We have used angle-resolved photoemission spectroscopy to investigate the band structure of ReS 2 , a transitionmetal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along k z . These experiments are in good agreement with our density-functional theory calculations, shedding light on the bulk electronic structure of ReS 2 , and how it can be expected to evolve when thinned to a single layer.

ACrX 2 (A = Cu, Ag; X = S, Se) compounds have a layered triangular structure similar to that of delafossites. In addition to a large variety of physical and chemical properties found in centrosymmetric delafossites, the broken inversion symmetry along the c-axis in this system leads to the emergence of additional functionalities such as piezoelectric, pyroelectric, and/ or nonlinear optical properties. We found that the chemical vapor transport (CVT) technique using CrCl 3 was the best growth method for ACrX 2 . In the case of A = Cu, however, a standard CVT procedure always produced a certain amount of a secondary phase, identified as the CuCr 2 X 4 spinel, which significantly affected magnetic and transport properties. By a modified CVT technique with appropriate heat treatments, pure single crystals of CuCrX 2 were successfully grown. The resistivity of ACrX 2 was systematically changed by the combinations of A and X atoms. In contrast to metallic selenides, sulfides were confirmed to be insulators with giant anisotropy (10 2 ≈ 10 3 ) between the out-of-plane (c-direction) and the in-plane (ab-plane) resistivity. In both X = S and Se, resistivity for A = Ag was higher than that for A = Cu. The drastic change in resistivity without carrier doping suggests that ionic conductivity and strong electron correlations play an important role in these materials.

Topological insulators with broken time-reversal symmetry and the Fermi level within the magnetic gap at the Dirac cone provides exotic topological magneto-electronic phenomena. Here, we introduce an improved magnetically doped topological insulator, Fedoped BiSbTe 2 Se (Fe-BSTS) bulk single crystal, with an ideal Fermi level. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements revealed that the surface state possesses a Dirac cone with the Dirac point just below the Fermi level by 12 meV. The normalized dI/dV spectra suggest a gap opening with Δ mag ∼55 meV, resulting in the Fermi level within the opened gap. Ionic-liquid gated-transport measurements also support the Dirac point just below the Fermi level and the presence of the magnetic gap. The chemical potential of the surface state can be fully tuned by ionic-liquid gating, and thus the Fe-doped BSTS provides an ideal platform to investigate exotic quantum topological phenomena.

The transport properties of a topological Josephson junction fabricated from a magnetically doped topological insulator (TI) were investigated. The conductance spectra of the Nb/Fe-Bi2Te2Se/Nb junction below 1 K showed an unusual trident-shaped zero bias conductance peak with a tiny peak width of ∼ 6 µV. The central peak of the trident peak represents the dc-Josephson current, and the side peaks may reflect an induced unconventional Cooper pairing. Additionally, unsaturated temperature dependence of critical current under 1 K may reflect the presence of an unconventional proximity effect. Furthermore, microwave irradiation derived a drastic change in the conductance spectra from the peak structure into oscillatory signals, a hallmark of the ac-Josephson supercurrent. The phase relation of the ac-Josephson effect under high power radiofrequency-irradiation was found to be 4π-periodic. The results suggest that the junction based on magnetically doped 3D TIs may realize an unconventional Cooper pairing, thus enabling access to the basic physics of Andreev bound states and unconventional superconductivity.