Composite to tilted vortex lattice transition in $Bi_2 Sr_2 CaCu_2 O_{8+\delta}$ in oblique fields

TL;DR

This study investigates a second phase transition in the vortex lattice of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+eta}$ superconductor under oblique magnetic fields, revealing a transition to a tilted vortex lattice state.

Contribution

The paper provides experimental evidence for a composite to tilted vortex lattice transition, identifying a tricritical point and characterizing the transition's first order nature.

Findings

Confirmation of a second, first order phase transition in vortex matter.

Identification of a tricritical point where the transition line intersects the melting line.

Evidence that the ground state near aligned fields is a tilted vortex lattice.

Abstract

Precision measurements of the vortex phase diagram in single crystals of the layered superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in oblique magnetic fields confirm the existence of a second phase transition, in addition to the usual first order vortex lattice melting line $H_{m}(T)$. The transition has a strong first order character, is accompanied by strong hysteresis, and intersects the melting line in a tricritical point ($H_{m}^{\perp}$, $H^{\parallel}_{cr}$). Its field dependence and the changing character of the melting line at the tricritical point strongly suggest that the ground state for magnetic fields closely aligned with the superconducting layers is a lattice of uniformly tilted vortex lines.