Search

Kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\mathrm{K}$, Rb, and Cs) exhibit superconductivity at 0.9--2.5 K and charge-density wave (CDW) at 78--103 K. Key electronic states associated with the CDW and superconductivity remain elusive. Here, we investigate low-energy excitations of ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ by angle-resolved photoemission spectroscopy. We found an energy gap of 50--70 meV at the Dirac-crossing points of linearly dispersive bands, pointing to an importance of spin-orbit coupling. We also found a signature of strongly Fermi-surface and momentum-dependent CDW gap characterized by the larger energy gap of maximally 70 meV for a band forming a saddle point around the $M$ point, the smaller (0--18 meV) gap for a band forming massive Dirac cones and a zero gap at the $\mathrm{\ensuremath{\Gamma}}/A$-centered electron pocket. The observed highly anisotropic CDW gap which is enhanced around the $M$ point signifies an importance of scattering channel connecting the saddle points, laying foundation for understanding the nature of CDW and superconductivity in $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$.

We have performed angle-resolved photoemission spectroscopy on epitaxial VTe2 films to elucidate the relationship between the fermiology and charge-density waves (CDW). We found that a two-dimensional triangular pocket in 1 monolayer (ML) VTe2 is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6ML counterpart, likely associated with the 1T-to-1T' structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6ML at low temperature distinct from anisotropic pseudogap in 1ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure caluculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides., Comment: To be published in Physical Review B

We have performed angle-resolved photoemission spectroscopy of AlB2 which is isostructural to high-temperature superconductor MgB2. Using soft-x-ray photons, we accurately determined the three-dimensional bulk band structure and found a highly anisotropic Dirac-cone band at the K point in the bulk hexagonal Brillouin zone. This band disperses downward on approaching the H point while keeping its degeneracy at the Dirac point, producing a characteristic Dirac nodal line along the KH line. We also found that the band structure of AlB2 is regarded as a heavily electron-doped version of MgB2 and is therefore well suited for fully visualizing the predicted Dirac nodal line. The present results suggest that (Al,Mg)B2 system is a promising platform for studying the interplay among Dirac nodal line, carrier doping, and possible topological superconducting properties., 6 pages, 3 figures

We have performed angle-resolved photoemission spectroscopy (ARPES) on CeBi which undergoes a two-step antiferromagnetic (AF) transition with temperature. Soft-x-ray ARPES has revealed the inverted band structure at the X point of bulk Brillouin zone for CeBi (and also for LaBi) as opposed to LaSb with non-inverted band structure. Low-energy ARPES on CeBi has revealed the Dirac-cone band at the Gamma point in the paramagnetic phase associated with the bulk band inversion. On the other hand, a double Dirac-cone band appears on entering the first AF phase at T = 25 K, whereas a single Dirac-cone band recovers below the second AF transition at T = 14 K. The present result suggests an intricate interplay between antiferromagnetism and topological surface states in CeBi., Comment: 8 pages, 6 figures

We have performed angle-resolved photoemission spectroscopy on a heterostructure consisting of topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and iron chalcogenide FeTe fabricated on ${\mathrm{SrTiO}}_{3}$ substrate by molecular-beam-epitaxy technique. This system was recently found to show the superconductivity albeit nonsuperconducting nature of each constituent material. Upon interfacing FeTe with two quintuple layers of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, we found that the Dirac-cone surface state of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ is shifted toward higher binding energy, while the holelike band at the Fermi level originating from FeTe moves toward lower binding energy. This suggests that electron charge transfer takes place from FeTe to ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ through the interface. The present result points to the importance of hole-doped FeTe interface for the occurrence of unconventional superconductivity.

We investigated the electronic structure of monolayer ${\mathrm{VTe}}_{2}$ grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer ${\mathrm{VTe}}_{2}$ takes the octahedral $1T$ structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi surface topology between ARPES results and first-principles band calculations for octahedral monolayer $1T\text{\ensuremath{-}}{\mathrm{VTe}}_{2}$. We have revealed that monolayer $1T\text{\ensuremath{-}}{\mathrm{VTe}}_{2}$ at low temperatures is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer ${\mathrm{VSe}}_{2}$ which undergoes a charge density wave (CDW) transition to an insulator at low temperatures. The present result suggests an importance of the Fermi surface topology for characterizing the CDW properties of monolayer transition-metal dichalcogenides.

