A. P. Orlov — Page 2

arXiv2005

Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow

We studied the commensurate semi-fluxon oscillations of Josephson flux-flow (JFF) in a Bi-2212 stacked structures near $T_c$ as a probe of melting of Josephson vortex lattice (JVL). We found that oscillations exist above 0.5T. The amplitude of oscillations is found to decrease gradually with temperature and to turn to zero without any jump at $T=T_0$, 3.5K below the resistive transition temperature $T_c$ indicating the phase transition of the second order. This characteristic temperature $T_0$ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, $T_{BKT}$, in elementary superconducting layers of Bi-2212 at zero magnetic field. On the base of these facts we infer that melting of triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and anti-vortices. The $B-T$ phase diagram of the BKT phase found out from our experiment is consistent with theoretical predictions.

cond-mat.supr-con↗

arXiv2004

The c-axis transport in naturally-grown Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ cross-whisker junctions

We studied the c-axis transport of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) cross-whisker junctions formed by annealing ``naturally'' formed whisker junctions. These frequently appear during growth when the $ab$-faces of neighboring whiskers come in contact. We obtained Fraunhofer patterns of the cross-junction critical currents in a parallel magnetic field, and found a sharp increase in the quasiparticle tunneling conductance at $eV=50-60$ mV, indicating high junction quality. For our weak junctions, the interface critical current density is about 0.03 of the critical current density across the stack of bulk intrinsic junctions, as is the room temperature conductivity, and is independent of the twist angle, in contrast to most of the data reported on ``artificial'' cross-whisker junctions [Y. Takano {\it et al.}, Phys. Rev. B {\bf 65}, 140513(R) (2002)]. Our results provide strong evidence of incoherent interface tunneling and for at least a small s-wave order parameter component in the bulk of Bi2212 for $T\le T_c$. They are also consistent with the bicrystal twist experiments of Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)].

cond-mat.supr-con↗