Photoemission Spectroscopy

The band structure of the layered perovskite SrBi2Ta2O9 (SBT) was calculated by tight binding and the valence band density of states was measured by x-ray photoemission spectroscopy. We find both the valence and conduction band edges to consist of states primarily derived from the Bi–O layer rather than the perovskite Sr–Ta–O blocks. The valence band maximum arises from O p and some Bi s states, while the conduction band minimum consists of Bi p states, with a wide band gap of 5.1 eV. It is argued that the Bi–O layers largely control the electronic response whereas the ferroelectric response originates mainly from the perovskite Sr–Ta–O block. Bi and Ta centered traps are calculated to be shallow, which may account in part for its excellent fatigue properties.

Nonlinear photoemission from a silver single-crystal is investigated by femtosecond laser pulses in a perturbative regime. A clear observation of above-threshold photoemission in solids is reported for the first time. The ratio between the three-photon above-threshold and the two-photon Fermi edges is found to be 10 -4 . This value constitues the only available test-bench for theories aimed to understand the mechanism responsible for above-threshold photoemission in solids.

We show that, whereas the present log spiral of revolution monochromator works well for Cr edges of 2.8% Cr 2 0 3 in a V 2 0 3 matrix, the device transmits noticeable V extended structure in the case of 0.365% Cr 2 0 3 . We demonstrate that the transmitted V extended structure is due to the V Kp line which is unresolved by the monochromator. It is suggested that this limitation may be overcome by designing a log spiral detector for the Cr Kp line rather than the Cr IQ line. Aspects of the design of this modified log spiral are discussed.

In this study, different electronic structure evolutions of perovskite single crystals are found via angle-resolved photoelectron spectroscopy (ARPES): (i) unchanged top valence band (VB) dispersions under different temperatures can be found in the CH3NH3PbI3, (ii) phase transitions induced the evolution of top VB dispersions, and even a top VB splitting with Rashba effects can be observed in the CH3NH3PbBr3. Combined with low-energy electron diffraction (LEED), metastable atom electron spectroscopy (MAES), and DFT calculation, we confirm different band structure evolutions observed in these two perovskite single crystals are originated from the cleaved top surface layers, where the different surface geometries with CH3NH3 + -I in CH3NH3PbI3 and Pb-Br in CH3NH3PbBr3 are responsible for finding band dispersion change and appearing of the Rashba-type splitting. Such findings suggest that the top surface layer in organic halide perovskites should be carefully considered to create functional interfaces for developing perovskite devices.

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?

Three dimensional (3D) topological insulators are quantum materials with a spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena in the absence of applied magnetic fields. They feature surface states that are topologically protected against scattering by time reversal symmetry. The proposed applications of topological insulators in device geometries rely on the ability to tune the chemical potential on their surfaces in the vicinity of the Dirac node. Here, we demonstrate a suite of surface control methods based on a combination of photo-doping and molecular-doping to systematically tune the Dirac fermion density on the topological (111) surface of Bi 2 Te 3 . Their efficacy is demonstrated via direct electronic structure topology measurements using high resolution angle-resolved photoemission spectroscopy. These results open up new opportunities for probing topological behavior of Dirac electrons in Bi 2 Te 3 . At least one of the methods demonstrated here can be successfully applied to other topological insulators such as the bulk-insulating-Bi 1-x Sb x , Sb 2 Te 3 and Bi 2 Se 3 which will be shown elsewhere. More importantly, our methods of topological surface state manipulation demonstrated here are highly suitable for future spectroscopic studies of topological phenomena which will complement the transport results gained from the traditional electrical gating techniques.

In this letter we report measurements of the coupling between Dirac fermion quasiparticles (DFQs) and phonons on the (001) surface of the strong topological insulator Bi2Se3. While most contemporary investigations of this coupling have involved examining the temperature dependence of the DFQ self-energy via angle-resolved photoemission spectroscopy (ARPES) measurements, we employ inelastic helium atom scattering to explore, for the first time, this coupling from the phonon perspective. Using a Hilbert transform, we are able to obtain the imaginary part of the phonon self-energy associated with a dispersive surface phonon branch identified in our previous work as having strong interactions with the DFQs. From this imaginary part of the self-energy we obtain a branch-specific electron-phonon coupling constant of 0.43, which is stronger than the values reported form the ARPES measurements.

Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects. T he use of angle-resolved photoemission spectroscopy (ARPES) to determine the electronic structure of materials is a much-used technique with many successes 1-3 . The electronic structure of a host of materials of fundamental interest, from prototypical transition metals such as W (refs 4-6), to more complex and technologically relevant substances, such as future semiconductors or topological compounds , or strongly correlated materials , have been successfully studied in the past. However, the inherent surface sensitivity of conventional ARPES, due to the low inelastic mean-free paths (IMFPs) of the photoemitted electrons, with kinetic energies usually ranging between about 20 and 150 eV, can lead to spectra that are strongly influenced by surface effects. This can of course be of great advantage in the description of new materials and their surfaces. Conversely, this surface sensitivity can become a major disadvantage if the bulk properties of the materials studied are of particular interest or significantly different from the surface properties, or if it is difficult to prepare a sufficiently good surface for standard ARPES measurements. This surface sensitivity has led recently to several studies in which ARPES has been carried out with photon energies up to the 1 keV range . These studies for a variety of systems, but especially for several strongly correlated materials, have shown the advantage of probing more deeply below the surface. One can thus also immediately ask whether going further into the multi-keV hard X-ray regime might also be possible and useful in this regard . The recent advent of new high-energy third-generation synchrotron facilities and the development of new high-energy electron analysers make it possible to attempt such novel angle-resolved experiments in the hard X-ray regime. The overall energy resolutions (down to ∼50 meV), as well as the angular resolutions of these newly developed facilities (down to ∼0.2 • -0.3 • ), in principle make it possible to meaningfully study dispersive energy bands at

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic AF insulator with a Néel temperature TN = 625 K and a large ordered moment µ = 3.9 µB. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and AF structures together with nearly the same high TN and large µ. Ba1-xKxMn2As2 is thus the first metallic ThCr2Si2-type M As-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba1-xKxMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.

Results are presented that have been obtained while operating the graphite hollow cathode duoplasmatron ion source in dual mode under constant discharge current. This dual mode operation enabled us to obtain the mass and emission spectra simultaneously. In mass spectra C3 is the main feature but C4 and C5 are also prominent, whereas in emission spectra C2 is also there and its presence shows that it is in an excited state rather than in an ionic state. These facts provide evidence that C3 is produced due to the regeneration of a soot forming sequence and leave it in ionic state. C3 is a stable molecule and the only dominant species among the carbon clusters that survives in a regenerative sooting environment at high-pressure discharges.

In order to improve the efficiency of perovskite solar cells (PSCs), careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (the MXene Ti3C2TX) with various termination groups (TX) to tune the work function (WF) of the perovskite absorber and the TiO2 electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and Density Functional Theory calculations show that the addition of Ti3C2TX to halide perovskite and TiO2 layers permits to tune the materials' WFs, without affecting other electronic properties. Moreover, the dipole induced by the Ti3C2TX at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified PSCs, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without Mxene.

The photodegradation mechanism due to synchrotron radiation exposure of crystalline poly[vinylidene fluoride–trifluoroetylene, P(VDF–TrFE)] copolymer thin films has been studied with ultraviolet photoemission spectroscopy (UPS) and mass spectroscopy. Upon increasing exposure to x-ray white light (hν⩽1000 eV), UPS measurements reveal that substantial chemical modifications occur in P(VDF–TrFE) 5 monolayer films, including the emergence of new valence band features near the Fermi level, indicating a semimetallic photodegradeted product. The photodetached fragments of the copolymer consist mainly of H2, HF, CHF, CH2. This x-ray exposure study demonstrates that P(VDF–TrFE) films, possessing unique technologically important properties, can be directly patterned by x-ray lithographic processes.

