We report the electronic structure of the thermoelectric semimetal Ta_2PdSe_6 with a large thermoelectric power factor and giant Peltier conductivity by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra reveal the coexistence of a sharp hole band with a light electron mass and a broad electron band with a relatively heavy electron mass, which originate from different quasi-one-dimensional (Q1D) chains in Ta_2PdSe_6. Moreover, the electron band around the Brillouin-zone (BZ) boundary shows a replica structure with respect to the energy originating from plasmonic polarons due to electron-plasmon interactions. The different scattering effects and interactions in each atomic chain lead to asymmetric transport lifetimes of carriers: a large Seebeck coefficient can be realized even in a semimetal. Our findings pave the way for exploring the thermoelectric materials in previously overlooked semimetals and provide a new platform for low-temperature thermoelectric physics, which has been challenging with semiconductors.

Layered ruthenium oxide Ca2RuO4 is a Mott insulator and its frozen state can be melted by various stimuli such as temperature variation, chemical substitution, pressure, and current application. In this study, we have investigated the variation of the electronic structure of Ca2RuO4 with temperature and electric current by means of hard x-ray photoemission spectroscopy to gain valuable insights into the origin of the insulator-metal transition (IMT). We have observed the reduction in the insulating gap as well as the suppression of the spectral weight for the lower Hubbard band with increasing temperature and electric current density. The variation of the insulating gap well accounts for the characteristic nonlinear conductivity. Our spectroscopic results demonstrate the close relationship between the octahedral distortion of RuO6 and the IMT in Ca2RuO4. Furthermore, to clarify the origin of the observed current-induced phenomena, the temperature increase of the sample due to electric current was evaluated on the basis of the Joule self-heating model. The simulated results revealed that the heating effects have a significant influence on the gap suppression and the spectral changes under current. However, this cannot explain all of the observed spectral changes under current, implying the possibility of an intrinsic current-induced effect.

The carrier dynamics of the ultra-high power factor semimetal Ta2PdSe6 are investigated using magnetotransport and anisotropic resistivity measurements. The magnetotransport analysis reveals a pronounced asymmetry in the mobilities of electrons and holes, which plays a key role in the thermoelectric performance. The effective mass and scattering time of the charge carriers are quantitatively determined through combined analyses of Shubnikov-de Haas oscillations and anisotropic resistivity. Furthermore, it is demonstrated that chemical substitution modifies the thermoelectric properties of Ta2PdSe6 primarily by altering the scattering time. These findings establish a new strategy-scattering engineering-for optimizing high power factor semimetals by controlling charge carrier lifetimes rather than solely relying on band structure engineering.

We performed angle-resolved photoemission spectroscopy studies on the triple-layer Bi2Sr2Ca2Cu3O10+δ over a wide doping range. Although the doping level of the inner CuO2 plane is extremely low in underdoped samples, the d-wave SC gap is enhanced to the unprecedentedly large value of Δ0 ~ 80-100 meV at the antinode. This gap persists well above Tc without a Fermi arc, indicating a "nodal metal". We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO2 plane, sandwiched by nearly optimally-doped two outer planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks show electron-hole symmetry, suggesting d-wave pairing fluctuations. Thus the proximity effect on the innermost CuO2 plane is the strongest in the triple-layer cuprates, which explains why the Tc reaches the maximum at the layer number of three in every multi-layer cuprate family.

Indium-tin oxide In2-ySnyO3 (ITO) is one of the most applied transparent conducting oxides (TCOs) because of the controllability of the electrical conductivity by Sn substitution. To explore additional functionalities such as ferromagnetism on In2O3 and ITO, the substitution of transition metals for In sites has been actively investigated. In this study, we found an anomalous carrier enhancement by a minute amount of Mn substitution in Mn and Sn co-doped In2O3 (In2-x-yMnxSnyO3) thin films. The increase of carrier concentration (nc) is higher than the conventional ITO films, indicating the important role for the transition metal substitution for the carrier generation. In order to clarify the effect of co-doping and the origin of the carrier injection by lightly Mn substitution, we have investigated the electronic structure of In2-x-yMnxSnyO3 thin films by means of hard x-ray photoemission spectroscopy (HAXPES). At lightly Mn substitution x = 0.05, the valence-band maximum (VBM) shifts toward higher binding energy and the spectral weight close to EF increases, corresponding to the remarkable enhancement of nc. The Sn doping dependence at x = 0.05 reveals that nc is maximized around y = 0.4, where the carrier enhancement has conventionally been difficult in ITO films. Moreover, the plasmon energy-loss features were observed in the core-level spectra for the samples with high nc. Our spectroscopic results provide a possibility of the increase in six-coordinated Sn4+ ions and/or oxygen vacancies which act as a dopant with lightly Mn doping in In2-x-yMnxSnyO3 and insights into the enhancement of nc for TCOs.

