arian, Nemšák, S, Gehlmann, M, Özdöl, VB, Bisti, F, Wijesekara, KD, Meyer-Ilse, J, Gullikson, E, Arenholz, E, Ciston, J, Schneider, CM, Strocov, VN, Suzuki, Y, and Gray, AX
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Engineering ,Physical Sciences ,Condensed Matter Physics ,Chemical sciences ,Physical sciences - Abstract
Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic LaNiO3 and antiferromagnetic CaMnO3 exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the LaNiO3/CaMnO3 interface. Our measurements reveal an increased concentration of Mn3+ and Ni2+ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the Mn4+-Mn3+ ferromagnetic double exchange and Ni2+-O-Mn4+ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.
- Published
- 2018
19. Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator
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Marinova, M., Rault, J. E., Gloter, A., Nemsak, S., Palsson, G. K., Rueff, J. -P., Fadley, C. S., Carretero, C., Yamada, H., March, K., Garcia, V., Fusil, S., Barthelemy, A., Stephan, O., Colliex, C., and Bibes, M.
- Subjects
Condensed Matter - Materials Science - Abstract
The electric field control of functional properties is a crucial goal in oxide-based electronics. Non-volatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab-initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects., Comment: Work supported by ERC Consolidator grant MINT (Contract No. 615759)
- Published
- 2017
- Full Text
- View/download PDF
20. The Effects of Spin-Excitons on the Surface States of SmB6: A Photoemission Study
- Author
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Arab, Arian, Gray, A. X., Nemsak, S., Evtushinsky, D. V., Schneider, C. M., Kim, Dae-Jeong, Fisk, Zachary, Rosa, P. F. S., Durakiewicz, T., and Riseborough, P. S.
- Subjects
Condensed Matter - Strongly Correlated Electrons - Abstract
We present the results of high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data is interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about -9 meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin-exciton may be responsible for the scattering which suppresses the formation of coherent surface quasi-particles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap.
- Published
- 2016
- Full Text
- View/download PDF
21. Characterization of free-standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy
- Author
-
Conti, G, Nemšák, S, Kuo, C-T, Gehlmann, M, Conlon, C, Keqi, A, Rattanachata, A, Karslıoğlu, O, Mueller, J, Sethian, J, Bluhm, H, Rault, JE, Rueff, JP, Fang, H, Javey, A, and Fadley, CS
- Subjects
cond-mat.mtrl-sci ,Electrical and Electronic Engineering ,Materials Engineering ,Mechanical Engineering - Abstract
Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS) and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with ångström precision. We determined that (i) the exposure to air induced the formation of an InAsO4 layer on top of the stoichiometric InAs(QM); (ii) the top interface between the air-side InAsO4 and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; (iii) the bottom interface between the InAs(QM) and the native oxide SiO2 on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO2/(Si/Mo) substrate was determined by HXPS. The value of VBO = 0.2 ± 0.04 eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO2 heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.
- Published
- 2018
22. Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO$_3$/SrTiO$_3$ Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
- Author
-
Eiteneer, D., Pálsson, G. K., Nemšák, S., Gray, A. X., Kaiser, A. M., Son, J., LeBeau, J., Conti, G., Greer, A. A., Keqi, A., Rattanachata, A., Saw, A. Y., Bostwick, A., Rotenberg, E., Gullikson, E. M., Ueda, S., Kobayashi, K., Janotti, A., Van de Walle, C. G., Blanca-Romero, A., Pentcheva, R., Schneider, C. M., Stemmer, S., and Fadley, C. S.
- Subjects
Condensed Matter - Strongly Correlated Electrons - Abstract
LaNiO$_3$ (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO$_3$ (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_3$ superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results.
- Published
- 2015
- Full Text
- View/download PDF
23. Effects of spin excitons on the surface states of SmB6: A photoemission study
- Author
-
Arab, A, Gray, AX, Nemšák, S, Evtushinsky, DV, Schneider, CM, Kim, DJ, Fisk, Z, Rosa, PFS, Durakiewicz, T, and Riseborough, PS
- Subjects
cond-mat.str-el - Abstract
We present the results of a high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data are interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about -9 meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin exciton may be responsible for the scattering, which suppresses the formation of coherent surface quasiparticles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap.
