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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.

The electronic properties of buried magnetic nano-layers were studied using the linear magnetic dichroism in the angular distribution of photoemitted Fe, Co, and Mn 2p electrons from a CoFe–Ir78Mn22 multi-layered sample. The buried layers were probed using hard X-ray photoelectron spectroscopy, HAXPES, at the undulator beamline P09 of the 3rd generation storage ring PETRA III. The results demonstrate that this magnetometry technique can be used as a sensitive element specific probe for magnetic properties suitable for application to buried ferromagnetic and antiferromagnetic magnetic materials and multilayered spintronics devices. Using the same instrument, spin-resolved Fe 2p HAXPES spectra were obtained from the buried layer with good signal quality.

Co2-based Heusler compounds with 27.8 valence electrons exhibit an exceptional electronic structure that makes them interesting materials for the application in spintronics. Co2Cr0.6Fe0.4Al is the most prominent example of this particular family of compounds. In this article new materials of this class are tested with respect to their structural and magnetic properties. X-ray diffraction, Mossbauer spectroscopy, energy dispersive X-ray spectroscopy, and SQUID magnetometry were carried out to characterize the compounds. The use of Co2Fe0.45Ti0.55Ge as a new material in spintronic devices is suggested.

The effect of the simultaneous substitution of cobalt by nickel and antimony by tin in the solid solution TiCo1–xNixSnxSb1–x was systematically investigated. The number of valence electrons does not change by this substitution and therefore the resistivity stays semimetallic or semiconducting. The series were synthesized by arcmelting and the thermoelectric properties were determined. It was found out that the substitution of cobalt and antimony by nickel and tin reduces the thermal conductivity to 2 W·m–1·K–1 at 400 K. The reduction is caused by titanium rich prolate micro structures that were found by energy dispersive X-ray spectroscopy investigations. The Seebeck coefficient and the resistivity increase with increasing amounts of cobalt and antimony. The highest Power factor was found for the composition TiCo0.8Ni0.2Sn0.8Sb0.2 at 900 K with the value of 9.2 μW·cm–1·K–1.

Atomically Controlled Surfaces, Interfaces and Nanostructures 13th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructures, ACSIN 2016, Frascati, Rome, Italy, Italy, 9 Oct 2016 - 14 Oct 2016 ; Science Series 9, 206-207(2016)., Published by Superstripes Press, Rome 2016, Rome, Italy

We use bulk sensitive hard x-ray photoelectron spectroscopy to provide unambiguous evidence for a pseudogap in the density of states around the Fermi level in icosahedral Zn-Mg-Y and Zn-Mg-Dy quasicrystals, in agreement with our density functional theory calculations. The pseudogap in these Zn-based quasicrystals is less pronounced compared to the Al-based ones such as Al-Pd-Mn and Al-Cu-Fe [J. Nayak et al., Phys. Rev. Lett. 109, 216403 (2012)]. This observation is in agreement with transport studies that indicate a larger charge carrier concentration in the Zn-based quasicrystals. Compared to Zn-Mg-Dy, the pseudogap is somewhat deeper in Zn-Mg-Y. The larger width of the Mg 1s and Zn 2p core-level spectra in Zn-Mg-Y is explained by different configurations of the local atomic surrounding compared to Zn-Mg-Dy.

The magnitude of the spin polarization at the Fermi level of ferromagnetic materials at room temperature is a key property for spintronics. Investigating the Heusler compound Co$_2$MnSi a value of 93$\%$ for the spin polarization has been observed at room temperature, where the high spin polarization is related to a stable surface resonance in the majority band extending deep into the bulk. In particular, we identified in our spectroscopical analysis that this surface resonance is embedded in the bulk continuum with a strong coupling to the majority bulk states. The resonance behaves very bulk-like, as it extends over the first six atomic layers of the corresponding (001)-surface. Our study includes experimental investigations, where the bulk electronic structure as well as surface-related features have been investigated using spin-resolved photoelectron spectroscopy (SR-UPS) and for a larger probing depth spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The results are interpreted in comparison with first-principles band structure and photoemission calculations which consider all relativistic, surface and high-energy effects properly.

