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Through Bi deposition on the single-crystalline $\mathrm{Mo}{\mathrm{S}}_{2}$ surface, we find that the density of the sulfur vacancy is a critical parameter for the growth of the crystalline Bi overlayer or cluster at room temperature. Also, the $\mathrm{Mo}{\mathrm{S}}_{2}$ band structure is significantly modified near \ensuremath{\Gamma} due to the orbital hybridization with an adsorbed Bi monolayer. Our experimental observations and analysis in combination with density functional theory calculation suggest the importance of controlling the sulfur vacancy concentration in realizing an exotic quantum phase based on the van der Waals interface of Bi and $\mathrm{Mo}{\mathrm{S}}_{2}$.

We report that the hydrogenation of a single crystal $2H\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ induces a novel-intermediate phase between 2H and 1T phases on its surface, i.e., the large-area, uniform, robust, and surface array of atomic stripes through the intralayer atomic-plane gliding. The total energy calculations confirm that the hydrogenation-induced atomic stripes are energetically most stable on the ${\mathrm{MoS}}_{2}$ surface between the semiconducting 2H and metallic 1T phase. Furthermore, the electronic states associated with the hydrogen ions, which is bonded to sulfur anions on both sides of the ${\mathrm{MoS}}_{2}$ surface layer, appear in the vicinity of the Fermi level $({E}_{\mathrm{F}})$ and reduces the band gap. This is promising in developing the monolayer-based field-effect transistor or vanishing the Schottky barrier for practical applications.

The electronic band structure of MoS${}_{2}$ single crystals has been investigated using angle-resolved photoelectron spectroscopy and first-principles calculations. The orbital symmetry and $k$ dispersion of these electronic states responsible for the direct and the indirect electronic band gaps have been unambiguously determined. By experimentally probing an increase of the electronic band gap, we conclude that a MoS${}_{2}$ (0002) surface localized state exists just below the valence band maximum at the $\ensuremath{\Gamma}$ point. This electronic state originates from the sulfur planes within the topmost layer. Our comprehensive study addresses the surface electronic structure of MoS${}_{2}$ and the role of van der Waals interlayer interactions.

Using the first-principles calculations, we explore the electronic structures of 2H-$M{X}_{2}$ ($M$ $=$ Mo, W; $X$ $=$ S, Se, Te). When the number of layers reduces to a single layer, the indirect gap of bulk becomes a direct gap with larger gap and the band curvatures are found to lead to the drastic changes of effective masses. On the other hand, when the strain is applied on the single layer, the direct gap becomes an indirect gap and the effective masses vary. Especially, the tensile strain reduces the gap energy and effective masses while the compressive strain enhances them. Furthermore, the much larger tensile stress leads to become metallic.

We have investigated the electronic structures of single- and double-layered MoS${}_{2}$, composing of heterojunction structures such as graphene, MoS${}_{2}$, and SiO${}_{2}$ and MoS${}_{2}$ and SiO${}_{2}$, using scanning photoelectron microscopy. Negative shifts of both core levels and valence bands toward the Fermi energy have been observed. In connection with first-principles calculations, we have confirmed that the direct gap of single-layer MoS${}_{2}$ is changed to an indirect gap by stacking additional layers via van der Waals interlayer interactions.

The evolution of the electronic properties of thin anthracene films on highly oriented pyrolytic graphite [0001] substrates as a function of temperature has been investigated using ultraviolet photoelectron spectroscopy. The change in the valence line shape with increasing substrate temperature has been univocally associated with the occurrence of different molecular orientations and structural phases (e.g., ``flat-lying'' mono- and multilayer films and a multilayer phase with a ``standing-up'' orientation). These thin-film phases are characterized by ionization energies varying up to 0.9 eV; the surface dipole and possibly to a minor extent polarization energy contributions are shown to be related to the specific molecular packing/molecular orientation and to the interplay between the molecule-molecule vs molecule-substrate interactions. Furthermore, for the monolayer system, a vibrational fine structure of the highest occupied molecular orbital (HOMO) is clearly revealed, thus allowing a detailed study of the HOMO hole-vibration coupling and the temperature-dependent broadening. Showing a large number of recently discussed contributions to the valence line shape, anthracene thin films emerge as ``benchmark'' systems to study the behavior of holes relevant for the charge transport in organic electronic devices.

The CeIr{sub 1-x}Rh{sub x}In{sub 5} series exhibits a range of interesting phenomena, including heavy-fermion superconductivity, non-Fermi liquid behavior, and concomitant antiferromagnetism (AF) and superconductivity (SC). In the low-Rh concentration range (0.1 {ge} x {ge} 0.5), specific heat measurements show a broad anomaly, suggestive of gross phase separation. We have performed x-ray absorption experiments at the Ce L{sub III}, Ir L{sub III}, and Rh K-edges as a function of Rh concentration and temperature. X-ray absorption near-edge structure (XANES) measurements indicate that cerium is close to trivalent in this system, with no measurable change with temperature from 20-300 K, consistent with a heavy-fermion material. Extended x-ray absorption fine structure (EXAFS) measurements as a function of temperature from all measured edges indicate the local crystal structure of all samples is well ordered, with no gross phase separation observed, even for samples with x = 0.125 and x = 0.25. These results therefore suggest that the anomalous specific heat behavior in the 0.1 {ge} x {ge} 0.5 range have some other explanation, and some possibilities are discussed.