- Published
- 2016
24. Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO$_3$ embedded in GdTiO$_3$
- Author
-
Nemšák, S., Conti, G., Pálsson, G. K., Conlon, C., Cho, S., Rault, J., Avila, J., Asensio, M. -C., Jackson, C., Moetakef, P., Janotti, A., Bjaalie, L., Himmetoglu, B., Van de Walle, C. G., Balents, L., Schneider, C. M., Stemmer, S., and Fadley`, C. S.
- Subjects
Condensed Matter - Materials Science ,Condensed Matter - Strongly Correlated Electrons - Abstract
For certain conditions of layer thickness, the interface between GdTiO$_3$ (GTO) and SrTiO$_3$ (STO) in multilayer samples has been found to form a two-dimensional electron gas (2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ (LSAT), with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved (ARPES) and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening., Comment: 12 pages, 4 figures
- Published
- 2015
- Full Text
- View/download PDF
25. Energetic, spatial and momentum character of a buried interface: the two-dimensional electron gas between two metal oxides
- Author
-
Nemšák, S., Conti, G., Gray, A. X., Pálsson, G. K., Conlon, C., Eiteneer, D., Keqi, A., Rattanachata, A., Saw, A. Y., Bostwick, A., Moreschini, L., Strocov, V., Kobayashi, M., Stolte, W., Ueda, S., Kobayashi, K., Gloskovskii, A., Drube, W., Jackson, C., Moetakef, P., Janotti, A., Bjaalie, L., Himmetoglu, B., Van de Walle, C. G., Borek, S., Minár, J., Braun, J., Ebert, H., Plucinski, L., Kortright, J. B., de Groot, F. M. F., Schneider, C. M., Balents, L., Stemmer, S., and Fadley, C. S.
- Subjects
Condensed Matter - Materials Science ,Condensed Matter - Other Condensed Matter ,Condensed Matter - Strongly Correlated Electrons - Abstract
The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications, and are the subject of intense study in many disciplines. We here apply novel experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO$_3$ (STO) and GdTiO$_3$ (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells STO/3 unit cells of GTO]$_{20}$ using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been introduced as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG with unprecedented detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO, and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG, and suggesting that oxygen vacancies are not the origin of it. Similar multi-technique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry., Comment: 34 pages, 10 figures
- Published
- 2015
- Full Text
- View/download PDF
26. Satellite Band Structure in Silicon Caused by Electron-Plasmon Coupling
- Author
-
Lischner, Johannes, Palsson, G. K., Vigil-Fowler, Derek, Nemsak, S., Avila, J., Asensio, M. C., Fadley, C. S., and Louie, S. G.
- Subjects
Condensed Matter - Materials Science - Abstract
We report the first angle-resolved photoemission measurement of the wave-vector dependent plasmon satellite structure of a three-dimensional solid, crystalline silicon. In sharp contrast to nanomaterials, which typically exhibit strongly wave-vector dependent, low-energy plasmons, the large plasmon energy of silicon facilitates the search for a plasmaron state consisting of resonantly bound holes and plasmons and its distinction from a weakly interacting plasmon-hole pair. Employing a first-principles theory, which is based on a cumulant expansion of the one-electron Green's function and contains significant electron correlation effects, we obtain good agreement with the measured photoemission spectrum for the wave-vector dependent dispersion of the satellite feature, but without observing the existence of plasmarons in the calculations., Comment: 7 pages, 5 figures, accepted for publication in PRB
- Published
- 2015
- Full Text
- View/download PDF
27. Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS.
- Author
-
Balogun, K., Chukwunenye, P., Anwar, F., Ganesan, A., Adesope, Q., Willadsen, D., Nemšák, S., Cundari, T. R., Bagus, P. S., D'Souza, F., and Kelber, J. A.