Time-of-flight photoemission electron microscopy was used to measure spatially resolved energy distribution curves of electrons emitted from Ag nanoparticle films with different mass thicknesses. Two-photon photoemission (2PPE) was induced by femtosecond laser pulse excitation with 3.1 eV photon energy and 200 fs pulse width. Regions of Ag nanoparticles with different average sizes and one region with a continuous 100 nm thick Ag film were deposited as a stepped wedge on a Si(1 1 1) substrate. Upon laser excitation the nanoparticle films exhibit a very high electron emission yield in the images, whereas the uncovered Si surface and the continuous Ag film are dark. The time-of-flight electron spectra obtained from the nanoparticle films are remarkably different from the spectra of the continuous film, well-known from literature. The nanoparticle spectra are up to a factor of 160 more intense than the spectrum of the continuous film. They reveal different widths, overall shape and a shift and broadening of the Fermi edge. The results are discussed in terms of Mie plasmon assisted two-photon photoemission of the nanoparticles.

The luminescent kinetic parameters of CsPbCl3 nanocrystals dispersed in matrices of the solid solutions Rb1−xCsxCl (x = 0.05–0.2) were studied under pulse synchrotron radiation excitation. Formation of CsPbCl3 nanocrystals is confirmed by the manifestation of the quantum size effect, which shows itself as a blue-shift of the exciton luminescence maximum and an essential shortening of the luminescence decay time for CsPbCl3 nanocrystals as compared to that for an excitonic luminescence of a bulk single crystal CsPbCl3. For the first time, using the atomic force microscopy method, the lead-containing nanocrystals embedded in a matrix of the Rb0.95Cs0.05Cl solid solution were directly observed.

Heusler compounds are promising candidates for future spintronics device applications. The electronic and magnetic properties of Co2Cr0.6Fe0.4Al, an electron-doped derivative of Co2CrAl, are investigated using circularly polarized synchrotron radiation and photoemission electron microscopy (PEEM). Element specific imaging reveals needle shaped Cr rich phases in a homogeneous bulk of the Heusler compound. The ferromagnetic domain structure is investigated on an element-resolved basis using x-ray magnetic circular dichroism (XMCD) contrast in PEEM. The structure is characterized by micrometre-size domains with a superimposed fine ripple structure; the lateral resolution in these images is about 100 nm. The domains look identical for Co and Fe giving evidence of a ferromagnetic coupling of these elements. No ferromagnetic contrast is observed at the Cr line. Magnetic spectroscopy exploiting XMCD reveals that the lack of magnetic moment, detected in a SQUID magnetometer, is mainly due to the moment of the Cr atom.

Investigations of luminescence kinetics properties of CSPbCl 3 microcrystals dispersed in RbPbCl 3 matrices have been carried out under excitation with synchrotron radiation in the energy region E exc = 4-20 eV. Different time components are registered in the decay kinetics of CsPbCl 3 microcrystals, appearing due to the direct excitation of the microcrystals or reabsorption of the intrinsic emission of the RbPbCl 3 matrix. A shortening of the luminescence decay time of the CsPbCl 3 microcrystals with respect to that observed for a CsPbCl 3 single crystal can be explained as evidence of the quantum size effect.

The luminescent kinetic parameters of the CsPbCl3 nanocrystals dispersed in matrices of solid solutions Rb1-xCsxCl (x=0.05-0.2) were studied under pulse synchrotron radiation excitation. Formation of CsPbCl3 nanocrystals is confirmed by manifestation of the quantum size effect, which is displayed by a short-wave shift of the exciton luminescence maximum and an essential shortening of the luminescence decay time for of CsPbCl3 nanocrystals as compared to that for an excitonic luminescence of a bulk single crystal CsPbCl3. For the first time, using the atomic force microscopy method, the lead-containing nanocrystals embedded in a matrix of the Rb0.95Cs0.05Cl solid solution have been directly observed.

