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The local structure of La(Fe1-xMnx)AsO, showing suppression of long-range magnetic order by Mn-substitution, has been studied. Extended X-ray absorption fine structure (EXAFS) measurements at As K-edge and Fe K-edge are used to probe the atomic correlations in the FeAs-layer. We have found that both Fe-As and Fe-Fe bondlengths tend to increase with increasing Mn concentration. The associated mean square relative displacements (MSRD) reveal a gradual increase indicating increased local disorder in the FeAs-layer with Mn content. The results reveal that the FeAs-layer thickness, measured by the As-height (hAs) from the Fe-Fe plane, is correlated with the Neel temperature. Furthermore, Fe Kβ X-ray emission spectroscopy (XES) shows a sharp increase in the Fe local magnetic moment with Mn substitution. The importance of the FeAs layer thickness in describing magnetic correlations in La(Fe1−xMnx)AsO is discussed.

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 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.

Recently, ammonia-thermal reaction has been used for molecular intercalation in layered FeSe, resulting a new Lix(NH3)yFe2Se2 superconductor with Tc ~ 45 K. Here, we have used temperature dependent extended x-ray absorption fine structure (EXAFS) to investigate local atomic displacements in single crystals of this new superconductor. Using polarized EXAFS at Fe K-edge we have obtained direct information on the local Fe-Se and Fe-Fe bondlengths and corresponding mean square relative displacements (MSRD). We find that the Se-height in the intercalated system is lower than the one in the binary FeSe, suggesting compressed FeSe4 tetrahedron in the title system. Incidentally, there is hardly any effect of the intercalation on the bondlengths characteristics, revealed by the Einstein temperatures, that are similar to those found in the binary FeSe. Therefore, the molecular intercalation induces an effective compression and decouples the FeSe slabs. Furthermore, the results reveal an anomalous change in the atomic correlations across Tc, appearing as a clear decrease in the MSRD, indicating hardening of the local lattice mode. Similar response of the local lattice has been found in other families of superconductors, e.g., A15-type and cuprates superconductors. This observation suggests that local atomic correlations should have some direct correlation with the superconductivity.

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

Many technological applications demand large amount of nanoparticles with well-defined properties, which is feasible only by using large-scale production methods. In this framework, we have performed structural and local geometric investigations of cobalt oxide nanoparticles synthesized by high temperature arc plasma route in helium and in air atmosphere with different arc currents, a competitive and low cost technological approach to synthesize large quantity of different types of nanoparticles. The complex scenario of phase fraction, shape, size distribution and hysteresis loop features of high temperature arc plasma synthesis of nanoparticles can be determined by the arc current and the selected gas. X-ray diffraction patterns reveal a multicomponent phase formation containing cubic cobaltous oxide (CoO), cobaltic oxide (Co3O4) and metallic cobalt phases. The synthesis of different phases is confirmed by x-ray absorption spectroscopy measurements at the Co K-edge. Both extended x-ray absorption fine structure and x-ray absorption near edge structure analyses show the presence of metallic nanoparticles in He ambient at high arc current. Moreover, high-resolution transmission electron microscopy images and magnetic hysteresis loop measurements show that the mean particle size increases and the coercivity decreases with increasing arc current in air ambient due to the intense particle–particle interaction. At variance, in He ambient synthesized samples due to the high quenching rate and the high thermal conductivity, a multi-domain formation in which the nanoparticles’ crystalline fraction decreases and a fluctuating coercivity due to core–shell structure is observed.

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.

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 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 the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

We report the experimental results of temperature dependent polarized As K-edge extended x-ray absorption fine structure (EXAFS) of LaFe1-xCoxAsO (x=0.0 and 0.11) single-crystals. By aligning the Fe-As bond direction in the direction of the x-ray beam polarization we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhombic transition at 150K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility, linear thermal expansion, etc indicating the emergence of the dynamical oscillations of the Fe - As pairs, 7 pages, 3 figures

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.

Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS$_{2-x}$Se$_x$ thermoelectric material. It is found that Se substitution effectively suppresses local distortion, that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials., Comment: 6 pages, 4 figures, 1 table, J. Appl. Phys. (in press)

The effect of electronic correlation on $\mathrm{WT}{\mathrm{e}}_{2}$ has been investigated by means of angle-resolved photoemission spectroscopy (ARPES) and band-structure calculation. In the ARPES results, the valence band top with Te $5p$ character reaches the Fermi level and forms complicated hole pockets around the zone center. In addition, two electron pockets are observed besides the hole pockets. The position of the electron pockets is inconsistent with the prediction of band calculation, suggesting the importance of electronic correlation in $\mathrm{WT}{\mathrm{e}}_{2}$. The present analysis indicates the correlation effect is associated with the excitonic coupling between the W $5d$ electron and Te $5p$ hole bands.

We have investigated the electronic properties of ${\mathrm{BiS}}_{2}$-based superconductors by using x-ray photoemission spectroscopy (XPS). In going from $x=0.3$ to 0.5 in ${\mathrm{PrO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the Pr $3d$ and Pr $4d$ peaks are shifted by $\ensuremath{\sim}0.10\ifmmode\pm\else\textpm\fi{}0.05$ eV from the Fermi level, partially consistent with the electron doping. In ${\mathrm{PrO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the ${\mathrm{Pr}}^{3+}\ensuremath{-}{\mathrm{Pr}}^{4+}$ mixed valence remains unchanged with the electron doping from $x=0.3$ to 0.5. In ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the doped electrons for $x=0.5$ almost suppress the ${\mathrm{Ce}}^{3+}\ensuremath{-}{\mathrm{Ce}}^{4+}$ valence fluctuation. Although the core-level peaks are also shifted by $\ensuremath{\sim}0.10\ifmmode\pm\else\textpm\fi{}0.05$ eV towards the higher-binding-energy side with the electron doping from $x=0$ to 0.5 in ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$, the Bi $4{f}_{7/2}$ binding-energy shift is higher in the Pr system compared with the Ce system. The present results suggest that the doped electrons increase orbital occupations in the rare-earth $4f$ orbitals at the valence band and show valence fluctuations differently in the two systems.

The local structures of 122-type paradium arsenides, namely BaPd2As2 and SrPd2As2, are examined by As K-edge extended x-ray absorption fine structure measurements to find a possible correlation between the variation of their superconducting transition temperature and the local structure. The local atomic distances are found to be consistent with average distances measured by diffraction techniques. The temperature dependence of mean square relative displacements reveal that, while BaPd2As2 is characterized by a local As-Pd soft mode, albeit with larger atomic disorder, SrPd2As2 shows anomalous As-Pd correlations with a kink at similar to 160 K due to hardening by raising temperature. We have discussed implications of these results and possible mechanisms of differing superconducting transition temperature in relation with the structural instability.

The local structure of La(Fe1−x Mn x )AsO has been investigated using temperature dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements. The EXAFS data reveal distinct behavior of Fe–As and Fe–Fe atomic displacements with a clear boundary between x ⩽ 0.02 and x > 0.02. The Fe–As bondlength shows a gradual thermal expansion while the Fe–Fe bond manifests a temperature dependent anomaly at ∼180 K for x > 0.02. It is interesting to find characteristically different nature of Fe–As and Fe–Fe bondlengths shown by the temperature dependent mean square relative displacements. Indeed, the Fe–As bond, stiffer than that of the Fe–Fe, gets softer for x ⩽ 0.02 and hardly shows any change for x > 0.02. On the other hand, Fe–Fe bond tends to be stiffer for x ⩽ 0.02 followed by a substantial softening for x > 0.02. Such a distinction has been seen also in the As K-edge x-ray absorption near edge structure, probing local geometry around As atom together with the valence electronic structure. The results suggest that local atomic displacements by Mn substitution inducing increased iron local magnetic moment that should be the main reason for its dramatic effect in iron-based superconductors.

