Vortex Nanoliquid in High-Temperature Superconductors

Abstract

Using a differential magneto-optical technique to visualize flow of transport currents, we reveal a new delocalization line within the reversible vortex liquid region in the presence of a low density of columnar defects. This line separates a homogeneous vortex liquid, in which all the vortices are delocalized, from a heterogeneous "nanoliquid" phase, in which interconnected nanodroplets of vortex liquid are caged in the pores of a solid skeleton formed by vortices pinned on columnar defects. The nanoliquid phase displays high correlation along the columnar defects but no transverse critical current.PACS numbers: 74.25. Qt, 74.25.Op, 74.25.Sv, 74.72.Hs Columnar defects (CDs) created in high-temperature superconductors by high-energy heavy ion irradiation act as very efficient pinning centers for vortices aligned along the CDs. The effects of this correlated disorder on the vortex properties have been investigated extensively mainly in the limit of relatively high density of CDs where the enhancement in the critical current for practical applications is most significant and the vortex matter is known to form the Bose glass phase [1][2][3][4]. In contrast, our understanding of the opposite limit, in which sparse CDs perturb a higher concentration of vortices, is still incomplete. Recent experimental studies [5,6] have shown that in this case the vortex matter can no longer be regarded as a homogeneously pinned medium. Instead, an intrinsically heterogeneous structure with two distinct sub-systems of vortices with very different characteristic energies is formed: vortices residing on CDs are strongly pinned and create a rigid skeleton, while the interstitial vortices form relatively ordered nanocrystals intercalated within the voids of this porous skeleton. This experimental finding of a "porous" vortex solid has contributed to a series of recent theoretical studies and numerical simulations [7][8][9][10][11][12]. Moreover, it was found that the melting line B m (T ) of the "porous" vortex solid has an unconventional behavior showing a pronounced kink [5,6]. This has led to the suggestion of the possible existence of two different liquid phases: at fields below the kink a usual homogeneous vortex liquid is formed upon melting, whereas at fields above the kink the "porous" solid melts into a heterogeneous state in which liquid vortices coexist with a skeleton of pinned vortices. It was proposed that in such a case a new transition line separating these two liquid phases should be expected [5,3,10]. Using a new imaging technique we present here the first experimental evidence for such a transition line within the reversible vortex liquid phase that separates a heterogeneous "nanoliquid" from a homogeneous liquid. One of the interesting properties of this novel nanoliquid phase is the absence of in-plane dissipationless superconductivity, while the coherence along the c-axis is apparently still preserved. The CDs were created along the c-axis in Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) single crystals (T c ≃ 90 K) using low ir...

Collective pinning of the vortex lattice by columnar defects in layered superconductors

“…It has a transverse scale for positional correlations, fig. 1 and that is listed in [5]. A direct sublimation transition between a vortex solid and a decoupled vortex-liquid phase is in fact consistent with previous experimental studies of the unirradiated system [16].…”

“…This phase is predicted to melt into a 3D vortex-line liquid that shows phase coherence across layers on length scales that are large compared to the spacing between adjacent layers. We believe that the phase transition observed recently inside of the vortex-liquid regime of high-temperature superconductors pierced by sparse columnar tracks [5] reflects layer decoupling by such a 3D vortex-line liquid [15]. This proposal is consistent with the absence of such a dimensional cross-over transition in the vortex-liquid phase of the unirradiated (pristine) superconductor [16].…”

“…A Bose glass phase with a divergent tilt-modulus is predicted to exist at relatively low fields such that each and every flux-line can be trapped by a correlated material defect [2]. Experimental studies of the opposite limit, where the number of flux lines exceeds the number of correlated pinning sites, have been made recently [3][4] [5]. Less is known theoretically about this regime in comparison to the former relatively low-field regime.…”

Imaging superconducting vortex cores and lattices with a scanning tunneling microscope

“…Generally, when vortex motion is produced on such a disordered lattice, the system reacts by leaving some vortices fixed. Others are mobile and give net motion [44,86,[247][248][249][250][251]. Similar phenomena have been observed in many particle systems, and they are termed plastic de-pinning.…”

“…For instance, in multiband superconductors and in nanostructures [84][85][86]. Of particular interest are recent advances in size reduction of SQUIDs.…”