Journal Article•
Theory of superconductivity
TL;DR: In this paper, the isotope effect has been used to show that superconductivity is essentially an extreme case of diamagnetism rather than a limit of infinite conductivity.
Abstract: Although superconductivity falls into the domain where one would expect ordinary non-relativistic quantum mechanics to be valid, it has proved to be extremely difficult to obtain an adequate theoretical explanation of this remarkable phenomenon. In spite of the large amount of excellent experimental and theoretical work devoted to the problem, there remain major unsettled questions. However, the area in which the answers are to be found has been narrowed considerably. There are very strong indications, if not quite a proof, that superconductivity is essentially an extreme case of diamagnetism rather than a limit of infinite conductivity. The isotope effect indicates that the superconducting phase arises from interactions between electrons and lattice vibrations.
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TL;DR: In this article, the phase of the order parameter is not important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition, and the phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties.
Abstract: THE superconducting state of a metal is characterized by a complex order parameter with an amplitude and a phase In the BCS-Eliashberg mean-field theory1, which is a very good approximation for conventional metals, the phase of the order parameter is un-important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition Here we argue that superconductors with low super-conducting carrier density (such as the organic and high-Tc oxide superconductors) are characterized by a relatively small phase 'stiffness9 and poor screening, both of which imply a significantly larger role for phase fluctuations As a consequence, in these mat-erials the transition to the superconducting state may not display typical mean-field behaviour, and phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties For some quasi-two-dimensional materials, notably underdoped high-temperature superconductors, the onset of long-range phase order controls the gross value of Tc as well as its systematic variation from one material to another
1,533 citations
TL;DR: In this paper, the authors studied the entanglement in the transverse Ising model, a special case of the one-dimensional infinite-lattice anisotropic XY model, which exhibits a quantum phase transition.
Abstract: What entanglement is present in naturally occurring physical systems at thermal equilibrium? Most such systems are intractable and it is desirable to study simple but realistic systems that can be solved. An example of such a system is the one-dimensional infinite-lattice anisotropic XY model. This model is exactly solvable using the Jordan-Wigner transform, and it is possible to calculate the two-site reduced density matrix for all pairs of sites. Using the two-site density matrix, the entanglement of formation between any two sites is calculated for all parameter values and temperatures. We also study the entanglement in the transverse Ising model, a special case of the XY model, which exhibits a quantum phase transition. It is found that the next-nearest-neighbor entanglement (though not the nearest-neighbor entanglement) is a maximum at the critical point. Furthermore, we show that the critical point in the transverse Ising model corresponds to a transition in the behavior of the entanglement between a single site and the remainder of the lattice.
1,274 citations
TL;DR: In this paper, a study of underdoped Bi2Sr2CaCu2O8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES) is presented.
Abstract: IT is well known that BCS mean-field theory is remarkably successful in describing conventional superconductors. A central concept of BCS theory is the energy gap in the electronic excitation spectrum below the superconducting transition temperature, Tc. The gap also serves as the order parameter: quite generally, long-range phase coherence and a non-zero gap go hand-in-hand1. But in underdoped high-Tc superconductors there is considerable evidence that a pseudogap (a suppression of spectral weight) is already formed in the normal state above Tc—first, from studies of the spin excitation spectrum2–5,24, which measure a 'spin gap', and later from a variety of other probes6–10. Here we present a study of underdoped Bi2Sr2CaCu2O8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES), which directly measures the momentum-resolved electron excitation spectrum of the CuO2 planes. We find that a pseudogap with d-wave symmetry opens up in the normal state below a temperature T* > Tc, and develops into the d-wave superconducting gap once phase coherence is established below Tc.
1,019 citations
905 citations
TL;DR: In this article, the authors present the most salient developments of research on organic conductors and superconductors during the past 10 years, including lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series.
Abstract: This review attempts to present the most salient developments of research on organic conductors and superconductors during the past 10 years. A theoretical introduction treats instabilities of quasi-one-dimensional electron systems and associated precursor effects which are relevant to the experimental results on organic conductors. We then describe the characterization of quasi-one-dimensional organic conductors by their transport, optical and magnetic properties. Finally, two sections are devoted to the experimental investigation of the low temperature instabilities: lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series. The importance of one-dimensional fluctuations is emphasized in both lattice and superconducting instabilities.
804 citations