The chemical interaction between the simple metals, aluminum and sodium, and crystalline copolymer thin films of vinylidene fluoride (70%) with trifluoroethylene (30%), has been studied using x-ray photoemission spectroscopy. Aluminum and sodium metalize the polymer differently and different binding sites for the two metals can be inferred from the corresponding core level shifts. Aluminum leads to enhanced screening of final photoemission states associated with the polymer, while sodium doping strongly influences the fluorine, but perturbs the carbon backbone only slightly.

The adsorption of trimesic acid (TMA) on Cu(110) has been studied in the temperature range between 130 and 550 K and for coverages up to one monolayer. We combine scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations to produce a detailed adsorption phase diagram for the TMA/Cu(110) system as a function of the molecular coverage and the substrate temperature. We identify a quite complex set of adsorption phases, which are determined by the interplay between the extent of deprotonation, the intermolecular bonding, and the overall energy minimization. For temperatures up to 280 K, TMA molecules are only partly deprotonated and form hydrogen-bonded structures, which locally exhibit organizational chirality. Above this threshold, the molecules deprotonate completely and form supramolecular metal-organic structures with Cu substrate adatoms. These structures exist in the form of single and double coordination chains, with the molecular coverage driving distinct phase transitions. † Part of the "Giacinto Scoles Festschrift".

University -With the advent of atomic resolution imaging techniques comes the challenge of disentangling the intrinsic electronic properties of materials from their stochastic atomic-scale disorder. In the past decade, machine learning (ML) image analysis techniques have rapidly evolved, while their applications in physics are just emerging. Here, we use ML to test local correlation hypotheses between spatially resolved measurements of disordered materials to overcome the limitations of standard Fourier analysis techniques. By training on a simulated density wave (DW) dataset, we develop a convolutional neural network (CNN) to uncover the doping-dependence of the DW in the cuprate superconductor (Pb,Bi) 2 (Sr,La) 2 CuO 6+ (Bi-2201) imaged via scanning tunneling microscopy. In Bi-based cuprates, the electronic inhomogeneity, caused by local variations in doping, limits the precision with which the DW wavevector can be measured. Our ML algorithm overcomes this limitation and allows clear differentiation between commensurate and incommensurate DW instabilities with physically distinct mechanisms. More broadly, our work lays the foundation for a ML approach to quantify intrinsic periodic order and correlations in datasets where these trends are masked by disorder.

As the MOSFET Si02-based gate dielectric layer approaches its fundamental physical limits, the investigation of high-k oxides is ongoing in order to determine which oxides can best continue the scaling of the MOSFET. Hf02 and hafnium silicates are leading candidates due to their relatively large band gaps, thermal stability in proximity to Si and relatively high dielectric constants. We have used a combination of x-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry (SE) to measure the band offsets between the high-k layers and the Si substrates. Shifts in band alignment that occur upon deposition of the Hf02 layer and annealing of the Hf02/Si02/Si fikn stack will be discussed in light of XPS spectra. Non-destructive compositional depth profiles constructed from angle resolved XPS data will also be presented and film thicknesses determined from them will be compared to thicknesses measured by SE.

The American Physical Society Electronic structure of N3 DYE molecules on the ZnO single crystal and epitaxial film surfaces JEAN-PATRICK THEISEN, ERIC BERSCH, SYLVIE RANGAN, YICHENG LU, ROBERT BARTYNSKI -Most dye-sensitized solar cells use TiO 2 nanoparticle films as the electrode, but ZnO offers an interesting alternative. We have used direct and inverse photoemission to measure the occupied and unoccupied electronic states, and their alignment with the band edges of the substrate, of N3 dye adsorbed on ZnO(0001), ZnO(11-2), epitaxial ZnO a-plane film surfaces, and ZnO nanopillars. As the unoccupied states of ZnO are of sp-character and of relatively low cross section, the LUMO of the dye is easily observed. Samples were prepared and passivated with a pivalate layer in UHV, then sensitized in air in a solution of N3 dye in acetonitrile. As opposed to the case of the TiO 2 (110) surface, STM measurements indicate that the pivalic acid does. From UPS, the N3 HOMO is found at ∼0.8 eV above the ZnO valence band edge, and the LUMO is found ∼1.5 eV above the conduction band edge for the epifilm. Differences in dye adsorption and orbital alignment for these different ZnO surfaces will be discussed.

