Upper critical magnetic field of LnO${}_{0.5}$F${}_{0.5}$BiS₂ (Ln = La, Nd) superconductors at ambient and high pressure

Abstract

The upper critical field H c2 of polycrystalline samples of LnO 0.5 F 0.5 BiS 2 (Ln = La, Nd) at ambient pressure (tetragonal structure) and high pressure (HP) (monoclinic structure) have been investigated via electrical resistivity measurements at various magnetic fields up to 8.5 T. The H c2 (T ) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below T c , which cannot be described by the conventional one-band Werthamer-Helfand-Hohenberg theory. For the LaO 0.5 F 0.5 BiS 2 sample under HP, as temperature is decreased, the upper critical field H onset , estimated from the onset of the superconducting transitions, increases slowly between 4.9 and 5.8 T compared with the slope of H onset (T ) below 4.9 T and above 5.8 T. This anomalous behavior reveals a remarkable similarity in superconductivity between LaO 0.5 F 0.5 BiS 2 samples measured under HP and synthesized under HP, although the crystal structures of the two samples were reported to be different. The experimental results support the idea that local atomic environment, which can be tuned by applying external pressure and can be quenched to ambient pressure via high temperature-pressure annealing, is possibly more essential to the enhancement of T c for BiS 2 -based superconductors than the structural phase transition. On the other hand, such anomalous behavior is very subtle in the case of NdO 0.5 F 0.5 BiS 2 under HP, suggesting that the anisotropy of the upper critical field in the ab-plane and the possible lattice deformation induced by external pressure is weak. This explains why the pressure-induced enhancement of T c for NdO 0.5 F 0.5 BiS 2 is not as large as that for LaO 0.5 F 0.5 BiS 2 .

High-pressure effects on nonfluorinated BiS2 -based superconductors La1−xFang1, Yazici2, Jeon3 et al. 2017Phys. Rev. B

“…By doping the BiS 2 -layers with electrons via appropriate chemical substitutions, superconductivity was found in Ln(O, F)BiS 2 (Ln = La-Nd, Yb), (Sr, La)FBiS 2 , (La, M )OBiS 2 (M = Ti, Zr, Hf, Th), Bi 3 O 2 S 3 , La(O, F)BiSe 2 , EuBiS 2 F, and Eu 3 Bi 2 S 4 F 4 with values of the superconducting transition temperature T c ranging from 2.7 to 10.6 K [2,[5][6][7][8][9][10][11]. One of the fascinating features in the behavior of several of the BiS 2 -based superconductors, including LnO 0.5 F 0.5 BiS 2 (Ln = La-Nd, Yb), (La,Sm)O 0.5 F 0.5 BiS 2 , Sr 0.5 R 0.5 FBiS 2 (R = La-Pr, Sm), LaO 0.5 F 0.5 BiSe 2 , and EuBiS 2 F, is the pressureinduced enhancement of T c that occurs at about 0.5-2.2 GPa, revealing the existence of two distinct superconducting phases [12][13][14][15][16][17][18]. This phenomenon is believed to be associated with a structural phase transition, which has been demonstrated in the case of LaO 0.5 F 0.5 BiS 2 , from a tetragonal structure to a monoclinic structure with a sudden change in unit cell volume at an external pressure (P ) of ∼0.5 GPa [19].…”

Pressure-induced enhancement of two-dimensionality in LaO1−xFxBi(Se/SSvitlyk1, Krztoń‐Maziopa2, Mézouar3 2019Phys. Rev. B2010View full textAdd to dashboardCite

“…However, upon a further compression a new phase with an increased Tc ~ 6 K emerges (SCII phase) [5][6][7]. Emergence of the HP SCII phase was observed in many other BiCh2-based materials [8][9][10][11][12][13][14][15][16][17], including LaO0.5F0.5BiS2 [17][18][19][20]. The SCII phase was reported to crystallize in a monoclinic structure (P21/m), as concluded from P-dependent powder diffraction data [19].…”