We have investigated the local structure of layered La 1 − x Ce x OBiSSe system by Bi L 3 -edge extended X-ray absorption fine structure (EXAFS) measurements for different Ce substitutions. Ce L 3 -edge X-ray absorption spectroscopy (XAS) has been used to evaluate the Ce valence responsible for the self-doping in this system. We have found that the local distortion, determined by the separation between two Bi-Ch distances within the BiCh 2 -layer (Ch=S,Se), is quickly suppressed by Ce substitution while the axial Bi-S2 bond elongates. Ce L 3 -edge XAS reveals a coexistence of Ce 3 + and Ce 4 + in which the Ce 4 + weight decreases, an indication of a partial breaking of R E -S-Bi ( R E =La/Ce) charge transfer channel with Ce substitution. The results suggest that interaction between R E O spacer layer and BiCh 2 layer, dictated by the out-of-plane Bi-S2 distance, has a significant role in triggering superconductivity in the title system, with the in-plane distortion controlling the charge mobility within the BiCh 2 -layer.

We report an angle-resolved photoemission spectroscopy study on NaSn2As2 in which a Sn 5p band is expected to provide interesting Fermi surface geometry with hole-like and electron-like curvatures....

We have investigated the local structure of differently charged NaxCoO2 cathode material as a function of temperature by Co K-edge X-ray absorption fine structure (EXAFS) measurements. We have found that the charge/discharge process has a direct effect on the bond characteristics of the cathode in the Na-ion batteries. The results reveal that the local Co–O bonds get softer, while the Co–Co bonds hardly show any change during discharge (sodiation). The present study underlines the key role of local atomic displacements in diffusion and the reversibility of ions in cathodes for batteries and points toward the feasibility of NaxCoO2 to be used as a cathode material. The results are discussed in comparison with the lithiation/delithiation of LixCoO2 battery materials.

Ab initio X-ray absorption near edge structure (XANES) calculations for FeSe x Te 1− x , using a structural model that combines FeSe and FeTe phases at the nanoscale, are compared with Fe K -edge XANES measurements in the “pre-edge” region. The important aspects of this model are (i) magnetic order in the FeTe phase; (ii) Se and Te atoms placed randomly in both FeSe and FeTe crystallographic positions and; (iii) the two distinct distances for Fe–Se and Fe–Te of the bulk phases. The calculated spectra reproduce the observed increase of spectral weight in the experiments on FeSe x Te 1− x with Se concentration. This is consistent with an inhomogeneous local electronic structure of FeSe x Te 1− x . Additionally, we have calculated projected electronic density of d -states for the Fe atom, revealing increased spectral weight in the “pre-edge” region of the XANES spectra, which correlates with the increase in Se concentration. The decrease of calculated Fe d -density of states for the Fermi level, N ( e F ), for high Te content is consistent with the suppression of superconductivity in the title system.

We have studied a charge-orbital driven metal-insulator transition (MIT) in hollandite-type ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$ by means of hard x-ray photoemission spectroscopy (HAXPES). The Ti $2p$ HAXPES indicates strong ${\mathrm{Ti}}^{3+}$/${\mathrm{Ti}}^{4+}$ charge fluctuation in the metallic phase above the MIT temperature. The metallic phase is characterized by a power-law spectral function near the Fermi level which would be a signature of bad metal with non-Drude polaronic behavior. The power-law spectral shape is associated with the large Seebeck coefficient of the metallic phase in ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$.

We have studied the valence electronic structure of Ag

We have performed scanning photoemission spectromicroscopy of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ across the metal-insulator transition at 123 K, which is accompanied by V $3d$ charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ single crystals, similar to Cr-doped ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ in which electronic configurations of ${\mathrm{Cr}}^{3+}$ and ${\mathrm{V}}^{3+}$ are the same as those of ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$. Typical domain size is $\ensuremath{\sim}5\text{--}10\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ is different from that of Cr-doped ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$, probably due to the charge degrees of freedom in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$.

