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Applied physics letters 120(18), 181601 (2022). doi:10.1063/5.0080289, We report on the soft x-ray absorption spectroscopy investigation of thin film capacitors using a modified total electron yield detection mode. This mode utilizes two ammeters instead of one as commonly employed in the classical total electron yield scheme to measure photocurrents of devices under soft x-ray irradiation. The advantage of this configuration over the surface sensitive classical total electron yield mode is that it can provide information from buried layers and interfaces up to a thickness equal to the penetration depth of soft x-rays. The method can be easily adapted to existing synchrotron end stations. We investigate dielectric capacitors with dissimilar electrodes to assess the feasibility of the modified total electron yield method. Furthermore, in operando soft x-ray absorption spectroscopy measurements are performed on ferroelectric capacitors under bias and using two ammeters. The experimental results are discussed in terms of the external and internal photoemission processes and their distribution in thin film capacitors under an external bias condition. The proposed detection method opens the way to perform electronic and chemical state analyses of the buried interfaces and layers in various devices like multiferroic tunnel junctions, memristive devices, etc., during operation under an applied bias., Published by American Inst. of Physics, Melville, NY

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00314-9

The journal of physical chemistry / C 125(25), 14105-14116 (2021). doi:10.1021/acs.jpcc.1c03527, Spin-crossover complexes in direct contact with substrates have sparked considerable interest, in particular, in view of their potential applications in molecular electronics. While a huge number of spin-crossover complexes is available, many of them are not robust enough to withstand the sample preparation and/or the interaction with the substrate. The techniques usually employed for these investigations, namely, near-edge X-ray absorption fine structure spectroscopy and low-temperature scanning tunneling microscopy, are not adapted for systematic studies because of the limited access to synchrotron-radiation facilities and complexity of the (indirect) spin determination, respectively. Here, we detail a methodology using a (more) commonly available technique, namely, vacuum ultraviolet (angle-resolved) photoemission spectroscopy, to determine the spin state of layers of three different spin-crossover complexes with thicknesses down to the submonolayer regime. We present an approach to determine the thicknesses of the investigated layers, relying on the inelastic mean free path of electrons determined from combined photoemission and X-ray absorption measurements. We report on the high-spin to low-spin relaxation dynamics of spin-crossover (SCO) layers and the influence of the ultraviolet light on these dynamics. While the observed relaxation processes occur on a timescale on the order of minutes, probing spin-state dynamics on the picosecond timescale is foreseeable with pump–probe photoemission spectroscopy., Published by Soc., Washington, DC

ECS journal of solid state science and technology 9(6), 061005 - (2020). doi:10.1149/2162-8777/ab9e18, The influence of the contracted corrole macrocycle, in comparison to the larger porphyrin macrocycle, on the electronic structure of nickel was studied with X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Synthesis and in situ characterization of the Ni complexes of octaethylporphyrin (NiOEP) and hexaethyldimethylcorrole (NiHEDMC) were performed in ultra-high vacuum. XPS and NEXAFS spectra reveal a +2 oxidation state and a low-spin d8 electron configuration of Ni in both complexes, despite the formal trianionic nature of the corrole ligand. UPS, in combination with density functional theory (DFT) calculations, support the electronic structure of a Ni(II) corrole with a π-radical character of the ligand. The NEXAFS spectra also reveal differences in the valence electronic structure, which are attributed to the size mismatch between the small Ni(II) center and the larger central cavity of NiOEP. Analysis of the gas-phase structures shows that the Ni−N bonds in NiOEP are 4%–6% longer than those in NiHEDMC, even when NiOEP adopts a ruffled conformation. The individual interactions that constitute the Ni−ligand bond are altogether stronger in the corrole complex, according to bonding analysis within the energy decomposition analysis and the natural orbitals for chemical valence theory (EDA-NOCV)., Published by ECS, Pennington, NJ

The journal of physical chemistry letters 9(7), 1491 - 1496 (2018). doi:10.1021/acs.jpclett.8b00338, Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF$_3$)$_2$)$_2$(phen)] (pypyr = 2-(2′-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex., Published by ACS, Washington, DC

The layered van der Waals antiferromagnet MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators such as Bi$_2$Te$_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi$_2$Te$_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure., Revised version

This corrects the article DOI: 10.1103/PhysRevLett.120.166401.