We have performed high-resolution angle-resolved photoemission spectroscopy of the quasi-one-dimensional (1D) topological superconductor candidate ${\mathrm{Tl}}_{2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{6}$ consisting of weakly coupled ${\mathrm{Mo}}_{3}{\mathrm{Se}}_{3}$ chains. We found a quasi-1D Fermi surface arising from a Dirac-like energy band, which is associated with the nonsymmorphic screw symmetry of the chains and predicted to trigger topological superconductivity. We observed a significant spectral-weight reduction over a wide energy range, together with a tiny Fermi-edge structure which exhibits a signature of a superconducting-gap opening below the superconducting-transition temperature. The observed quasi-1D Dirac-like band and its very small density of states point to an unconventional nature of superconductivity in ${\mathrm{Tl}}_{2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{6}$.

We have performed angle-resolved photoemission spectroscopy of the layered ternary telluride ${\mathrm{Ta}}_{3}{\mathrm{SiTe}}_{6}$ which is predicted to host nodal lines associated with nonsymmorphic crystal symmetry. We found that the energy bands in the valence-band region show Dirac-like dispersions which present a band degeneracy at the $R$ point of the bulk orthorhombic Brillouin zone. This band degeneracy extends one dimensionally along the whole $SR$ high-symmetry line, forming nodal lines protected by the glide mirror symmetry of the crystal. We also observed a small band splitting near ${E}_{\mathrm{F}}$ which supports the existence of hourglass-type dispersions predicted by the calculation. The present results provide an excellent opportunity to investigate the interplay between exotic nodal fermions and nonsymmorphic crystal symmetry.

In the one-monolayer FeSe film on SrTiO${}_{3}$, which is a new member of high-temperature superconductor family, there exists two competing proposals on the nature of the ground state in the parent undoped film: strongly correlated insulator (as in copper-oxide superconductors) versus moderately correlated metal (as in iron-based superconductors). Here, the authors provide experimental evidence for the metallic nature by performing $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ angle-resolved photoemission spectroscopy on an undoped one-monolayer FeSe film prepared by careful post-growth annealing. The authors demonstrate the materialization of an ideal two-dimensional Dirac electron system, where only the Dirac-cone band crosses the Fermi level, unlike in the multilayer counterpart.

We have performed angle-resolved photoemission spectroscopy on HfSiS, which has been predicted to be a topological line-node semimetal with square Si lattice. We found a quasi-two-dimensional Fermi surface hosting bulk nodal lines, alongside the surface states at the Brillouin-zone corner exhibiting a sizable Rashba splitting and band-mass renormalization due to many-body interactions. Most notably, we discovered an unexpected Dirac-like dispersion extending one dimensionally in $k$ space---the Dirac-node arc---near the bulk node at the zone diagonal. These novel Dirac states reside on the surface and could be related to hybridizations of bulk states, but currently we have no explanation for its origin. This discovery poses an intriguing challenge to the theoretical understanding of topological line-node semimetals.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on noncentrosymmetric Weyl semimetal candidate NbP, and determined the electronic states of both Nb- and P-terminated surfaces corresponding to the "opposite" surfaces of a polar crystal. We revealed a drastic difference in the Fermi-surface topology between the opposite surfaces, whereas the Fermi arcs on both surfaces are likely terminated at the surface projection of the same bulk Weyl nodes. Comparison of the ARPES data with our first-principles band calculations suggests notable difference in electronic structure at the Nb-terminated surface between theory and experiment. The present result opens a platform for realizing exotic quantum phenomena arising from unusual surface properties of Weyl semimetals., 5 pages, 4 figures

We report a systematic angle-resolved photoemission spectroscopy on topological insulator (TI) TlBi1-xSbxTe2 which is bulk insulating at 0.5 < x < 0.9 and undergoes a metal-insulator-metal transition with the Sb content x. We found that this transition is characterized by a systematic hole doping with increasing x, which results in the Fermi-level crossings of the bulk conduction and valence bands at x~ 0 and x~1, respectively. The Dirac point of the topological surface state is gradually isolated from the valence-band edge, accompanied by a sign reversal of Dirac carriers. We also found that the Dirac velocity is the largest among known solid-solution TI systems. The TlBi1-xSbxTe2 system thus provides an excellent platform for Dirac-cone engineering and device applications of TIs.

