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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 ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$.

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 investigate superconductivity in the candidate topological superconductor ${\mathrm{Nb}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ from $x=0$ to 1. We observe superconductivity for all $xg0$ with a superconducting transition temperature ${T}_{C}\ensuremath{\sim}$ 3 K similar to that reported in Cu- and Sr-intercalated ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. Structurally, for small $x$ we find introduction of Nb modifies the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ separation while for larger $x$ the system is driven to the BiSe structure. While all compounds are found to be superconducting, two distinct behaviors for the upper critical field ${H}_{c2}$ are observed. From photoemission spectroscopy we find that the enhanced ${H}_{c2}$ corresponds to compounds with a low surface Dirac point energy ($\ensuremath{-}0.67$ eV) relative to the Fermi level; in absolute scale this value is lower than previously reported superconductors in this materials family. We comment on the utility of this system for the study of topological superconductivity in a new structure type combining a doping (${\mathrm{Bi}}_{2}$ type) layer and layers with topologically protected gapless state (${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ type).

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 use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce 4f electronic states of a new BiS2 layered superconductor CeO1-xFxBiS2, for polycrystalline and single-crystal samples. The Ce 3d spectrum of the single crystal of nominal composition x = 0.7 has no f(0) component and the spectral shape closely resembles the ones observed for Ce trivalent insulating compounds, strongly implying that the CeO layer is still in an insulating state even after the F doping. The Ce 3d-4f resonant SXPES for both polycrystalline and single-crystal samples shows that the prominent peak is located around 1 eV below the Fermi level (E-F) with negligible spectral intensity at EF. The F-concentration dependence of the valence band spectra for single crystals shows the increases of the degeneracy in energy levels and of the interaction between Ce 4f and S 3p states. These results give insight into the nature of the CeO1-xFx layer and the microscopic coexistence of magnetism and superconductivity in CeO1-xFxBiS2.

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 of layered Nax(NH3)yMoSe2 system by Mo K-edge extended X-ray absorption fine structure (EXAFS) measurements performed as a function of temperature. We find that molecular intercalation in MoSe2 largely affects the Mo-Se network while Mo-Mo seems to sustain small changes. The Einstein temperature ( Θ E ) of Mo-Mo distance hardly changes ( ∼ 264 K) indicating that bond strength of this distance remains unaffected by intercalation. On the other hand, Mo-Se distance suffers a softening, revealed by the decrease of Θ E from ∼ 364 K to ∼ 350 K. The results indicate that Na+ ion transported by NH3 molecules may enter between the two MoSe-layers resulting reduced Se-Se coupling. Therefore, increased hybridization between Se 4p and Mo 4d orbitals due to inter-layer disorder is the likely reason of metallicity in intercalated MoSe2 and superconductivity at low temperature.

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

Photoelectron spectroscopy is a powerful experimental tool for direct observation of electronic states in solids. Since the discovery of the superconductivity in LaO1−xFxBiS2 at 2012, variety of ph...

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.

We report syntheses of new superconducting metal-doped MoSe2 materials (MxMoSe2). The superconducting MxMoSe2 samples were prepared using a liquid NH3 technique and can be represented as ‘(NH3)yMxMoSe2’. The Tcs of these materials were approximately 5.0 K, independent of x and the specific metal atom. X-ray diffraction patterns of (NH3)yNaxMoSe2 were recorded using polycrystalline powders. An increase in lattice constant c showed that the Na atom was intercalated between MoSe2 layers. The x-independence of c was observed in (NH3)yNaxMoSe2, indicating the formation of a stoichiometric compound in the entire x range, which is consistent with the x-independence of Tc. A metallic edge of the Fermi level was observed in the photoemission spectrum at 30 K, demonstrating its metallic character in the normal state. Doping of MoSe2 with Li and K also yielded superconductivity. Thus, MoSe2 is a promising material for designing new superconductors, as are other transition metal dichalcogenides.