10 mars 2015; International audience; no abstract

The half-metallic Heusler compound Co2MnSi is a very attractive material for spintronic devices because it exhibits very high tunnelling magnetoresistance ratios. This work reports on a spectroscopic investigation of thin Co2MnSi films as they are used as electrodes in magnetic tunnel junctions. The investigated films exhibit a remanent in-plane magnetization with a magnetic moment of about 5 μBwhen saturated, as expected. The low coercive field of only 4 mT indicates soft magnetic behavior. Magnetic dichroism in emission and absorption was measured at the Co and Mn 2p core levels. The photoelectron spectra were excited by circularly polarized hard X-rays with an energy of 6 keV and taken from the remanently magnetized film. The soft X-ray absorption spectra were taken in an induction field of 4 T. Both methods yielded large dichroism effects. An analysis reveals the localized character of the electrons and magnetic moments attributed to the Mn atoms, whereas the electrons related to the Co atoms contribute an itinerant part to the total magnetic moment.

Luminescence spectral and kinetic characteristics have been studied under the synchrotron excitation in the energy range of 4–20 eV for CsPbCl 3 microcrystals dispersed in a PbCl 2 crystal matrix. In the luminescence decay kinetics of the CsPbCl 3 microcrystals, different components have been recorded and connected with the direct excitation of the microcrystals and reabsorption of the self-trapped exciton emission of a PbCl 2 matrix. The decrease of the luminescence decay time of the CsPbCl 3 microcrystals as compared with that characteristic of a CsPbCl 3 single crystal may be caused by the quantum size effect.

We present an electronic structure study of a magnetic oxide/ semiconductor model system, EuO on silicon, which is dedicated for efficient spin injection and spin detection in silicon-based spintronics devices. A combined electronic structure analysis of Eu core levels and valence bands using hard X-ray photoemission spectroscopy was performed to quantify the nearly ideal stoichiometry of EuO “spin filter” tunnel barriers directly on silicon, and the absence of silicon oxide at the EuO/Si interface. These results provide evidence for the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Hard X-ray photoemission spectroscopy of an Al/EuO/Si heterostructure probing the buried EuO and EuO/Si interface. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Ti/HfO2 interface plays a major role for resistance switching performances. However, clear interface engineering strategies to achieve reliable and reproducible switching have been poorly investigated. For this purpose, we present a comprehensive study of the Ti/HfO2 interface by a combined experimental-theoretical approach. Based on the use of oxygen-isotope marked Hf*O2, the oxygen scavenging capability of the Ti layer is clearly proven. More importantly, in line with ab initio theory, the combined HAXPES-Tof-SIMS study of the thin films deposited by MBE clearly establishes a strong impact of the HfO2 thin film morphology on the Ti/HfO2 interface reactivity. Low-temperature deposition is thus seen as a RRAM processing compatible way to establish the critical amount of oxygen vacancies to achieve reproducible and reliable resistance switching performances.

Luminescence spectral and kinetic characteristics of CsPbCl3 nanocrystals, dispersed in perovskite-like CsSrCl3 matrix, have been studied under excitation by synchrotron radiation in the UV–visible spectral region (4 to 20 eV). Various time components in the decay kinetics of CsPbCl3 nanocrystals appear due to direct excitation of nanocrystals or reabsorption of the luminescence of other emitting centres. Shortening of the luminescence decay time of the CsPbCl3 nanocrystals with respect to that observed for a CsPbCl3 single crystal has been explained as evidence of quantum-size effect.

The luminescence spectral and kinetic characteristics of CsPbCl3 nanocrystals, thermally created in a CsCl:Pb single crystal and dispersed in a CsCl matrix, have been studied under excitation by synchrotron radiation in the VUV spectral region (4 to 20 eV). Various components found in the decay kinetics of the 2.98 eV emission have been connected with the direct excitation of the CsPbCl3 luminescence and with the reabsorption of luminescence of other emitting centers by the CsPbCl3 nanocrystals. A high-energy shift of the free exciton emission band maximum of the CsPbCl3 nanocrystals with respect to that observed for a CsPbCl3 single crystal as well as the shortening of the luminescence decay time are considered from the point of view of quantum size effects.

