skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Evidence for structural and electronic instabilities at intermediate temperatures in K-(BEDT-TTF){sub 2}X for X=Cu[N(CN){sub 2}]Cl, Cu[N(CN){sub 2}]Br and Cu(NCS){sub 2} : implications for the phase diagram of these quasi-2D organic superconductors.

Abstract

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 the 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. Mostmore » 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

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
949358
Report Number(s):
ANL/MSD/JA-40225
Journal ID: ISSN 0163-1829; PRBMDO; TRN: US1003500
DOE Contract Number:  
DE-AC02-06CH11357
Resource Type:
Journal Article
Journal Name:
Phys. Rev. B
Additional Journal Information:
Journal Volume: 65; Journal Issue: 14 ; Apr. 1, 2002; Journal ID: ISSN 0163-1829
Country of Publication:
United States
Language:
ENGLISH
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; BEDT-TTF; DEGREES OF FREEDOM; FERMI LEVEL; GROUND STATES; ORDER PARAMETERS; ORGANIC SUPERCONDUCTORS; PHASE DIAGRAMS; PRESSURE COEFFICIENT; SUPERCONDUCTORS; THERMAL EXPANSION; TRANSITION TEMPERATURE

Citation Formats

Muller, J., Lang, M., Steglich, F., Schlueter, J. A., Kini, A. M., Sasaki T., Materials Science Division, Max-Planck-Inst. fur Chemische Physik fester Stoffe, Physikalisches Inst. der Univ. Frankfurt, and Tohoku Univ. Evidence for structural and electronic instabilities at intermediate temperatures in K-(BEDT-TTF){sub 2}X for X=Cu[N(CN){sub 2}]Cl, Cu[N(CN){sub 2}]Br and Cu(NCS){sub 2} : implications for the phase diagram of these quasi-2D organic superconductors.. United States: N. p., 2002. Web. doi:10.1103/PhysRevB.65.144521.
Muller, J., Lang, M., Steglich, F., Schlueter, J. A., Kini, A. M., Sasaki T., Materials Science Division, Max-Planck-Inst. fur Chemische Physik fester Stoffe, Physikalisches Inst. der Univ. Frankfurt, & Tohoku Univ. Evidence for structural and electronic instabilities at intermediate temperatures in K-(BEDT-TTF){sub 2}X for X=Cu[N(CN){sub 2}]Cl, Cu[N(CN){sub 2}]Br and Cu(NCS){sub 2} : implications for the phase diagram of these quasi-2D organic superconductors.. United States. doi:10.1103/PhysRevB.65.144521.
Muller, J., Lang, M., Steglich, F., Schlueter, J. A., Kini, A. M., Sasaki T., Materials Science Division, Max-Planck-Inst. fur Chemische Physik fester Stoffe, Physikalisches Inst. der Univ. Frankfurt, and Tohoku Univ. Mon . "Evidence for structural and electronic instabilities at intermediate temperatures in K-(BEDT-TTF){sub 2}X for X=Cu[N(CN){sub 2}]Cl, Cu[N(CN){sub 2}]Br and Cu(NCS){sub 2} : implications for the phase diagram of these quasi-2D organic superconductors.". United States. doi:10.1103/PhysRevB.65.144521.
@article{osti_949358,
title = {Evidence for structural and electronic instabilities at intermediate temperatures in K-(BEDT-TTF){sub 2}X for X=Cu[N(CN){sub 2}]Cl, Cu[N(CN){sub 2}]Br and Cu(NCS){sub 2} : implications for the phase diagram of these quasi-2D organic superconductors.},
author = {Muller, J. and Lang, M. and Steglich, F. and Schlueter, J. A. and Kini, A. M. and Sasaki T. and Materials Science Division and Max-Planck-Inst. fur Chemische Physik fester Stoffe and Physikalisches Inst. der Univ. Frankfurt and Tohoku Univ.},
abstractNote = {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 the 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.},
doi = {10.1103/PhysRevB.65.144521},
journal = {Phys. Rev. B},
issn = {0163-1829},
number = 14 ; Apr. 1, 2002,
volume = 65,
place = {United States},
year = {2002},
month = {4}
}