Experimental determination of the superconducting pairing state in YBCO from the phase coherence of YBCO-Pb dc SQUIDs

Wollman, David A.; Harlingen, D. J. Van; Lee, W. C.; Ginsberg, D. M.; Leggett, Anthony J.

doi:10.1103/physrevlett.71.2134

Cited by 1,087 publications

(659 citation statements)

References 16 publications

(8 reference statements)

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“…However, use of π-rings for a model spin system requires a large number of rings. The first superconducting π-rings, 8,9,10,11 which depended on the momentum dependence of the pairing wavefunction in the high-T c cuprate perovskite superconductors, were made using technologies which would be difficult to extend to many devices. Recently a technology that allows for photolithographic fabrication of π-shift devices and arrays of great complexity, using YBa 2 Cu 3 O 7−δ -AuNb (YBCO-Au-Nb) ramp-edge tunneling junctions has been demonstrated.…”

Section: Introductionmentioning

confidence: 99%

Antiferromagnetic ordering in arrays of superconductingπ-rings

et al. 2005

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We report experiments in which one dimensional (1D) and two dimensional (2D) arrays of YBa2Cu3O 7−δ -Nb π-rings are cooled through the superconducting transition temperature of the Nb in various magnetic fields. These π-rings have degenerate ground states with either clockwise or counter-clockwise spontaneous circulating supercurrents. The final flux state of each ring in the arrays was determined using scanning SQUID microscopy. In the 1D arrays, fabricated as a single junction with facets alternating between alignment parallel to a [100] axis of the YBCO and rotated 90 o to that axis, half-fluxon Josephson vortices order strongly into an arrangement with alternating signs of their magnetic flux. We demonstrate that this ordering is driven by phase coupling and model the cooling process with a numerical solution of the Sine-Gordon equation. The 2D ring arrays couple to each other through the magnetic flux generated by the spontaneous supercurrents. Using π-rings for the 2D flux coupling experiments eliminates one source of disorder seen in similar experiments using conventional superconducting rings, since π-rings have doubly degenerate ground states in the absence of an applied field. Although anti-ferromagnetic ordering occurs, with larger negative bond orders than previously reported for arrays of conventional rings, long-range order is never observed, even in geometries without geometric frustration. This may be due to dynamical effects. Monte-Carlo simulations of the 2D array cooling process are presented and compared with experiment.

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Section: Introductionmentioning

confidence: 99%

Antiferromagnetic ordering in arrays of superconductingπ-rings

et al. 2005

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“…This state is characterised by an unusual vortex pattern in the form of a necklace of fractional vortices around the perimeter of the material, where neighbouring vortices have opposite current circulation. This vortex pattern is a result of a spectral rearrangement of current carrying states near the surfaces.The phase-sensitive experiments [1,2] carried out in the early 1990's showed that the high-temperature superconductors have predominantly d-wave pairing symmetry. Ever since, there has been an ongoing debate whether there exists a low-temperature phase that breaks time-reversal (T ) symmetry, in addition to the reflection symmetry of the crystal broken by the d-wave order parameter itself.…”

mentioning

confidence: 99%

Spontaneously broken time-reversal symmetry in high-temperature superconductors

Håkansson¹,

Löfwander²,

Fogelström³

2015

Nature Phys

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Conventional superconductors are strong diamagnets that through the Meissner effect expel magnetic fields. It would therefore be surprising if a superconducting ground state would support spontaneous magnetics fields. Such time-reversal symmetry broken states have been proposed for the high-temperature superconductors, but their identification remains experimentally controversial. Here we show a route to a low-temperature superconducting state with broken time-reversal symmetry that may accommodate currently conflicting experiments. This state is characterised by an unusual vortex pattern in the form of a necklace of fractional vortices around the perimeter of the material, where neighbouring vortices have opposite current circulation. This vortex pattern is a result of a spectral rearrangement of current carrying states near the surfaces.The phase-sensitive experiments [1,2] carried out in the early 1990's showed that the high-temperature superconductors have predominantly d-wave pairing symmetry. Ever since, there has been an ongoing debate whether there exists a low-temperature phase that breaks time-reversal (T ) symmetry, in addition to the reflection symmetry of the crystal broken by the d-wave order parameter itself. Several experiments on tunnelling and charge transport support a phase transition into a state with broken T -symmetry at low temperatures [3][4][5][6][7][8]. On the other hand efforts to detect the concomitant spontaneous magnetic field have failed, or at best have put severe restrictions on how strong the subdominant order can be [9][10][11][12][13]. Below we show how to reconcile these two sets of experiments and how the route to a low-temperature superconducting state with broken Tsymmetry may occur. We emphasize that the superconducting state with broken T -symmetry we describe here does not necessarily imply a multicomponent superconducting order-parameter [14,15].The superconducting state in unconventional superconductors, such as the cuprates, is fragile to scattering off impurities, defects and surfaces [16]. Scattering leads to pair-breaking and formation of so-called Andreev states [16][17][18]. These states are formed by constructive normal and Andreev reflection processes, have energies ε A within the superconducting energy gap ∆, and are spatially bound to the scattering centers. For superconducting grains, when the size of the superconducting material is comparable with the superconducting coherence length, the whole superconducting state of the grain is affected by boundary scattering and the formation of Andreev states leads to properties that are not fully understood yet. For the realization of real devices, a deeper understanding of the ground state of grains is therefore called for.We consider meso-scaled grains of a d-wave superconductor and relax the assumption of translational invariance along the surface. The typical grain sizes we consider correspond to side lengths D ∼ 10ξ 0 to 100ξ 0 , where ξ 0 = v F /(2πk B T c ) is the superconducting coherence length (v F is the...

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“…3 We take the d x 2 −y 2 order parameter as having its positive lobe along the the crystalâ axis and its negative lobe along thê b-axis. 4 For symmetries obeyed by propagators and self energies, see 28 .…”

Section: A the Self-consistent Pinholementioning

confidence: 99%

Pinhole junctions in d-wave superconductors

Fogelström

Kurkijärvi

1996

Czech J Phys

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We present a self consistent treatment of pinhole junctions in d x 2 −y 2 superconductors. The current-phase relation js(χ) is studied at different temperatures and at different angles α between the crystalâ-axis and the junction (surface) normal. We show that the critical current of a junction can be reduced by pair-breaking effects at the separation. We also study the Josephson energy of a pinhole as a function of the phase difference across the junction. In particular are mapped the positions of the energy minima χmin at different temperatures as functions of (αL, αR), the crystal orientations of the left and right superconductors. With decreasing temperature there is an increasing range of crystal orientations where χmin varies continuously from 0 to π.

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Experimental determination of the superconducting pairing state in YBCO from the phase coherence of YBCO-Pb dc SQUIDs

Cited by 1,087 publications

References 16 publications

Antiferromagnetic ordering in arrays of superconductingπ-rings

Antiferromagnetic ordering in arrays of superconductingπ-rings

Spontaneously broken time-reversal symmetry in high-temperature superconductors

Pinhole junctions in d-wave superconductors

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