We have made $^{53}\mathrm{Cr}$ NMR measurements on polycrystalline and single-crystalline film samples to study the local electronic state of a half-metallic ferromagnet ${\mathrm{CrO}}_{2}$ which has the rutile structure with one crystallographically equivalent Cr site. We observe two kinds of $^{53}\mathrm{Cr}$ NMR spectra in the ferromagnetic state of both the samples. An analysis on the hyperfine field shows the presence of two Cr sites with different orbital occupancies, suggesting that a local orbital order takes place with breaking the local symmetry in the rutile structure. This may be ascribed to the negative charge transfer between chromium and oxygen ions.

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.

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors., Comment: 15 pages, 4 figures, + supplemental material

A soft X-ray angle-resolved photoemission system applicable to 100 µm crystals has been developed., A system for angle-resolved photoemission spectroscopy (ARPES) of small single crystals with sizes down to 100 µm has been developed. Soft X-ray synchrotron radiation with a spot size of ∼40 µm × 65 µm at the sample position is used for the excitation. Using this system an ARPES measurement has been performed on a Si crystal of size 120 µm × 100 µm × 80 µm. The crystal was properly oriented on a sample stage by measuring the Laue spots. The crystal was cleaved in situ with a microcleaver at 100 K. The cleaved surface was adjusted to the beam spot using an optical microscope. Consequently, clear band dispersions along the Γ–X direction reflecting the bulk electronic states were observed with a photon energy of 879 eV.

How annealing influences the morphology of a highly regioregular poly(3-hexylthiophene) (RR-P3HT) film at the substrate interface as well as the lateral inhomogeneity in the electronic structure of the film are elucidated. Whereas previous studies have reported that high-molecular-weight (MW) RR-P3HT films tend to show low crystallinity even after annealing, it is found that high-MW RR-P3HT does show high crystallinity after annealing at high temperature for a long time. Photoemission electron microscopy (PEEM), X-ray photoemission spectroscopy, and ultraviolet photoemission spectroscopy results clearly resolve a considerable lateral inhomogeneity in the morphology of RR-P3HT film, which results in a variation of the electronic structure depending on the local crystallinity. The PEEM results show how annealing facilitates crystal growth in a high-MW RR-P3HT film.

Synchrotron-based angle-resolved photoemission spectroscopy is used to determine the electronic structure of layered SnSe, which was recently turned out to be a potential thermoelectric material. We observe that the top of the valence band consists of two nearly independent hole bands, whose tops differ by ~20 meV in energy, indicating the necessity of a multivalley model to describe the thermoelectric properties. The estimated effective masses are anisotropic, with in-plane values of 0.16-0.39 m$_0$ and an out-of-plane value of 0.71 m$_0$, where m$_0$ is the rest electron mass. Information of the electronic structure is essential to further enhance the thermoelectric performance of hole-doped SnSe., Comment: 14 pages including 2 figures + 2 pages of supplementary data

We have performed temperature ( T )-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi 2 B 2 C to study the electronic structure, especially the effect of AF ordering in T -dependent superconducting (SC) gap. To estimate the values of T -dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s -wave SC gap. From the fitting results, we find a sudden deviation from the BCS prediction just below T N . This observation can be well explained by the theoretical model, indicating that the origin of anomalous T -dependence is competition between rapid evolution of AF molecular field and SC condensation energy.

We have measured Ca-intercalated graphite superconductor CaC 6 (T c = = 11.2 K) by soft X-ray photoemission spectroscopy in order to understand the electronic structure. For the valence band, we observed several structures that correspond to those of calculated density of states with the partial density of states of Ca 3d at the Fermi level (E F ). We also observed core level spectra that are a very large asymmetric Ca 2p and asymmetric C 1s for CaC 6 , suggesting the existence of conduction electrons derived from Ca 3d and a charge transfer from Ca to graphene layer. These results provide spectroscopic evidence for PDOS of Ca 3d at E F . From a comparison of electronic structure of CaC 6 and other graphite intercalation compounds (GlCs), we found the difference between CaC 6 and other superconducting GlCs, which provides deeper understanding of the superconductivity of CaC 6 .

