Specific heat measurementsof the lattice contribution and the spin-density-wave andanion-ordering transitions in the (TMTSF)2ClO4 salt (TMTSF≡tetramethyltetraselenafulvalene)

Abstract

The specific heat of the quasi-one-dimensional organic compound (TMTSF)2ClO4 was measured between 1.9 and 35 K for two statescorresponding to the metallic/superconducting ground state and theinsulating spin-density-wave (SDW) ground state at low temperature,by varying the cooling conditions through the anion-ordering(AO) transition at 24 K. These are the first thermodynamicalmeasurements of the SDW ground state. Like in the previouslyinvestigated compounds (TMTSF)2X, with X = PF6 andAsF6, and (TMTTF)2Br (where TMTTF≡tetramethyltetrathiafulvalene), the lattice specific heat showsstrong deviations from a Debye behaviour, which can be explained bythe quasi-one-dimensional structural character. Bumps in C/T3occur at T = 3.5-4 K and 5 K for the quenched SDW and relaxedmetallic states respectively; these are in good agreement with thefrequencies of low-lying modes detected by infrared spectroscopy forthe metallic sample. In addition, analysis of the latticecontribution suggests a Tα (α = 2.7) acousticcontribution for T≳2 K, ascribed to the lowdimensionality of the structure. The SDW transition was detected atTc = 4.5 K, as expected from the high value of thequenching rate (of 3.3 K s-1) through the AO transition(the amplitude of the anomaly represents about 3% of the total heatcapacity). The AO transition was investigated either by decreasingthe temperature for the slow-cooled sample or by reheating, at asimilar rate, for the quenched sample; the specific heat anomalies areidentical for the two procedures, due to reordering annealingeffects on reheating, a phenomenon previously investigated by x-rayexperiments. Surprisingly, the entropy under the AO anomaly isRln (4/3), much lower than the Rln 2 value expected for the twopossible configurational orientations of the perchlorate ion in thedisordered state. Similar low values of the entropy have beenpreviously measured for the perrhenate (ReO4) and BF4 salts.