Antiferromagnetic spin fluctuations in the metallic phase of quasi-two-dimensional organic superconductors

Yusuf, Eddy, Powell, Benjamin and McKenzie, Ross H. (2007) Antiferromagnetic spin fluctuations in the metallic phase of quasi-two-dimensional organic superconductors. Physical Review B (Condensed Matter and Materials Physics), 75 21: 214515-1-214515-11. doi:10.1103/PhysRevB.75.214515

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Author Yusuf, Eddy
Powell, Benjamin
McKenzie, Ross H.
Title Antiferromagnetic spin fluctuations in the metallic phase of quasi-two-dimensional organic superconductors
Journal name Physical Review B (Condensed Matter and Materials Physics)   Check publisher's open access policy
ISSN 1098-0121
1550-235X
Publication date 2007-06-29
Year available 2007
Sub-type Article (original research)
DOI 10.1103/PhysRevB.75.214515
Open Access Status File (Publisher version)
Volume 75
Issue 21
Start page 214515-1
End page 214515-11
Total pages 11
Place of publication College Park, MD, United States
Publisher American Physical Society
Language eng
Subject 240401 Optics and Opto-electronic Physics
C1
780102 Physical sciences
01 Mathematical Sciences
02 Physical Sciences
09 Engineering
Abstract We give a quantitative analysis of the previously published nuclear magnetic resonance (NMR) experiments in the kappa-(ET)(2)X family of organic charge-transfer salts. The temperature dependence of the nuclear-spin relaxation rate 1/T-1, the Knight shift K-s, and the Korringa ratio K is compared to the predictions of the phenomenological spin-fluctuation model of Moriya and Millis, Monien, and Pines (M-MMP), that has been used extensively to quantify antiferromagnetic spin fluctuations in the cuprates. For temperatures above T-NMR similar or equal to 50 K, the model gives a good quantitative description of the data in the metallic phases of several kappa-(ET)(2)X materials. These materials display antiferromagnetic correlation lengths which increase with decreasing temperature and grow 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 neglects spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase the nuclear-spin relaxation rate, the Knight shift, and the Korringa ratio 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, similar to that observed in the underdoped cuprate superconductors, opens up in the density of states below T-NMR. Such a pseudogap has recently been predicted to occur in the dimerized organic charge-transfer salts materials by the resonating valence bond (RVB) theory. We propose specific experiments on organic superconductors to elucidate these issues. For example, measurements to see if high magnetic fields or high pressures can be used to close the pseudogap would be extremely valuable.
Formatted abstract
We give a quantitative analysis of the previously published nuclear magnetic resonance (NMR) experiments in the ĸ-(ET)2X family of organic charge-transfer salts. The temperature dependence of the nuclear-spin relaxation rate 1/T1, the Knight shift K-s, and the Korringa ratio К is compared to the predictions of the phenomenological spin-fluctuation model of Moriya and Millis, Monien, and Pines (M-MMP), that has been used extensively to quantify antiferromagnetic spin fluctuations in the cuprates. For temperatures above TNMR similar or equal to  50 K, the model gives a good quantitative description of the data in the metallic phases of several κ-(ET)2X materials. These materials display antiferromagnetic correlation lengths which increase with decreasing temperature and grow to several lattice constants by TNMR. 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 neglects spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase the nuclear-spin relaxation rate, the Knight shift, and the Korringa ratio 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, similar to that observed in the underdoped cuprate superconductors, opens up in the density of states below T-NMR. Such a pseudogap has recently been predicted to occur in the dimerized organic charge-transfer salts materials by the resonating valence bond (RVB) theory. We propose specific experiments on organic superconductors to elucidate these issues. For example, measurements to see if high magnetic fields or high pressures can be used to close the pseudogap would be extremely valuable.
Keyword Physics, Condensed Matter
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

 
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Created: Tue, 29 Apr 2008, 21:29:16 EST by Fiona Krohn on behalf of School of Mathematics & Physics