- Subjects
VANADIUM oxide ,WATER vapor ,MOLECULAR interactions ,X-ray photoelectron spectroscopy ,OXIDE coating ,VANADIUM - Abstract
Interactions of N
2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction (NRR) mechanisms. Interactions of N2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10−1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N2 and equal pressures of N2 and H2 O vapor. In pure N2 , broad, partially resolved N1s features were observed at binding energies of 401.0 and 398.7 eV, with a relative intensity of ∼3:1, respectively. These features remained upon subsequent pumpdown to 10−9 Torr. The observed maximum N surface coverage was ∼1.5 × 1013 cm−2 —a fraction of a monolayer. In the presence of equal pressures of H2 O, the adsorbed N intensity at 10−1 Torr is ∼25% of that observed in the absence of H2 O. The formation of molecularly adsorbed H2 O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N2 bonding to both V(IV) and V(III) cation sites but less so for V(V) sites. Hartree–Fock-based cluster calculations for N2 –V end-on adsorption show that experimental XPS doublet features are consistent with the calculated shake-up and normal, final ionic configurations for N2 end-on bonding to V(III) sites but not V(IV) sites. The XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to the electrolyte under NRR-relevant conditions. [ABSTRACT FROM AUTHOR]- Published
- 2022
- Full Text
- View/download PDF
28. Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
- Author
-
Eiteneer, D., Pálsson, G.K., Nemšák, S., Gray, A.X., Kaiser, A.M., Son, J., LeBeau, J., Conti, G., Greer, A.A., Keqi, A., Rattanachata, A., Saw, A.Y., Bostwick, A., Rotenberg, E., Gullikson, E.M., Ueda, S., Kobayashi, K., Janotti, A., Van de Walle, C.G., Blanca-Romero, A., Pentcheva, R., Schneider, C.M., Stemmer, S., and Fadley, C.S.
- Published
- 2016
- Full Text
- View/download PDF
29. A photoemission study of carbon monoxide interaction with the Ga–Pd(110) system
- Author
-
Skála, T., Bača, D., Libra, J., Tsud, N., Nehasil, V., Nemšák, S., Prince, K.C., and Matolín, V.
- Published
- 2008
- Full Text
- View/download PDF
30. Structure of Pd/tungsten oxide epitaxial system
- Author
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Mašek, K., Nemšák, S., and Matolín, V.
- Published
- 2007
- Full Text
- View/download PDF
31. Study of the growth of supported Pd–Sn bimetallic nanoclusters
- Author
-
Mašek, K., Mixa, M., Nemšák, S., and Matolín, V.
- Published
- 2006
- Full Text
- View/download PDF
32. Structural study of epitaxial tungsten oxide nanoclusters
- Author
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Mašek, K., Nemšák, S., and Matolín, V.
- Published
- 2005
- Full Text
- View/download PDF
33. RHEED study of Pd/Al bimetallic thin film growth on NaCl (0 0 1) substrate
- Author
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Mašek, K, Moroz, V, Nemšák, S, and Matolı́n, V
- Published
- 2004
- Full Text
- View/download PDF
34. Boron Doped diamond films as electron donors in photovoltaics: An X-ray absorption and hard X-ray photoemission study.
- Author
-
Kapilashrami, M., Conti, G., Zegkinoglou, I., Nemšák, S., Conlon, C. S., Tärndahl, T., Fjällström, V., Lischner, J., Louie, Steven G., Hamers, R. J., Zhang, L., Guo, J.-H., Fadley, C. S., and Himpse, F. J.
- Subjects
DIAMOND films ,DOPED semiconductors ,BORON ,SOLAR cells ,TRANSPARENT electronics ,ELECTRON donors ,PHOTOVOLTAIC cells ,X-ray photoelectron spectroscopy - Abstract
Highly boron-doped diamond films are investigated for their potential as transparent electron donors in solar cells. Specifically, the valence band offset between a diamond film (as electron donor) and Cu(In,Ga)Se
2 (CIGS) as light absorber is determined by a combination of soft X-ray absorption spectroscopy and hard X-ray photoelectron spectroscopy, which is more depthpenetrating than standard soft X-ray photoelectron spectroscopy. In addition, a theoretical analysis of the valence band is performed, based on GW quasiparticle band calculations. The valence band offset is found to be small: VBO = VBMCIGS - VBMdiamond = 0.3 eV ± 0.1 eV at the CIGS/Diamond interface and 0.0 eV±0.1 eV from CIGS to bulk diamond. These results provide a promising starting point for optimizing the band offset by choosing absorber materials with a slightly lower valence band maximum. [ABSTRACT FROM AUTHOR]- Published
- 2014
- Full Text
- View/download PDF
35. Effects of spin excitons on the surface states of SmB6 : A photoemission study
- Author
-
Arab, Arian, Gray, AX, Nemšák, S, Evtushinsky, DV, Schneider, CM, Kim, Dae-Jeong, Fisk, Zachary, Rosa, PFS, Durakiewicz, T, and Riseborough, PS
- Subjects
cond-mat.str-el - Abstract
© 2016 American Physical Society. We present the results of a high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data are interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about -9 meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin exciton may be responsible for the scattering, which suppresses the formation of coherent surface quasiparticles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap.