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide $1T$-$\mathrm{Ta}{\mathrm{S}}_{2}$, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping $\ensuremath{\hbar}/J\ensuremath{\approx}14\text{ }\text{ }\mathrm{fs}$. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.

We investigate the perturbation and subsequent recovery of the correlated electronic ground state of the Mott insulator 1T-TaS 2 by means of femtosecond time-resolved photoemission spectroscopy in normal emission geometry. Upon an increase of near-infrared excitation strength, a considerable collapse of the occupied Hubbard band is observed, which reflects a quench of short-range correlations. It is furthermore found that these excitations are directly linked to the lifting of the periodic lattice distortion which provides the localization centers for the formation of the insulating Mott state. We discuss the observed dynamics in a localized real-space picture.

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS_{2}, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.

Physical review / B 96(24), 241408 (2017). doi:10.1103/PhysRevB.96.241408, $MoTe_2$ has recently been shown to realize in its low-temperature phase the type-II Weyl semimetal (WSM). We investigated by time- and angle- resolved photoelectron spectroscopy (tr-ARPES) the possible influence of the Weyl points on the electron dynamics above the Fermi level $E_F$, by comparing the ultrafast response of $MoTe_2$in the trivial and topological phases. In the low-temperature WSM phase, we report an enhanced relaxation rate of electrons optically excited to the conduction band, which we interpret as a fingerprint of the local gap closure when Weyl points form. By contrast, we find that the electron dynamics of the related compound $WTe_2$is slower and temperature independent, consistent with a topologically trivial nature of this material. Our results shows that tr-ARPES is sensitive to the small modifications of the unoccupied band structure accompanying the structural and topological phase transition of $MoTe_2$., Published by Inst., Woodbury, NY

Die elektronische Struktur des FeII-Spincrossover-Komplexes [Fe(H2bpz)2(phen)], deponiert als ultradunner Film auf Au(111), wird mithilfe von UV-Photoelektronenspektroskopie (UPS) im High-Spin- und im Low-Spin-Zustand bestimmt. Dies ermoglicht es auch, den thermischen sowie den photoinduzierten Spinubergang in diesem System zu verfolgen. Der Komplex wird ebenfalls durch Bestrahlung mit Vakuum-UV-Licht in den metastabilen High-Spin-Zustand uberfuhrt. Relaxationsraten nach Photoanregung werden als Funktion der Temperatur bestimmt. Sie zeigen einen Ubergang von thermisch aktiviertem zu Tunnelverhalten und sind zwei Grosenordnungen hoher als im Volumenmaterial.

The electronic structure of the iron(II) spin crossover complex [Fe(H2bpz)2(phen)] deposited as an ultrathin film on Au(111) is determined by means of UV-photoelectron spectroscopy (UPS) in the high-spin and in the low-spin state. This also allows monitoring the thermal as well as photoinduced spin transition in this system. Moreover, the complex is excited to the metastable high-spin state by irradiation with vacuum-UV light. Relaxation rates after photoexcitation are determined as a function of temperature. They exhibit a transition from thermally activated to tunneling behavior and are two orders of magnitude higher than in the bulk material.

A previous time-resolved optical study reported on a metastable hidden electronic state in 1T-TaS2, which is only accessible upon photoexcitation and created under non-equilibrium conditions [1]. The properties of such a state are distinct from those of any other state in the equilibrium phase diagram and it is possible to revert to the thermodynamic initial state either by illuminating with picosecond laser pulses or by applying other thermal erase procedures. In this work we show photoinduced switching to a metastable hidden state on the same material, and probe it by means of both static and time-resolved photoemission spectroscopy, thus having direct access to the electronic structure of the system. From our experimental findings and comparison with other studies, we conclude that we obtain partial switching, leading to a hidden state with persisting insulating nature but significant modifications in the electronic structure and CDW ordering.

