Antiferromagnetic Spin Fluctuations and the Pseudogap in the Paramagnetic Phases of Quasi-Two-Dimensional Organic Superconductors

Abstract

We give a quantitative analysis of the published results of nuclear magnetic resonance (NMR) experiments for k-(ET)2X family of organic charge transfer salts by using the phenomenological spin fluctuation model of Moriya, and Millis, Monien and Pines (M-MMP). For temperatures above T_NMR ~ 50 K, the model gives a good quantitative description of the data for the paramagnetic metallic phase of several k-(ET)2X materials, with an antiferromagnetic correlation length which increases with decreasing temperature; growing to several lattice constants by T_NMR. It is shown that the fact that the dimensionless Korringa ratio is much larger than unity is inconsistent with a broad class of theoretical models (such as dynamical mean-field theory) which neglect spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase, 1/T1, Ks, and K all decrease significantly with decreasing temperature below T_NMR. This cannot be described by the M-MMP model and the most natural explanation is that a pseudogap opens up in the density of states below T_NMR, as in, for example, the cuprates. We show that the NMR measurements reported for k-(ET)2Cu2(CN)3 are qualitatively inconsistent with this material having a ground state with long range magnetic order. A pseudogap in the metallic state of organic superconductors is an important prediction of the resonating valence bond theory of superconductivity. We propose specific new experiments on organic superconductors to elucidate the nature, origin, and momentum dependence of the pseudogap and its relationship to superconductivity.

Two-column format; 24 pages, 6 figures. Longer and unpublished version of arXiv:0705.1382 [Phys. Rev. B 75, 214515 (2007)]

References in corpus (8)

• Unconventional critical behaviour in a quasi-two-dimensional organic conductor
• Strong electronic correlations in superconducting organic charge transfer salts
• Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
• The effect of irradiation-induced disorder on the conductivity and critical temperature of the organic superconductor $\kappa$-(BEDT-TTF)${}_2$Cu(SCN)${}_2$
• Pairing Symmetry Competition in Organic Superconductors
• Symmetry of the superconducting order parameter in frustrated systems determined by the spatial anisotropy of spin correlations
• Real space imaging of the metal - insulator phase separation in the band width controlled organic Mott system $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br
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Cited by in corpus (12)

• Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors
• Near-degeneracy of extended $s+d_{x^2-y^2}$ and $d_{xy}$ order parameters in quasi-two-dimensional organic superconductors
• Evidence for eight node mixed-symmetry superconductivity in a correlated organic metal
• Spin Fluctuations and the Pseudogap in Organic Superconductors
• Dynamical reduction of the dimensionality of exchange interactions and the "spin-liquid" phase of $\kappa$-(BEDT-TTF)${}_{2}X$
• Antiferromagnetic fluctuations in the quasi-two-dimensional organic superconductor detected by Raman spectroscopy
• Superconductivity and Mottness in Organic Charge Transfer Materials
• Fate of the Hebel-Slichter peak in superconductors with strong antiferromagnetic fluctuations
• Spin diffusion and spin conductivity in the 2d Hubbard model
• Vertex Corrections and the Korringa Ratio in Strongly Correlated Electron Materials
• Superconductivity and fluctuating magnetism in quasi two-dimensional kappa-(BEDT-TTF)2Cu[N(CN)2]Br probed with implanted muons
• Possible observation of the signature of the bad metal phase and its crossover to a Fermi liquid in K(BEDT-TTF)2Cu(NCS)2 bulk and nanoparticles by Raman scattering