Stabilisation of the metallic state at low temperatures in HMTTF-TCNQ under pressure

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Abstract
Measurements are reported of the b axis conductivity of HMTTF-TCNQ under hydrostatic pressures up to 33 kbar, and the room temperature, ambient pressure conductivity anisotropies. The two transitions seen in the conductivity at 49K and 43K at ambient pressure fall in temperature when pressure is applied; the lower transition is absent above 3 kbar and the upper transition falls linearly at 1.5K kbar-1. The low-temperature insulating behaviour at ambient pressure is rapidly replaced by weakly metallic behaviour, which at 10 kbar is very similar to that seen in HMTSF-TCNQ at ambient pressure. At higher pressures the resistivity was still falling at 1.2K. It is suggested that HMTTF-TCNQ above 3 kbar and HMTSF-TCNQ at ambient pressure undergo distortions on predominantly one type of stack. A unified picture of the pressure behaviour of this series of TCNQ salts is presented; the Peierls distortions in TTF-TCNQ and TSF-TCNQ occur when the transverse conduction is diffusive and the system is in the one-dimensional regime, whereas HMTTF-TCNQ and HMTSF-TCNQ distort when transverse motion is coherent and an anisotropic three-dimensional band structure picture is appropriate.