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The lower Ti–Tx disorder for the Ti2CTx than the Ti3C2Tx system results in a higher specific capacitance.

PbCrO3 features an unusual charge distribution Pb0.52+Pb0.54+Cr3+O3 with Pb charge disproportionation at ambient pressure. A charge transfer between Pb and Cr is induced by the application of pressure resulting in Pb2+Cr4+O3 charge distribution and a large volume collapse. Here, structural and charge distribution changes in PbCr1-xVxO3 are investigated. Despite a cubic crystal structure in 0 ≤ x ≤ 0.60, discontinuous reduction in the unit cell volume was observed between x = 0.35 and 0.40. Hard X-ray photoemission spectroscopy confirmed the change in Pb charge state from the coexisting Pb2+ and Pb4+ at x = 0.35 to single Pb2+ at x = 0.40. This indicates that V substitution stabilizes the high pressure cubic Pb2+Cr4+O3-type phase. With further increase in the V substitution, the PbVO3-type polar tetragonal phase appeared at x = 0.80.

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFexO3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1-x)Ni2+1-xFe3+xO3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.

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 respon...

We have investigated band structure and edge states of single and AA bilayer honeycomb lattice systems with p orbital degeneracy which would be relevant for relatively heavy metals such as Pb and Bi. The p electrons, which are triply degenerate in the atomic limit, form quadruple degeneracy at K point in a single layer honeycomb lattice. The spin-orbit interaction splits the quartet at K point into two singlets and one doublet which are mixtures of p x , p y and p z orbitals. In the AA bilayer honeycomb lattice, the Dirac node at K point is deformed into circular line node around it. With the intermediate interlayer coupling, there is a degenerate point along Γ-K with p z character. A p x /p y type degenerate point exists under the strong interlayer coupling. This degenerate point is robust against the spin-orbit interaction and the antiferromagnetic spin arrangement only along the intralayer bonds. Without SOI, localized edge states with p x /p y or p z character are created at the zigzag edges both in the single and AA bilayer systems. In the AA bilayer system with SOI, only the p x /p y edge states exhibit the Dirac-like dispersion.

Recently the structural transition in Na2Ti3Cl8 at 200 K with the formation of triangular Ti3 clusters was studied by ab-initio calculations in PRL 124, 167203 (2020). It was concluded on the basis of the DFT+U results that the usual band effects are not sufficient to explain this transition. The authors then invoked magnetic mechanisms, including complicated factors such as biquadratic and ring exchanges. We point out that this overcomplicated description is unnecessary, and that the formation of trimers, or breathing kagome lattice in Na2Ti3Cl8, and in many other systems, has very simple, almost trivial explanation basing on the orbital-driven Peierls transition.

We report x-ray and optical absorption spectroscopy as well as hard x-ray photoemission spectroscopy on hexagonal ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$ in which a Cu $3d$ spin-orbital liquid state is suggested from absence of the Jahn-Teller distortion of ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{6}$ octahedra and of the magnetic ordering. The experimental results and their cluster model analysis indicate that O $2p$ holes play a crucial role in realizing the spin-orbital liquid state of hexagonal ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$. These oxygen holes appear due to the ``reaction'' ${\mathrm{Sb}}^{5+}\ensuremath{\rightarrow}{\mathrm{Sb}}^{3+}\phantom{\rule{4pt}{0ex}}+$ two O $2p$ holes, with these holes being able to attach to Cu ions. The present work opens avenues towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.

Perovskite-oxide Bi1–xPbxNiO3 for 0.60 ≤ x ≤ 0.80 was found to show a polar orthorhombic-to-nonpolar orthorhombic phase transition accompanied by negative thermal expansion. Bi1–xPbxNiO3 showed suc...

A spin transition between high-spin (HS) and low-spin (LS) states in a solid can occur when the energies of two spin configurations intersect, which is usually caused by external perturbations such...