- Published
- 2016
36. Electronic structure of the dilute magnetic semiconductor Ga1−xMnxP from hard x-ray photoelectron spectroscopy and angle-resolved photoemission
- Author
-
Keqi, A., primary, Gehlmann, M., additional, Conti, G., additional, Nemšák, S., additional, Rattanachata, A., additional, Minár, J., additional, Plucinski, L., additional, Rault, J. E., additional, Rueff, J. P., additional, Scarpulla, M., additional, Hategan, M., additional, Pálsson, G. K., additional, Conlon, C., additional, Eiteneer, D., additional, Saw, A. Y., additional, Gray, A. X., additional, Kobayashi, K., additional, Ueda, S., additional, Dubon, O. D., additional, Schneider, C. M., additional, and Fadley, C. S., additional
- Published
- 2018
- Full Text
- View/download PDF
37. Energetic, spatial, and momentum character of the electronic structure at a buried interface: The two-dimensional electron gas between two metal oxides
- Author
-
Nemšák, S., Conti, G., Gray, A. X., Palsson, G. K., Conlon, C., Eiteneer, D., Keqi, A., Rattanachata, A., Saw, A. Y., Bostwick, A., Moreschini, L., Rotenberg, E., Strocov, V. N., Kobayashi, M., Schmitt, T., Stolte, W., Ueda, S., Kobayashi, K., Gloskovskii, A., Drube, W., Jackson, C. A., Moetakef, P., Janotti, A., Bjaalie, L., Himmetoglu, B., Van De Walle, C. G., Borek, S., Minar, J., Braun, J., Ebert, H., Plucinski, L., Kortright, J. B., Schneider, C. M., Balents, L., De Groot, F. M F, Stemmer, S., Fadley, C. S., Sub Inorganic Chemistry and Catalysis, Inorganic Chemistry and Catalysis, Sub Inorganic Chemistry and Catalysis, and Inorganic Chemistry and Catalysis
- Subjects
Physics ,Condensed matter physics ,Inverse photoemission spectroscopy ,Angle-resolved photoemission spectroscopy ,02 engineering and technology ,Electronic structure ,Condensed Matter Physics ,021001 nanoscience & nanotechnology ,01 natural sciences ,Resonance (particle physics) ,Electronic, Optical and Magnetic Materials ,Characterization (materials science) ,Momentum ,0103 physical sciences ,Electronic ,ddc:530 ,Optical and Magnetic Materials ,010306 general physics ,0210 nano-technology ,Fermi gas ,Excitation - Abstract
The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO3 (STO) and GdTiO3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells (u.c.) STO/3 u.c. of GTO](20) using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.
