Evidence for structural and electronic instabilities at intermediate temperatures inκ−(BEDT−TTF)2Xfor

Jens Müller

¹

,

Michael Lang

²

,

F. Steglich

³

et al.

Abstract

We present high-resolution measurements of the coefficient of thermal expansion ␣(T)ϭ‫ץ‬ ln l(T)/‫ץ‬T of the quasi-two-dimensional ͑quasi-2D͒ salts -(BEDT-TTF) 2 X with XϭCu(NCS) 2 , Cu͓N(CN) 2 ͔Br, and Cu͓N(CN) 2 ͔Cl in the temperature range Tр150 K. Three distinct kinds of anomalies corresponding to different temperature ranges have been identified. These are ͑A͒ phase-transition anomalies into the superconducting "XϭCu(NCS) 2 , Cu͓N(CN) 2 ͔Br… and antiferromagnetic "XϭCu͓N(CN) 2 ͔Cl… ground state, ͑B͒ phasetransition-like anomalies at intermediate temperatures ͑30-50͒ K for the superconducting salts, and ͑C͒ kinetic, glasslike transitions at higher temperatures, i.e., ͑70-80͒ K for all compounds. By a thermodynamic analysis of the discontinuities at the second-order phase transitions that characterize the ground state of system ͑A͒, 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 ͑B͒, distinct anomalies reminiscent of second-order phase transitions have been found at T* ϭ38 K and 45 K for the superconducting XϭCu(NCS) 2 and Cu͓N(CN) 2 ͔Br salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of T*, ‫ץ‬T*/‫ץ‬p i (iϭa,b,c), are strictly anticorrelated with those of T c . Based on comparative studies including the nonsuperconducting X ϭCu͓N(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 ͑C͒, all compounds show ␣(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 Cu͓N(CN) 2 ͔Br salts reported in the literature.

Pump- and Probe-polarization Analyses of Ultrafast Carrier Dynamics in Organic Superconductors

“…However, any electronic ordered states accompanied by SB such as CDW have not been suggested for κ-NCS and κ-Br, respectively. Instead, the recent thermal expansion measurements suggest that the glasslike structural transition occurs at T g ∼ 80 K for κ-Br salt and 70 K for κ-NCS [11], where the T g s agree well with our results. In this report the glasslike transition may correspond to order-disorder transition of end terminal ethylene group of BEDT-TTF molecules.…”

“…These compounds consist of alternate stacking of the BEDT-TTF conducting layers and the insulating anion X layers, forming highly two-dimensional electron system. In order to avoid structural disorder of BEDT-TTF molecules [11] the samples were slowly cooled at rates of 0.1-1 K/min. The electronic state is dominated by effective electron correlation t/U, where t and U are the transfer integral between dimers of BEDT-TTF molecules and the on-site Coulomb repulsion, respectively.…”

Magnetic field induced lattice effects in a quasi-two-dimensional organic conductor close to the Mott metal-insulator transition

“…Upon cooling, an anomaly at T g ≈ 77 K is observed. This anomaly has been attributed to a glasslike transition, which has been discussed in the literature in connection with the freezing out of the ethylene end groups 25 and anion ordering. 24,26 Upon further cooling, a second anomaly around T P = 30 K is observed.…”

“…In order to reduce cooling-rate-dependent effects associated with disorder of the ethylene end groups of the ET molecules, a cooling rate of ∼−3 K/h (thermal expansion) and ∼−6 K/h (resistance) through the glasslike transition around 77 K (Ref. 25) was applied. After the initial controlled cooldown, the sample was kept at temperatures below 40 K. Measurements of α(T ,B = const) were performed at temperatures ranging from 4.5 K up to about 14 K except for B = 0.5 T, where the measurements were limited to T 11 K.…”

Spin-current injection and detection in κ-(BEDT-TTF)2Cu[N(CN)2]Br

“…We also mention that the temperature at which the ISHE suppression was observed coincides with a glass transition temperature of κ-Br films. 37,38 However, at the present stage, there is no framework to discuss the relation between the spin-current injection efficiency and such lattice fluctuations. To obtain the full understanding of the temperature dependence of the spin-pumping-induced ISHE voltage in the κ-Br/YIG systems, more detailed experimental and theoretical studies are necessary.…”