We report appearance of superconductivity at 5 K in NdO_0.5F_0.5BiS₂ and supplement the discovery [Demura et al., e-print arXiv:1207.5248] of the same in layered sulfide based compound. Detailed structural analysis showed that the studied compound is crystallized in tetragonal P4/nmm space group with lattice parameters a = 3.9911(3) Å and c = 13.3830(2) Å. Bulk superconductivity is established in NdO_0.5F_0.5BiS₂ at 5 K by both transport and magnetic measurements. Electrical transport measurements showed superconducting T_c onset at 5.2 K and T_c (ρ = 0) at 4.7 K. Under applied magnetic field, both T_c onset and T_c (ρ = 0) decrease to lower temperatures and an upper critical field [H_c2(0)] of above 23 kOe is estimated. Isothermal magnetization exhibited typical type-II behavior with lower critical field (H_c1) of around 25 Oe. Specific heat [C_p(T)] is investigated in the temperature range of 1.9–50 K in zero external magnetic field. A Schottky-type anomaly is observed at low temperature below 7 K.

Transition-metal dichalcogenides (${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$) have recently drawn much attention, because of the nonsaturating extremely large magnetoresistance (XMR) observed in these compounds in addition to the predictions of likely type-II Weyl semimetals. Contrary to the topological insulators or Dirac semimetals where XMR is linearly dependent on the field, in ${\mathrm{WTe}}_2$ and ${\mathrm{MoTe}}_2$ the XMR is nonlinearly dependent on the field, suggesting an entirely different mechanism. Electron-hole compensation has been proposed as a mechanism of this nonsaturating XMR in ${\mathrm{WTe}}_2$, while it is yet to be clear in the case of ${\mathrm{MoTe}}_2$ which has an identical crystal structure of ${\mathrm{WTe}}_2$ at low temperatures. In this Rapid Communication, we report low-energy electronic structure and Fermi surface topology of ${\mathrm{MoTe}}_2$ using angle-resolved photoemission spectrometry (ARPES) technique and first-principles calculations, and compare them with that of ${\mathrm{WTe}}_2$ to understand the mechanism of XMR. Our measurements demonstrate that ${\mathrm{MoTe}}_2$ is an uncompensated semimetal, contrary to ${\mathrm{WTe}}_2$ in which compensated electron-hole pockets have been identified, ruling out the applicability of charge compensation theory for the nonsaturating XMR in ${\mathrm{MoTe}}_2$. In this context, we also discuss the applicability of other existing conjectures on the XMR of these compounds.

The carrier dynamics and electronic structures of type-II Weyl semimetal candidates ${\mathrm{MoTe}}_2$ and ${\mathrm{WTe}}_2$ have been investigated by using temperature-dependent optical conductivity $\left[\sigma \right(\omega \left)\right]$ spectra. Two kinds of Drude peaks (narrow and broad) have been separately observed. The width of the broad Drude peak increases with elevating temperature above the Debye temperature of about 130 K in the same way as those of normal metals, on the other hand, the narrow Drude peak becomes visible below 80 K and the width is rapidly suppressed with decreasing temperature. Because the temperature dependence of the narrow Drude peak is similar to that of a type-I Weyl semimetal TaAs, it was concluded to originate from Dirac carriers of Weyl bands. The result suggests that the conductance has the contribution of two kinds of carriers, normal semimetallic and Dirac carriers, and this observation is evidence of the type-II Weyl semimetals ${\mathrm{MoTe}}_2$ and ${\mathrm{WTe}}_2$. The obtained carrier mass of the semimetallic bands and the interband transition spectra suggest the weak electron correlation effect in both materials.