We report an angle-resolved photoemission spectroscopy (ARPES) study of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ single crystals which have a hole-doped ${\mathrm{CoO}}_{2}$ triangular lattice. Similar to ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, the Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ band crosses the Fermi level with strongly renormalized band dispersion while the Co $3d\phantom{\rule{4pt}{0ex}}{e}_{g}^{\ensuremath{'}}$ bands are fully occupied in ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ ($x=0.46$ and 0.71). At $x=0.46$, the Fermi surface area is consistent with the bulk hole concentration indicating that the ARPES result represents the bulk electronic structure. On the other hand, at $x=0.71$, the Fermi surface area is larger than the expectation which can be associated with the inhomogeneous distribution of Li reported in the previous scanning tunneling microscopy study by Iwaya et al. [Phys. Rev. Lett. 111, 126104 (2013)]. However, the Co $3d$ peak is systematically shifted towards the Fermi level with hole doping excluding phase separation between hole rich and hole poor regions in the bulk. Therefore, the deviation of the Fermi surface area at $x=0.71$ can be attributed to hole redistribution at the surface avoiding polar catastrophe. The bulk Fermi surface of Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ is very robust around $x=0.5$ even in the topmost ${\mathrm{CoO}}_{2}$ layer due to the absence of the polar catastrophe.

The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr2IrO4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds.

We have studied the nanoscale structure and unoccupied electronic states in FeSe1-xTex by a combined analysis of Se K, Te L1 and Fe K-edges X-ray absorption measurements. Extended X-ray absorption fine structure (EXAFS) results show that iron-chalcogen (Fe-Se and Fe-Te) distances in ternary FeSe1-xTex are similar to those measured for binary FeSe and FeTe. The local Fe-Se/Te distances determined by different absorption edges fit well in the characteristic Z-plot of random alloys, providing unambiguous support to the inhomogeneous nanoscale structure of the ternary FeSe1-xTex system. X-ray absorption near-edge structure (XANES) spectra reveal a gradual evolution of the unoccupied valence electronic states as a function of Te-substitution in FeSe1-xTex. The Fe 3d-Se 4p/Te 5p hybridization is found to decrease with Te-substitution, accompanied by an increase in unoccupied Se 4p states and a decrease in unoccupied Te 5p states. The results are discussed in the frame of local inhomogeneity in the FeSe1-xTex system driven by random alloying of Se/Te atoms.

We have studied the effect of Se substitution on Eu valence in a layered ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ superconductor using a combined analysis of x-ray absorption near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu ${L}_{3}$-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. The average Eu valence decreases sharply from $\ensuremath{\sim}2.3$ for $x=0.0$ to $\ensuremath{\sim}2.1$ for $x=0.4$. Consistently, Eu $3d$ XPS shows a clear decrease in the average valence by Se substitution. Bi $4f$ XPS indicates that effective charge carriers in the ${\mathrm{BiCh}}_{2}$ (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.

We have studied the electronic structure of ${\mathrm{Eu}}_{3}{\mathrm{F}}_{4}{\mathrm{Bi}}_{2}{\mathrm{S}}_{4}$ using a combination of Eu ${L}_{3}$-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu ${L}_{3}$-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of ${\mathrm{Eu}}^{2+}/{\mathrm{Eu}}^{3+}$. The bulk charge doping was estimated to be $\ensuremath{\sim}0.3$ per Bi site in ${\mathrm{Eu}}_{3}{\mathrm{F}}_{4}{\mathrm{Bi}}_{2}{\mathrm{S}}_{4}$, which corresponds to the nominal $x$ in a typical RE${\mathrm{O}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$ system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the $X$ point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in ${\mathrm{Eu}}_{3}{\mathrm{F}}_{4}{\mathrm{Bi}}_{2}{\mathrm{S}}_{4}$, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi

Archimedes is a feasibility study to a future experiment to ascertain the interaction of vacuum fluctuations with gravity. The future experiment should measure the force that the earth's gravitational field exerts on a Casimir cavity by using a balance as the small force detector. The Archimedes experiment analizes the important parameters in view of the final measurement and experimentally explores solutions to the most critical problems.