Metal-ferroelectric interfaces form the basis of novel electronic devices. A key effect determining the device functionality is the bias-dependent change of the electronic energy-level alignment at the interface. Here, hard x-ray photoelectron spectroscopy (HAXPES) is used to determine the energy-level alignment at two metal-ferroelectric interfaces—Au versus SrRuO3 on the relaxor ferroelectric Pb(Mg1/3 Nb2/3 )0.72 Ti0.28 O3 (PMN-PT)- directly in situ as a function of electrical bias. The bias-dependent average shifts of the PMN-PT core levels are found to have two dominant contributions on the 0.1–1-eV energy scale: one depending on the metal electrode and the remanent electric polarization and the other correlated with electric-field-induced strain. Element-specific deviations from the average shifts are smaller than 0.1 eV and appear to be related to predicted dynamical charge variations in PMN-PT. In addition, the efficiency of ferroelectric polarization switching is shown to be reduced near the coercive field under x-ray irradiation. The results establish HAXPES as a tool for the in operando investigation of metal-ferroelectric interfaces and suggest electric-field-induced modifications of the polarizationdistribution as a novel way to control the barrier height at such interfaces.

The manipulation of low-energy beams of neutral atoms and molecules via their de Broglie wavelength is a branch of atom optics often referred to as de Broglie matter wave optics. The application areas include fundamental quantum mechanics, atom interferometry, and the development of new microscopy instrumentation. The focusing of de Broglie matter waves with a Fresnel zone plate was used to demonstrate the first neutral helium microscopy imaging. The ultimate resolution of such a microscope is limited by the width of the outermost zone. Because a Fresnel zone plate for atoms cannot be fabricated on a substrate (the low-energy atom beams would not be able to penetrate the substrate material), this gives a fabrication determined limit for the first-order focus of around 30--50 nm. Therefore, it is important to search for alternative optical elements that enable higher resolution. Photon sieves consist of a large number of pinholes, arranged suitably relative to the Fresnel zones. The great advantages are that the width of the pinholes can be larger than the respective Fresnel zones and a free-standing pinhole is much easier to fabricate than a free-standing zone. Thus, with a photon-sieve structure applied for de Broglie matter wave manipulation, the fabrication limit for focusing is reduced to potentially around 3--5 nm. Here we present a realization of such an ``atom sieve,'' which we fabricated out of a silicon nitride (SiN) membrane, using electron-beam lithography and reactive ion etching. Our atom sieve is 178 $\ensuremath{\mu}\mathrm{m}$ in diameter and has 31 991 holes. The diameter of the holes varies from 1840 to 150 nm. Using a beam of neutral, ground-state helium atoms with an average wavelength of 0.055 nm, we demonstrate helium atom focusing down to a spot size of less than 4 $\ensuremath{\mu}\mathrm{m}$. The focus size is limited by the intrinsic velocity spread of the helium beam.

Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the Mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time- and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. Specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-TaS(2) is a Peierls insulator and that the ultrafast response of 1T-TiSe(2) is highly suggestive of an excitonic insulator.

In situ ultraviolet photoelectron spectroscopy is used to study the growth of ultrathin films of azobenzene-based functional molecules (azobenzene, Disperse Orange 3 and a triazatriangulenium platform with an attached functional azo-group) on the layered metal TiTe(2) and on the layered semiconductor HfS(2) at liquid nitrogen temperatures. Effects of intermolecular interactions, of the substrate electronic structure, and of the thermal energy of the sublimated molecules on the growth process and on the adsorbate electronic structure are identified and discussed. A weak adsorbate-substrate interaction is particularly observed for the layered semiconducting substrate, holding the promise of efficient molecular photoswitching.

Angle-resolved photoemission spectroscopy at a temperature of 1 K is used to determine the wave-vector dependence of the spectral gap and band renormalization due to electron-phonon coupling in the layered chargedensity-wave superconductor 2H -NbSe2. The measured gap size and coupling parameter are Fermi-surface-sheet dependent and anisotropic. The largest energy gap, highest coupling strength, and strongest variation in both quantities are found on the double-walled Nb 4d-derived Fermi-surface sheet that is centered on the corners of the hexagonal Brillouin zone. On this sheet, the spectral gap has two distinct anticorrelated components associated with superconductivity and the charge-density wave. The results establish 2H -NbSe2 as a moderately correlated intermediate-coupling anisotropic multigap superconductor.