Angle-resolved photoemission spectromicroscopy on IrTe2 reveals evolution of mesoscopic striped domains across its first order phase transition at about 280 K. The striped texture of the domains is characterized by a herringbone arrangement of the electronic anisotropy axes. Under further cooling down to 47 K, the striped domains evolve into trijunction domains with the electronic anisotropy in three directions. Each domain harbors quasi one-dimensional surface bands forming Fermi arcs with peculiar spin polarization. The Fermi arc corresponds to an edge state of the two-dimensional bulk electronic states truncated at the surface, indicating an interesting interplay between the symmetry breaking and the surface electronic states., Comment: 11 pages + 5 figures

We review recent advances in strongly correlated oxides as thermoelectric materials in pursuit of energy harvesting. We discuss two topics: one is the enhancement of the ordinary thermoelectric properties by controlling orbital degrees of freedom and orbital fluctuation not only in bulk but also at the interface of correlated oxides. The other topic is the use of new phenomena driven by spin-orbit coupling (SOC) of materials. In 5d electron oxides, we show some SOC-related transport phenomena, which potentially contribute to energy harvesting. We outline the current status and a future perspective of oxides as thermoelectric materials., Graphical Abstract

PbCrO

We have investigated the electronic structure evolution with F doping in $\mathrm{Pr}{\mathrm{O}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}\mathrm{Bi}{\mathrm{S}}_{2}$ ($x=0.0,0.3,0.5$) by means of angle-resolved photoemission spectroscopy. Undoped $\mathrm{Pr}\mathrm{O}\mathrm{Bi}{\mathrm{S}}_{2}$ exhibits Fermi surface (FS) pockets around the $X$ point which can be associated with electron doping due to the ${\mathrm{Pr}}^{3+}/{\mathrm{Pr}}^{4+}$ mixed valence. At $x=0.3$, the FS pockets are expanded, and their area is almost consistent with the F doping level. Interestingly, horizontal segments facing each other in the rectangular FS pockets lose their spectral weight. The partial suppression of the FS pockets suggests possible anisotropic charge fluctuations around $x=0.3$. At $x=0.5$, the FS area is much smaller than the expected value as commonly observed for $\mathrm{Ce}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}\mathrm{Bi}{\mathrm{S}}_{2}$ and $\mathrm{Nd}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}\mathrm{Bi}{\mathrm{S}}_{2}$. In addition, the highly depleted spectral weight at the FS pockets would be consistent with evident charge fluctuations driven by atomic disorder in the $\mathrm{Bi}{\mathrm{S}}_{2}$ network.

Photo-induced phase transitions have been intensively studied owing to the ability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it has currently been a central issue how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photo-induced phase transitions. Here, we develop a new analysis method, frequency-domain angle-resolved photoemission spectroscopy, to gain precise insight into electron-phonon couplings during photo-induced insulator-to-metal transitions for Ta$_2$NiSe$_5$. We demonstrate that multiple coherent phonons generated by displacive excitations show band-selective coupling to the electrons. Furthermore, we find that the lattice modulation corresponding to the 2 THz phonon mode, where Ta lattice is sheared along the a-axis, is the most relevant for the photo-induced semimetallic state.

The heavy fermion state with Kondo-hybridization (KH), usually manifested in f-electron systems with lanthanide or actinide elements, was recently discovered in several 3d transition metal compounds without f-electrons. However, KH has not yet been observed in 4d/5d transition metal compounds, since more extended 4d/5d orbitals do not usually form flat bands that supply localized electrons appropriate for Kondo pairing. Here, we report a doping- and temperature-dependent angle-resolved photoemission study on 4d Ca2-xSrxRuO4, which shows the signature of KH. We observed a spectral weight transfer in the γ-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localized γ-band induces the KH with an itinerant β-band, resulting in spectral weight suppression around the Fermi level. Our work is the first to demonstrate the evolution of the OSMP with possible KH among 4d electrons, and thereby expands the material boundary of Kondo physics to 4d multi-orbital systems.

The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps = 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc = 8 GPa. Superconductivity with a maximum Tsc = 1.2 K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5., Comment: 6 pages, 4 figures

Temperature-dependent Fe K-edge extended X-ray absorption fine structure (EXAFS) and Fe Kβ X-ray emission spectroscopy (XES) measurements are used to study the local structure and local Fe magnetic...