Changes of the electronic structure accompanied by charge localization and a transition to an antiferromagnetic ground state were observed in the (DOEO)$_4$[HgBr$_4$]TCE organic semiconductor. Localization starts in the region of about 150 K and the antiferromagnetic state occurs below 60 K. The magnetic moment of the crystal contains contributions of antiferromagnetic inclusions (droplets), individual paramagnetic centers formed by localized holes and free charge carriers at 2 K. Two types of inclusions of 100-400 nm and 2-5 nm sizes were revealed by transmission electron microscopy. Studying the symmetry of the antiferromagnetic droplets (100-400 nm inclusions) and individual localized holes by electron spin resonance (ESR) revealed fingerprints of the antiferromagnetic resonance spectra of the spin correlated droplets as well as paramagnetic resonance spectra of the individual localized charge carriers. Photoelectron spectroscopy in the VUV, soft and hard X-ray range shows temperature-dependent effects upon crossing the critical temperature. The substantially different probing depths of soft and hard X-ray photoelectron spectroscopy yield information on the surface termination. The combined investigation using soft and hard X-ray photons to study the same sample results in details of electronic structure including structural aspects at the surface., Comment: 22 pages, 9 figures. arXiv admin note: text overlap with arXiv:0710.5586 by other authors

Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure., Spintronics requires materials in which most of the spins at the Fermi edge are aligned with each other at room temperatures. Jourdan et al. observe such a spin polarization of 93% in Co2MnSi—a Heusler alloy amenable to many spintronic applications; evidence of the material’s half-metallicity.

We present a detailed study of the electronic structure and band alignment in LaAlO${}_{3}$/SrTiO${}_{3}$ oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO${}_{3}$ film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO${}_{3}$ film and the SrTiO${}_{3}$ substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO${}_{3}$/SrTiO${}_{3}$ heterostructures.

Journal of applied physics 114(19), 194306 - (2013). doi:10.1063/1.4831953, Published by American Institute of Physics, Melville, NY

The bulk sensitivity of hard x-ray photoelectron spectroscopy in combination with circularly polarized radiation of the P09 beamline at PETRA III enables the investigation of the magnetic properties of capped films. We have determined the temperature dependence of the magnetic circular dichroism in the Fe 2$p$ and in the Gd 3$d$ states in amorphous GdFe films. The magnetic dichroism reflects the stronger temperature dependence of Gd moments compared to Fe moments in agreement with mean-field models. We resolved the exchange split Gd 3${d}_{5/2}$ substates and found a significant temperature dependence of the splitting which is attributed to a temperature dependent part of the exchange energy caused by Stoner-like itinerant states. A comparison of experimental and simulated x-ray photoemission spectra with corresponding data for x-ray absorption spectroscopy exciting the identical core holes clearly demonstrates that the core hole exchange interaction is the origin of the observed multiplet features for Gd.

The PERCIVAL soft X-ray imager is being developed by DESY, RAL, Elettra, DLS, and PAL to address the challenges at high brilliance Light Sources such as new-generation Synchrotrons and Free Electron Lasers. Typical requirements for detector systems at these sources are high frame rates, large dynamic range, single-photon counting capability with low probability of false positives, high quantum efficiency, and (multi)-mega-pixel arrangements. PERCIVAL is a monolithic active pixel sensor, based on CMOS technology. It is designed for the soft X-ray regime and, therefore, it is post-processed in order to achieve high quantum efficiency in its primary energy range (250 eV to 1 keV) . This work will report on the latest experimental results on charge collection efficiency obtained for multiple back-side-illuminated test sensors during two campaigns, at the P04 beam-line at PETRA III, and the CiPo beam-line at Elettra, spanning most of the primary energy range as well as testing the performance for photon-energies below 250 eV . In addition, XPS surface analysis was used to cross-check the obtained results.

The electronic structure of Heusler alloys having mixed magnetic phases, comprising of vicinal anti-ferromagnetic and ferromagnetic orders, is of great significance. We present the results of an electronic structure study on NixCu1−xMnSb Heusler alloys, using Mn-2p core-level photoemission spectroscopy. Room temperature data in the paramagnetic phase reveal a non-monotonic variation of both electron correlation strength and conduction-band hybridization such that the former enhances while the latter weakens for compositions showing a mixed phase relative to compositions at the phase boundaries to the ordered phases. The results suggest a possible electronic driving force for settling mixed-magnetic phases.

