Inderheeset al.Reply

Abstract

Inderhees etal Reply:In our Letter, we reported on measurements of the specific heat of two single crystals of YBa2Cu307-<5 near the superconducting transition temperature T c . We observed a deviation from the usual mean-field discontinuity, which we attributed to Gaussian thermodynamic fluctuations. For a conventional superconductor, the amplitude ratio C + IC~ would be expected to be that of an O(n) Ginzburg-Landau theory, with «=2. In this case, the amplitude ratio is a universal number, 2 allowing us to test the hypothesis that YBa2Cu3C>7-5 is a conventional superconductor. Analyzing our data as if they corresponded to an OOi) model, we found that 5.6 <n < 10.2 for one crystal, and 6.2 < n < 8 for the other. From this we concluded that the 0(2) Ginzburg-Landau theory did not describe our data, and thus the order parameter in Yl^CuaCh-a has more than two components.Once this point has been established, the next step is to explore the possible allowed symmetries of the order parameter and their implications for the value of C + /C~. This has been carried out in work performed by Annett, Randeria, and Renn (ARR), 3 who have made an exhaustive study of the amplitude ratio expected for a variety of possible pairing states in both orthorhombic and tetragonal environments. Special cases of their results were mentioned in the preceding Comments by Brand and Doria and by Muzikar. The results of these authors are correct as far as they go, but they misstate our conclusions-we sought to establish that the 0(2) Ginzburg-Landau theory was inadequate to describe the experimental results, not to assert that a large-Ai OOz) model was appropriate.Some additional remarks are in order, based partly on the work of ARR. First, if we assume for the moment that the crystal structure of YBa2Cu307-5 only affects C + /C~ through the "mass anisotropy," then the O(n) model gives an upper bound on C*/C ~. This is because the internal symmetry must contain an 0(2) sector which couples to electromagnetism and gives rise to superconductivity. In d dimensions the longitudinal mode will always give a factor 2 d/1 in the denominator of C + /C~; for internal symmetry groups other than 0(«), not all of the remaining modes will be Goldstone modes, in general, and so they can be expected to make a positive contribution to the denominator too. This can be

Symmetry of the order parameter for high-temperature superconductivity

“…Some authors report observations of a split transition [50], while recent studies on largely untwinned single crystals find no evidence for a splitting (S. E. lnderhees, private communication). It is possible that features seen for some twinned crystals [51] may be due to a superconducting transition occurring on the twinning boundaries [52].…”

Zero-temperature equation-of-motion of electron pair in the BCS theory of superconductivity

“…2. Experimentally, Tb = 93 K and T, = 89 K for that sample (23). We cannot predict how high the Tb is, compared with the T,, until the detailed data pertinent to the sample are available.…”

“…[23]. Imagine for the sake of simplicity the jellium model of the background medium for the electron pair and the associated homogeneous electron density for pN-2.…”

“…The potential u@(l, 2) for this equation is given as an explicit functional of electron density, eq. [23], so that Indeed, in our density functional theory of superconductivity (10, 1 l), the crucial mathematical strategy was to find the zeros of the order parameter G k (defined for the quasi particles in eq.…”

Electronic Properties of High-Temperature Superconductor DyBa2Cu3O7from Critical Fields and Specific Heat Studies

“…We analyzed the specific heat data in the vicinity of Tc taking into account also the Gaussian fluctuations and following the formula derived in [12]. The detailed analysis of our data for DyBa2Cu3O7 is described elsewhere [9] according to the formula where t = TTc 1 -1, γ is the electronic specific heat coefficient, a and b are the coefficients of the linear temperature dependent background mainly of the lattice specific heat, CT are the coefficients of Gaussian fluctuations terms below (-) and above (+) Tc and the third term describes the BCS-like contribution to the specific heat.…”