As the MOSFET Si02-based gate dielectric layer approaches its fundamental physical limits, the investigation of high-k oxides is ongoing in order to determine which oxides can best continue the scaling of the MOSFET. Hf02 and hafnium silicates are leading candidates due to their relatively large band gaps, thermal stability in proximity to Si and relatively high dielectric constants. We have used a combination of x-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry (SE) to measure the band offsets between the high-k layers and the Si substrates. Shifts in band alignment that occur upon deposition of the Hf02 layer and annealing of the Hf02/Si02/Si fikn stack will be discussed in light of XPS spectra. Non-destructive compositional depth profiles constructed from angle resolved XPS data will also be presented and film thicknesses determined from them will be compared to thicknesses measured by SE.

Valence-and conduction-band edges of ultrathin oxides ͑SiO 2 , HfO 2 , Hf 0.7 Si 0.3 O 2 , ZrO 2 , and Al 2 O 3 ͒ grown on a silicon substrate have been measured using ultraviolet photoemission and inverse photoemission spectroscopies in the same UHV chamber. The combination of these two techniques has enabled the direct determination of the oxide energy gaps as well as the offsets of the oxide valence-and conduction-band edges from those of the silicon substrate. These results are supplemented with synchrotron x-ray photoemission spectroscopy measurements allowing further characterization of the oxide composition and the evaluation of the silicon substrate contribution to the spectra. The electron affinity has also been systematically measured on the same samples. We find reasonably good agreement with earlier experiments where assumptions regarding energygap values were needed to establish the conduction-band offsets. The systematics of our photoemission and inverse photoemission results on different ultrathin films provide a comprehensive comparison of these related systems.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 and SrTiO3 has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO3 thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO3 /SrTiO3 samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1-2 uc of LaAlO3 and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

Recently discovered (CaFe1-xPtxAs)10Pt3As8 and (CaFeAs)10Pt4-yAs8 superconductors are very similar materials having the same elemental composition and structurally similar superconducting FeAs slabs. Yet the maximal critical temperature achieved by changing Pt concentration is approximately twice higher in the latter. Using angle-resolved photoemission spectroscopy(ARPES) we compare the electronic structure of their optimally doped compounds and find drastic differences. Our results highlight the sensitivity of critical temperature to the details of fermiology and point to the decisive role of band-edge singularities in the mechanism of high-Tc superconductivity.

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe2As2, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 K and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the Γ-point and an electron pocket at each of the X-points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation of the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No dierences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

Swapnil Patil, Sudhir Pandey, VRR Medicherla, RS Singh, R. Bindu, EV Sampathkumaran, and Kalobaran Maiti∗ Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India (Dated: ...

Initiated by the discovery of topological insulators, topologically non-trivial materials, more specifically topological semimetals and metals have emerged as new frontiers in the field of quantum materials. In this work, we perform a systematic measurement of EuMg2Bi2, a compound with antiferromagnetic transition temperature at 6.7 K, observed via electrical resistivity, magnetization and specific heat capacity measurements. By utilizing angle-resolved photoemission spectroscopy in concurrence with first-principles calculations, we observe Dirac cones at the corner and the zone center of the Brillouin zone. From our experimental data, multiple Dirac states at Γ and K points are observed, where the Dirac nodes are located at different energy positions from the Fermi level. Our experimental investigations of detailed electronic structure as well as transport measurements of EuMg2Bi2 suggest that it could potentially provide a platform to study the interplay between topology and magnetism.

We have reflected on the ratio of specific heat and temperature (γorCel) of selected electron and hole doped highTccuprates. The electronic specific heat rises slowly and there is a sudden drop at the critical temperature and then it gradually decreases for temperatures above TC. The behavior of electronic specific heat below TC is in good agreement with the BCS result for d-wave superconductor. The electronic contribution to the total specific heat of these materials is less than 1 %.The relevance to high-Tc superconductivity in the cupratesis

