Observation of superconductivity and its enhancement at the charge density wave critical point in LaAgSb 2

We discover superconductivity (SC) in LaAgSb2 at ambient pressure and its close correlation with a charge density wave (CDW) under pressure. The superconducting transition temperature (Tc) exhibits a sharp peak at a CDW critical pressure of 3.2 GPa. We demonstrate that the carriers inhabiting the Sb-square net are crucial not only in the formation of CDW but also in SC for their relatively strong electron-phonon coupling (EPC). Furthermore, theoretical EPC strength in pristine LaAgSb 2 cannot explain the observed peak with Tc∼1 K, which indicates that an additional mechanism reinforces SC only around the CDW critical pressure

Magnetotransport studies of the Sb square-net compound LaAgSb2 under high pressure and rotating magnetic fields

Square-net-layered materials have attracted attention as an extended research platform of Dirac fermions and of exotic magnetotransport phenomena. In this study, we investigated the magnetotransport properties of LaAgSb2, which has Sb-square-net layers and shows charge density wave (CDW) transitions at ambient pressure. The application of pressure suppresses the CDWs, and above a pressure of 3.2 GPa a normal metallic phase with no CDWs is realized. By utilizing a mechanical rotator combined with a high-pressure cell, we observed the angular dependence of the Shubnikov–de Haas (SdH) oscillation up to 3.5 GPa, and we confirmed the notable two-dimensional nature of the Fermi surface. In the normal metallic phase, we also observed a remarkable field-angular-dependent magnetoresistance (MR), which exhibited a “butterflylike” polar pattern. To understand these results, we theoretically calculated the Fermi surface and conductivity tensor at the normal metallic phase. We showed that the SdH frequency and Hall coefficient calculated based on the present Fermi surface model agree well with the experiment. The transport properties in the normal metallic phase are mostly dominated by the anisotropic Dirac band, which has the highest conductivity due to linear energy dispersions.We also proposed that momentum-dependent relaxation time plays an important role in the large transverse MR and negative longitudinal MR in the normal metallic phase, which is experimentally supported by the considerable violation of Kohler’s scaling rule. Although quantitatively complete reproduction was not achieved, the calculation showed that the elemental features of the butterfly MR could be reasonably explained as the geometrical effect of the Fermi surface

Successive destruction of charge density wave states by pressure in LaAgSb2

We comprehensively studied the magnetotransport properties of LaAgSb2 under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at TCDW1∼210 K and TCDW2∼190 K at ambient pressure. With the application of pressure, both TCDW1 and TCDW2 were suppressed and disappeared at the critical pressures of PCDW1=3.0–3.4 GPa and PCDW2=1.5–1.9 GPa, respectively. At PCDW1, the Hall conductivity showed a steplike increase, which is consistently understood by the emergence of a two-dimensional hollow Fermi surface at PCDW1. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the c axis. The negative contribution was observed in the whole pressure region from 0 to 4 GPa. Shubnikov–de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In PPCDW1, we observed a single frequency of ∼48 T with a cyclotron effective mass of 0.066m0, whose cross section in the reciprocal space corresponded to only 0.22% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of ∼9.2 T, which is significantly enhanced in the limited pressure range of PCDW2<P<PCDW1. The amplitude of this oscillation was anomalously suppressed in the high-field and low-temperature region, which cannot be explained by the conventional Lifshitz-Kosevich formula