- Published
- 2016
38. Contact-Free Mapping of Electronic Transport Phenomena of Polar Domains inSrMnO3Films
- Author
-
Schaab, J., primary, Krug, I. P., additional, Doğanay, H., additional, Hackl, J., additional, Gottlob, D. M., additional, Khan, M. I., additional, Nemšák, S., additional, Maurel, L., additional, Langenberg, E., additional, Algarabel, P. A., additional, Pardo, J. A., additional, Schneider, C. M., additional, and Meier, D., additional
- Published
- 2016
- Full Text
- View/download PDF
39. Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO3 embedded in GdTiO3
- Author
-
Nemšák, S., primary, Conti, G., additional, Palsson, G. K., additional, Conlon, C., additional, Cho, S., additional, Rault, J. E., additional, Avila, J., additional, Asensio, M.-C., additional, Jackson, C. A., additional, Moetakef, P., additional, Janotti, A., additional, Bjaalie, L., additional, Himmetoglu, B., additional, Van de Walle, C. G., additional, Balents, L., additional, Schneider, C. M., additional, Stemmer, S., additional, and Fadley, C. S., additional
- Published
- 2015
- Full Text
- View/download PDF
40. Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy
- Author
-
Karslıoğlu, O., primary, Nemšák, S., additional, Zegkinoglou, I., additional, Shavorskiy, A., additional, Hartl, M., additional, Salmassi, F., additional, Gullikson, E. M., additional, Ng, M. L., additional, Rameshan, Ch., additional, Rude, B., additional, Bianculli, D., additional, Cordones, A. A., additional, Axnanda, S., additional, Crumlin, E. J., additional, Ross, P. N., additional, Schneider, C. M., additional, Hussain, Z., additional, Liu, Z., additional, Fadley, C. S., additional, and Bluhm, H., additional
- Published
- 2015
- Full Text
- View/download PDF
41. Band offsets in complex-oxide thin films and heterostructures of SrTiO3/LaNiO3 and SrTiO3/GdTiO3 by soft and hard X-ray photoelectron spectroscopy
- Author
-
Conti, G., primary, Kaiser, A. M., additional, Gray, A. X., additional, Nemšák, S., additional, Pálsson, G. K., additional, Son, J., additional, Moetakef, P., additional, Janotti, A., additional, Bjaalie, L., additional, Conlon, C. S., additional, Eiteneer, D., additional, Greer, A. A., additional, Keqi, A., additional, Rattanachata, A., additional, Saw, A. Y., additional, Bostwick, A., additional, Stolte, W. C., additional, Gloskovskii, A., additional, Drube, W., additional, Ueda, S., additional, Kobata, M., additional, Kobayashi, K., additional, Van de Walle, C. G., additional, Stemmer, S., additional, Schneider, C. M., additional, and Fadley, C. S., additional
- Published
- 2013
- Full Text
- View/download PDF
42. Depth profiling of ultra-thin alumina layers grown on Co(0001)
- Author
-
Nemšák, S, primary, Skála, T, additional, Yoshitake, M, additional, Prince, K C, additional, and Matolín, V, additional
- Published
- 2013
- Full Text
- View/download PDF
43. Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO3 embedded in GdTiO3.
- Author
-
Nemšák, S., Conti, G., Palsson, G. K., Conlon, C., Cho, S., Rault, J. E., Avila, J., Asensio, M.-C., Jackson, C. A., Moetakef, P., Janotti, A., Bjaalie, L., Himmetoglu, B., Van de Walle, C. G., Balents, L., Schneider, C. M., Stemmer, S., and Fadley, C. S.
- Subjects
- *
STRONTIUM compounds , *GADOLINIUM compounds , *PHOTOEMISSION , *RESONANCE , *ELECTRON gas , *OPTICAL properties of metals - Abstract
For certain conditions of layer thickness, the interface between GdTiO3 (GTO) and SrTiO3 (STO) in multilayer samples has been found to form a two-dimensional electron gas(2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7, with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening. [ABSTRACT FROM AUTHOR]
- Published
- 2015
- Full Text
- View/download PDF
44. Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO3 embedded in GdTiO3.
- Author
-
Nemšák, S., Conti, G., Palsson, G. K., Conlon, C., Cho, S., Rault, J. E., Avila, J., Asensio, M.-C., Jackson, C. A., Moetakef, P., Janotti, A., Bjaalie, L., Himmetoglu, B., Van de Walle, C. G., Balents, L., Schneider, C. M., Stemmer, S., and Fadley, C. S.
- Subjects
STRONTIUM compounds ,GADOLINIUM compounds ,PHOTOEMISSION ,RESONANCE ,ELECTRON gas ,OPTICAL properties of metals - Abstract
For certain conditions of layer thickness, the interface between GdTiO
3 (GTO) and SrTiO3 (STO) in multilayer samples has been found to form a two-dimensional electron gas(2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO3 )0.3(Sr2 AlTaO6 )0.7 , with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening. [ABSTRACT FROM AUTHOR]- Published
- 2015
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45. The growth of Au/Pd on alumina/Cu-Al system
- Author
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Nemšák, S, primary, Skála, T, additional, Libra, J, additional, Mašek, K, additional, Cabala, M, additional, and Matolín, V, additional
- Published
- 2008
- Full Text
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46. Photoemission spectroscopy and electron diffraction study of Pd/tungsten oxide/W(110) epitaxial system
- Author
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Mašek, K, primary, Nemšák, S, additional, Mravcáková, M, additional, Blumentrit, P, additional, Skála, T, additional, Škoda, M, additional, and Matolín, V, additional
- Published
- 2008
- Full Text
- View/download PDF
47. Effects of spin excitons on the surface states of SmB6: A photoemission study.
- Author
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Arab, Arian, Gray, A. X., Nemšák, S., Evtushinsky, D. V., Schneider, C. M., Dae-Jeong Kim, Fisk, Zachary, Rosa, P. F. S., Durakiewicz, T., and Riseborough, P. S.