Transition metal oxides (TMOs) exhibit a broad spectrum of functional electronic, magnetic, and optical properties, making them attractive for various technological applications. The scale and impact of surface defects and inhomogeneity can extend many unit cells below the surface. Overlooking this aspect of TMO surfaces can result in an incorrect interpretation of their physics and inhibit their maturation into device technology. Soft x-ray absorption spectroscopy (XAS) is a common technique for TMO studies, and different XAS acquisition modes can be used to measure different depth regimes in the sample. Here, we demonstrate a substantial disparity between the near-surface region and the “bulk” of the prototypical TMO SrVO3. By driving the system across two scenarios of orbital polarization, we illustrate how a common XAS surface-sensitive acquisition technique fails to detect the intrinsic orbital polarization. By stark contrast, a “bulk”-sensitive technique successfully captures this effect, elucidating the expected orbital occupation inversion. These results not only underscore the impact of the near-surface region on the correct interpretation of TMO fundamental physics, but further highlight the scale of surface inhomogeneity, a critical aspect of nanoscale functional devices.

We report the electronic structure of the ternary chalcopyrite CdSnAs2 using angle-resolved photoemission spectroscopy (ARPES) combined with the band-structure calculation. The tiny Fermi surface (FS) with the Fermi wave number kF = 0.012 & Aring;-1 was observed, and the carrier density n = 1.2 x 1017 cm-3 was estimated. The deduced carrier density n indicates the electron density parameter rs = 240, which corresponds to the extremely low density limit of the three-dimensional (3D) electron gas. On the other hand, the calculated band structure of CdSnAs2 well reproduced the band gap and the effective mass reported by the Hall measurement, the Shubnikov- de Haas (SdH) oscillation, and the optical measurement, quantitatively. Therefore, the ARPES results indicate that the carrier density n decreases and there is a large deviation between the band calculation and the ARPES band structure. These results reveal that the extremely low electron density can be realized near the surface due to the bulk band-bending effect. Moreover, we found the high Fermi velocity of VF = 2.55 x 106 m/s and the extremely light mass of m*/m0 similar to 0.005 comparable to the Dirac materials. This suggests that the effective mass m*/m0 is reduced due to the effect of the long-range Coulomb interaction in the extremely low-density limit. Our findings provide a venue to investigate the physics of the electron correlation in the extremely low-density electron gas as well as the Wigner crystallization or Anderson localization.

Understanding how an antiferromagnetic Mott insulator develops into a paramagnetic metal and/or superconductor by carrier doping is a fundamental problem in high-Tc cuprate superconductor. The impact of the chemical potential mu controlled by doping has been extensively investigated to solve this issue. The chemical potential shift Delta mu in La-2-xSrxCuO4 (LSCO) exhibits the different behavior from the other high-T-c cuprates. In this work, we have re-examined Delta mu of LSCO showing the anomalous behavior by using the hard and soft x-ray photoemission spectroscopies (HAXPES and SXPES). We found that Delta mu reveals downward behavior by small hole doping in HAXPES and SXPES, which is distinct from the anomalous pinning behavior over a wide doping range reported previously in LSCO but is consistent with the weak Delta mu pinning in the vicinity of the Mott insulator as in other hole-doped cuprates. Our spectroscopic findings demonstrate that the previously considered strong pinning effect in LSCO does not occur. Based on our results, we compare the bulk Delta mu from HAXPES with that estimated from the thermodynamic quantity and the three-dimensional tight-binding calculation and discuss the mechanism of Delta mu with doping.

