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Register a new userPressure effects on the superconducting transition in nH-CaAlSi
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We present a combined experimental and theoretical study of the effects of pressure on T_c of the hexagonal layered superconductors nH-CaAlSi (n = 1, 5, 6), where nH labels the different stacking variants that were recently discovered. Experimentally, the pressure dependence of T_c has been investigated by measuring the magnetic susceptibility of single crystals up to 10 kbar. In contrast to previous results on polycrystalline samples, single crystals with different stacking sequences display different pressure dependences of T_c. 1H-CaAlSi shows a decrease of T_c with pressure, whereas 5H and 6H-CaAlSi exhibit an increase of T_c with pressure. Ab-initio calculations for 1H, 5H and 6H -CaAlSi reveal that an ultrasoft phonon branch associated to out-of-plane vibrations of the Al-Si layers softens with pressure, leading to a structural instability at high pressures. For 1H-CaAlSi the softening is not sufficient to cause an increase of T_c, which is consistent with the present experiments, but adverse to previous reports. For 5H and 6H the softening provides the mechanism to understand the observed increase of T_c with pressure. Calculations for hypothetical 2H and 3H stacking variants reveal qualitative and quantitative differences.
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La3Co4Sn13 is a superconducting material with transition temperature at Tc = 2.70 K, which presents a superlattice structural transition at T* ~ 150 K, a common feature for this class of compounds. However, for this material, it is not clear that at T* the lattice distortions arise from a charge density wave (CDW) or from a distinct microscopic origin. Interestingly, it has been suggested in isostructural non-magnetic intermetallic compounds that T* can be suppressed to zero temperature, by combining chemical and external pressure, and a quantum critical point is argued to be observed near these critical doping/pressure. Our study shows that application of pressure on single-crystalline La3Co4Sn13 enhances Tc and decreases T*. We observe thermal hysteresis loops for cooling/heating cycles around T* for P > 0.6 GPa, in electrical resistivity measurements, which are not seen in x-ray diffraction data. The hysteresis in electrical measurements may be due to the pinning of the CDW phase to impurities/defects, while the superlattice structural transition maintains its ambient pressure second-order transition nature under pressure. From our experiments we estimate that T* vanishes at around 5.5 GPa, though no quantum critical behavior is observed up to 2.53 GPa.
By means of synchrotron X-ray diffraction, we studied the effect of high pressure, P, up to 13 GPa on the room temperature crystal structure of superconducting CaC6. In this P range, no change of the pristine space group symmetry, textit{R=3m}, is found. However, at 9 GPa, i.e. close to the critical value at which a large T_c reduction was reported recently, we observed a compressibility jump concomitant to a large broadening of Bragg peaks. The reversibility of both effects upon depressurization and symmetry arguments give evidence of an order-disorder phase transition of second order, presumably associated with the Ca sublattice, which provides a full account for the above Tc reduction.
The effects of pressure on the superconducting properties of a Bi-based layered superconductor La2O2Bi3Ag0.6Sn0.4S6, which possesses a four-layer-type conducting layer, have been studied through the electrical resistance and magnetic susceptibility measurements. The crystal structure under pressure was examined using synchrotron X-ray diffraction at SPring-8. In the low-pressure regime, bulk superconductivity with a transition temperature Tc of ~ 4.5 K was induced by pressure, which was achieved by in-plane chemical pressure effect owing to the compression of the tetragonal structure. In the high-pressure regime above 6.4 GPa, a structural symmetry lowering was observed, and superconducting transitions with a Tc ~ 8 K were observed. Our results suggest the possible commonality on the factor essential for Tc in Bi-based superconductors with two-layer-type and four-layer-type conducting layers.
Investigating the pressure dependence of the superconducting (SC) transition temperature $T_{rm c}$ is crucial for understanding the SC mechanism. Herein, we report on the pressure dependence of $T_{rm c}$ in the nonmagnetic topological line-nodal material CaSb$_2$, based on measurements of electric resistance and alternating current magnetic susceptibility. $T_{rm c}$ initially increases with increasing pressure and peaks at $sim$ 3.1~GPa. With a further increase in pressure, $T_{rm c}$ decreases and finally becomes undetectable at 5.9~GPa. Because no signs of phase transition or Lifshitz transition are observed in the normal state, the peculiar peak structure of $T_{rm c}$ suggests that CaSb$_2$ has an unconventional SC character.
Our experiments show that for two or more pieces of a wire, of different lengths in general, combined in parallel and connected to a dc source, the current ratio evolves towards unity as the combination is cooled to the superconducting transition temperature Tc, and remains pinned at that value below it. This re-distribution of the total current towards equipartition without external fine tuning is a surprise. It can be physically understood in terms of a mechanism that involves the flux-flow resistance associated with the transport current in a wire of type-II superconducting material. It is the fact that the flux-flow resistance increases with current that drives the current division towards equipartition.
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