Studies on superconductivity arising from doping a Mott insulator have become critical in the area of superconductivity. Within the framework of the Bose-Fermi-Hubbard model, we discuss the thermodynamic phase transition via specific heat in hole and electron doped high-TC cuprate superconductors. By considering the interplay between the electrons (fermions) and cooper pairs (bosons), the main features of the temperature dependence of the specific-heat, are well reproduced. It is shown that a hump-like feature in the specific-heat appears at the superconducting transition temperature TC, and then the specific-heat varies exponentially as a function of temperature for the temperatures T<TC. This is in consistency that at lower temperatures, a superconducting gap seems to open progressively. In particular, quantitatively, we report a specific heat value ~4.6661×10 -3 eV/K for YBa2Cu3O6 (YBCO), 4.6419×10 -3 eV/K for La2xSrxCuO4 (LSCO), 4.67×10 -3 eV/K for Nd2-xCexCuO4 (NCCO) and 4.6...

3d-5d perovskite oxides, ABO3 (where A and B are 3d or 5d elements), form polar surfaces in the (001)-stacked thin films. As a result, the polar-polar (001) interface between two ABO3 insulators could create polar discontinuity potentially producing a two-dimensional electron gas of higher density and stronger spatial localization compared to the widely studied polar-nonpolar oxide interfaces, such as (001) LaAlO3/SrTiO3. Here, as a model system, we explore the interface between polar (001) TbScO3 and polar (001) KTaO3 using first-principles density functional theory. We find that the intermixed interface Ta0.75Sc0.25O2/Tb0.75K0.25O maintaining the bulk perovskite charge stacking (e.g., …+1/-1/+1…) is insulating and has a lower energy than the metallic interface TbO/TaO2 breaking such stacking. This intermixed interface is however prone to the formation of oxygen vacancies which make it conducting. We emphasize that the driving force for the formation of the 2DEG here is not a built-in electric field stemming from the polar discontinuity but the interface stoichiometry. We find that the ratio of oxygen vacancy concentration is a factor of 30 times larger at the interface than in bulk KTO at room temperature. The oxygen vacancy-induced two-dimensional electron gas resides on the Ta-5d electronic orbitals with dxy and dxz/yz occupation dominating overall charge density near and far away from the interface.

Ladies and gentlemen, the following theory has been concluded, quantified and is in a position to explain freely and easily the phenomena connected to the High-temperature-superconductivity. I therefore ask you to forward my paper to your referee committee. Theory of high-temperature-superconductivity in Cuprates by Diplom-Chemist Hans Christian Haunschild, private scholar, born Frankfurt am Main 2/20/1964.

Recently, it has been shown that experimental data from angle-resolved photoemission spectroscopy on oriented molecular films can be utilized to retrieve real-space images of molecular orbitals in two dimensions. Here, we extend this orbital tomography technique by performing photoemission initial state scans as a function of photon energy on the example of the brickwall monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on Ag(110). The overall dependence of the photocurrent on the photon energy can be well accounted for by assuming a plane wave for the final state. However, the experimental data, both for the highest occupied and the lowest unoccupied molecular orbital of PTCDA, exhibits an additional modulation attributed to final state scattering effects. Nevertheless, as these effects beyond a plane wave final state are comparably small, we are able, with extrapolations beyond the attainable photon energy range, to reconstruct three-dimensional images for both orbitals in agreement with calculations for the adsorbed molecule.

Longer acenes such as heptacene are promising candidates for optoelectronic applications but are unstable in their bulk structure as they tend to dimerize. This makes the growth of well-defined monolayers and films problematic. In this article, we report the successful preparation of a highly oriented monolayer of heptacene on Ag(110) by thermal cycloreversion of diheptacenes. In a combined effort of angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations, we characterize the electronic and structural properties of the molecule on the surface in detail. Our investigations allow us to unambiguously confirm the successful fabrication of a highly oriented complete monolayer of heptacene and to describe its electronic structure. By comparing experimental momentum maps of photoemission from frontier orbitals of heptacene and pentacene, we shed light on differences between these two acenes regarding their molecular orientation and energy-level alignment on the metal surfaces.

