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.
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.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 949358
- Report Number(s):
- ANL/MSD/JA-40225; PRBMDO; TRN: US1003500
- Journal Information:
- Phys. Rev. B, Vol. 65, Issue 14 ; Apr. 1, 2002; ISSN 0163-1829
- Country of Publication:
- United States
- Language:
- ENGLISH
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