Diffuse x-ray scattering measurements on the cubic paraelectric phase of single-crystal PbTiO{sub 3} are presented. No diffuse scattering sheets are found, in contrast to sheets observed in the cubic paraelectric phases of BaTiO{sub 3} and KNbO{sub 3}. A quantitative analysis of the diffuse scattering indicates that the soft-mode branch contribution to sheets is negligible in all three ferroelectric perovskites. Differences in the diffuse scattering are explained by differences in the disordering of the local displacements, thus resolving a long-standing controversy.

Although materials that exhibit nearest-neighbor-only antiferromagnetic interactions and geometrical frustration theoretically should not magnetically order in the absence of disorder, few such systems have been observed experimentally. One such system appears to be the pyrochlore ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}.$ However, previous structural studies indicated that ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is an imperfect pyrochlore. To clarify the situation, we performed neutron powder-diffraction (NPD) and x-ray absorption fine-structure (XAFS) measurements on samples that were prepared identically to those that show no magnetic order. The NPD measurements show that the long-range structure of ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is well ordered with no structural transitions between 4.5 and 600 K. In particular, mean-squared displacements ${u}^{2}\mathrm{'}\mathrm{s}$ for each site follow a Debye model with no offsets. No evidence for Tb/Ti site interchange was observed within an upper limit of 2%. Likewise, no excess or deficiency in the oxygen stoichiometry was observed, within an upper limit of 2% of the nominal pyrochlore value. Tb ${L}_{\mathrm{III}}$- and Ti K-edge XAFS measurements from 20 K to 300 K similarly indicate a well-ordered local structure. Other aspects of the structure are considered. We conclude that ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ has, within experimental error, an ideal, disorder-free pyrochlore lattice, thereby allowing the system to remain in a dynamic, frustrated spin state to the lowest observed temperatures.

High-quality thin Nb and NbN films (60-100 A) are grown on (100) n + -InAs (n = 10 1 9 cm - 3 ) substrates by dc-magnetron sputter deposition. Studies of the electronic properties of interfaces between the superconductor and the semiconductor are done by Raman scattering measurements. The superconducting proximity effect at superconductor-semiconductor interfaces is observed through its impact on inelastic light scattering intensities originating from the near-interface region of InAs. The InAs longitudinal optical phonon LO mode (237 cm - 1 ) and the plasmon-phonon coupled modes L_ (221 cm - 1 ) and L + (1100 to 1350 cm - 1 ), for n + =1×10 1 9 - 2×10 1 9 cm - 3 are measured. The intensity ratio of the LO mode (associated with the near-surface charge accumulation region, in InAs) to that of the L_ mode (associated with bulk InAs), is observed to increase by up to 40% below the superconducting transition temperature. This temperature-dependent change in light scattering properties is only observed with high quality superconducting films and when the superconductor and the semiconductor are in good electrical contact. A few possible mechanisms of the observed effect are proposed.

We describe the local structure of crystalline ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{0.4}{\mathrm{CuO}}_{4}$ $(x=0.12,$ $0.16)$ in the temperature range 10--300 K as determined from orientation-dependent La K-edge x-ray absorption fine structure (XAFS) measurements. Such XAFS measurements of c-axis-aligned powders permit distinguishing like atoms at similar bonding distances because the measurement determines the angle of the bonds relative to the c axis. The local structure of $x=0.12$ about La atoms up to their fourth nearest neighbors, the distance which can reliably be probed by XAFS, does not correspond to any of the average periodic structures determined for this material by diffraction. The $\mathrm{Cu}\ensuremath{-}{\mathrm{O}}_{6}$ octahedra for $x=0.12$ are found to tilt $4.6\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.4\ifmmode^\circ\else\textdegree\fi{}$ from the c axis along an axis in the $a\ensuremath{-}b$ plane $20.4\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$ from the a axis. This suggests the $\mathrm{Pccn}$ (LTO2) structure, though the octahedra become somewhat distorted and the more distant atoms do not fit the LTO2 structure. In contrast, the local structure of $x=0.16$ does fit the LTO2 structure and has corresponding tilt values of $3.8\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}0.5\ifmmode^\circ\else\textdegree\fi{}$ and $19.5\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}7.5\ifmmode^\circ\else\textdegree\fi{}.$ The local structures for both concentrations are found to be independent of temperature, indicating that the various crystal phase transitions found in diffraction are due to long-range averaging of local structure regions with orientational disorder. The local structure correlation length appears to be longer than the high-${T}_{c}$ coherence length, indicating that the local structure is the relevant one when considering the pairing mechanism.