- Subjects
- *
EXCITON theory , *SAMARIUM compounds , *SURFACE states - Abstract
We present the results of a high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data are interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about --9 meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin exciton may be responsible for the scattering, which suppresses the formation of coherent surface quasiparticles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
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48. Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.
- Author
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Kim P, Weeraratna C, Nemšák S, Dias N, Lemmens AK, Wilson KR, and Ahmed M
- Abstract
The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch
+ ) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch+ into the anti-conformer in relatively lower ChCl concentrations. Finally, the depletion of Ch+ from the interface (probing depth = 0.4 nm) is demonstrated by aerosol-based velocity map imaging XPS measurements of glyceline and water mixtures. The nanostructure of liquid-vapor interfaces and structural rearrangement by hydration can provide critical insight into the molecular origin of the deep eutectic behavior and gas-capturing application of DESs.- Published
- 2024
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49. Oxidation and Reduction of Polycrystalline Cerium Oxide Thin Films in Hydrogen.
- Author
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Ben Yaacov A, Falling LJ, Ben David R, Attia S, Andrés MA, Nemšák S, and Eren B
- Abstract
This study investigates the oxidation state of ceria thin films' surface and subsurface under 100 mTorr hydrogen using ambient pressure X-ray photoelectron spectroscopy. We examine the influence of the initial oxidation state and sample temperature (25-450 °C) on the interaction with hydrogen. Our findings reveal that the oxidation state during hydrogen interaction involves a complex interplay between oxidizing hydride formation, reducing thermal reduction, and reducing formation of hydroxyls followed by water desorption. In all studied conditions, the subsurface exhibits a higher degree of oxidation compared to the surface, with a more subtle difference for the reduced sample. The reduced samples are significantly hydroxylated and covered with molecular water at 25 °C. We also investigate the impact of water vapor impurities in hydrogen. We find that although 1 × 10
-6 Torr water vapor oxidizes ceria, it is probably not the primary driver behind the oxidation of reduced ceria in the presence of hydrogen.- Published
- 2023
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50. Vanadium oxide, vanadium oxynitride, and cobalt oxynitride as electrocatalysts for the nitrogen reduction reaction: a review of recent developments.
- Author
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Balogun K, Ganesan A, Chukwunenye P, Gharaee M, Adesope Q, Nemšák S, Bagus PS, Cundari TR, D'Souza F, and Kelber JA
- Abstract
The electrocatalytic reduction of molecular nitrogen to ammonia-the nitrogen reduction reaction (NRR)-is of broad interest as an environmentally- and energy-friendly alternative to the Haber-Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies that counter several commonly held assumptions regarding transition metal oxynitrides and oxides as NRR catalysts. Specifically, we find that for the vanadium oxide, vanadium oxynitride, and cobalt oxynitride systems, (a) there is no Mars-van Krevelen mechanism and that the reduction of lattice nitrogen and N
2 to NH3 occurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice; and (b) that NRR and the hydrogen evolution reaction do occur in concert under the conditions studied for Co oxynitride, but not for V oxynitride. Additionally, these results highlight the importance of both O-ligation of the V or Co center for metal-binding of dinitrogen, and the importance of N in stabilizing the transition metal cation in an intermediate oxidation state, for effective N≡N bond activation. This review also highlights the importance and limitations of ex situ and in situ photoemission-involving controlled transfer between ultra-high vacuum and electrochemistry environments, and of operando near ambient pressure photoemission coupled with in situ studies, in elucidating the complex chemistry relevant to the electrolyte/solid interface., (© 2023 IOP Publishing Ltd.)- Published
- 2023
- Full Text
- View/download PDF
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