The technique of time-resolved pump-probe x-ray photoelectron spectroscopy using the free-electron laser in Hamburg (FLASH) is described in detail. Particular foci lie on the macrobunch resolving detection scheme, the role of vacuum space-charge effects and the synchronization of pump and probe lasers. In an exemplary case study, the complete Ta 4f core-level dynamics in the layered charge-density-wave (CDW) compound 1T-TaS2 in response to impulsive optical excitation is measured on the sub-picosecond to nanosecond timescale. The observed multi-component dynamics is related to the intrinsic melting and reformation of the CDW as well as to extrinsic pump-laser-induced vacuum space-charge effects.

Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics of the charge-density wave in the Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1 ps, the system is driven across a phase transition from a long-range charge ordered state to a quasiequilibrium state with domainlike short-range charge and lattice order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic-site selectivity.

Angle-resolved photoelectron spectroscopy (ARPES) is widely used to study the electronic structure of crystalline solids such as high-temperature superconductors, topological insulators and graphene-based materials. Time-resolved ARPES has opened the door to the study of the response of such electronic features on ultrafast timescales. Now Rohwer et al. add a new dimension. Using high photon energies, they are able to study ultrafast dynamics at high momenta, at which some of the most interesting fundamental phenomena occur. Applying the technique to the charge density wave material 1T-TiSe2, they obtain stroboscopic images of the electronic band structure at high momentum and show that atomic-scale periodic long-scale order collapses on a surprisingly short timescale of 20 femtoseconds. This work reveals rapid response times in photoinduced properties that could stimulate research into new types of ultrafast switching device. Angle-resolved photoemission spectroscopy (ARPES) is widely used to study the electronic structure of a wide range of correlated materials. Time-resolved ARPES allows the study of the response of such electronic features on ultrafast timescales; this paper now adds an exciting new dimension by using high photon energies that allow the study of ultrafast dynamics at high momenta, where often the most interesting fundamental phenomena occur. The technique is applied to the charge density wave material 1T-TiSe2 and it is shown with stroboscopic imaging of the electronic band structure at high momentum that atomic-scale periodic long-range order collapses on a surprisingly short timescale of 20 femtoseconds. Intense femtosecond (10−15 s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion1,2,3. This technique is particularly useful in the study4,5 and in the control6 of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context a direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom7,8,9,10. Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy10,11. Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe2 (refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solid-state transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems14, we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.

In situ photoelectron spectroscopy is used to study the adsorption and photoisomerization of azobenzene multilayers on the layered semiconductor HfS 2 at liquid nitrogen temperatures. The measured valence band spectra indicate weak molecule–substrate coupling and provide evidence for reversible switching of azobenzene multilayers by light with different wavelengths. The photoswitching manifests itself in spectral shifts due to changes in the electrical surface conductance and in modifications of the electronic structure consistent with the results of outer valence Green’s function calculations. The photoemission results appear to establish azobenzene as an optoelectrical molecular switch.

Photoemission microspectroscopy on the layered misfit compound $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_{2}$ provides direct evidence for Ta substitution into PbS layers as well as for Pb substitution into ${\mathrm{TaS}}_{2}$ layers. This metal cross substitution alters the charge balance between alternating layers and can explain the remarkable stability of $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_{2}$ and, possibly, of analogous misfit compounds. It is suggested that even formally stoichiometric misfit compounds can be stabilized by this mechanism.

Time- and angle-resolved photoemission spectroscopy with a high–harmonic- generation source is used to classify the potential excitonic insulator 1T -TiSe2 and the reference Peierls-Mott insulator 1T -TaS2 on the basis of the melting times of “spectroscopic order parameters”.

An advanced type of diffractive optical element is presented that combines the concept of the photon sieve with an off-axis, off-normal incidence reflection geometry. Compared to transmission optical elements, the signal-to-background ratio is significantly increased by separating the first from other diffraction orders without drastically reducing the size of the smallest diffractive element. The reflection photon sieve produces sharp foci at maximum contrast and offers the advantages of effective heat dissipation and a large working space above the focal plane. Experimental results for a device working at a photon energy of 100 eV are presented and compared to theory.

Nature Communications 12(1), 3650 (2021). doi:10.1038/s41467-021-23727-3, Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids., Published by Nature Publishing Group UK, [London]