## Abstract

We present high-resolution measurements of the coefficient of thermal expansion (formula presented) of the quasi-two-dimensional (quasi-2D) salts (formula presented) with (formula presented) (formula presented) and (formula presented) in the temperature range (formula presented) Three distinct kinds of anomalies corresponding to different temperature ranges have been identified. These are (A) phase-transition anomalies into the superconducting (formula presented) (formula presented) and antiferromagnetic (formula presented) 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 (formula presented) 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 (formula presented) and 45 K for the superconducting (formula presented) and (formula presented) salts, respectively. Most interestingly, we find that the signs of the uniaxial pressure coefficients of (formula presented) (formula presented) (formula presented) are strictly anticorrelated with those of (formula presented) Based on comparative studies including the nonsuperconducting (formula presented) salt as well as isotopically labeled compounds, we propose that (formula presented) 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 (formula presented) anomalies that can be identified with a kinetic, glasslike transition where, below a characteristic temperature (formula presented) 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 (formula presented) salts reported in the literature.

Original language | English |
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Pages (from-to) | 1-14 |

Number of pages | 14 |

Journal | Physical Review B - Condensed Matter and Materials Physics |

Volume | 65 |

Issue number | 14 |

DOIs | |

Publication status | Published - 2002 |

## ASJC Scopus subject areas

- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics