Spin Fluctuation-Induced Superconductivity Research Articles

Developments of the theory of spin fluctuations and spin fluctuation-induced superconductivity.

Theory of spin fluctuations as developed in the past 30 years have played important roles in the theory of magnetism in metals, particularly in elucidating the properties around the magnetic instability or quantum critical points. Recently the theory has been extended to deal with the spin fluctuaion-mediated superconductivity with anisotropic order parameters in strongly correlated electron systems. These theoretical developments are briefly reviewed and the high temperature superconductivity of cuprates and organic and heavy electron superconductors are discussed in the light of these theories.

Spin fluctuation-induced superconductivity in organic compounds

Spin fluctuation-induced superconductivity in quasi-two-dimensional organic compounds such as κ-(BEDT-TTF) 2X is investigated within a fluctuation exchange approximation using a half-filled dimer Hubbard model with right-angled isosceles triangular lattice. The calculated value for T c is in good agreement with experiment and the ( U/ τ)-dependence of T c compares qualitatively well with the observed pressure dependence of T c. The pairing symmetry is of ( x 2− y 2)-type and the energy gap develops with decreasing temperature below T c more rapidly than in the BCS model.

Theory of Spin Fluctuation-Induced Superconductivity Based on ad-pModel. II. –Superconducting State–

The superconducting state of a two-dimensional d-p model is studied from the spin fluctuation point of view by using a strong coupling theory. The fluctuation exchange (FLEX) approximatoin is employed to calculate the spin fluctuations and the superconducting gap functions self-consistently in the optimal- and over-doped regions of hole concentration. The gap function has a symmetry of d_{x^2 - y^2} type and develops below the transition temperature T_c more rapidly than in the BCS model. Its saturation value at the maximum is about 10 T_c. When the spin fluctuation-induced superconductivity is well stabilized at low temperatures in the optimal regime, the imaginary part of the antiferromagnetic spin susceptibility shows a very sharp resonance peak reminiscent of the 41 meV peak observed in the neutron scattering experiment on YBCO. The one-particle spectral density around k=(pi,0) shows sharp quasi-particle peaks followed by dip and hump structures bearing resemblance to the features observed in the angle-resolved photoemission experiment. With increasing doping concentration these features gradually disappear.

Spin Fluctuation-Induced Superconductivity in Organic Compounds

Spin fluctuation-induced superconductivity in two-dimensional organic compounds such as \kappa-(ET)_2-X is investigated by using a simplified dimer Hubbard model with right-angled isosceles triangular lattice (transfer matrices -\tau, -\tau^\prime). The dynamical susceptiblity and the self-energy are calculated self-consistently within the fluctuation exchange approximation and the value for T_c as obtained by solving the linearized Eliashberg-type equations is in good agreement with experiment. The pairing symmetry is of d_{x^2-y^2} type. The calculated (U/\tau)-dependence of T_c compares qualitatively well with the observed pressure dependence of T_c. Varying the value for \tau^\prime/\tau from 0 to 1 we interpolate between the square lattice and the regular triangular lattice and find firstly that values of T_c for \kappa-(ET)_2-X and cuprates scale well and secondly that T_c tends to decrease with increasing \tau^\prime/\tau and no superconductivity is found for \tau^\prime/\tau=1, the regular triangular lattice.

Theory of Spin Fluctuation-Induced Superconductivity Based on ad-pModel

Spin fluctuation-induced superconductivity is studied on a two-dimensional d - p model by using a strong coupling theory, and is applied to interpret high- T c cuprates. A self-consistent renormalization scheme is employed within the so-called fluctuation exchange (FLEX) approximation or the renormalized random phase approximation (RRPA). Using the band parameter values which approximately reproduce the observed Fermi surface and the d - and p -hole numbers estimated from NMR experiments, we get reasonable values for T c in an optimum and overdoped concentration range. The dynamical susceptibility, the nuclear spin-lattice relaxation rate and the one electron spectral density are well compared with the existing experimental results.

Spin Fluctuation-Induced Superconductivity in Two and Three Dimensional Systems

The strong coupling theory of unconventional superconductivity due to spin fluctuations around the antiferromagnetic instability, previously developed for two dimensional (square lattice) systems is extended to three dimensional (simple cubic lattice) systems. The values for T c are found to be similar for the 2- and 3-dimensional systems provided the band widths and the spin fluctuation parameters take similar values. The dependences of T c on various parameters are investigated. In both dimensions the value of T c is most sensitive to the frequency spread of antiferromagnetic fluctuations; the larger the frequency spread the higher T c . In the light of the present results a systematic scaling analysis is given for the values of T c for possible spin fluctuation-induced superconductors including high- T c cuprates, heavy electron- and organic superconductors, etc.

Cu NMR and NQR Studies of Impurities-Doped YBa2(Cu1-xMx)3O7(M=Zn and Ni)

The normal and superconducting properties in impurity-doped YBa 2 (Cu 1- x M x ) 3 O 7 (M=Zn and Ni) have been investigated by 63 Cu NMR and NQR techniques. The nonmagnetic Zn-doping into the CuO 2 plane in YBa 2 Cu 3 O 7 (YBCO 7 ) causes a local collapse of antiferromagnetic (AF) spin correlation near Zn impurities and induces a gapless superconductivity with a finite density of states at the Fermi surface. On the other hand, in the case of Ni doping, there is no appreciable modification except for a small decrease of T c , although the Ni spins behave actually as local moments. The gapless feature in Zn-doped YBCO 7 is shown to be consistently interpreted within the framework of the d -wave model with gap zeros of lines at the Fermi surface as argued in heavy fermion superconductors. It is pointed out that the AF spin fluctuation-induced superconductivity is applicable to high- T c cuprates.