We have performed high-resolution angle-resolved photoemission spectroscopy on layered polymorph series of $1H\ensuremath{-}, 5H\ensuremath{-}$, and $6H\ensuremath{-}\mathrm{CaAlSi}$, which exhibit superconductivity below 6.5, 5.7, and 7.7 K, respectively. While the overall band structure in the valence-band region is similar among these compounds, the volume of the Fermi surface at the $M$ point and the magnitude of the superconducting gap are markedly different from each other. Implications of such variation in the electronic structure due to the superstructure along the $c$ axis are discussed in relation to the physical properties of CaAlSi.

We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)${}_{2}$Te${}_{4}$ and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators [Pb(Bi,Sb)${}_{2}$Te${}_{4}$, Bi${}_{2}$Te${}_{3}$, Bi${}_{2}$Se${}_{3}$, and TlBiSe${}_{2}$] and elucidated that the out-of-plane spin polarization ${P}_{z}$ is systematically correlated with the warping strength. However, the magnitude of ${P}_{z}$ is found to be only half that expected from the $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ theory when the warping is strong. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.

We have performed angle-resolved photoemission spectroscopy (ARPES) of KFeAgTe${}_{2}$, and revealed the absence of band crossing at the Fermi level (${E}_{\mathrm{F}}$) indicative of the unconventional insulating nature of this material. Comparison of the ARPES-derived band dispersions with the first-principles calculations based on local density approximation and the inclusion of electron correlation $U$ demonstrated that the ground state of KFeAgTe${}_{2}$ is not a simple band insulator. And also, our fitting result on the ARPES experimental density of states near ${E}_{\mathrm{F}}$ plausibly excludes the possibility of Anderson insulator. We suggest that KFeAgTe${}_{2}$ is most likely a Mott insulator or a Hund insulator, providing a deep insight into the insulating ground state.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of layered chalcogenide 1$T$-TaS${}_{2\ensuremath{-}x}$Se${}_{x}$ to elucidate the electronic states especially relevant to the occurrence of superconductivity. We found a direct evidence for a Ta-5$d$-derived electron pocket associated with the superconductivity, which is fragile against a Mott-gap opening observed in the insulating ground state for S-rich samples. In particular, a strong electron-electron interaction-induced Mott gap driven by a Ta 5$d$ orbital also exists in the metallic ground state for Se-rich samples, while finite ARPES intensity near the Fermi level likely originating from a Se 4$p$ orbital survives, indicative of the orbital-selective nature of the Mott transition. Present results suggest that effective electron correlation and $p$-$d$ hybridization play a crucial role to tune the superconductivity and Mott metal-insulator transition.

We have fabricated Rb-intercalated bilayer graphene on 6H-SiC(0001) and performed low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). In LEED experiments, we observed a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 pattern identical to that of the bulk graphite intercalation compound C${}_{8}$Rb. The existence of a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 Rb layer is also confirmed by ARPES, where band folding due to the periodic potential of intercalated atoms was observed. All these results indicate that Rb atoms are intercalated between the two graphene monolayers in a regular manner similar to that of bulk C${}_{8}$Rb. Further, we observed a parabolic metallic band at the Brillouin-zone center, analogous to the so-called interlayer band, which is thought to play an important role for the superconductivity in graphite intercalation compounds.

We have performed systematic angle-resolved photoemission spectroscopy of the topological crystalline insulator (TCI) Pb${}_{1\ensuremath{-}x}$Sn${}_{x}$Te to elucidate the evolution of its electronic states across the topological phase transition. As previously reported, the band structure of SnTe ($x=1.0$) measured on the (001) surface possesses a pair of Dirac-cone surface states located symmetrically across the $\overline{X}$ point in the (110) mirror plane. Upon approaching the topological phase transition into the trivial phase at ${x}_{c}\ensuremath{\approx}0.25$, we discovered that Dirac cones gradually move toward the $\overline{X}$ point with its spectral weight gradually reduced with decreasing $x$. In samples with $x\ensuremath{\le}0.2$, the Dirac-cone surface state is completely gone, confirming the occurrence of the topological phase transition. Also, the evolution of the valence band feature is found to be consistent with the bulk band inversion taking place at ${x}_{c}$. The tunability of the location of the Dirac cones in the Brillouin zone would be useful for applications requiring Fermi-surface matching with other materials, such as spin injection.

