Low-Dimensional Critical Behaviors and Competition Between Order Parameters in the Organic Metal (Tmtsf) 2 ClO 4
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LOW-DIMENSIONAL CRITICAL BEHAVIORS AND COMPETITION BETWEEN ORDER PARAMETERS IN THE ORGANIC METAL (TMTSF) 2 CIO4 F. PESTY, P. GAROCHE AND M. HERITIER Laboratoire de Physique des Solides, Associ6 au C.N.R.S., U.P.S., Bat 510 91405 Orsay Cedex (FRANCE) ABSTRACT In low-dimensional conductors, the instability of the metallic state can lead to the formation at low temperature of a spin density wave induced by the magnetic field (FISDW) . The transition results from the complex interplay between the one dimensional instability of the electronic gas and the quantization of the magnetic field's flux. This second-order phase-transition line has been investigated by measuring both specific heat and thermal conductivity along the c* direction. The mean-field jump and the gap value have been deduced respectively from the anomaly and the exponential decay of the electronic specific heat. The coupling strength khas been evaluated, and the X>0.3 value indicates clearly a strong coupling behavior at high field. Below 8 teslas, the specific heat displays a double anomaly in relation with the competition between subphases. Above the second-order transition line, critical fluctuations are observed on both specific heat and lattice thermal conductivity. Along this line, one-dimensional fluctuations increase with increasing magnetic field. It is proposed that the very high field reentrance of the metal is to be related to enhancement of the 1D fluctuations. INTRODUCTION In the Bechgaard salts (TMTSF) 2 X, the FISDW states have been extensively studied during the past decade, from both experimental and theoretical points of view [1). The appearance of the SDW in a moderate magnetic field (about 3 teslas) is so far understood as resulting from an orbital effect of the field which, by lowering the effective dimensionality of the electronic motion, leads at low enough temperature to the usual instability of the low-D metallic state. A description of the cascade of field induced transitions has been achieved within the "standard" model [2-6]. But number of experimental facts remain unclear. Among them, the high-field total reentrance of the metallic phase into the FISDW states (7-9], and the low-field partial ones [10), observed in the very well ordered (TMTSF) 2 CIO 4 . Here, we will focus on the anomalous behavior of the metal-FISDW transition line in the intermediate field range. MEASUREMENT OF THE HEAT CONDUCTION ALONG THE c* AXIS The 2mg single crystal, of a L=0.3mm thickness and a S=6.5mm2 cross-section area is glued onto a sapphire slab (Figure 1) . The transverse direction, the c* axis, is the least conducting one. The sample can thus be considered as resulting from the stacking of conducting planes roughly parallel to the sapphire's surface. Our basic assumption is that the planes are perfectly isothermal and that the heat conduction just takes place between neighboring planes. In such a case we are left with a purely one-dimensional Mat. Res. Soc. Symp. Proc. Vol. 173. 01990 Materials Research Society
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heat diffusion problem.
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