Phonon-mediated superconductivity in the Sb square-net compound LaCuSb2

We investigated the electronic structure and superconducting properties of single-crystalline LaCuSb2. The resistivity, magnetization, and specific heat measurements showed that LaCuSb2 is a bulk superconductor. The observed Shubnikov–de Haas oscillation and magnetic field dependence of the Hall resistivity can be reasonably understood assuming a slightly hole-doped Fermi surface. An electron-phonon coupling calculation clarified the difference from the isostructural compound LaAgSb2, indicating that (i) low-frequency vibration modes related to the interstitial layer sandwiched between the Sb square nets significantly contribute to the superconductivity and (ii) carriers with sizable electron-phonon coupling distribute isotropically on the Fermi surface. These are assumed to be the origin of the higher superconducting transition temperature compared with LaAgSb2. We conclude that the superconducting properties of LaCuSb2 can be understood within the framework of the conventional phonon-mediated mechanism

Lesions of the nucleus accumbens core modulate development of matching behavior

BACKGROUND: The development of choice is a crucial determinant in the performance of appetitive responses. Given two options with different reinforcement rates, animals match their relative rate of responding to the relative rates of reinforcement (i.e., matching behavior). A previous study has shown that the nucleus accumbens core (AcbC) is involved in the performance of matching behavior in trained animals. However, the role of the AcbC in the acquisition of matching behavior has not been addressed. RESULTS: We conducted a series of experimental sessions to examine the role of the AcbC on the development of matching behavior. Instrumental responding was measured in rats with excitotoxic lesions of the AcbC. Rats were given two options that differed in the relative rate of reinforcement under concurrent variable-interval schedules. The locations of the more frequently reinforced option and the alternative option were randomly switched between sessions. Lesions of the AcbC accelerated the development of matching behavior compared to the sham-operated group. The AcbC-lesioned rats exhibited closer conformity to the matching law than shams when the options were in the same positions as in the previous session (the same condition), but not when the option locations had been switched (the different condition). The AcbC rats showed smaller probabilities of switching behavior between alternatives than shams. Post-reinforcement pausing was not affected by the AcbC lesion. Neither numbers of rewards obtained nor number of lever presses were different between the AcbC-lesioned rats and shams over session blocks. CONCLUSIONS: Our results suggest that the AcbC plays a regulatory role in the development of matching behavior through switching probabilities rather than perception of reward magnitude. The differential effect of AcbC lesions on the matching behavior between the same and different conditions suggests influence of the spontaneous recovery, that is, reversion to a previously reinforced choice at the beginning of the next session, on the development of matching behavior in the AcbC-lesioned rats

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

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

Pressure-Temperature Phase Diagram of α\alpha-Mn

Electrical resistivity and ac-susceptibility measurements under high pressure were carried out in high-quality single crystals of $\alpha$-Mn. The pressure-temperature phase diagram consists of an antiferromagnetic ordered phase (0<$P$<1.4 GPa, $T<T_{\rm N}$), a pressure-induced ordered phase (1.4<$P$<4.2-4.4 GPa, $T<T_{\rm A}$), and a paramagnetic phase. A significant increase was observed in the temperature dependence of ac-susceptibility at $T_{\rm A}$, indicating that the pressure-induced ordered phase has a spontaneous magnetic moment. Ferrimagnetic order and parasitic ferromagnetism are proposed as candidates for a possible magnetic structure. At the critical pressure, where the pressure-induced ordered phase disappears, the temperature dependence of the resistivity below 10 K is proportional to $T^{5/3}$. This non-Fermi liquid behavior suggests the presence of pronounced magnetic fluctuation.Comment: 6 pages, 4 figure

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

Anomalous Hall effect triggered by pressure-induced magnetic phase transition in α-Mn

Recent interest in topological nature in condensed matter physics has revealed the essential role of Berry curvature in the anomalous Hall effect (AHE). However, since a large Hall response originating from Berry curvature has been reported in quite limited materials, the detailed mechanism remains unclear at present. Here, we report the discovery of a large AHE triggered by a pressure-induced magnetic phase transition in elemental α-Mn. The AHE is absent in the noncollinear antiferromagnetic phase at ambient pressure, whereas a large AHE is observed in the weak ferromagnetic phase under high pressure despite the small magnetization of ≈0.02μB/Mn. Our results indicate that the emergence of the AHE in α-Mn is governed by the symmetry of the underlying magnetic structure, providing a direct evidence of a switch between a zero and nonzero contribution of the Berry curvature across the phase boundary. α-Mn can be an elemental and tunable platform to reveal the role of Berry curvature in AHE