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Title: Experimental and semiempirical method to determine the Pauli-limiting field in quasi-two-dimensional superconductors as applied to κ-(BEDT-TTF)2Cu(NCS)2: strong evidence of a FFLO state

Abstract

We present upper critical field data for {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2} with the magnetic field close to parallel and parallel to the conducting layers. We show that we can eliminate the effect of vortex dynamics in these layered materials if the layers are oriented within 0.3-inch of parallel to the applied magnetic field. Eliminating vortex effects leaves one remaining feature in the data that corresponds to the Pauli paramagnetic limit (H{sub p}). We propose a semiempirical method to calculate the H{sub p} in quasi-2D superconductors. This method takes into account the energy gap of each of the quasi-2D superconductors, which is calculated from specific-heat data, and the influence of many-body effects. The calculated Pauli paramagnetic limits are then compared to critical field data for the title compound and other organic conductors. Many of the examined quasi-2D superconductors, including the above organic superconductors and CeCoIn{sub 5}, exhibit upper critical fields that exceed their calculated H{sub p} suggesting unconventional superconductivity. We show that the high-field low-temperature state in {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2} is consistent with the Fulde-Ferrell-Larkin-Ovchinnikov state.

Authors:
; ; ; ; ; ; ; ; ; ; ;  [1];  [2];  [2];  [2]
  1. (Materials Science Division)
  2. (
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1050177
Report Number(s):
ANL/MSD/JA-72068
Journal ID: ISSN 1098-0121; TRN: US201218%%530
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 85; Journal Issue: 21
Country of Publication:
United States
Language:
ENGLISH
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; CRITICAL FIELD; ENERGY GAP; MAGNETIC FIELDS; ORGANIC SUPERCONDUCTORS; SPECIFIC HEAT; SUPERCONDUCTIVITY; SUPERCONDUCTORS

