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We have investigated the electronic structures of metal-to-insulator transitions (MIT) due to charge-density-wave phase transition in 1T- TaS x Se 2-x by angle-resolved photoemission spectroscopy. In the insulating phase of the sample with x = 1.5, we observe a gap formation at the Fermi energy (E F ), while in the metallic phase of the sample with x = 1.2, we do not observe a drastic change in the spectra between at room temperature and at low temperature. We observe a peak originating from the lower Hubbard band in the insulating phase of the sample with x = 1.5. In the metallic phase of the sample with x = 1.2, the peak intensity is smaller and the peak energy shifts near E F , but the lower Hubbard band peak still remains. These results suggest that MIT in 1T- TaS x Se 2-x is due to the Mott localization.

On using scanning tunneling microscopy we observed charge-density-wave (CDW) structures in layered compounds 1T-TaS2−xSex (x=0, 0.3, 0.5) at room temperature, and found a new domain structure for the x=0.3 compound. The new domain structure has a mesh-like pattern, in which the domain boundaries are imaged with brighter contrast only at specific sample voltages, implying that the domain structure has an electronic origin. Moreover, the CDW arrangement is irregularly distorted, while its amplitude is spatially unchanged. These features are in clear contrast to those of the hexagonal domains in the x=0 compound, suggesting that the structural transformation with respect to x is not a well-defined phase transition with abrupt disappearance of the hexagonal domains, but a continuous restructuring of CDW. We argue for the mesh-like structure to be a kind of moire pattern, originating from the local variation of CDW stacking along the c-axis.

STM images of subsurface impurities were found to change abruptly at the metal–insulator transition temperature in 1T-TaS2. In the metallic phase the impurities were imaged as less-conducting regions with diameters less than a few nanometers; in the insulating phase, however, characteristic ring-shape structures were observed, which typically showed 1 nm in corrugation amplitude and several tens of nanometers in diameter. Based on a recent theoretical investigation, this phenomenon can be interpreted as a long-range standing wave formation caused by a significant increase of the effective wavelengths of the electronic low energy excitations associated with the metal–insulator transition.

Surface etching of the layered compound 1T-TaS2 with a STM tip was observed in an ultrahigh vacuum where the true vacuum tunneling gap was achieved. The etching did not always progress in a layer-by-layer manner, but etch-pits often had the depths of multiples of the unit layer. The etching rate on a clean surface was much slower than that on a surface once exposed to the ambient air, which showed a rough and irregular nano-scale morphology, implying that the surface of 1T-TaS2 is chemically degraded by the water vapor in air. On a clean surface, the STM etching started from the defect-like sites seen on the STM image, which possibly reflect the substrate impurities located on the Ta planes or van der Waals gaps, and progressed preferably in the scanning direction. The etching rate tended to increase with the bias voltage, regardless of its sign. This suggests that the electrostatic attractive force between tip atoms and surface atoms and/or the current-induced local heating contribute significantly to the STM etching of 1T-TaS2 in ultrahigh vacuum.

New families of barium-free mercury-based oxide superconductors were found in the HgSr2Can-1CunOy (n=1, 2) systems. The 1201-type compounds (n=1) were structurally stabilized by partial substitution of Mo for the Hg site or Re for the Cu site. These 1201-type compounds showed a superconducting transition at 60–70 K. The 1212-type compounds (n=2) with Tc of approximately 100 K were also synthesized by substitution of Mo or Re for the Hg or Cu, and Y for the Ca sites. Such a simultaneous Y doping was necessary to obtain 1212-type single-phase samples, while it seemed to result in electron carrier doping to the system. As was recently pointed out by the present authors in (Hg, Cr)Sr2CuOy, the Sr based compounds prepared in this study were also very stable during the synthesis and no deterioration was recognized after the preparations in sharp contrast to Ba containing compounds.

The layered compound 1T–TaS1.7Se0.3 forms a nanoscale domain structure, separated by mesh-like domain walls, above its bulk metal–insulator transition temperature TMI of ∼180 K. Scanning tunneling microscopy and spectroscopy of the compound demonstrated that each metallic domain can be converted to insulating one by successive scans of the probe tip just above TMI. This tip-assisted phenomenon is consistently explained by assuming that the domain structure arises from irregular distortion of charge density waves, and that the stacking pattern of charge density waves plays an essential role in the metal–insulator transition.

Scanning tunnelling microscopy (STM) and spectroscopy (STS) have been performed on layered materials, cuprate superconductors and transition metal dichalcogenides, in order to elucidate the properties of high temperature superconductivity and electron localization respectively, in close correlation with their low dimensional electron properties.

We present a detailed angle-resolved photoemission study on the layered transition-metal dichalcogenide 1T-TaS1.2Se0.8 in the commensurate charge-density-wave (CDW) phase. A drastic reduction in the spectral weight along the high symmetry line GammaM, particularly around the point M, is observed when s-polarized light was used. This implies that the initial state must be symmetric with respect to a mirror plane perpendicular to the line GammaK, which is consistent with conventional band calculations in the absence of the CDW. We conclude that there is only a limited amount of modification of the electronic structure of 1T-TaS1.2Se0.8 in the commensurate CDW phase due to the CDW-related potential.

We have investigated the electronic structure of the commensurate charge-density-wave (CDW) phase for $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ by Ta $4f$ core-level and the angle-resolved photoemission. From the energy shifts of Ta $4f$ core levels due to a CDW formation, it is found that the CDW amplitude in the $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ is smaller than that in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ reported previously. The band dispersion along the normal to the two-dimensional layers indicates that $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ has a three-dimensional (3D) character in the Fermi surface compared with $1T\ensuremath{-}{\mathrm{TaS}}_{2},$ which has a two-dimensional Fermi surface. The 3D Fermi surface of $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ is thought to originate from the larger interaction between layers due to a large charge transfer between Ta $5d$ and Se $4p$ orbitals.

Mott transition in the solid solution of 1T-TaSxSe2−x was investigated by the scanning tunneling microscopy/spectroscopy (STM/STS) at 77 K. For x>1.5, a clear Mott-Hubbard gap structure of ∼0.5 eV, accompanied with two conductance peaks was observed. In TaS1.2Se0.8, on the other hand, it was found that the gap structure was collapsed. The x dependence of the tunneling spectrum is consistent with the Mott transition picture where the density of states near the Fermi level goes to zero with increasing x.

We have synthesized new Ba-free Hg-based superconductors (Hg1−xMx)Sr2(Ca1−yYy)n−1CunOz (n=1,2). These compounds are chemically and structurally stabilized by partial substitution of the elements MCr, Mo and Re. While M-free compounds were multi-phased and superconducting volume fractions were less than 1%, M-doped Hg 1201-type compounds, (Hg1−xMx)Sr2CuOz, were nearly single phased and found to be superconductors with Tc's of 50∼70K. Furthermore, by partially substituting Y for Ca, we have synthesized 1212-type compounds (Hg1−xMx)Sr2(Ca1−yYy)Cu2Oz with Tc's∼100K. In contrast to the Ba-containing compounds, the present materials are insensitive to open air during the synthesis and also quite stable after the preparation.