SUPERCONDUCTING AND DIELECTRIC INSTABILITIES IN TL2MO6SE6 - UNUSUAL TRANSPORT-PROPERTIES AND UNSATURATING CRITICAL-FIELD

The Hall effect of the quasi-one-dimensional compound Tl2Mo6Se6 undergoes a progressive transition between an electronlike metallic regime at 300 K and a holelike semimetallic regime just above the superconducting transition (T(c) almost-equal-to 6.5 K). We show that, above almost-equal-to 80 K, the paradoxical transport properties can be explained by an unusual anisotropic electron scattering related to the low dimensionality of the system and to its instability. A spin-density-wave (SDW) or charge-density-wave (CDW) state certainly builds up below approximately 80 K-leading also to a change of regime in the thermopower but not in the conductivity-which is accompanied, at lower temperatures, by a magnetic-field dependence of the Hall coefficient and by a significant transverse magnetoresistance. The superconducting (SC) state therefore arises after the electron spectrum has been partly dielectrized by the CDW (or SDW) condensation, and it involves a very small density of states. A large paraconductivity which we attribute to superconducting fluctuations is detected just above T(c) and the transverse upper critical field H(c)2 perpendicular-to does not saturate down to 50 mK, which confirms that we are dealing with a very unusual superconducting state, intricately connected with the other competing instabilities.