We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method., Comment: 15 pages, 7 figures, accepted for publication in Journal of Applied Physics

We use hard x-ray photoemission to resolve a controversial issue regarding the mechanism for the formation of quasicrystalline solids, i.e., the existence of a pseudogap at the Fermi level. Our data from icosahedral fivefold Al-Pd-Mn and Al-Cu-Fe quasicrystals demonstrate the presence of a pseudogap, which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase near the surface. In contrast to Al-Pd-Mn, we find that in Al-Cu-Fe the pseudogap is fully formed; i.e., the density of states reaches zero at E(F) indicating that it is close to the metal-insulator phase boundary.

The Heusler compound CoTiSb was synthesized and investigated theoretically and experimentally with respect to electronic structure and optical, mechanical, and vibrational properties. The optical properties were investigated in a wide spectral range from 10 meV to 6.5 eV and compared with ab initio calculations. The optical spectra confirm the semiconducting nature of CoTiSb, with a strong exciton absorption at 1.83 eV. The calculated phonon dispersion as well as elastic constants verify the mechanical stability of CoTiSb in the cubic $C{1}_{b}$ system. Furthermore, solid solution series of CoTi${}_{1\ensuremath{-}x}$${M}_{x}$Sb ($M=\text{Sc}$, V and $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.2$) were synthesized and investigated. The transport properties were calculated by all-electron ab initio methods and compared to the measurements. The thermoelectric properties were investigated by measuring the temperature dependence of electrical resistivity, Seebeck coefficient, and thermal conductivity. The thermal conductivity of the substituted compounds was significantly reduced. Sc substitution resulted in a $p$-type behavior with a high Seebeck coefficient of +177.8 $\ensuremath{\mu}$V/K (350 K) at 5$%$ Sc substitution. This value is in good agreement with the calculations. Fully relativistic Korringa--Kohn--Rostoker calculations in combination with the coherent potential approximation clarify the different contribution of states in the (001) plane of the Fermi surface for Sc- or V-substituted compounds CoTi${}_{0.95}$${M}_{x}$Sb ($M=\text{Sc}$, V).

The effect of charge transfer (CT) in complexes of the donors tetra - and hexamethoxyprene ( TMP and HMP ) with the classical acceptor tetracyanoquinodimethane ( TCNQ ) was studied using hard X-ray photoemission (HAXPES). Microcrystals of the complex were grown via vapour diffusion from donor–acceptor mixtures. The bulk sensitivity of HAXPES at a photon energy of 6 keV completely eliminates the problem of surface contamination for such delicate organic materials grown from solution. The donor molecules were produced using a novel synthesis route functionalizing polycyclic aromatic hydrocarbons at their periphery. For comparison, spectra were also taken from thin-film samples of the same compounds produced via co-deposition in UHV. Upon complex formation, the oxygen 1s core-level spectra (being a fingerprint of the methoxy -group of the donors) change from the single-line spectrum of pure HMP ( TMP ) to a spectrum with two distinct lines shifted by 1.4 (0.9) eV and 2.6 (2.3) eV with respect to the position of the oxygen 1s line of the pure donors. The nitrogen 1s spectra (being a fingerprint of the cyano -group in the acceptor) show two peaks as well with a corresponding shift of 0.9 eV and 2.0 eV in comparison with the leading line of pure TCNQ in opposite direction to the oxygen 1s spectra. These values are substantially larger than shifts in near edge X-ray absorption fine structure (NEXAFS) and ultraviolet photoelectron spectroscopy (UPS) spectra of the same complexes. The changes in the spectra are discussed in terms of the CT in the complexes. Residues of pure donor and acceptor materials in the microcrystal fractions of the complexes are evident from the presence of non-shifted lines. Peak-area analysis reveals that charge is transferred to a fraction of 60% of the molecules in the complexes.