Experimental Proof of a Time-Reversal-Invariant Order Parameter with aπshift inYBa2Cu3

Abstract

To determine the symmetry of the order parameter in superconducting YBazCu&07 z (YBCO), we use a scanning SQUID microscope at 4.2 K to perform independent experiments on six YBCO-Ag-Pb SQUIDs. We find completely unambiguous evidence for a time-reversal-invariant order parameter with a phase shift of m-between the a and b directions of YBCO. Our results are inconsistent with purely s-wave symmetric pairing and are strongly suggestive of d 2 -y2 symmetric pairing in YBCO. PACS numbers: 74.50.+r, 74.20.Mn, 74.72.Bk At present, there is enormous controversy concerning the symmetry of the order parameter in the high transition temperature (T,) superconductor YBa2Cu307 s (YBCO) [1]. The work we report here greatly extends the pioneering experiment of Wollman et al. [2], who first reported results on YBCO-Pb SQUIDs. Their conclusion of d .~2 symmetric pairing has been supported by some recent work on Josephson junctions and SQUIDs [3 -5], but not by other work [6,7]. In our own work, we have found that common effects produced by trapped vortices, magnetic field gradients, measuring currents, or asymmetries in the SQUIDs can mimic those produced by d-wave superconductivity.In light of this, the disagreements in prior work are neither surprising nor reassuring.In this Letter, we describe experiments which use a scanning SQUID microscope and a time-reversal-invariance test to provide consistent unambiguous evidence that YBCO has a time-reversal-invariant order parameter with a~shift between the crystal a and b directions.To understand how a SQUID can be used to find the pairing symmetry of a superconductor, consider Fig. 1(a), which shows a schematic of our type a bSQUID. O-ne half of the SQUID is made from the s-wave superconductor Pb and the other half from YBCO [2]. The two Josephson junctions allow tunneling of pairs between the superconductors, with one junction oriented normal to the YBCO a axis and the other normal to the b axis. If no magnetic field 8 is present and YBCO has a d 2 -y2 symmetry, then pairs tunneling through the a-axis junction have a phase shift of m with respect to pairs tunneling through the b-axis junction. Thus, a pair which travels once around the SQUID loop acquires an intrinsic phase shift of m. Such a~shift produces a current J circulating around the loop. If P = 2LIo/4O» 1, then a vr shift generates LJ = 4'o/2 of flux in the loop, where = h/2e is the flux quantum, L is the SQUID loop inductance, and Io is the average critical current of the junctions at 8 = 0. No such intrinsic phase shift, circulating current, or half quantized Aux would be produced at B = 0 if YBCO had s-wave symmetry or if the SQUID had a type a-a geometry, i.e. , both junctions oriented normal to the a axis [see Fig. 1(b)].In principle, the above ideas can be used to determine the pairing symmetry.However, in practice, great care is required because such a circulating current can arise from any small magnetic field. Such fields can be created by the measuring apparatus, by vortices in the superconducting films, or...

d-wave pairing symmetry in cuprate superconductors

“…Nevertheless, these phase-insensitive techniques have produced a large body of evidence for d-wave pairing in the cuprates. The recent development of phase-sensitive pairing symmetry test [2][3][4][5][6][7], has yielded compelling evidence for predominantly dwave pairing symmetry in a number of optimally doped cuprates [8]. A question naturally arises: How universal is the d x 2 −y 2 pairing in cuprate superconductors?…”

Theory for the interdependence of high-Tc superconductivity and dynamical spin fluctuations

“…During the last years the symmetry of the superconducting order parameter in momentum space was studied intensively, because it probably holds the key for an understanding of the High-T c systems and the role of spin fluctuations within the cuprates [2]. Recent angular resolved photoemission (ARPES) experiments and phase sensitive measurements of the gap function [3][4][5][6][7] clearly favor a d x 2 −y 2 or an anisotropic s x 2 +y 2 symmetry of the gap in momentum space over an isotropic s-wave scenario. Moreover the maximum of T c upon doping, the occurrence of an additional dip in the ARPES spectra of Bi 2 Sr 2 CaCu 2 O 8+δ found by Dessau et al [8] and the dip at ω = ±3∆ in superconductor-insulator-superconductor (SIS) tunneling measurements [9] are important experiments that may be significant clues for the pairing interaction.…”

Josephson (001) tilt grain boundary junctions of high-temperature superconductors

“…In those experiments, one of the junctions functioned as a 0-junction, and the other functioned as a π-junction. 49 The beauty of the recent experiment is that the zigzag array of alternating 0-/π-junctions was observed by a SQUID microscope to exhibit an ordered array of fractional vortices, the magnitude of which was within the experimental error of Φ 0 /2, where Φ 0 is the flux quantum. Although no microscopic analysis of the edges of the YBCO was presented, the ion milling technique employed necessarily leaves a rough surface on a scale of at least 1-5 nm, so that the tunneling across each of these edges is most likely incoherent.…”

“…After deposition of a thick layer of Au, a thicker layer of Nb was applied. This experiment is a variation of the earlier θ = 90 • YBCO/Pb superconducting quantum interference device (SQUID) experiment of Mathai et al, 49 which contained YBCO/Pb junctions with a thin Ag interstitial layer along the ramped nominally (100) and (010) edges of YBCO, respectively. In those experiments, one of the junctions functioned as a 0-junction, and the other functioned as a π-junction.…”