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (Tc=7.2 K) in order to study the electronic structure near the Fermi level (EF). Careful determination of measured momentum space that across Γ point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that EF is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at EF were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations.

Charge ordering and superconductivity are observed in the phase diagrams of a variety of materials such as NbSe3, layered transition-metal dichalcogenides and high-temperature copper oxide superconductors, low-dimensional organics, Ba1−xKxBiO3 and so forth. Although both conventional charge-density-wave (CDW) and superconducting transitions show an energy gap in the single-particle density of states at the Fermi level (EF), their physical properties are poles apart: insulating behaviour for the CDW and zero resistivity in superconductors. Consequently, these two ground states are believed to compete with each other. Here we provide evidence for maximized superconductivity at points in momentum (k) space that are directly connected by the CDW ordering vector. Temperature-dependent angle-resolved photoemission spectroscopy of 2H-NbSe2 across the CDW and superconducting transitions (TCDW∼33 K and Tc=7.2 K, respectively) shows CDW-induced spectral-weight depletion at the same Fermi-surface-crossing k points, which evolve into the largest superconducting gaps. These k points also show the highest electron–phonon coupling and lowest Fermi velocities. Our results demonstrate that charge order can boost superconductivity in an electron–phonon coupled system, in direct contrast to the prevailing view that it only competes with superconductivity.

MgB 2 shows the highest superconducting transition temperature among intermetallic compounds. Angle-resolved photoemission spectroscopy revealed their electronic structure, and the result is well explained within a simple band picture except for one band. Other photoemission studies on related materials of AlB 2 and CaAlSi also show an extra band similar to that in MgB 2 . The direct observation of the superconducting gap confirmed a two-gap superconductivity. The larger and smaller gaps open at the σ and π bands, respectively. The carbon substitution effect on the two gaps was evaluated by using sub-meV photoemission spectroscopy.

We have observed zero resistivity above 10 K and an onset of resistivity reduction at 25.2 K in a heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower Tc. We found that the carrier has a longer mean free path and lifetime than in the previous report, indicating that this highest Tc diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20 K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high Tc diamond.

We present laser-excited ultrahigh-resolution photoemission spectroscopy of YNi 2 B 2 C and ErNi 2 B 2 C. The extremely high energy resolution enables us to discuss the superconducting (SC) gap function of YNi 2 B 2 C and to measure the density of states which is coexisted between superconductivity and antiferromagnetism in ErNi 2 B 2 C. For YNi 2 B 2 C, s + g-wave SC gap symmetry cannot reproduce the experimental data, implying importance of taking the complicated electronic structures into consideration. For ErNi 2 B 2 C, the observed coherent peak is very broad and finite density of state exists at Fermi level.

Valence band spectra of icosahedral quasicrystals and their 1/1 cubic approximants in the Cd-Yb and Cd-Ca systems are investigated systematically. A sharp Fermi edge, indicative of sizable density of states at the Fermi level (E F ), is clearly observed for all the compounds and E F is located not at the bottom of the pseudogap but at its shoulder. In addition, a peak structure appears just below E F for the Cd-Ca system, which is also found for the Cd-Yb system when subtracting a Yb 4f derived doublet. Ca 2p-3d resonant photoemission spectroscopies on the Cd-Ca compounds reveal the existence of Ca 3d states at E F , indicating that the 3d states do contribute to the peak structure at E F , which is attributed to hybridization with the Cd sp states. The low-temperature structural properties of the Cd-based compounds are also reported. An occurrence of a structural phase transition at 110 K and 100 K for Cd6Yb and Cd6Ca, respectively, is well documented. The transitions are understood as a consequence of a...

