A Bragg glass phase in the vortex lattice of a type II superconductor

Abstract

Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of crystalline order. The resulting state of matter has been a long-standing puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into 'crystallites'. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The 'lattice' of vortices that contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron-diffraction data of the vortex lattice provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transitions and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover, the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and the conductivity of electronic crystals.

Transition between the Bragg glass and the disordered phase inNb3Sndetected by third harmonics of the ac magnetic susceptibility

“…In this phase diagram a transition in the vortex lattice has been predicted, between a disordered phase and the Bragg Glass Phase [3,4]. This latter is characterized by a quasi long range order and a perfect topological order, in which the vortex lattice stills survives despite the presence of the pinning [3], as it has also been experimentally evidenced [5]. The disordered phase has been supposed to be again a glass phase ("multidomain glass"), but with a topological disorder at the largest length scales [4].…”

“…the temperature corresponding to the local minimum in the dip, at various DC magnetic fields. In the H-T region below the T p line the vortex lattice still survives (Bragg glass) [3,5], whereas the disorder destroys the lattice in the temperature range between T p and T c . It can be seen that the T p line stops at a certain point in the high field/low temperature region of the phase diagram.…”

SANS study of vortex lattice structural transition in optimally doped (Ba1−xKx)Fe2As2

“…Depending on the system (weakly disordered/or strong pinning) this structural transition drives the vortex lattice to a liquid phase with a melting transition or to a disordered solid phase with a glass transition respectively [19,18,15,1,5,11]. In conventional superconductors like in Nb single crystal and in NbSe 2 an anomalous phenomena is found in the vicinity of the peak effect where the critical current j c increases sharply below the upper critical field H c2 [17,11].…”

“…The change in current-voltage characteristics that this entails leads to the so-called "second magnetization peak" (SMP) phenomenon in numerous type II superconductors. The first order transition at the SMP onset field, B sp [12,11], has been interpreted either in terms of a structural change from a dislocation-free vortex "Bragg glass" to a plastically disordered phase [13,14,15], or in terms of the loss of vortex integrity along the field direction [6,16]. Thus, the understanding of the vortex phase diagram in disordered superconductors is incomplete at best, unsettled questions being the mechanism of the SMP transition, the link between vortex solid structure and dynamics, the nature of the vortex glass, and whether the high-field vortex solid is, in all cases, distinct from the liquid.…”

“…by correction for the vortex form factor F , that reflects the magnetic field distribution around a vortex [15]. Here, K 0 is the vortex reciprocal lattice vector and ω is the rocking angle.…”