The occupied and the unoccupied electronic structure of CeAg 2 Ge 2 single crystal has been studied using high resolution photoemission and inverse photoemission spectroscopy respectively. High resolution photoemission reveals the clear signature of Ce 4f states in the occupied electronic structure which was not observed earlier due to the poor resolution. The coulomb correlation energy in this system has been determined experimentally from the position of the 4f states above and below the Fermi level. Theoretically the correlation energy has been determined by using the first principles density functional calculations within the generalized gradient approximations taking into account the strong intra-atomic (on-site) interaction Hubbard U ef f term. Although the valence band calculated with different U ef f does not show significant difference, but the substantial changes are observed in the conduction band. The estimated value of correlation energy from both the theory and the experiment is ≈4.2 eV for CeAg 2 Ge 2 .

Three dimensional (3D) topological insulators are quantum materials with a spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena in the absence of applied magnetic fields. They feature surface states that are topologically protected against scattering by ...

We calculate the normal state Nernst signal in the cuprates resulting from a reconstruction of the Fermi surface due to spin density wave order. An order parameter consistent with the reconstruction of the Fermi surface detected in electron-doped materials is shown to sharply enhance the Nernst signal close to optimal doping. Within a semiclassical treatment, the obtained magnitude and position of the enhanced Nernst signal agrees with Nernst measurements in electron-doped cuprates. Our result is mainly caused by the role of Fermi surface geometry under influence of a spin density wave gap. We discuss also possible roles of short-ranged magnetic order in the normal state Nernst effect and the Fermi surface reconstruction observed by photoemission spectroscopy.

After a brief review of current ideas on stripe order in cuprate high-temperature superconductors, we discuss the quasiparticle Nernst effect in the cuprates, with focus on its evolution in nonsuperconducting stripe and related nematic states. In general, we find the Nernst signal to be strongly enhanced by nearby van-Hove singularities and Lifshitz transitions in the band structure, implying that phases with translation symmetry breaking often lead to a large quasiparticle Nernst effect due to the presence of multiple small Fermi pockets. Open orbits may contribute to the Nernst signal as well, but in a strongly anisotropic fashion. We discuss our results in the light of recent proposals for a specific Lifshitz transition in underdoped YBa2Cu3Oy and make predictions for the doping dependence of the Nernst signal. I.

Band ofFsets at lattice-mismatched heterojunctions can be tuned owing to their dependence on macroscopic strain, and hence on the substrate composition. The system studied here, GaAs/Si(001), is lattice mismatched aud heterovaleut, offering thus an additional flexibility, due to the intrinsic nonbulk character of the band offset at heterovalent junctions. Starting with a study of macroscopic and microscopic elasticity, we evaluate the band offset for several fully relaxed inequivalent interfaces. Both macroscopic strain and microscopic morphology strongly affect the offset between the topmost Si and GaAs valence bands, which, consequently, is tunable, in principle, by as much as 1.1 eV.

Higher-order topology in non-Hermitian (NH) systems has recently become one of the most promising and rapidly developing fields in condensed matter physics. Many distinct phases that were not present in the Hermitian equivalents are shown in these systems. In this work, we examine how higher-order Weyl semimetals are impacted by NH perturbation. We identify a new type of topological semimetal, i.e., non-Hermitian higher-order Weyl semimetal (NHHOWS) with surface diabolic points. We demonstrate that in such an NHHOWS, new exceptional points inside the bulk can be created and annihilated, therefore allowing us to manipulate their number. At the boundary, these exceptional points are connected through unique surface states with diabolic points and hinge states. For specific system parameters, the surface of NHHOWS behaves as a Dirac phase with linear dispersion or a Luttinger phase with a quadratic dispersion, thus paving a way for Dirac-Luttinger switching. Finally, we employ the biorthogonal technique to reinstate the standard bulk boundary correspondence for NH systems and compute the topological invariants. The obtained quantized biorthogonal Chern number and quadruple moment topologically protect the unique surface and hinge states, respectively.

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We have performed hard x-ray and soft xray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm 3d core-level spectra, we have estimated the valence of Sm to be ∼2.96, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm 5d-derived band. The present finding is consistent with the theory suggesting a possible topological state in SmO and show that rare-earth monoxides or their heterostructures can be a new playground for the interplay of strong electron correlation and spin-orbit coupling.

