We review a variety of experimental data capable of indicating the pairing state of YBa2Cu3O7-δ. Using the results of a group theoretic analysis we summarise the possible interpretations of these data, and concluded that, at present, there is no firm evidence that the pairing is conventional.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Energy Engineering and Power Technology
- Electrical and Electronic Engineering
TY - JOUR
T1 - Unconventional pairing in YBa2Cu3O7-d?
AU - Annett, James F.
AU - Goldenfeld, Nigel
AU - Renn, S. R.
N1 - Funding Information: 1. ORDER PARAMETER STRUCTURE This article briefly assesses the available experimental data with a view to determining the pairing state of YBa2Cu307-$. Our full analysis, based upon a rigorous group theoretical study will appear elsewhere 1. In ref. 2 the fluctuation specific heat was measured, and its temperature dependence in an interval approaching the critical region was found to be consistent with weak Gaussian fluctuations. The early measurements of ref. 2 did indeed show a significantly larger amplitude ratio than could be explained by the conventional Ginzburg-Landau theory, apparently ruling out a conventional pairing state. Subsequent measurements on less-twinned samples indicate an amplitude ratio which is consistent with that of a conventional Ginzburg-Landau theory3; however, this result is inconclusive since both conventional and unconventional pairing are consistent with this new data 4. Another consequence of non-trivial internal structure for the order parameter is the splitting of the transition due to external symmetry breaking perturbations 5. For example, for some unconventional states, an applied strain which lowers the symmetry can split the phase transition into two transitions at slightly different temperatures 1 as has been observed in UPt36. In one report 7 of a split transition in powder samples of YBa2Cu3OT-8, a second superconducting phase in addition to the ordinary 90 K orthorhombic phase was sought using high resolution X-ray diffraction, but not found. 2. LOW-LYING EXCITATIONS We start with a brief discussion of the results of Raman scattering from YBa2Cu3OT..6e. Tl~e-most striking feature of the intensity plotted as a function of energy shift is the presence of a background which is approximately linear in slope, extending out to (e.g.) about 600 cm -1 at a temperature of 15 K before peaking and ultimately decreasing. The presence of Fano anti-resonances shows that the background is an intrinsic, bulk electronic effect, not associated with the surface. The fact that this continuum persists in the super-conducting state is prima facie evidence for low-lying excitations in the gap. From a detailed analysis, which assumes that the continuum is an electron-hole continuum associated with an interband transition, it has been proposed that the pairing state has nodes on the YS boundary of the Bdllouin zone 9. Perhaps the most conclusive evidence for low-lying excitations comes from the temperature dependence of the linewidth o1 the 340 cm -1 phonon. Assuming that this phonon couples to the electron-hole background, then the linewidth is proportional to the single particle density of states. If there were no low-lying excitations in the gap, then as the temperature is reduced, the linewidth at low temperatures should be lower than that in the normal state, due to the reduction of the decay channel. However, it is observed to increase. This implies the existence of low-lying excitations in the gap. These low-lying :~Work supported by NSF grants DMR-86-12860 and DMR-84-15063. *Alfred P. Sloan Foundation Fellow.
PY - 1989/12
Y1 - 1989/12
N2 - We review a variety of experimental data capable of indicating the pairing state of YBa2Cu3O7-δ. Using the results of a group theoretic analysis we summarise the possible interpretations of these data, and concluded that, at present, there is no firm evidence that the pairing is conventional.
AB - We review a variety of experimental data capable of indicating the pairing state of YBa2Cu3O7-δ. Using the results of a group theoretic analysis we summarise the possible interpretations of these data, and concluded that, at present, there is no firm evidence that the pairing is conventional.
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U2 - 10.1016/0921-4534(89)91063-0
DO - 10.1016/0921-4534(89)91063-0
M3 - Article
AN - SCOPUS:0024933630
SN - 0921-4534
VL - 162-164
SP - 377
EP - 378
JO - Physica C: Superconductivity and its applications
JF - Physica C: Superconductivity and its applications
IS - PART 1