SUPERCONDUCTIVITY UNDER PRESSURE IN LINEAR CHALCOGENIDES

We have measured the electrical resistivity Of several quasi 1-D chalc ogenides between 1.2 and 300K and from ambient pressure up to 25GPa. O n NbSe3 we find that the high temperature charge density wave (CDW) tr ansition T1 disappears at a pressure of about 3.5GPa, where the superc onducting transition temperature T(c) jumps to approximately 5K. The s imilar pressure dependence of the logarithmic pressure derivative of b oth T1 and T(c) above this pressure suggests the existence of independ ent superconducting states. On m-TaS3 we observe a maximum on the T1-P variation at approximately 3.5GPa, that we attribute to a CDW transit ion temperature strongly diminished by 1-D fluctuations that am reduce d at low applied pressures, with T2 behaving in a monotonous decreasin g way with pressure. While on o-TaS3 the CDW that develops at 210K at ambient pressure, gives way to a superconducting state of T(c) approxi mately 3K at a pressure of 6.5GPa. The behaviour of (TaSe4)2I is scale d to the precedent , with a higher critical pressure of 15GPa that ind uces the appearance of a superconducting state with T(c)=2.5K.