We have performed angle-resolved photoemission spectroscopy of Co-based boride superconductor LaCo1.73Fe0.27B2 (Tc = 4.1 K), which is isostructural to the 122-type Fe-pnictide superconductor with the pnictogen atom being replaced with boron. We found that the Fermi level is located at a dip in the density of states (DOS) in contrast to Co-pnictide ferromagnets. This reduction in DOS together with the strong Co 3d-B 2p covalent bonding removes the ferromagnetic order and may cause the superconductivity. The energy bands near the Fermi level show higher three dimensionality and a weaker electron-correlation effect than those of Fe pnictides. The Fermi surface topology is considerably different from that of Fe pnictides, suggesting the difference in the superconducting mechanism between boride and pnictide superconductors.

We have performed high-resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe0.95Co0.05As (T-c similar to 18 K). We observed a holelike Fermi surface around the zone center and two electronlike Fermi surfaces around the M point, which can be connected by the Q = (pi, pi) wave vector, suggesting that scattering over the near-nested Fermi surfaces is important in this 111 pnictide. Nearly isotropic superconducting gaps with sharp coherent peaks are observed below T-c on all three Fermi surfaces. Upon increasing temperature through T-c, the gap size shows little change while the coherence vanishes. Large ratios of 2 Lambda/k(B)T(c) similar to 8 are observed for all the bands, indicating a strong coupling in this system. These results are not expected from a classical phonon-mediated pairing mechanism.

We performed high-resolution angle-resolved photoemission spectroscopy on single-layered cuprate Bi${}_{2}$Sr${}_{2}$CuO${}_{6}$ to clarify the origin of the pseudogap. By using various photon energies, we succeeded in directly observing two different pseudogaps with two different energy scales which coexist in the antinodal region: one reflects the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave pairing strength while the other has a larger energy scale suggesting an origin distinct from superconductivity. The observed two-pseudogap behavior provides a key to fully understanding the pseudogap phenomena in cuprates.

We performed a systematic, high-resolution, angle-resolved photoemission spectroscopy of Bi${}_{1\ensuremath{-}x}$Sb${}_{x}$ alloys to study the evolution of electronic states as a function of Sb concentration $x$. We revealed that the number of Fermi-level crossings is different between the topologically trivial ($x=$ 0.023) and nontrivial ($x=$ 0.17) phases: four and five times for the former and latter cases, respectively. We also found a systematic expansion of the bandwidth of spin-split surface bands at the $\mathrm{\ensuremath{\Gamma}\ifmmode \bar{}\else \={}\fi{}}$ point upon substituting Bi with Sb, which is indicative of the monotonous reduction of the spin-orbit coupling strength.

We have performed a systematic photoemission study of the chemical-potential shift as a function of carrier doping in a pnictide system based on ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$. The experimentally determined chemical-potential shift is consistent with the prediction of a rigid band shift picture by renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) in the Fermi surface (FS) nesting condition due to different effective masses for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA, which matches well the observed asymmetric superconducting domes in the phase diagram, strongly supports FS quasinesting driven superconductivity in iron pnictides.

We have performed angle-resolved photoemission spectroscopy on the overdoped Ba${}_{0.3}$K${}_{0.7}$Fe${}_{2}$As${}_{2}$ superconductor (${T}_{c}=22$ K). We demonstrate that the superconducting (SC) gap on each Fermi surface (FS) is nearly isotropic whereas the gap value varies from 4.4 to 7.9 meV on different FSs. By comparing with under- and optimally doped Ba${}_{1\ensuremath{-}x}$K${}_{x}$Fe${}_{2}$As${}_{2}$, we find that the gap value on each FS nearly scales with ${T}_{c}$ over a wide doping range ($0.25\ensuremath{\leqslant}x\ensuremath{\leqslant}0.7$). Although the FS volume and the SC gap magnitude are strongly doping dependent, the multiple nodeless gaps can be commonly fitted by a single gap function assuming pairing up to the second nearest neighbor, suggesting the universality of the short-range pairing states with the ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave symmetry.

