Extended thermodynamics, effective elastic coefficients and electromagnetoelastic waves in superconducting layer (original) (raw)
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A QUALITATIVE OVERVIEW OF THE MECHANISMS OF SUPERCONDUCTIVITY SHAILAJ KUMAR SHRIVASTAVA
The mechanism of superconductivity continues to be one of the most fascinating and challenging topics in condense matter physics. The discovery of high-Tc cuprate superconductors and iron based superconductors has challenged the classical theories of condensed matter physics and opened a new chapter of strongly correlated electron systems. The key question of superconductivity is the nature of mechanism of pairing of carriers. The electron phonon interaction or spin fluctuations are considered to be central to the mechanism of superconductivity. In this article attempt has been made to highlights the brief outcome of various models and theories on the mechanism of superconductivity. I.Introduction Dutch scientist Heike Kammerlingh Onnes [1] discovered that electrical resistance of various metals e.g mercury, lead, tin and many others disappeared when the temperature was lowered below some critical value Tc. Meissner and Oschenfeld [2] observed that when a material is cooled in the presence of a magnetic field, on reaching its superconducting transition temperature (Tc) the magnetic flux is suddenly completely expelled from its interior. It means it exhibits perfect diamagnetism. Gorter [3, 4] put forward the idea of a two fluid model, in which the electron gas within the superconductor has two components. One component has no entropy and carries the supercurrent while the other component has all the entropy and behaves like a normal electron gas. Below the super conducting transition temperature, the superconducting electrons short out the normal electrons so that the electrical resistance is zero. These two features were captured in the equation proposed by London brothers [5], who first realized the quantum character of the phenomenon. Ginzburg and Landau [6] created a theory describing the transition between the superconducting and normal phases. Although the Ginzburg and Landau theory explained the macroscopic properties of superconductors, the microscopic properties remain unsolved. Bardeen, Cooper and Schrieffer created microscopic theory (BCS theory) [7] which describe conventional superconductors in the low temperature and low magnetic field regime. According to BCS theory, the superconductors at below Tc have an energy gap equal to binding energy of the Cooper pair, which dominates the transition temperature. The binding energy of the Cooper pair depends on the density of electron states at the Fermi surface, and on the strength of electron phonon interaction. High temperature superconductors are characterized by a layered two dimensional superconducting condensate and unique features that are very different from conventional superconducting materials. Recent studies [8, 9] reveal that the theoretical explanation for copper and iron superconductors could be the same and could even apply to other materials. The spin fluctuation mechanism of high-Tc superconductivity in copper oxide compound is determined by the high intensity of the antiferromagnetic exchange interaction. According to spin fluctuation mechanism [10], the pairing wave function of cuprate high-Tc superconductor should have d-wave symmetry. But unfortunately, some reports supported the d-symmetry for the high-Tc superconductors whereas others supported the s-symmetry. The survey of the mechanism of superconductivity [11] emphasized that all models used the conception of pairing with the subsequent formation of Bose-Condensate at Tc irrespective of the nature of the resulting attraction.
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Journal of Physics-condensed Matter, 2003
Using thermal Green's function technique the thermodynamics of superconductors with charge- and spin-density waves is considered. The Δ/Tc ratio and the paramagnetic limit Hp are calculated for various values of the dielectric gap σ and the parameter ν = Nnd(0)/Nd(0), where Nnd(0) and Nd(0) are the electron densities of states for non-dielectrized and dielectrized Fermi surface sections, Δ(0) is the zero-temperature value of the superconducting order parameters, and Tc is the critical temperature. It is shown that for the charge-density wave-superconductors the values Δ(0)/Tc are substantially less than in the conventional BCS theory. The theoretical conclusions are in a good agreement with the experiment.[Russian Text Ignored.]
Thermal Properties of Superconductors
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