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

J. Müller; M. Lang; F. Steglich; J. A. Schlueter; A. M. Kini; T. Sasaki

doi:10.1103/PhysRevB.65.144521

paper

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

arXiv:cond-mat/0107480 · doi:10.1103/PhysRevB.65.144521

Abstract

We present high-resolution measurements of the coefficient of thermal expansion $α (T) = \partial ln l (T) / \partial T$ of the quasi-twodimensional (quasi-2D) salts $κ$ -(BEDT-TTF) $_{2}$ X with X = Cu(NCS) $_{2}$ , Cu[N(CN) $_{2}$ ]Br and Cu[N(CN) $_{2}$ ]Cl. 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 $T^{*}$ are strictly anticorrelated with those of $T_{c}$ . We propose that $T^{*}$ marks the transition to a spin-density-wave (SDW) 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, glass-like transition where, below a characteristic temperature $T_{g}$ , disorder in the orientational degrees of freedom of the terminal ethylene groups becomes frozen in. We argue that the degree of disorder increases on going from the X = Cu(NCS) $_{2}$ to Cu[N(CN) $_{2}$ ]Br and the Cu[N(CN) $_{2}$ ]Cl salt. Our results provide a natural explanation for the unusual time- and cooling-rate dependencies of the ground-state properties in the hydrogenated and deuterated Cu[N(CN) $_{2}$ ]Br salts reported in the literature.

22 pages, 7 figures

References in corpus (1)

Magnetic and Electronic Phase Diagram and Superconductivity in the Organic Superconductors k-(BEDT-TTF)2X

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

Abstract

References in corpus (1)

Cited by in corpus (58)

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