High-pressure resistance of PuTe up to 25 GPa

Abstract

The electrical resistance of a single crystal of PuTe has been measured up to $\ensuremath{\sim}25 \mathrm{GPa}$ and as a function of temperature down to 1.3 K. At low pressure, the resistance R displays the expected activated behavior, i.e., an upturn at low temperature and a change of curvature at $\ensuremath{\sim}200 \mathrm{K}$ as at ambient pressure. Surprisingly, the resistance at 1.5 K is enhanced by increasing pressure. For the NaCl phase, we propose a model where the conductivity \ensuremath{\sigma} is composed of three contributions; a constant 0 K term $\ensuremath{\sigma}(0)$ as expected in a semimetal, the Mott's law $\ensuremath{\sigma}\ensuremath{\propto}\mathrm{exp}\ensuremath{-}{(T}_{0}{/T)}^{1/4}$ indicating variable-range hopping conductivity below 100 K, and an exponential term $\mathrm{exp}(\ensuremath{-}{E}_{g}{/2k}_{B}T)$ due to a band gap. At 0.2 GPa, the activation energies ${T}_{0}$ and ${E}_{g}$ equal 23 meV and 0.185 eV, and increase up to 110 meV and $\ensuremath{\sim}0.4 \mathrm{eV}$ at 4.2 GPa, respectively. As a possible origin of the Mott's law behavior, we suggest a mechanism of hopping conduction involving the $5f$ states of the Pu ions. Between 10.7 GPa and 12.7 GPa, R strongly decreases over the whole temperature range. We attribute this collapse to the NaCl-CsCl structural transition. This effect indicates a strong increase of the state density and is in agreement with theoretical predictions. Above 12.7 GPa, the upturn tends to disappear and the temperature variation of R progressively approaches that of a metal. Our data suggest a magnetic transition below 15 K in the CsCl phase.