Three-dimensional electronic structure of highly metallic sodium tungsten bronze, ${\text{Na}}_{0.8}{\text{WO}}_{3}$, is investigated by high-resolution angle-resolved photoemission spectroscopy. The experimentally determined valence-band structure along the momentum directions both parallel and perpendicular to the surface has been compared with the results of ab initio band-structure calculation. The angle-resolved photoemission spectroscopy spectra for different photon energies reveal that possibly the oxygen vacancies in the system are responsible for the evolution of density of states at the top of $\ensuremath{\Gamma}$ point in experimental valence band. The band dispersion around $\ensuremath{\Gamma}(X)$ point leading to an electronlike Fermi surface is well predicted by the band calculation. As we move from bulk-sensitive to more-surface-sensitive photon energy, we found emergence of Fermi surfaces at $X(M)$ and $M(R)$ points similar to the one at $\ensuremath{\Gamma}(X)$ point, suggesting the reconstruction of surface due to rotation/deformation of ${\text{WO}}_{6}$ octahedra.

We have performed ultrahigh energy- and momentum-resolved angle-resolved photoemission spectroscopy of impurity-doped ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$. We found that the point node of the superconducting gap near the $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$ point in a pristine sample gradually evolves into a small gapless Fermi arc upon Zn or Ni doping. The shape of impurity-induced density of states inside the superconducting gap shows a marked difference between Zn and Ni doping, demonstrating that the low-energy excitation properties are strongly sensitive to the character of impurities.

We have performed high-resolution angle-resolved photoemission spectroscopy on quasi-one-dimensional indium chains on the Si(111) surface to study the temperature-induced metal-insulator (MI) transition accompanied by the $4\ifmmode\times\else\texttimes\fi{}1$ to $8\ifmmode\times\else\texttimes\fi{}2$ structural change. The band dispersion near ${E}_{F}$ shows an abrupt change at $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, giving a relatively large energy gap compared to the energy scale of the transition temperature. The band dispersion of the $8\ifmmode\times\else\texttimes\fi{}2$ phase shows no discernible temperature evolution, in sharp contrast to the conventional simple Peierls or structural transition. The experimental results suggest that the Peierls instability is important in the MI transition, while other structural effects are cooperatively involved.

High-resolution angle-resolved photoemission spectroscopy has been performed on Zn-substituted ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$ to study the impurity effect on the electronic structure near the Fermi level. Upon Zn substitution, the kink in the band dispersion at $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and the coherence peak in the off-nodal region in the superconducting state are significantly weakened and, at the same time, additional electronic states systematically evolve inside the superconducting gap. The suppression of the kink with Zn substitution shows a good correspondence to the low-energy incommensurate magnetic excitations.

We have performed low-energy ultrahigh-resolution photoemission spectroscopy on $\mathrm{Co}{({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x})}_{2}$ $(x=0.075)$ to elucidate the bulk electronic states responsible for the ferromagnetic transition. By using a newly developed plasma-driven low-energy xenon (Xe) discharge lamp $(h\ensuremath{\nu}=8.436\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, we clearly observed a sharp quasiparticle peak at the Fermi level together with the remarkable temperature dependence of the electron density of states across the transition temperature. Comparison with the experimental result by the $\mathrm{He}I\ensuremath{\alpha}$ resonance line $(h\ensuremath{\nu}=21.218\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ indicates that the sharp quasiparticle is of bulk origin and is produced by the Fermi-level crossing of the Co $3d$ ${e}_{g\ensuremath{\downarrow}}$ subband.

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on quasi-one-dimensional (1D) compounds ${\mathrm{Nb}}_{3}{X}_{4}$ $(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$. We have experimentally determined the band structure of these compounds and compared with several different band calculations. We found a fairly straight 1D Fermi surface in ${\mathrm{Nb}}_{3}{\mathrm{Te}}_{4}$ (charge density wave transition temperature ${T}_{\mathit{CDW}}=110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, superconducting transition temperature ${T}_{c}=1.9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), while the Fermi surface of ${\mathrm{Nb}}_{3}{\mathrm{Se}}_{4}$ $({T}_{c}=2.4\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and ${\mathrm{Nb}}_{3}{\mathrm{S}}_{4}$ $({T}_{c}=3.8\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ shows a remarkable wiggling, indicative of the more two-dimensional-like (three-dimensional-like) character. We also found a sharp quasiparticle peak near the Fermi level $({E}_{F})$ at low temperatures for ${\mathrm{Nb}}_{3}{\mathrm{S}}_{4}$ and ${\mathrm{Nb}}_{3}{\mathrm{Se}}_{4}$, while the spectral weight near ${E}_{F}$ of ${\mathrm{Nb}}_{3}{\mathrm{Te}}_{4}$ is considerably suppressed. These experimental results indicate that the anisotropy of the electronic structure remarkably changes with varying the chalcogen atom in the compounds.

