Spin fluctuations and high temperature superconductivity

Abstract

Theory of spin¯uctuations for itinerant magnetism and its application to high temperature superconductivity are reviewed. After a brief introduction to the whole subject the developments of the self-consistent renormalization theory of spin¯uctuations are summarized with particular emphasis on critical properties at the quantum phase transitions. Most of the anomalous properties in the normal state of high-T c cuprates are understood as due to the critical behaviours for the two dimensional antiferromagnetic metals. By analysing the nuclear magnetic relaxation rate and the T -linear term of resistivity, the set of parameters to specify the spin¯uctuations are determined. It is shown that by using the parameters thus obtained one can describe other quantities as well, e.g. optical conductivity. Then we proceed to the theory of superconductivity by the spin¯uctuation mechanism. After some discussion on the weak coupling treatments, the strong coupling theory is reviewed. It is shown that the set of parameters determined by the normal state properties of the high-T c cuprates just give a transition temperature of the right order of magnitude. Among the parameters, the most sensitive one for T c is the frequency spread of the spin¯uctuations. This fact enables us to present a possible uni® ed picture of the antiferromagnetic spin¯uctuation-induced superconductors, including heavy fermion superconductors and organic superconductors. This point of view may be con® rmed to a certain extent by microscopic calculations based on the¯uctuation exchange approximation for the two-dimensional Hubbard models representing not only the cuprates but also organic and trellis lattice compounds. The review is concluded with some discussions on future problems, e.g. the pseudo spin-gap in the under-doped region.

Pseudogap Phenomena and Superconducting Fluctuations in Hubbard Model for High-TcCuprates

“…The antiferromagnetic spin fluctuations described by χ s (q, iΩ n ) play a dominant role in the FLEX approximation. The characteristic results of the nearly anti-ferromagnetic Fermi liquid theory 17,53,54,55,56,59,60,72,73,75) are qualitatively reproduced within the FLEX approximation. The superconducting critical temperature T c is determined as the temperature below which the linearized Dyson-Gor'kov equation has a non-trivial solution ( Fig.…”

“…57) The FLEX approximation is a conserving approximation 58) and describes the anti-ferromagnetic spin fluctuations including the mode coupling effects partly. 59,60) It has been shown that the superconducting critical temperature with a relevant order T c ∼ 100K is obtained by the FLEX approximation for High-T c cuprates. 61,62,63,64,65,66,67) The relevant results are also obtained for the organic superconductor κ-(BEDT-TTF) compounds 68,69,70) and for the ladder type compound Sr 14−x Ca x Cu 24 O 41 .…”

Relationship between Superconductivity and Antiferromagnetism in LaFe(As1−xPx)O Revealed by 31P-NMR

“…It has been believed that superconductivity is mediated by antiferromagnetic (AFM) fluctuations in cuprates and heavy-fermion superconductors. [1][2][3][4] The relationship between superconductivity and AFM fluctuations in recent-discovered iron-based superconductors has also been discussed. [5][6][7][8][9][10][11][12][13][14][15][16] Up to now, it has been reported that superconductivity appears around the AFM quantum critical point in the Ba122 system, indicative of the strong relationship between superconductivity and low-energy AFM fluctuations.…”

Quantum critical behavior in the asymptotic limit of high disorder in the medium entropy alloy NiCoCr0.8

“…[1][2][3][4][5] One key to making progress in this area is the identification of model material systems that are complex enough to exhibit the physics of interest but simple enough to be compared to theory. Studies of several low dimensional magnetic materials have greatly improved our understanding of QCP physics in insulating solids, [6][7][8] but our understanding of QCP phenomena in metallic systems is much less clear.…”