Superconducting and dielectric instabilities in Tl2Mo6Se6: Unusual transport properties and unsaturating critical field

Abstract

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 (Tc≊6.5 K). We show that, above ≊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 ∼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 Tc and the transverse upper critical field Hc2⊥ 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.

Source-reported materials — not catalogue approval

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