We review our angle-resolved photoemission spectroscopy (ARPES) results across superconducting transitions in 2 H –NbSe 2 and MgB 2 , and also show recent results on YNi 2 B 2 C. For 2 H –NbSe 2 and MgB 2 , we observe that the superconducting gap exhibits clear Fermi surface (FS) sheet dependence for two types of FS sheets with different characters, thus providing evidence that the FS sheet dependence is a general feature for a superconductor having multiple FS sheets of different character. In YNi 2 B 2 C, we show experimental FSs, and momentum-dependent superconducting gap showing large anisotropy observed on a single FS. These ARPES results provide direct evidence for and importance of momentum-dependent superconducting gaps of phonon-mediated superconductors.

We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na 0.35 CoO 2 ·1.3H 2 O using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of ∼20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Δ of 0.5–0.7 meV. The energy scale of the SC gap is ∼20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures.

We have investigated the low-energy electronic state of boron-doped diamond thin film by the laser-excited photoemission spectroscopy. A clear Fermi-edge is observed for samples doped above the semiconductor–metal boundary, together with the characteristic structures at 150 � n meV possibly due to the strong electron–lattice coupling effect. In addition, for the superconducting sample, we observed a shift of the leading edge below T c indicative of a superconducting gap opening. We discuss the electron–lattice coupling and the superconductivity in doped diamond. r 2006 NIMS and Elsevier Ltd. All rights reserved.

The electronic structure of nearly optimally doped novel superconductor LaO1−xFxBiS2(x = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (EF), which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near EF shows a squarelike distribution around the (Z) point in addition to electronlike Fermi-surface (FS) sheets around the X(R) point, indicating that FS of LaO0.54F0.46BiS2 is in close proximity to the theoretically predicted topological change.

We study the bulk electronic structure of Na x CoO 2 .yH 2 O using Hard X-ray (HX, h ν = 5.95 KeV) synchrotron photoelectron spectroscopy (PES). The Co 2 p core level spectra show well-separated Co 3 + and Co 4 + ions. Cluster calculations suggest low spin Co 3 + and Co 4 + character, and a moderate on-site Coulomb correlation energy U dd ∼ 3–5.5 eV. Photon-dependent valence band PES identifies Co 3 d and O 2 p derived states, in near agreement with band structure calculations. We discuss the importance of HX-PES for studying correlated transition metal oxides.

We have developed an ultrahigh-resolution photoemission system using a quasi continuous wave (quasi-CW) VUV laser. The photon energy of the VUV laser is 6.994 eV. It has a line width of 260 μeV and a repetition rate of 80 MHz. The photon energy of this laser is the highest for a quasi-CW laser. Using a quasi-CW laser removes the space charge effect and hence does not degrade the energy resolution. For the electron analyser, we use the newly designed ultrahigh-resolution photoelectron analyser GAMMADATA-SCIENTA R4000. The analyser resolution is about 250 μeV with a wide collecting angle of about 38°. The total energy resolution of this system is measured to be 360 μeV for the Fermi edge of gold. Using this system, we have measured several low transition temperature superconductors and succeeded to observe the bulk superconducting state of these materials.

Relaxation time of Bloch states at the Fermi level ( τ F ), Fermi velocity ( v F ), and mean free path ( l F ) of the conduction electrons in the (Bi, Pb)2(Sr, La)2CuO6+δ (Bi2201) superconductors with various hole-concentration (p) were determined by the angle-resolved photoemission spectroscopy (ARPES) measurement with high energy- and momentum-resolutions; Δ E = 10 meV and Δ k = 0.005 A−1. We found that τ F at k F along ( 0 , 0 ) – ( π , π ) cut is kept constant over a wide Hole-concentration range while it in the vicinity of ( π , 0 ) is drastically shortened with decreasing p and eventually reaches nearly zero in the under-doped sample with the transition temperature of 27 K. By calculating electrical resistivity and comparing it with the measured one, we confirmed that τ F , v F , and l F determined by the ARPES measurement can be used for the quantitative estimation of the electrical transport properties, such as the electrical resistivity, in high- T c cuprates.