Recently, hole-doped superconducting cuprates with the T ′ -structure La1.8-xEu0.2SrxCuO4 (LE-SCO) have attracted a lot of attention. We have performed x-ray photoemission and absorption spectroscopy measurements on as-grown and reduced T ′ -LESCO. Results show that electrons and holes were doped by reduction annealing and Sr substitution, respectively. However, it is shown that the system remains on the electron-doped side of the Mott insulator or that the charge-transfer gap is collapsed in the parent compound.

We have studied the electronic structure of the spinel-type V oxides LiV 2 O 4 , ZnV 2 O 4 , and CdV 2 O 4 using x-ray photoemission spectroscopy. The p-d charge-transfer energy was estimated from the valence-band spectra and the exchange interaction due to d-d transfer T was found to be larger than that due to p-d transfer T in these systems, consistent with the existing theoretical models. Based on the V 2p core-level spectra, we discuss the oxidation state of V ions in the surface region. Interestingly, only in ZnV 2 O 4 does the oxidation state of V ions strongly depend on the procedure of sample preparation.

We have studied the electronic structure of the Ni triangular lattice in NiGa 2 S 4 using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2p core-level spectrum shows that the S 3p to Ni 3d charge-transfer energy is ∼ -1 eV and the ground state is dominated by the d 9 L configuration (L is a S 3p hole). Cell perturbation analysis for the NiS 2 triangular lattice indicates that the strong S 3p hole character of the ground state provides the enhanced superexchange interaction between the third nearest neighbor sites.

An angle-resolved photoemission spectroscopy (ARPES) study is reported on a Mott insulator NiGa 2 S 4 in which Ni 2þ (S ¼ 1) ions form a triangular lattice and the Ni spins do not order even in its ground state. The first ARPES study on the two-dimensional spin-disordered system shows that low-energy hole dynamics at high temperatures is characterized by wave vectors Q E which are different from wave vectors Q M dominating low-energy spin excitations at low temperatures. The unexpected difference between Q E and Q M is deeply related to charge fluctuation across the Mott gap in the frustrated lattice and is a key issue to understand the spin-disordered ground states in Mott insulators.

We have studied the electronic structure of the spinel-type compound CuIr 2 S 4 using x-ray photoemission spectroscopy (XPS). CuIr 2 S 4 undergoes a metal-insulator transition (MIT) at 226 K. In going from the metallic to insulating states, the valence-band photoemission spectrum shows a gap opening at the Fermi level and a rigid-band shift of 0:15 eV. In addition, the Ir 4f core-level spectrum is dramatically changed by the MIT. The Ir 4f line shape of the insulating state can be decomposed into two contributions, consistent with the charge disproportionation of Ir 3 :Ir 4 1:1. XPS measurements under laser irradiation indicate that the charge disproportionation of CuIr 2 S 4 is very robust against photoexcitation in contrast to Cs 2 Au 2 Br 6 which shows photo-induced valence transition.

We have studied the electronic structure of the diluted magnetic semiconductor Ga1-xMnxN (x = 0.0, 0.02 and 0.042) grown on Sn-doped n-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn L-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new state were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2p core level and the Mn 3d partial density of states were analyzed using configurationinteraction calculation on a MnN4 cluster model. The deduced electronic structure parameters reveal that the p-d exchange coupling in Ga1-xMnxN is stronger than that in Ga1-xMnxAs.

We report on a photoemssion study of A-site ordered perovskite oxides ACu 3 Co 4 O 12 ͑A = Ca and Y͒ which are metallic for A = Ca and insulating for A = Y. If the Cu valence is +2 as stabilized by the square planar coordination, the Co valence for A = Ca is formally +4 and the system could be a Mott insulator while that for A = Y should be +3.75 and the system is a doped Mott insulator. The photoemission study reveals that the Cu valence for A = Ca and Y is formally +3 with the d 9 L configuration ͑L: an O 2p hole͒ which corresponds to the Zhang-Rice singlet state. In addition, it is found that the Co valence for A = Y is formally +3 with the low-spin d 6 configuration consistent with its insulating behavior.