Evidence for structural and electronic instabilities at intermediate temperatures in κ-(BEDT-TTF)₂X for X=Cu[N(CN)₂]Cl, Cu[N(CN)₂]Br and Cu(NCS)₂: Implications for the phase diagram of these quasi-two-dimensional organic superconductors

We present high-resolution measurements of the coefficient of thermal expansion α(T)=∂ In l(T)/∂T of the quasi-two-dimensional (quasi-2D) salts κ-(BEDT-TTF)₂X with X=Cu(NCS)₂, Cu[N(CN)₂]Br, and Cu[N(CN)₂]Cl in the temperature range T≤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)₂, Cu[N(CN)₂]Br) and antiferromagnetic (X=Cu[N(CN)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 T_c 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)₂ and Cu[N(CN)₂]Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of T*, ∂T*/∂p_i (i=a,b,c), are strictly anticorrelated with those of T_c. Based on comparative studies including the nonsuperconducting X=Cu[N(CN)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 α(T) anomalies that can be identified with a kinetic, glasslike transition where, below a characteristic temperature T_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)₂]Br salts reported in the literature.