Cuprate Superconductors May Be Conventional After All

Abstract

The copper oxides present the highest superconducting temperature and properties at odds with other compounds, suggestive of a fundamentally different superconductivity. In particular, the Abrikosov vortices fail to exhibit localized states expected and observed in all clean superconductors. We have explored the possibility that the elusive vortex-core signatures are actually present but weak. Combining local tunneling measurements with large-scale theoretical modeling, we positively identify the vortex states in YBa 2 Cu 3 O 7−δ . We explain their spectrum and the observed variations thereof from one vortex to the next by considering the effects of nearby vortices and disorder in the vortex lattice. We argue that the superconductivity of copper oxides is conventional, but the spectroscopic signature does not look so because the superconducting carriers are a minority. DOI: 10.1103/PhysRevLett.119.237001 Type-II superconductors immersed in a magnetic field let quantized flux tubes perforate them: the Abrikosov vortices. This remarkable property underlies and often limits many applications of superconductors. In 1964, Caroli, de Gennes, and Matricon used the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity to predict that vortices in type-II superconductors host a collection of localized electrons bound to their cores [1]. The direct observation of these localized states 25 years later by scanning tunneling spectroscopy (STS) is a spectacular verification of the BCS theory [2]. The formation of vortex-core bound states is an immediate consequence of the superconducting condensate being composed of electron pairs, while excitations in the vortex, being unpaired, have a different topology. Core states are, therefore, a robust property of superconductors, like edge states in topological insulators, irrespective of the origin and symmetry of the force that glues the electrons into pairs. In spectroscopy, they appear in the clean limit l ≫ ξ as a zero-bias peak in the local density of states (LDOS) at the vortex center, where l and ξ are the electron mean free path and superconducting coherence length, respectively, or as a structureless LDOS in the dirty limit l ≲ ξ [3]. Next to NbSe 2 [2,4], the core states were seen by STS in several superconducting materials [5][6][7][8][9][10], including the pnictides, which are believed to host unconventional pairing [11][12][13][14][15]. The high-T c cuprates stand out as the only materials in which the vortex-core states have been looked for but not found. In YBa 2 Cu 3 O 7−δ (Y123), discrete finite-energy structures initially believed to be vortex states [16,17] were recently shown to be unrelated to vortices [18]. In Bi 2 Sr 2 CaCu 2 O 8þδ (Bi2212), the vortex cores present no trace of a robust zero-bias peak, but instead very weak finite-energy features apparently related to a charge-density wave order [19][20][21][22][23][24].The absence of vortex states in cuprates is challenging the existing theories. Because these states are topological they are robust [25...

Bound states in a superfluid vortex: A detailed study along the BCS-BEC crossover

“…We have further verified that bound states occur in the interior of a vortex for couplings on the BCS side of unitary, and rapidly disappear when entering the BEC regime past unitarity. Consistently with this theoretical finding, a clear experimental finding for the occurrence of bound states in a vortex in a superconducting material [1] can indeed be greeted as a signature that the superfluid phase of that material may be described by the conventional BCS theory [2]. Yet, it is also known from the analysis of Ref.…”

“…Recently, interest has arisen in the internal structure of vortices in type-II superconductors, owing to the presence of bound states in the vortex core region of a Y123 superconductor that were detected experimentally [1]. The relevance of this finding has been highlighted [2], as an indication that the superconducting state of hightemperature superconducting materials should be well described by the conventional BCS pairing theory [3], in spite of the fact that the normal state of these materials is highly unconventional.…”

“…(iii) The number of bound states is found to be nonnegligible even in the intermediate-coupling region of the crossover, where the Cooper pair size becomes comparable with the inter-particle distance. To the extent that this small size is compatible with the occurrence of hightemperature superconductivity [12], there is thus no a priori reason to associate the occurrence of bound states in a vortex with the superconductivity being of the conventional BCS type as recently asserted [2]. (iv) Finally, it is proposed that the shape of the local density of states vs energy, taken about the center of the vortex, can serve as a guide to distinguish where a given fermionic superfluid stands in the coupling-temperature phase diagram of the BCS-BEC crossover.…”

Superconductivity and Microwaves

“…It is generally believed that the HTSC cuprates are not conventional, because their energy gap is characterized as d-wave in contrast to the s-wave energy gap of conventional SC [50]. However, such belief has been challenged very recently [51] and the debate follows. The point is that we do not know fully the whole SC state of these HTSC cuprates.…”

Interband Electron Pairing for Superconductivity from the Breakdown of the Born‐Oppenheimer Approximation

“…[13] Another class of superconductors breaking the McMillan limit is the layered ferropnictides, with the highest T c = 55 K found for SmFeAsO 1-x F x in 2008. [15,16] The layered compound MgB 2 , found to be superconducting at 39 K in 2001, [17] is a conventional superconductor breaking the Mc-Millan limit. However, it has recently been found [15] that the high-T c cuprate YBa 2 Cu 3 O 7-d adopts vortex states in a magnetic field, a signature predicted for conventional type-II superconductors by the BCS theory, suggesting that the superconductivity of cuprate superconductors may turn out to be conventional.…”