We report the electronic structure of In1.8-xMnxSn0.2O3 (x = 0.0, 0.2, and 0.3), which combines transparency, ferromagnetism, and semiconducting properties, by means of hard x-ray photoemission spectroscopy (HAXPES), soft x-ray resonant photoemission spectroscopy (RPES), and x-ray absorption spectroscopy (XAS). The spectral shape of the Mn L-edge XAS spectra indicates that the valence of the substitutional Mn ions is divalent. The peak position of the In 3d and O 1s core-level spectra shifts toward lower binding energy with Mn concentration suggest the hole doping due to the Mn2+ ions. The valence-band spectra exhibit the well-defined features associated with the donor states across the Fermi level E-F and the valence-band edge. The valence-band maximum (VBM) shifts to lower binding energy and the spectral weight near E-F decreases with increasing Mn concentration, which is consistent with the hole doping nature observed in the core-level shift. The Mn 2p-3d RPES reveals that the hump structure around 1.9 eV above the VBM originates from the Mn 3d impurity band and the valence-band state consists of the O 2p band strongly hybridized with the Mn 3d orbital. Furthermore, there is no contribution of the Mn 3d orbital to the spectral weight close to E-F. Our results reveal the entire valence-band structure of In1.8-xMnxSn0.2O3 and the effect of the Mn substitution, which provides the important information related to the physical properties such as transparency, electrical conductivity, and ferromagnetism for the functional materials.

We report the electronic structure of BaIr2Ge7 with two types of cage structure by means of angle-resolved photoemission spectroscopy (ARPES) and hard x-ray photoemission spectroscopy. ARPES spectra reveal the three-dimensional and multiband Fermi surfaces (FSs) originating from the hybridized Ir 5d and Ge 4p orbitals. The observed FSs show C2 symmetry, reflecting the orthorhombic Ammm crystal structure of BaIr2Ge7. The temperature dependence of the ARPES spectra exhibits the thermal spectral broadening, and the width of the spectral peak shows a concave-downward behavior with temperature. Considering the effect of anharmonic phonon modes, we have reproduced the temperature dependence of the electrical resistivity as well as the thermal spectral broadening. The resultant renormalized phonon frequencies mr0(1) = 146.9 K and m(2) r0 = 70.6 K are comparable to the Einstein temperatures estimated from the previous specific heat measurement. Our results suggest the existence of the weak anharmonic phonon modes in BaIr2Ge7.

Transition metal oxides (TMOs) exhibit a broad spectrum of electronic, magnetic, and optical properties, making them intriguing materials for various technological applications. Soft x-ray absorption spectroscopy (XAS) is widely used to study TMOs, shedding light on their chemical state, electronic structure, orbital polarization, element-specific magnetism, and more. Different XAS acquisition modes feature different information depth regimes in the sample. Here, we employ two XAS acquisition modes, having surface-sensitive versus bulk probing depths, on the prototypical TMO SrVO3. We illustrate and elucidate a strong apparent discrepancy between the different modes, emphasizing the impact of the near-surface region on the interpretation of XAS data. These findings highlight the importance of the acquisition mode selection in XAS analysis. Moreover, the results highlight the role of the near-surface region not only in the characterization of TMOs, but also in the design of future nanoscale oxide electronics.

Received 13 February 2023DOI:https://doi.org/10.1103/PhysRevResearch.5.019003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSuperconductivityPhysical SystemsStrongly correlated systemsTechniquesMachine learningPhotoemission spectroscopyCondensed Matter, Materials & Applied Physics

We report the electronic structure of Ca2-xSrxRuO4 (CSRO) for x = 0.06 by means of angle-resolved photoemission spectroscopy (ARPES) and hard x-ray photoemission spectroscopy (HAXPES). The surface-sensitive ARPES results show the Fermi surfaces (FSs) consisting of the dxz/yz and dxy orbitals even in the low temperature bulk insulating phase, indicating the surface metallic state. In order to investigate the surface electronic structure as well as the bulk electronic structure, we have performed the angular dependent HAXPES and revealed the electronic structure depending on the probing depth. The HAXPES results suggest the bulk insulating state and the surface metallic state of CSRO (x = 0.06). Moreover, the observed band dispersion derived from the dxz/yz orbital exhibits kink structures with the energy scales of 35 and 60 meV. The distinct kink structures suggest the strong electron-phonon coupling compared with Sr2RuO4. Our results reveal that the surface metallic state with the large electron-lattice coupling is realized in the lightly Sr-doped region.