We report time- and angle-resolved photoemission spectroscopy on SnSe which currently attracts great interest due to its extremely high thermoelectric performance. Laser-assisted photoemission signals are observed within $\pm$20 fs of the pump pulse arrival. Around 30-50 fs after the photoexcitation, the conduction band minima are not populated by the photoexcited electrons while the valence bands are considerably broadened. In going from 90 fs to 550 fs after the photoexcitation, the photoexcited carriers are decayed into the multiple conduction band minima. The observed conduction bands are consistent with the band structure calculations. The multiple conduction minima suggest possibility of high and anisotropic thermoelectric performance of n-type SnSe single crystal if it is realized., Comment: 5 pages, 4 figures

We have investigated the electronic structure of protect annealed Pr1.3−xLa0.7CexCuO4 (x = 0.05) with small superconducting volume fraction by means of angle-resolved photoemission spectroscopy (AR...

The transition metal chalcogenide ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ undergoes a second-order phase transition at ${T}_{c}=328\phantom{\rule{0.16em}{0ex}}\text{K}$ involving a small lattice distortion. Below ${T}_{c}$, a band gap at the center of its Brillouin zone increases up to about 0.35 eV. In this work, we study the electronic structure of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ in its low-temperature semiconducting phase, using resonant inelastic x-ray scattering (RIXS) at the Ni ${L}_{3}$ edge. In addition to a weak fluorescence response, we observe a collection of intense Raman-like peaks that we attribute to electron-hole excitations. Using density functional theory calculations of its electronic band structure, we identify the main Raman-like peaks as interband transitions between valence and conduction bands. By performing angle-dependent RIXS measurements, we uncover the dispersion of these electron-hole excitations that allows us to extract the low-energy boundary of the electron-hole continuum. From the dispersion of the valence band measured by angle-resolved photoemission spectroscopy, we derive the effective mass of the lowest unoccupied conduction band.

The ${\mathrm{BiS}}_{2}$-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the ${\mathrm{EuFBiS}}_{2}$ compound, showing a superconducting transition temperature ${T}_{c}\ensuremath{\sim}0.3$ K at ambient pressure. Upon increasing external pressure, ${T}_{c}$ goes through a large amplification, accompanied by a structural phase transition (SPT) from tetragonal to monoclinic symmetry. Here, we use a combination of Eu ${\phantom{\rule{4pt}{0ex}}L}_{3}$ - edge x-ray absorption spectroscopy and synchrotron x-ray diffraction to unveil the evolution of the Eu valence and lattice symmetry under high pressure. We find that the average Eu valence increases gradually with pressure, exhibiting a pressure plateau near the SPT, at which the ${T}_{c}$ increases sharply. Since in ${\mathrm{EuFBiS}}_{2}$ the charge carriers are introduced via self-doping induced by the mixed valence of the Eu ions, our findings clearly indicate that the role of the charge doping is marginal in the ${T}_{c}$ enhancement. On the other hand, the structural distortions, taking place at the SPT, play a central role in enhancing the superconducting properties of the ${\mathrm{EuFBiS}}_{2}$ system.

Electronic properties of ${\mathrm{V}}_{2}{\mathrm{OPO}}_{4}$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and subsequent theoretical calculations. The V $1s$ and $2p$ HAXPES spectra are consistent with the charge ordering of ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$. The binding energy difference between the ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ components is unexpectedly large, indicating large bonding-antibonding splitting between them in the final states of core level photoemission. The V $1s$ HAXPES spectrum exhibits a charge transfer satellite which can be analyzed by configuration interaction calculations on a ${\mathrm{V}}_{2}{\mathrm{O}}_{9}$ cluster. The V $3d$ spectral weight near the Fermi level is assigned to the $3d\phantom{\rule{4pt}{0ex}}{t}_{2g}$ orbitals of the ${\mathrm{V}}^{2+}$ site. The broad V $3d$ spectral distribution is consistent with the strong hybridization between ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ in the ground state. The core level and valence band HAXPES results indicate substantial charge transfer from the ${\mathrm{V}}^{2+}$ site to the ${\mathrm{V}}^{3+}$ site.

