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[en] Conventional superconductivity is well-explained in the framework of BCS theory by the formation of spin-singlet Cooper pairs. However, other exotic types of superconductivity involving, e.g., spin-triplet pairs exist as well. In general, superconducting correlations can be characterized by a nonvanishing pair amplitude which has a definite symmetry in spin, momentum and time or frequency. While the spin and momentum symmetry have been probed experimentally for different classes of superconductivity, the odd-frequency nature of certain superconducting correlations has so far been probed only indirectly. Here, we propose the thermopower as an unambiguous way to assess odd-frequency superconductivity. This is possible since the thermoelectric coefficient given by Andreev-like processes is only finite in the presence of odd-frequency superconductivity. We illustrate our general findings with a simple example of a superconductor-quantum dot-ferromagnet hybrid.
[en] The magnetic susceptibility of a BCS superconductor with partially depaired states (Fulde state) is investigated as a possible explanation of deviations from the Chandrasekar-Clogston limit
[en] Bardeen's knowledge of the experimental data had bounded the theory of superconductivity quite tightly before B, C and S developed their theory. When one speaks with John Bardeen's friends about him, one frequently hears words such as brilliant, quiet, persistent, generous, visionary, athletic, kind, thoughtful and remarkable. It is the author's good fortune to have the chance to recount some incidents from his life that are connected with the theory of superconductivity. This article draws on the author's personal memories; his many other friends and colleagues will set down their own recollections elsewhere. The evolution of the microscopic theory of superconductivity closely parallels the scientific life of Joh Bardeen. Starting with his PhD dissertation, done under the guidance of Eugene Wigner, he spent much of his life developing an understanding of electron interaction effects and transport properties of metals, semiconductors and superconductors. His fascination with the remarkable phenomenon of superconductivity goes back to his graduate student days at Princeton. Although interrupted during the war years and in the late 1940's at Bell Labs, he returned to this perplexing topic when he moved to the University of Illinois in 1951. 20 refs., 7 figs
[en] A generalized BSC Hamiltonian, in which the interaction strengths (V11, V12, V22) among and between 'electron' (1) and 'hole' (2) Cooper pairs are differentiated, is proposed. The ground state energy W of the Hamiltonian with respect to that of the non-interacting system W0 is found by variational calculations W0 - W = 1/4[N1(O)Δ21 + N2(O)Δ22], where N1(0) and N2(0) represent the 'electron' and 'hole' densities of states at the Fermi energy εF, and Δ1 and Δ2 are the solutions of the simultaneous equations, Δj = 1/2 Vj1N1(0)Δ1 sinh-1 (ℎωD/Δ1) + 1/2 Vj2N2(0)Δ2 sinh-1 (ℎωD/Δ2), with ωD denoting the Debye frequency. The usual BCS results are obtained in the limits: (all)Vjt = V0, N1(o) = N2(0). The supercondensate is composed of zero momentum 'electron' and 'hole' Cooper pairs, which are neither bosons nor fermions. Any excitations generated through the BCS interaction Hamiltonian containing Vjl must involve Cooper pairs of antiparallel spins and nearly opposite momenta. Non-zero momentum or excited Cooper pairs above the critical temperature Tc move like free bosons with the energy-momentum relation ε(j) = ω(j)0 + 1/2 υ(j)Fq, where υ(j)F is the Fermi velocity and ω(j)(<0) is the band edge, which is lower than that of free-electron pairs (ε = 0). They undergo a Bose-Einstein condensation at Tc = 1.00856ℎυFn1/3/kB, where n = N(0)ℎωD is the density of Cooper pairs
[en] We present results of numerical simulations of the 3+1d Nambu-Jona-Lasinio model with a non-zero baryon chemical potential μ, with particular emphasis on the superfluid diquark condensate and associated susceptibilities. The results, when extrapolated to the zero diquark source limit, are consistent with the existence of a non-zero BCS condensate at high baryon density. The nature of the infinite volume and zero temperature limits are discussed
[en] We present a derivation of a previously announced result for matrix elements between exact eigenstates of the pairing Hamiltonian. Our results, which generalize the well-known Bardeen–Cooper–Schrieffer (BCS) (Bardeen et al 1957 Phys. Rev. 108 1175; 1957 Phys. Rev. 106 162) expressions for what are known as ‘coherence factors’, are derived based on the Slavnov (1989 Theor. Math. Phys. 79 502) formula for overlaps between Bethe-ansatz states, thus making use of the known connection between the exact diagonalization of the BCS Hamiltonian, due to Richardson (1963 Phys. Lett. 3 277; 1964 Nucl. Phys. A 52 221), and the algebraic Bethe ansatz. The resulting formula has a compact form after a suitable parameterization of the energy plane. Although we apply our method here to the pairing Hamiltonian, it may be adjusted to study what is termed the ‘Sutherland limit’ (Sutherland 1995 Phys. Rev. Lett. 74 816) for exactly solvable models, namely where a macroscopic number of rapidities form a large string. (paper)
[en] The time-tested Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity is generally accepted to be the correct theory of conventional superconductivity by physicists and, by extension, by the world at large. There are, however, an increasing number of 'red flags' that strongly suggest the possibility that BCS theory may be fundamentally flawed. An ever-growing number of superconductors are being classified as 'unconventional', not described by the conventional BCS theory and each requiring a different physical mechanism. In addition, I argue that BCS theory is unable to explain the Meissner effect, the most fundamental property of superconductors. There are several other phenomena in superconductors for which BCS theory provides no explanation. Furthermore, BCS theory has proven unable to predict any new superconducting compounds. This paper suggests the possibility that BCS theory itself as the theory of 'conventional' superconductivity may require a fundamental overhaul. I outline an alternative to conventional BCS theory proposed to apply to all superconductors, 'conventional' as well as 'unconventional', that offers an explanation for the Meissner effect as well as for other puzzles and provides clear guidelines in the search for new high temperature superconductors.