Search

A decade ago, Alleno et al. [Physica C 242 169 (1995)] reported measurements under hydrostatic pressure P of the superconducting temperature Tc (P), with P<2 GPa, in the compounds RNi2B2C, where R = Y, Ho, Er, Tm. We utilise an extrapolation they propose for their data to higher pressures to address the question posed in the title. We then find critical pressures Pc ranging from ∼ 23 GPa for Y down to ∼ 12 GPa for Ho. The corresponding slopes [image omitted] range from ∼ -2 GPa/GPa for Er to ∼ -1 GPa/GPa for Tm. Crucial to the discussion is whether a structural phase transition is induced in RNi2B2C by pressure before this critical pressure Pc is reached. Theory presently cannot answer such a question, but the compound for say R = Er should be experimentally accessible. Though [image omitted] is predicted to be finite above, all important is whether Tc (P) is proportional to (Pc -P) as Pc is approached, or whether the behaviour is non-analytic, such as (Pc -P)γ, where γ is non-integer. If the former situation obtains, important consequences follow from the Ehrenfest relation for dTc/dP in terms of (a) thermal expansion, and (b) specific heat. [ABSTRACT FROM AUTHOR]

In earlier work, we have been concerned with the scaling properties of some classes of superconductors, specifically with heavy Fermion materials and with five bcc transition metals of BCS character. Both of these classes of superconductors were three-dimensional but here we are concerned solely with quasi-two-dimensional high- T c cuprates in the underdoped region of their phase diagram. A characteristic feature of this part of the phase diagram is the existence of a pseudogap (pg). We therefore build our approach around the assumption that is the basic dimensionless ratio on which to focus, where the energy E pg introduced above is a measure of the pseudogap. Since anyon fractional statistics apply to two-dimensional assemblies, we expect the fractional statistics parameter allowing ‘interpolation’ between Fermi-Dirac and Bose-Einstein statistical distribution functions as limiting cases to play a significant role in determining and experimental data are analyzed with this in mind. [ABSTRACT FROM AUTHOR]

Two complementary mechanisms have been proposed for relatively high temperature superconductor MgB 2 . While the first is the electron–phonon mechanism of BCS theory, advocated strongly by Pickett and co-workers, the second, by Bianconi et al ., invokes Feshbach shape resonances. While we cannot presently discount the second mechanism, and while both proposals exploit the multiband nature of the electronic structure of MgB 2 , we show here that five body-centred cubic (bcc) transition metals, whose superconducting transition temperature correlate intimately with elastic constants and therefore are plainly BCS-like in character, lie on a curve which has MgB 2 at the high T c end. Any alternative mechanism to electron–phonon interaction in MgB 2 will need to account quantitatively for this circumstance. [ABSTRACT FROM AUTHOR]

Examines the theoretical relations between the superconducting transition temperatures of metallic elements related to elastic constants. Principles of the Bardeen-Cooper-Schrieffer theory; Dependence of metallic elements on atomic number Z; Characterization of elements by the Cauchy discrepancy.

Phillips and Jung [J.C. Phillips and J. Jung (2002). Phil. Mag. B , 82 , 1163] refute nodal properties of the gap function Δ k and propose an isotropic s -wave picture with electron-phonon interaction as the mechanism for high- T c superconductivity in the cuprates. Here, we compare and contrast predictions of various physical properties with the gap function Δ k reflecting three scenarios: (i) isotropic s -wave pairing, (ii) extended s -wave with eight line nodes as championed quite recently by Zhao [G.M. Zhao (2001). Phys. Rev. B , 64 , 024503], and (iii) d -wave pairing. By referring to (a) scanning tunnelling microscopy imaging of a Zn impurity, supported by our own subsequent theoretical study, (b) linear decrease of T c with non-magnetic impurity concentration, and (c) calculations on the extended s -wave scenario applied to tunnelling conductance, strong evidence for nodal properties of Δ k is given. The contentions of Phillips and Jung [J.C. Phillips and J. Jung (2002). Phil. Mag. B , 82 , 1163] should therefore be treated with considerable caution, isotropic s -wave behaviour being truly exceptional among the high- T c cuprates. [ABSTRACT FROM AUTHOR]

Superconductors, classified by materials, embrace at least four broad groups: (i) BCS metals and alloys; (ii) heavy Fermion materials; (iii) high- T c cuprates and (some) organic compounds, and (iv) fullerides. Broadly speaking, in classes (i) and (iv), with (i) possibly embracing the recent discovery of superconductivity in MgB 2 with T c ∼ 40 K, electron liquids flow through essentially non-magnetic lattices and the electron-phonon interaction is a key component of the mechanism for Cooper pairing. In classes (ii) and (iii), plus the low- T c material Sr 2 RuO 4 , electron or hole liquids flow through assemblies with magnetic spin fluctuations. The nature of the normal state in class (iii) is not yet universally agreed, both Fermi or Luttinger liquids remaining viable to date, the former, however, with the formation of precursor 2 e Bosons somewhat above T c . Our own studies reveal some common ground between classes (ii) and (iii), involving coherence lengths and effective masses, as well as non- s -wave pairing, though the interactions leading to pairing almost certainly have different physical origins in these two groups. Finally, topological superconductivity is reviewed. It is argued that such a treatment of a topological superfluid could eventually deepen the understanding of the class (iv) fullerides. Resonating valence bond theory, used by Anderson and co-workers as an, of course, approximate strongly-correlated electron technique for high- T c cuprates, can itself be re-written in the form of topological superconductivity, as discussed especially by Wiegmann and collaborators. [ABSTRACT FROM AUTHOR]

By solution of the Bethe–Goldstone equation for the Cooper pairing problem, an approximate analytic relation is derived between coherence length ξ and the binding energy of the Cooper pair. This relation is then qualitatively confirmed by numerically solving the corresponding self-consistent gap equations, following the crossover from weak to strong coupling, in non-s-wave superconductors. The relation applies to non-conventional superconductors, and in particular to heavy Fermions and to high-Tc cuprates. Utilizing in addition a phenomenological link between kBTc and a characteristic energy &epsiv;c=ℎ2/2m&ast;ξ2, with m&ast; the effective mass, major differences are exposed in the functional relation between kBTc and &epsiv;c for s-wave materials and for non-conventional superconductors. The relation between critical temperature and &epsiv;c thereby proposed correctly reflects the qualitative properties of heavy Fermion superconductors. [Copyright &y& Elsevier]

Imaging the effects of an impurity like Zn in high-Tc superconductors [Nature 61 (2000) 746] has rekindled interest in defect problems in the superconducting phase. This has prompted us here to re-examine the early work of March and Murray [Phys. Rev. 120 (1960) 830] on the linear response function in an initially translationally invariant Fermi gas. In particular, we present corresponding results for a superconductor at zero temperature, both in the s- and in the d-wave case, and mention their direct physical relevance in the case when the impurity potential is highly localized. [Copyright &y& Elsevier]