The Fermi surface and superconducting gap of MgB2 have been studied by angle-resolved photoemission spectroscopy. Two Fermi surfaces composed of the σ band and the π band are clearly observed without contaminated by the “surface-derived” band. The high-resolution photoemission spectra show that superconducting gaps with values of 5.5 and 2.2 meV open on the σ band and the π band, respectively, both of which close at the bulk transition temperature (Tc). This result provides an experimental evidence for the two-band superconductivity on MgB2 and, moreover, validates the role of momentum-dependent electron–phonon coupling as the origin of multiple-gap superconductivity as well as the high transition temperature (Tc) of MgB2.

Energy–momentum ( E – k ) dispersion and the shape of the Fermi surface (FS) in the (Bi,Pb) 2 (Sr,La) 2 CuO 6+ δ (Bi2201) superconductors of various hole-concentrations were investigated by the angle-resolved photoemission spectroscopy (ARPES) with high energy and momentum resolutions (Δ E =10 meV and Δ k =0.005 A −1 ). The E – k dispersion is kept rigid and the chemical potential moves towards lower or higher energies with increasing or decreasing hole-concentration, respectively, over a wide hole-concentration range from the nearly optimal-doped condition with a transition temperature of 32 K to a heavily over-doped condition where the superconducting transition disappears. By comparing the shape of the FS determined for the present Bi2201 with that reported for the Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212), we found that the area surrounded by the FS in the Bi2201 is fairly larger than that in the Bi2212. Estimated hole-concentration in the Bi2201 was found to vary from 0.25 to 0.43 holes/Cu, that is more than twice as large as that in the optimally doped La 2− x Sr x CuO 4 (LSCO) and Bi2212 superconductors of 0.17 holes/Cu.

Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (ΔE≈10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca3Co4O9, which has attracted interests as one of the potential thermoelectric materials because of its possession of high thermoelectric power S, relatively low electrical resistivity ρ, and small thermal conductivity κ. A narrow band of 1.5–2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level (EF). This peak turns out to be less significant when Co 2p–3d resonance takes place, indicating that it consists mainly of O 2p and of small amount of Co 3d component. Since EF is located near the high-energy edge of this narrow band, the density of states at EF is finite but negligibly small at room temperature. An energy-gap across EF opens below 50 K with decreasing temperature. This development of the energy-gap causes the insulating behavior; divergence both in electrical resistivity and Hall coefficient. We calculated thermoelectric power S(T) using the photoemission spectra near EF. The calculated S(T) shows almost half of the measured value. Large thermoelectric power up to 200 μV/K observed for the Ca3Co4O9 is closely related to the metallic electron transport in a less dispersive band with EF near its band-edge.

We have performed X-ray absorption and valence band photoemission spectroscopy on ZrB 2 , which has the same AlB 2 structure of MgB 2 , to study the electronic structure. From X-ray absorption spectra near the B 1s–2p threshold, we found a small absorption edge at 188.23 eV with a negligible angular dependence upto 190 eV photon energy. This indicates that B 2p partial density of states near the Fermi level ( E F ) is three dimensional, in sharp contrast to that of MgB 2 . Resonant photoemission study across the B 1s–2p threshold was found to be difficult due to a large contribution of Auger electrons. On the other hand, resonant photoemission across the Zr 3p–4d threshold does show small change in intensity within 4 eV of E F . This indicates that the occupied states near E F have substantial Zr 4d character. These observations are in relatively good agreement with band structure calculations.