We have performed Ce ${L}_{3}$-edge x-ray absorption spectroscopy (XAS) and Ce $4d\ensuremath{-}4f$ resonant photoemission spectroscopy (PES) on single crystals of ${\mathrm{CeO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}{\mathrm{BiS}}_{2}$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$ electronic states. In Ce ${L}_{3}$-edge XAS, a mixed valence of Ce was found in the $x=0.0$ sample, and F doping suppressed it, which is consistent with the results on polycrystalline samples. As for resonant PES, we found that the Ce $4f$ electrons in both $x=0.0$ and 0.5 systems respectively formed a flat band at 1.0 and 1.4 eV below the Fermi level and there was no contribution to the Fermi surfaces. Interestingly, Ce valence in ${\mathrm{CeOBiS}}_{2}$ deviates from ${\mathrm{Ce}}^{3+}$ even though Ce $4f$ electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce $4f$ in ${\mathrm{CeOBiS}}_{2}$ is mixed with unoccupied Bi $6{p}_{z}$, which is consistent with a previous local structural study. Based on the analysis of the Ce ${L}_{3}$-edge XAS spectra using Anderson's impurity model calculation, we found that the transfer integral becomes smaller, increasing the number of Ce $4f$ electrons upon the F substitution for O.

A quantitative determination of the statistical distribution of the Mn-O instantaneous bond lengths in the La0.75Ca0.25MnO3 system by Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy shows that the giant magneto-resistance (GMR) transition occurs at a crossover from a metallic phase with large (or intermediate) polarons to an insulating phase where small polarons coexist with large (or intermediate) polarons. In the GMR region, the two Mn polaronic domains coexist and share half of the space. © 1998 Published by Elsevier Science Ltd. All rights reserved.

We have studied local structure of LiMnO2, LiMn0.65Cr0.35O2 and LiMn0.5Ni0.5O2 compounds by Mn K-edge extended X-ray absorption fine structure measurements. The local structure of LiMnO2 is found to be consistent with Jahn Teller distorted MnO6 octahedra characterized by two different Mn-O bond distances. The Jahn Teller distortions are suppressed in the Cr and Ni substituted compounds, resulting a single Mn-O distance. However, the Cr atoms tend to occupy a site at a longer distance from Mn in the host lattice (Mn-Cr distance is longer than Mn-Mn distance), unlike the Ni atoms which prefer a site closer to the Mn atoms (Mn-Ni distance is shorter than Mn Mn distance). Incidentally, Mn-O and Mn Mn bonds are substantially stiffer in the Cr and Ni substituted compounds. In addition, the static atomic disorder is confined around Cr atoms in the LiMn0.65Cr0.35O2, that is different from the case of LiMn0.5Ni0.5O2 in which larger static disorder appears in the proximity of the Mn atoms. The results suggest that the differences in the local structure of different compounds should be the likely reason for their differing battery characteristics. (C) 2013 Elsevier B.V. All rights reserved.

Ab initio Fe K-edge X-ray absorption near edge structure (XANES) calculations on FeTe1−x Se x , based on its crystallographic structure, are compared with experimental data. The calculated XANES spectra are inconsistent with the observed increase of spectral weight in the preedge region of the experimental spectra on the ternary system in which Te is substituted by Se. However, the weighted average of the XANES of FeTe and FeSe binaries reproduce well the observed trend in the experimental spectra of the ternary systems. This suggests that the FeTe1−x Se x is characterized by an inhomogeneous local electronic structure, resulting from a random-alloy like local atomic structure. The calculated unoccupied Fe d-density of states at the Fermi level increases monotonically with Te content unlike the observed change in T c with Te content. T c increases with increasing Te content up to x=0.5; however, the superconductivity is suppressed in the FeTe due to magnetic order, which is not considered in these calculations.