Citation Formats

Agosta, C. C., Jin, J., Coniglio, W. A., Smith, B. E., Cho, K., Mihut, I., Martin, C., Tozer, S. W., Murphy, T. P., Palm, E. C., Schlueter, J. A., Kurmoo, M., Clark Univ.), Nat. High Field Magnet Lab.), and The Royal Inst.). Experimental and semiempirical method to determine the Pauli-limiting field in quasi-two-dimensional superconductors as applied to κ-(BEDT-TTF)2Cu(NCS)2: strong evidence of a FFLO state. United States: N. p., 2012. Web. doi:10.1103/PhysRevB.85.214514.
Agosta, C. C., Jin, J., Coniglio, W. A., Smith, B. E., Cho, K., Mihut, I., Martin, C., Tozer, S. W., Murphy, T. P., Palm, E. C., Schlueter, J. A., Kurmoo, M., Clark Univ.), Nat. High Field Magnet Lab.), & The Royal Inst.). Experimental and semiempirical method to determine the Pauli-limiting field in quasi-two-dimensional superconductors as applied to κ-(BEDT-TTF)2Cu(NCS)2: strong evidence of a FFLO state. United States. doi:10.1103/PhysRevB.85.214514.
Agosta, C. C., Jin, J., Coniglio, W. A., Smith, B. E., Cho, K., Mihut, I., Martin, C., Tozer, S. W., Murphy, T. P., Palm, E. C., Schlueter, J. A., Kurmoo, M., Clark Univ.), Nat. High Field Magnet Lab.), and The Royal Inst.). 2012. "Experimental and semiempirical method to determine the Pauli-limiting field in quasi-two-dimensional superconductors as applied to κ-(BEDT-TTF)2Cu(NCS)2: strong evidence of a FFLO state". United States. doi:10.1103/PhysRevB.85.214514.
@article{osti_1050177,
title = {Experimental and semiempirical method to determine the Pauli-limiting field in quasi-two-dimensional superconductors as applied to κ-(BEDT-TTF)2Cu(NCS)2: strong evidence of a FFLO state},
author = {Agosta, C. C. and Jin, J. and Coniglio, W. A. and Smith, B. E. and Cho, K. and Mihut, I. and Martin, C. and Tozer, S. W. and Murphy, T. P. and Palm, E. C. and Schlueter, J. A. and Kurmoo, M. and Clark Univ.) and Nat. High Field Magnet Lab.) and The Royal Inst.)},
abstractNote = {We present upper critical field data for {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2} with the magnetic field close to parallel and parallel to the conducting layers. We show that we can eliminate the effect of vortex dynamics in these layered materials if the layers are oriented within 0.3-inch of parallel to the applied magnetic field. Eliminating vortex effects leaves one remaining feature in the data that corresponds to the Pauli paramagnetic limit (H{sub p}). We propose a semiempirical method to calculate the H{sub p} in quasi-2D superconductors. This method takes into account the energy gap of each of the quasi-2D superconductors, which is calculated from specific-heat data, and the influence of many-body effects. The calculated Pauli paramagnetic limits are then compared to critical field data for the title compound and other organic conductors. Many of the examined quasi-2D superconductors, including the above organic superconductors and CeCoIn{sub 5}, exhibit upper critical fields that exceed their calculated H{sub p} suggesting unconventional superconductivity. We show that the high-field low-temperature state in {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2} is consistent with the Fulde-Ferrell-Larkin-Ovchinnikov state.},
doi = {10.1103/PhysRevB.85.214514},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 21,
volume = 85,
place = {United States},
year = 2012,
month = 1
}
  • We present high-resolution measurements of the coefficient of thermal expansion {alpha}(T)={partial_derivative} ln l(T)/{partial_derivative}T of the quasi-two-dimensional (quasi-2D) salts {kappa}-(BEDT-TTF){sub 2}X with X=Cu[N(CN){sub 2}]Cl, Cu[N(CN){sub 2}]Br and Cu(NCS){sub 2} in the temperature range T<{approx}150 K. Three distinct kinds of anomalies corresponding to different temperature ranges have been identified. These are (A) phase-transition anomalies into the superconducting (X=Cu(NCS){sub 2}, Cu[N(CN){sub 2}]Br) and antiferromagnetic (X=Cu[N(CN){sub 2}]Cl) ground state, (B) phase-transition-like anomalies at intermediate temperatures (30-50) K for the superconducting salts, and (C) kinetic, glasslike transitions at higher temperatures, i.e., (70-80) K for all compounds. By a thermodynamic analysis of the discontinuities at themore » second-order phase transitions that characterize the ground state of system (A), the uniaxial-pressure coefficients of the respective transition temperatures could be determined. We find that in contrast to what has been frequently assumed, the intraplane-pressure coefficients of Tc for this family of quasi-2D superconductors do not reveal a simple form of systematics. This demonstrates that attempts to model these systems by solely considering in-plane electronic parameters are not appropriate. At intermediate temperatures (B), distinct anomalies reminiscent of second-order phase transitions have been found at T*=38 K and 45 K for the superconducting X=Cu(NCS){sub 2} and Cu[N(CN){sub 2}]Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of T*, {partial_derivative}T*/{partial_derivative}p{sub i} (i=a,b,c), are strictly anticorrelated with those of {Tc}. Based on comparative studies including the nonsuperconducting X=Cu[N(CN){sub 2}]Cl salt as well as isotopically labeled compounds, we propose that T* marks the transition to a density-wave state forming on minor, quasi-1D parts of the Fermi surface. Our results are compatible with two competing order parameters that form on disjunct portions of the Fermi surface. At elevated temperatures (C), all compounds show {alpha}(T) anomalies that can be identified with a kinetic, glasslike transition where, below a characteristic temperature T{sub g}, disorder in the orientational degrees of freedom of the terminal ethylene groups becomes frozen in. Our results provide a natural explanation for the unusual time- and cooling-rate dependences of the ground-state properties in the hydrogenated and deuterated Cu[N(CN){sub 2}]Br salts reported in the literature.« less
  • We determine the phase diagram of a quasi-one-dimensional superconductor (weakly coupled chains system with an [ital open] [ital Fermi] [ital surface]) in a magnetic field. The usual Ginzburg-Landau regime is followed, when the field is increased, by a [ital cascade] of superconducting phases separated by first-order transitions, which ends in a strong reentrance of the superconducting phase. These new phases show a novel kind of symmetry of a laminar type. The Zeeman splitting does not completely suppress the reentrance in very strong field, the ground state being in this case a Larkin-Ovchinnikov-Fulde-Ferrell state.
  • Measurements of both the magnetization and magnetotransport of {kappa}-(BEDT-TTF){sub 2}I{sub 3} (BEDT-TTF is bisethylenedithio-tetrathiafulvalene) in magnetic fields extending to 60 T at 0.4 K and 20 T at 35 mK are reported. Strong eddy currents observed in the magnetization are found to exhibit critical currentlike behavior. This might be connected with the breakdown of the quantum Hall effect, as proposed previously for {alpha}-phase salts. The strong two dimensionality leads to an apparent fall of the effective mass together with an overall suppression of the amplitude of the magnetic quantum oscillations at high magnetic fields or very low temperatures. These effectsmore » are more pronounced for the Shubnikov{endash}de Haas (SdH) effect but clearly visible also for the de Haas{endash}van Alphen (dHvA) oscillations. The apparent fall of the effective mass and the deviations of the dHvA signal from the behavior predicted by the standard Lifshitz-Kosevich theory can quantitatively be explained by the influence of chemical-potential oscillations on the wave form in a two-dimensional, spin-split Fermi liquid. The much stronger deviations from the conventional behavior in the transport data hint to an additional mechanism unique to the SdH effect. thinsp {copyright} {ital 1998} {ital The American Physical Society}« less
  • We have calculated the effect of Pauli limiting on the upper critical magnetic field {ital H}{sub {ital c}2} of a {ital d}-wave superconductor stabilized by antiferromagnetic spin fluctuations. This calculation includes the effect of arbitrary impurity concentrations. Strong-coupling effects are considered explicitly.