Soft x-ray energy-dependent angle-resolved photoemission measurements are performed on the heavy-fermion superconductor CeIrIn5 to study the Fermi surface (FS) and the electronic structure near the Fermi level in the ΓMZA plane. The non-dispersive band designated as the Ce 4f7/21 final state is found at 280 meV along with the main body of the valence band consisting of highly dispersive In 5p bands, which are in good qualitative agreement with the band structure calculation based on the itinerant 4f electron model. Furthermore, we found two electron-like and one hole-like cylindrical FSs at the M (A) and Γ (Z) points. The Fermi momenta of the observed FSs are consistent with the de Haas–van Alphen experiment, while they contradict the stronger anisotropy of the FS along the kz direction predicted by the band calculation. The results suggest stronger two-dimensionality of the electronic structure of CeIrIn5 and point to the need for its theoretical verification.

We have investigated the effectiveness of a hot-filament chemical vapor deposition (HFCVD) method for preparation of a boron-doped superconducting diamond film with higher superconducting transition temperature ( T c ). A boron-doped superconducting diamond film has been fabricated on a diamond (111) substrate using the HFCVD method, and studied by means of scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and resistivity. The film consists of grains with an average size of 200 mm. Analyses of valence band and boron 1 s core-level XPS spectra indicate the formation of boron-doped diamond film on the substrate. From the resistivity measurements, the film is found to be a superconductor with onset T c of 7.1 K. Carrier concentration determined by Hall conductivity measurements is 1.1 × 10 21 cm − 3 . The value of T c is higher compared with that in boron-doped superconducting diamond films prepared by a commonly used microwave plasma-assisted chemical vapor deposition (MPCVD) method, at the same carrier concentration [A. Kawano et al., Phys. Rev. B 82 (2010) 085318]. The result of higher T c in the film by the HFCVD method is consistent with the previous one [Wang et al., Diamond Relat. Mater. 15 (2006) 659], suggesting that the HFCVD method is effective for preparation of boron-doped superconducting diamond films showing higher T c .

We study the superconducting electronic structures of Nb 3 Al and MgB 2 using high-resolution spectroscopy. The obtained spectrum of Nb 3 Al measured below T c shows clear opening of the superconducting gap with a sharp pile up in the density of states and a shift of the leading edge. In addition, the spectrum shows a peak–dip–hump line shape expected from the strong-coupling theory. On the other hand, for MgB 2 , the superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a single isotropic gap. The superconducting spectral shape of MgB 2 can be explained in terms of a multicomponent gap.

Recent advances in the energy resolution and cryogenic techniques used in photoemission spectroscopy have enabled us to directly observe the superconducting electronic structure of superconductors with a relatively lower transition temperature (low- T c ). In this paper, we report photoemission results on low- T c superconductors measured with an ultrahigh-resolution low-temperature photoemission spectrometer. The result of Nb 3 Al exhibits a superconducting gap and a phonon-induced peak–dip–hump structure characteristic of the strong-coupling superconducting spectral function. From measurements of Ba 0.67 K 0.33 BiO 3 , we observe isotropic s-wave superconducting gap opening below T c as well as depletion of intensity near the Fermi level above T c , which can be attributed to a phonon-induced pseudogap. We will compare these photoemission results with those of high- T c superconductors.

Superconducting transitions of elemental metals and Ni borocarbides are studied using ultrahigh-resolution and low- temperature photoemission spectroscopy. Photoemission spectra of elemental metals (Nb and Pb) show the appearance of a sharp peak just below the Fermi level and a shift of the leading edge, indicating clear opening of the superconducting gap. Furthermore, Pb spectrum shows a dip and hump structure beyond the peak, characteristic of the strong-coupling superconductivity. On the other hand, comparative study of Y(Ni Pt ) B C (x50.0 and 0.2) shows a difference in the 12 xx 22 slope of the leading edge, which can be explained in terms of a highly anisotropic s-wave gap in YNi B C, consistent to 22 magnetic field dependent specific heat measurements. These observations demonstrate that photoemission spectroscopy is a useful and reliable technique to study electronic structures and phase transitions at very small energy scales. © 2002 Elsevier Science B.V. All rights reserved.

