We present specific-heat measurements as a function of field (3-6 T) and temperature (0.35-1.7 K) of a single crystal of the organic conductor tetramethyltetraselenafulvalenium perchlorate [(TMTSF)2C104]. The small electronic contribution, 40 nJ/K, shows clearly above a temperaturedependent threshold field, a series of jumps and peaks. The phase diagram deduced from this magnetocalorimetric investigation exhibits in the threshold region indications of a partial reentrance of the normal metal in between semimetallic regions.PACS numbers: 71.25.Hc The organic superconductors tetramethyltetraselenafulvalenium fluorophosphate [ (TMTSF) 2PF6 ] and tetramethyltetraselenafulvalenium perchlorate [ (TMTSF) 2CIO4] display a very unusual phase transition at low temperature if a magnetic field is applied along the least-conducting axis c*. The normal metal observed above the superconducting critical field Hciic*) transforms into a field-induced spin-densitywave state above a temperature-dependent threshold field Ho(T). An indication of this transition was first reported in the pressure-stabilizing metallic phase of the PF6 salt.^ The magnetoresistance displays above the transition Shubnikov-de Haas-type oscillations.^ The angular variation of their frequency implies closed orbits in the (a,b) plane and open orbits in the c* direction.^ Therefore, the Fermi surface is supposed to be essentially two dimensional at high field. The same oscillations have been observed in the magnetotransport of the CIO4 salt.^'"^ Specific-heat measurement indicates that a phase transition separates the low-field quasi one-dimensional metallic phase from the high-field semimetallic state.^'^ In this phase the observation of steplike Hall voltage at low temperature pointed out similarities to the quantum Hall effect observed in two-dimensional semiconductors. However, the temperature dependence of the transport properties suggests that these steps are related to a series of phase transitions.^"^ Various models have been proposed in order to explain the anomalous properties of the high-field phase.^^"^"^ In this Letter we report the first investigation of the field-induced spin-densitywave state by direct measurement of the specific heat at constant temperature as a function of a magnetic field /flic*, and we wish to demonstrate that real phase transitions, induced by quantum effects, separate different semimetallic states. Moreover, we focus our attention on the threshold-field region and propose a complex phase diagram with a partial reentrance of the normal metal in between the semimetallic state. The applied magnetic field was oriented along the c* axis, i.e., perpendicular to the higher-conducting plane (a,b). A microlithography process and a thin-film technique on sapphire were used as a means to produce a sample holder with a very low heat capacity [c(l K,6 T) = 2 nJ/K, dashed line in the insert of Fig. 1]; its contribution was subtracted from the total heat capacity. It represents between 7% and 20% of the sample electronic heat capaci...
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