8,893 research outputs found
Failed theories of superconductivity
Almost half a century passed between the discovery of superconductivity by
Kamerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen,
Cooper and Schrieffer. During the intervening years the brightest minds in
theoretical physics tried and failed to develop a microscopic understanding of
the effect. A summary of some of those unsuccessful attempts to understand
superconductivity not only demonstrates the extraordinary achievement made by
formulating the BCS theory, but also illustrates that mistakes are a natural
and healthy part of the scientific discourse, and that inapplicable, even
incorrect theories can turn out to be interesting and inspiring.Comment: 14 pages, 3 figures (typos fixed), to appear in: Bardeen Cooper and
Schrieffer: 50 YEARS, edited by Leon N Cooper and Dmitri Feldma
BCS as Foundation and Inspiration: The Transmutation of Symmetry
The BCS theory injected two powerful ideas into the collective consciousness
of theoretical physics: pairing and spontaneous symmetry breaking. In the 50
years since the seminal work of Bardeen, Cooper, and Schrieffer, those ideas
have found important use in areas quite remote from the stem application to
metallic superconductivity. This is a brief and eclectic sketch of some
highlights, emphasizing relatively recent developments in QCD and in the theory
of quantum statistics, and including a few thoughts about future directions. A
common theme is the importance of symmetry {\it transmutation}, as opposed to
the simple {\it breaking} of electromagnetic symmetry in classic
metallic superconductors.Comment: 25 pages, 4 figures. Contribution to "Fifty Years of Bardeen, Cooper,
and Schrieffer'', to be published by World Scientific. Also to appear in
IJMP
Damping in a Superconducting Mechanical Resonator
We study a mechanical resonator made of aluminum near the normal to super
conductivity phase transition. A sharp drop in the rate of mechanical damping
is observed below the critical temperature. The experimental results are
compared with predictions based on the Bardeen Cooper Schrieffer theory of
superconductivity and a fair agreement is obtained
Superconducting loop quantum gravity and the cosmological constant
We argue that the cosmological constant is exponentially suppressed in a
candidate ground state of loop quantum gravity as a nonperturbative effect of a
holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar
connection components, corresponding to degenerate gravitational configurations
breaking large gauge invariance and CP symmetry, behave as composite fermions
that condense as in Bardeen-Cooper-Schrieffer theory of superconductivity.
Cooper pairs admit a description as wormholes on a de Sitter boundary.Comment: 10 pages; v2 matches the published versio
Semiclassical Theory of Bardeen-Cooper-Schrieffer Pairing-Gap Fluctuations
Superfluidity and superconductivity are genuine many-body manifestations of
quantum coherence. For finite-size systems the associated pairing gap
fluctuates as a function of size or shape. We provide a parameter free
theoretical description of pairing fluctuations in mesoscopic systems
characterized by order/chaos dynamics. The theory accurately describes
experimental observations of nuclear superfluidity (regular system), predicts
universal fluctuations of superconductivity in small chaotic metallic grains,
and provides a global analysis in ultracold Fermi gases.Comment: 4 pages, 2 figure
Microscopic Derivation of the Ginzburg-Landau Model
We present a summary of our recent rigorous derivation of the celebrated
Ginzburg-Landau (GL) theory, starting from the microscopic
Bardeen-Cooper-Schrieffer (BCS) model. Close to the critical temperature, GL
arises as an effective theory on the macroscopic scale. The relevant scaling
limit is semiclassical in nature, and semiclassical analysis, with minimal
regularity assumptions, plays an important part in our proof.Comment: Typo in Eq. (14) corrected, references update
Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation
Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the
coherent splitting of Cooper pairs from a superconductor to two spatially
separated quantum dots has been predicted to generate nonlocal pairs of
entangled electrons. In order to test this hypothesis, we propose a scheme to
transfer the spin state of a split Cooper pair onto the polarization state of a
pair of optical photons. We show that the produced photon pairs can be used to
violate a Bell inequality, unambiguously demonstrating the entanglement of the
split Cooper pairs.Comment: 11 pages, 9 figures, v3 with added reference
The oxygen isotope effect on critical temperature in superconducting copper oxides
The isotope effect provided a crucial key to the development of the BCS
(Bardeen-Cooper-Schrieffer) microscopic theory of superconductivity for
conventional superconductors. In superconducting cooper oxides (cuprates)
showing an unconventional type of superconductivity, the oxygen isotope effect
is very peculiar: the exponential coefficient strongly depends on doping level.
No consensus has been reached so far on the origin of the isotope effect in the
cuprates. Here we show that the oxygen isotope effect in cuprates is in
agreement with the bisoliton theory of superconductivity.Comment: 3 pages including 4 figures; version 2 is with minor correction
Microwave Response of V3Si Single Crystals: Evidence for Two-Gap Superconductivity
The investigation of the temperature dependences of microwave surface
impedance and complex conductivity of V3Si single crystals with different
stoichiometry allowed to observe a number of peculiarities which are in
remarkable contradiction with single-gap Bardeen-Cooper-Schrieffer theory. At
the same time, they can be well described by two-band model of
superconductivity, thus strongly evidencing the existence of two distinct
energy gaps with zero-temperature values Delta1~1.8Tc and Delta2~0.95Tc in
V3Si.Comment: Submitted to Europhysics Letter
Self-Consistent Approximations for Superconductivity beyond the Bardeen-Cooper-Schrieffer Theory
We develop a concise self-consistent perturbation expansion for
superconductivity where all the pair processes are naturally incorporated
without drawing "anomalous" Feynman diagrams. This simplification results from
introducing an interaction vertex that is symmetric in the particle-hole
indices besides the ordinary space-spin coordinates. The formalism
automatically satisfies conservation laws, includes the Luttinger-Ward theory
as the normal-state limit, and reproduces the Bardeen-Cooper-Schrieffer theory
as the lowest-order approximation. It enables us to study the thermodynamic,
single-particle, two-particle, and dynamical properties of superconductors with
competing fluctuations based on a single functional of Green's
function in the Nambu space. Specifically, we derive closed equations
in the FLEX-S approximation, i.e., the fluctuation exchange approximation for
superconductivity with all the pair processes, which contains extra terms
besides those in the standard FLEX approximation.Comment: 14 pages, 6 figure
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