The electronic structure of Heusler-type Fe(2)VAl has been studied by high-resolution photoelectron spectroscopy with the excitation photon energy hnu ranging from 21.2 eV (the He I laboratory light source) to 904 eV (the soft X-ray synchrotron light source) for clean surfaces prepared by scraping or fracturing polycrystalline and single crystalline specimens. Photoelectron spectra recorded for the fractured surfaces show a 10 eV-wide valence band with fine structures and a clear decrease in the intensity towards the Fermi level E(F), while a high intensity at E(F) and no fine structures are observed for the scraped surface. Comparison with the theoretical density of states (DOS) indicates that the vacuum ultraviolet photoelectron spectra emphasize the transition-metal 3d bands but the soft X-ray photoelectron spectra agree remarkably well with the DOS including the fine structures and the pseudogap at E(F). The present results suggest that the electronic structure of Fe(2)VAl is highly sensitive to possible strain and defects induced by scraping. Bulk electronic structures of Fe(2)VAl are discussed in relation to the reported fascinating transport properties.

High-resolution HeI photoemission spectroscopy (UPS), Mn 2p-3d resonant photoemission spectroscopy (RPES) and Mn 2p X-ray absorption spectroscopy (XAS) have been performed to investigate the electronic structure and its effect on the electrical resistivity in (La(1-z)Nd(z))(0.46)Sr(0.54)MnO(3) (z = 0, 0, 2, 0.6 and 1.0). It was found that in the UPS and RPES spectra the Fermi edge persisted over the temperature range 15or = Tor = 340 K regardless of the magnetic structure or the composition of the samples. The density of states at the Fermi level [N(E(F))] in the samples where 0or = zor = 0.6 was increased drastically at the Curie temperature (T(C)) with decreasing temperature, but essentially kept unchanged across the Néel temperature (T(N)). A fairly large reduction at T(C) and a small increase at T(N) in the electrical resistivity with decreasing temperature are found to be well accounted for in terms of the temperature dependence of N(E(F)). The presence of a finite N(E(F)) in the insulating Nd(0.46)Sr(0.54)MnO(3) was also found. Thus the origin of the insulating behavior in this sample can be regarded as the Anderson localization associated with the small density of states and the chemical disorder between Nd and Sr.

Ultrahigh-resolution photoemission spectroscopy reveals the crossover behavior of CeRu 4 Sb 12 into a low carrier state. The density of states (DOS) around the Fermi level ( E F ) decreases on lowering the temperature. The temperature dependence of the DOS at E F is consistent with the results of the transport measurements and explained in terms of the development of the coherence.

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E(F)) up to similar to 5k(B)T can be observed (k(B) is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E(F). Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E(F), can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.

We use core level and valence band soft x-ray photoemission spectroscopy (SXPES) to investigate electronic structure of new BiS$_{2}$ layered superconductor LaO$_{1-x}$F$_{x}$BiS$_{2}$. Core level spectra of doped samples show a new spectral feature at the lower binding energy side of the Bi 4${f}$ main peak, which may be explained by core-hole screening with metallic states near the Fermi level ($E_{\rm F}$). Experimental electronic structure and its ${x}$ dependence (higher binding energy shift of the valence band as well as appearance of new states near $E_{\rm F}$ having dominant Bi 6${p}$ character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near $E_{\rm F}$ from that of calculations might give insight into the interesting physical properties. These results provide first experimental electronic structure of the new BiS$_{2}$ layered superconductors., Comment: 14 pages, 4 figures

We carried out ultrahigh-resolution photoemission (URPE) and resonant-inverse photoemission spectroscopy on CePd3 and CePd2.9Cu0.1 compounds. The URPE spectra of CePd3 and CePd2.9Cu0.1 clearly show small humps in the vicinity of Fermi level well below the Kondo temperature. The hump is gradually formed below the characteristic coherent temperature T*. We conclude that it gives direct evidence of the appearance of the small pseudogap below T* in CePd3 and CePd2.9Cu0.1.