We report the electronic structure of a natural mineral calaverite AuTe2 under high pressure by means of the infrared spectroscopy. The optical conductivity at ambient pressure shows a Drude response and a hump structure around 0.2 eV. These characteristic results are more prominent in going from high temperature to low temperature. The Drude response increases with pressure, which corresponds to the reduction of the electrical resistivity. Meanwhile, the hump structure is suppressed and merged into the Drude response with increasing pressure. Further applying pressure up to 3 GPa, the hump structure almost disappears. We have fitted the optical conductivity by using the Drude-Lorentz model and obtained the plasma frequency omega(p) and the scattering rate gamma. The squared plasma frequency omega(2)(p) increases and the scattering rate gamma decreases with pressure. Our results suggest that the suppression of the electrical resistivity under pressure is mainly due to the increase of the carrier density n and the reduction of the scattering rate gamma. By comparing the optical conductivity with the band structure calculation and the photoemission spectroscopy, we discuss the low-energy excitation corresponding to the hump structure and the variation of the Drude component in the optical conductivity.

Experimental data are the source of understanding matter. However, measurable quantities are limited and theoretically important quantities are often hidden. Nonetheless, recent progress of machine-learning techniques opens possibilities of exposing them only from available experimental data. In this article, the Boltzmann-machine method is applied to the angle-resolved photoemission spectroscopy spectra of cuprate superconductors. We find prominent peak structures both in normal and anomalous self-energies, but they cancel in the total self-energy making the structure apparently invisible, while the peaks make dominant contributions to superconducting gap, hence providing a decisive testimony for the origin of superconductivity. The relation between superfluid density and critical temperature supports involvement of universal carrier relaxation time associated with dissipative strange metals. The present achievement opens avenues for innovative machine-learning spectroscopy method.

We investigated the electronic structures of mono- and few-layered Ru nanosheets ( N layers (L) with N = 1, ~6, and ~9) on Si substrate by ultra-violet and x-ray photoemission spectroscopies. The spectral density of states (DOS) near E F of ~6 L and 1 L is suppressed as it approaches E F in contrast to that of ~9 L, which is consistent with the Ru 3 d core-level shift indicating the reduction of the metallic conductivity. A power law g ( ε ) ∝ | ε − ε F | α well reproduces the observed spectral DOS of ~6 L and 1 L. The evolution of the power factor α suggests that the transition from the metallic state of ~9 L to the 2-dimensional insulating state with the soft Coulomb gap of 1 L through the disordered 3-dimensional metallic state of ~6 L.

We investigate the metal to nonmagnetic insulator (MI) transition of MnP-type Ru pnictide RuP using hard x-ray and ultraviolet photoemission spectroscopies. The spectral weight at E-F is suppressed below the MI transition temperature T-MI, while there is no appreciable change across the pseudogap temperature T-PG. The estimated energy scale of the gap is similar to 110 meV, which is in good correspondence to the spin gap opening observed in the previous NMR study. According to the band structure calculation, the density of states at E-F mainly originates from narrow bands of the Ru 4d(xy), orbitals. Our results suggest that the fourfold-degenerate Ru 4d(xy), orbitals are deeply related to the MI transition. Based on the photoemission results and the band structure calculation, we argue the possible origin of the MI transition for polycrystalline RuP.

Ag-loaded TiO2 photocatalysts prepared by photodeposition method in an argon atmosphere exhibited highly selective photocatalytic activity for CO2 reduction with water to produce CO, while the sample prepared under an air atmosphere predominantly promoted water splitting.

The isovalent-substituted iron pnictide compound SrFe 2 (As 1− x P x ) 2 exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe 2 (As 0.65 P 0.35 ) 2 in three-dimensional momentum space by synchrotron and laser-based angle-resolved photoemission spectroscopy. The three hole Fermi surfaces (FSs) at the zone center have SC gaps with different magnitudes, whereas the SC gaps of the electron FSs at the zone corner are almost isotropic and k z -independent. As a possible nodal SC gap structure, we propose that the SC gap of the outer hole FS changes sign around the Z-X [(0, 0, 2 π ) − ( π , π , 2 π )] direction.

We have performed a photoemission study of the Mott-Hubbard system Nd1-xSrxVO3 (x = 0.20 and 0.30) to investigate the electronic structure in the vicinity of the metal-insulator transition. By using bulk sensitive hard X-ray photoemission spectroscopy, we have observed a large coherent spectral weight near the Fermi level compared to those observed with surface-sensitive low photons. In particular, a pseudogap with an energy of similar to 0.2 eV has been observed near the Fermi level, which is consistent with a prediction with a dynamical cluster approximation calculation. In order to understand the characteristic features in the Mott-Hubbard-type metal-insulator transition, particularly the pseudogap opening at x = 0.2 and 0.3, a phenomenological model of the self-energy has been proposed.