Stress Induced Electron Transition in Monoclinic TANTALUM-SELENIUM-3

A very large room-temperature piezoresistance is observed in TaSe(,3) ((DELTA)R((epsilon))/R (TURNEQ) 1.5 at (epsilon) = 1%) as compared with simple metals such as Cu or Al ((DELTA)R((epsilon))/R (TURNEQ) 0.015 at (epsilon) = 1%). The strain (epsilon) is defined as (epsilon) = (DELTA)L/L and (DELTA)R((epsilon))/R = R((epsilon)) - R((epsilon) = 0) /R((epsilon) = 0). As the temperature is lowered the piezoresistance becomes much larger (At T = 22 K, (DELTA)R((epsilon))/R (TURNEQ) 10('3) at (epsilon) = 1%). The room temperature thermopower also changes drastically with strain, going from a few (mu)V/K at zero strain to approximately 50-60 (mu)V/K at (epsilon) (TURNEQ) 1%. Below T = 100 K a sharp transition occurs in both the resistance and the thermopower at a uniaxial strain of (epsilon) (TURNEQ) 1%. For TaSe(,3), a strain of (epsilon) = 1% corresponds to a uniaxial stress ((sigma)) of 2.5 GPa. In the zero stress state ((sigma) = 0), TaSe(,3) is metallic with dR/dT > 0 at all temperatures within the measurement range (T = 1.6 K to T = 300 K). In the "high stress state", ((sigma) (GREATERTHEQ) 2.5 GPa); dR/dT < 0 down to 100 K in all samples; but, below 100 K, dR/dT became positive at some temperature in all but one sample. In that sample dR/dT < 0 down to the lowest temperature measured (T (TURNEQ) 1.5 K). Analogous to the resistance the thermopower also has a very different temperature dependence in the "high stress state" than the zero stress state. In addition, preliminary measurements of the magnetoresistance show a much different behavior in the two states. These measurements are consistent with a Lifshitz -type electron transition (a change in topology of the Fermi surface) in TaSe(,3) when a high stress is applied, although a structural phase transition can not be ruled out. Some samples of TaSe(,3) undergo a superconducting transition at T = 2.1 K. When stress is applied to the sample, the supercon- ductivity is suppressed. In the "high stress state" superconductivity is totally suppressed.