V $3d$ charge and orbital states in ${\mathrm{V}}_{2}{\mathrm{OPO}}_{4}$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of ${\mathrm{V}}_{2}{\mathrm{OPO}}_{4}$ is very unique in that the charge transfer between ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ in face-sharing ${\mathrm{VO}}_{6}$ chains provides negative thermal expansion as reported by E. Pachoud et al. [J. Am. Chem. Soc. 140, 636 (2018)]. The near-edge region of O $1s$ XAS exhibits the three features which can be assigned to transitions to O $2p$ mixed into the unoccupied V $3d\phantom{\rule{4pt}{0ex}}{t}_{2g}$ and ${e}_{g}$ orbitals of ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$. The V $2p$ XAS line shape can be reproduced by multiplet calculations for a mixed-valence state with ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$. The polarization dependence of the O $1s$ and V $2p$ XAS spectra indicates V $3d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the ${\mathrm{V}}^{3+}$ site in the face-sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ sites along the chains, while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ sites between the chains.

Physical review / B 101(2), 020508 (2020). doi:10.1103/PhysRevB.101.020508, The coexistence of phases, characterized by different electronic degrees of freedom, commonly occurs in layered superconductors. Among them, alkaline intercalated chalcogenides are model systems showing the microscale coexistence of paramagnetic (PAR) and antiferromagnetic (AFM) phases, however, the temporal behavior of different phases is still unknown. Here, we report a visualization of the atomic motion in the granular phase of KxFe2−ySe2 using x-ray photon correlation spectroscopy. Unlike the PAR phase, the AFM texture reveals an intermittent dynamics with avalanches as in martensites. When cooled down across the superconducting transition temperature Tc, the AFM phase goes through an anomalous slowing behavior, suggesting a direct relationship between the atomic motions in the AFM phase and the superconductivity. In addition to providing compelling evidence of avalanchelike dynamics in a layered superconductor, the results provide a basis for different theoretical models to describe the quantum states in inhomogeneous solids., Published by Inst., Woodbury, NY

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 ${E}_{\mathrm{F}}$ is suppressed below the MI transition temperature ${T}_{\mathrm{MI}}$, while there is no appreciable change across the pseudogap temperature ${T}_{\mathrm{PG}}$. The estimated energy scale of the gap is $\ensuremath{\sim}110$ 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 ${E}_{\mathrm{F}}$ mainly originates from narrow bands of the Ru $4{d}_{xy}$ orbitals. Our results suggest that the fourfold-degenerate Ru $4{d}_{xy}$ 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.

We have investigated electronic structure evolution by cation substitution in ${\mathrm{Ba}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ by means of hard x-ray photoemission spectroscopy. Localization of Nb $4d$ electrons manifests as spectral weight transfer from a coherent component at the Fermi level to an incoherent one at 1--2 eV below it. This behavior is similar to that of electron-doped ${\mathrm{SrTiO}}_{3}$. On the other hand, Nb $3d$ and $4p$ core level spectra exhibit a screening effect with an energy scale of 1--2 eV by the coherent Nb $4d$ electrons similar to $4d$ electron systems near Mott transitions. The energy scale indicates that electron correlation is involved in the metal to insulator transition in the present system, although the Nb $4d$ band is about 1/10 filled. The present results suggest a mechanism of electron localization due to atomic disorder and electron correlation.