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

We have carried out high-resolution angle-resolved and resonant photoemission spectroscopy (RPES) on heavy-fermion superconductors Ce2CoIn8 and Ce2RhIn8 to study the electronic band structure and the nature of the Ce 4f electrons. We have experimentally determined the valence-band structure and compared them with the full-potential linear augmented plane-wave band calculations. We found that both compounds have quasi-two-dimensional cylindrical Fermi surfaces centered at the M(A) point in the Brillouin zone, which may be an essential parameter for the development of the superconductivity. Comparison with the band calculations based on the itinerant and localized models suggests that the Ce 4f electrons are essentially localized in both compounds at a measured temperature of 40 K. RPES results have confirmed the localized character of the Ce 4f electrons in both compounds, with a relatively stronger localized nature in Ce2RhIn8 than in Ce2CoIn8. This difference in the strength of localized character well explains the difference in the magnetic properties between the two compounds.

We have performed an x-ray angle-resolved photoemission spectroscopy (ARPES) on ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{B}}_{6}$ $(x=0.005)$ to study the band structure relevant to the anomalous high-temperature ferromagnetism. The obtained band structure shows a good agreement with the $GW$ band calculation as well as the previous ARPES results with ultraviolet light. This indicates that ${\mathrm{CaB}}_{6}$ is not an excitonic insulator, but is intrinsically a semiconductor with a $1\text{\ensuremath{-}}\mathrm{eV}$ gap at the $X$ point.

One of the interesting features of the photoemission spectra of the high temperature cuprate superconductors is the presence of a large signal (referred to as the ``background'') in the unoccupied region of the Brillouin zone. Here we present data indicating that the background signal is a component of the photocurrent that is separate from the spectral function and is most likely due to strong scattering of the photoelectrons. We also present an analytical method that can be used to subtract the background signal.

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been performed on a layered transition-metal dichalcogenide (TMDC) $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ to study the $(4\ifmmode\times\else\texttimes\fi{}4)$ charge-density wave (CDW) mechanism. We observed a partial Fermi-surface (FS) nesting on the electronlike FS centered at the M (L) point. The spectral weight near ${E}_{F}$ is considerably suppressed below the transition temperature ${(T}_{c}=110\mathrm{K})$ around the nested portion, while a negligible spectral change is observed even across ${T}_{c}$ in other portions of FS. This suggests that the CDW transition in $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ is caused by the three-dimensional FS nesting. Implications are discussed in relation to the physical properties of $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ as well as the ARPES results of other TMDC's.

We have performed systematic angle-resolved photoemission spectroscopy on the high-${T}_{c}$ superconducting family of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{2n+4}$ $(n=1--3).$ In addition to the generic features of the large Fermi surface and d-wave superconducting gap, we have found that there exists a scaling of the doping dependence of the energy gap. Moreover, comparison of the nodal dispersion in the superconducting state shows that the ``kink'' (the sudden change of band dispersion), while occurring at a similar binding energy, becomes more pronounced with increasing layer number n, indicating stronger coupling between electrons and a collective mode.

We report temperature-dependent angle-resolved photoemission spectroscopy (ARPES) on superstructure-free Bi1.80Pb0.38Sr2.01CuO6-delta (overdope, T-c ,0)-(pi,pi) cut, indicating a holelike Fermi surface centered at (pi,pi). We also found that the spectral intensity is strongly affected by k-dependent matrix elements but the spectral function itself suffers no photon-energy dependence.

We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6}$ (Bi2201) and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously; it is a single large hole barrel centered about $(\ensuremath{\pi},\ensuremath{\pi})$ in both materials.

We examine the momentum and energy dependence of the scattering rate of the high temperature cuprate superconductors using angle resolved photoemission spectroscopy. The scattering rate is of the form a + bw around the Fermi surface for under and optimal doping. The inelastic coefficient "b" is found to be isotropic. The elastic term, "a", however, is found to be highly anisotropic for under and optimally doped samples, with an anisotropy which correlates with that of the pseudogap. This is contrasted with heavily overdoped samples, which show an isotropic scattering rate and an absence of the pseudogap above T_c. We find this to be a generic property for both single and double layer compounds., Comment: Extended version of cond-mat/0404385