Isovalent substitution of S by Se in LaOBiS2-x Se x has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS2 structural instability, we have studied the local structure of LaOBiS2-x Se x ([Formula: see text]) using temperature dependent Bi L3-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS2 sub-lattice is largely distorted in LaOBiS2 (x = 0.0), with two in-plane Bi-S1 distances separated by ∼0.4 A instead LaOBiSSe (x = 1.0) showing much smaller local disorder with two in-plane Bi-Se distances in the plane being separated by ∼0.2 A. Temperature dependent study shows that the two Bi-S1 distances are characterized by different bond strength in LaOBiS2 (x = 0.0) while it is similar for the Bi-Se distances in LaOBiSSe (x = 1.0). The out of plane Bi-S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS2 layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS2-x Se x is an important factor to describe differing electronic and thermal transport of the two compounds.

In our previous studies on the tolerance of living organisms such as planktons and spores of mosses to the high hydrostatic pressure of 7.5 GPa, we showed that all the samples could be borne at this high pressure. These studies have been extended to the extreme high pressure of 20 GPa by using a Kawai-type octahedral anvil press. It was found that the average diameter of the spores of Venturiella exposed to 20 GPa for 30 min was 25.5 μm, which is 16.5% smaller (40.0% smaller in volume) than that of the control group which was not exposed to high pressure. The inner organisms showed a further extent of plastic deformation. As a result, a gap appeared between the outer cover and the cytoplasm. A relationship has been obtained between the survival ratio and plastic deformation of spores of moss Venturiella caused by the application of ultra high pressure.

Temperature dependent Co K-edge extended X-ray absorption fine structure is used to investigate local disorder in LiCoO 2 nanoparticles. We find that the nanostructuring has direct influence on the bondlength characteristics. The results reveal a substantial decrease in the force constant of Co–O bonds (the Co–O bonds becoming more flexible), while that for the Co–Co bonds showing hardly any change (or increases slightly) in LiCoO 2 nanoparticles with respect to the bulk. Therefore, both random disorder and Co–O bondlength flexibility should be the factors to limit the battery characteristics of the LiCoO 2 nanoparticles.

We investigated the magnetic flux-pinning in iron-based 1111-type (REFeAsO, RE= rare earth) and 11- type (FeSe1−xTex) superconductors by third harmonic acmagnetic susceptibility measurements, a technique sensitive to the dynamics of magnetization, probing non-linear processes in flux pinning transport. Despite the larger thermal fluctuations due to the high critical temperature and large anisotropy, flux dynamics of 1111-type pnictides points out a more efficient pinning mechanism than the one in the 11-type chalcogenides. We have associated the stronger pinning in the 1111-family to the presence of REO (RE=rare earth) spacer layers separating the active FeAslayers unlike the 11-chalcogenides in which no spacer layers are present. Therefore, the disorder in the FeAs layers induced by the misfit strain due to spacer layers has an important role in the enhanced flux pinning of 1111-type superconductors.

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi4Te6 and superconducting CsBi3.5Pb0.5Te6 (T-c similar to 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L-3 edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L-3 edge XANES is hardly affected by Pb substitution while the Te-L-1 edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi4-xPbxTe6 preferentially goes on Te sites. Similarly, the Cs-L-3 edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L3 edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi4Te6 into a bulk superconductor by Pb-substitution in CsBi4-xPbxTe6.

We have studied the electronic structure of FeSe1−xTex and Ir1−xPtxTe2 using photoemission spectroscopy. For FeSe1−xTex, angle-resolved photoemission results indicate that the Fe 3d yz/zx orbital degeneracy at Γ point and orbitally induced Peierls effect in the tetragonal lattice play important roles for the superconductivity. It is suggested that the Jahn-Teller instability of the yz/zx states couples with local lattice distortion derived from the Te substitution for Se and provides an inhomogeneous electronic state. Photoemission results of IrTe2 with triangular lattice are also consistent with the orbitally induced Peierls scenario. The Pt substitution for Ir suppresses the static band Jahn–Teller effect and induces an inhomogeneous electronic state in which orbital (or bond or nematic) fluctuations may help superconductivity through the Peierls effect.