Using light to manipulate materials into desired states is one of the goals in condensed matter physics, since light control can provide ultrafast and environmentally friendly photonics devices. However, it is generally difficult to realise a photo-induced phase which is not merely a higher entropy phase corresponding to a high-temperature phase at equilibrium. Here, we report realisation of photo-induced insulator-to-metal transitions in Ta2Ni(Se1−xSx)5 including the excitonic insulator phase using time- and angle-resolved photoemission spectroscopy. From the dynamic properties of the system, we determine that screening of excitonic correlations plays a key role in the timescale of the transition to the metallic phase, which supports the existence of an excitonic insulator phase at equilibrium. The non-equilibrium metallic state observed unexpectedly in the direct-gap excitonic insulator opens up a new avenue to optical band engineering in electron–hole coupled systems., Ultrafast light pulses can manipulate and probe materials faster than relaxation timescales, leading to new electronic states and insights into equilibrium properties. Okazaki et al. use the properties of photo-induced metallic states to investigate unconventional correlated behaviour in Ta2NiSe5 and Ta2NiS5.

Charge distribution changes in Bi- and Pb-3d transition metal perovskite type oxides were examined by comprehensive precise structural analysis, spectroscopy, and theoretical investigations. The change in the depth of the d level of the transition metal caused the intermetallic charge transfer. A temperature-induced charge-transfer transition in chemically modified BiNiO3 results in technologically important negative thermal expansion.

Recently CeOBiS2 system without any fluorine doping is found to show superconductivity posing question on its origin. Using space resolved ARPES we have found a metallic phase embedded in the morphological defects and at the sample edges of stoichiometric CeOBiS2. While bulk of the sample is semiconducting, the embedded metallic phase is characterized by the usual electron pocket at X point, similar to the Fermi surface of doped BiS2-based superconductors. Typical size of the observed metallic domain is larger than the superconducting correlation length of the system suggesting that the observed superconductivity in undoped CeOBiS2 might be due to this embedded metallic phase at the defects. The results also suggest a possible way to develop new systems by manipulation of the defects in these chalcogenides with structural instability.

We have studied temperature-driven effects on photoemission spectra of hollandite-type ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$ across its metal-insulator transition (MIT) at 220 K by using surface-sensitive photon energies from 27 to 70 eV. The surface Ti $3d$ spectral weight was suppressed and shifted towards higher binding energy from the bulk if the surface was obtained by cleavage above the MIT temperature. The Ti $3d$ spectral weight was recovered once the sample was cooled across the MIT, indicating a transfer of Ti $3d$ electrons from the bulk to the surface across the MIT. The recovered Ti $3d$ spectral shape is consistent with the bulk-sensitive result and indicates strong localization character of the Ti $3d$ electron even in the metallic phase. Resonant photoemission spectroscopy with absorption from the Ti $3p$ core level exhibits resonances of the O $2p$-Ti $3d$ and O $2p$-Ti $4s,p$ hybridized states. The result suggests that the O $2p$-Ti $4s,p$ hybridization plays a role in the interplay between the bulk and the surface in ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$.

We have studied electronic properties of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ in the intermediate-temperature phase as well as in the high-temperature metallic phase by using hard x-ray photoemission spectroscopy (HAXPES). The V $2p$ HAXPES show a shift in the high-temperature phase between 300 and 245 K which is similar to the shift of spectral weight near the Fermi edge. The binding energy of the O $1\mathit{s}$ HAXPES peak does not change except a slight shift of the lower binding energy edge in the opposite direction between 300 and 180 K across the transition to the intermediate-temperature phase. This behavior is in sharp contrast to the transition to the low-temperature insulating phase where V $2p$ and O $1s$ HAXPES show dramatic shifts in the same direction. This indicates that the charge-orbital change in the intermediate-temperature phase is driven by the correlated V $3d$ electrons and is electronic. The ${\mathrm{V}}^{2.5+}\text{\ensuremath{-}}{\mathrm{V}}^{2.5+}$ bond ordering is related to the metallic contribution and gradually decreases with cooling from 300 K. The valence-band HAXPES show the metallic features of the pseudogap behavior near the Fermi edge in and above the intermediate-temperature phase due to ${\mathrm{V}}^{2.5+}\text{\ensuremath{-}}{\mathrm{V}}^{3+}$ charge fluctuation. The magnitude of the pseudogap increases from 300 to 180 K in parallel with the gradual breaking of the ${\mathrm{V}}^{2.5+}\text{\ensuremath{-}}{\mathrm{V}}^{2.5+}$ bond and formation of trimers.