J.R. Cooper

Semi-metallic behaviour of HMTSF-TCNQ at low temperatures under pressure☆

Abstract Some results of electrical resistivity (ϱ) and transverse magnetoresistance measurements on single crystals of the charge transfer compound HMTSF-TCNQ under pressure are reported. There is evidence that dϱ/d T remains positive over the whole temperature range under pressure, and that a T 2 law is obeyed from 0.19 to 2 K at 14 kbar. Together with a relatively large magnetoresistance this is an indication of semi-metallic behaviour.

Evidence for a sharp metal-semiconductor phase transition in K2Pt(CN)4Br0.33H2O under pressure☆

Abstract A well defined metal-semiconductor has been observed at high pressure in K2Pt(CN)4Br0.33H2O by resistivity measurements up to 32 kbar. The transition temperature corresponds to a narrow peak in the logarithmic derivative ∂ 1n ϱ/∂ ( 1 T ) and its width is connected with the pressure enhanced 3-d coupling. The 32 kbar data are discussed in terms of a mean-field treatment of the Peierls transition (Horovitz, Gutfreund, Weger). This high pressure investigation suggest a possible suppression of the Peierls transition in KCP, above 70 kbar. Under the same conditions a superconductor, Tc ≅ 6Kcould possible exist.

Nuclear Spin-Lattice Relaxation and EPR Studies of TTF-TCNQ and TMTTF-TCNQ Under Hydrostatic Pressure

Abstract The quasi one-dimensional metallic character of TTF-TCNQ has been established by measurements of electrical conductivity, magnetic susceptibility, thermoelectric power, microwave conductivity and optical reflectivity at ambient pressure1. The metallic character of TMTTF-TCNQ is discussed in the preceding report2 in comparison with that of TTF-TCNQ. The metallic state results from the strong n-orbital overlap along the TCNQ and TTF chains. In TTF-TCNQ both chains are metallic with transfer integrals of order 0.1 eV. The intetchain transfer integral is of the order of 1 meV, which shows that the interchain hopping time of electrons is much shorter than the characteristic time scale of electron spin dynamics (Ωs, T ls) and nuclear spin relaxation. Therefore, the ratio of the spin-lattice relaxation times of each chains observed by Rybaczewski et al.3 using deuterated TTF-TCNQ which is nearly independent of the temperature down to 60 K indicates a temperature independent ratio of the static susceptib...

Magnetism of TTF-TCNQ and related compounds: Pressure effects

Abstract Pressure coefficients of the spin susceptibility, nuclear relaxation rates and resistivity have been measured in TTF-TCNQ, TMTTF-TCNQ and HMTSF-TCNQ. We point out that the very large pressure coefficients are not in agreement with the significantly smaller coefficients measured in optical absorption experiments under pressure. We propose an explanation of the magnetic properties of TTF-TCNQ and related compounds in terms of itinerant one dimensional antiferromagnetism. Using the Shiba-Pincus Hubbard model calculation we derive a value U/4t∥ close to unity which points towards the importance of correlations in the TTF-TCNQ family.

On the behaviour of TSeF-TCNQ under pressure

Abstract We report b-axis electrical conductivity data for TSeF-TCNQ single crystals from 12 to 300 K under hydrostatic pressures up to 9 kbar. The single anomaly visible in the conductivity at 29 K and the low temperature conductivity gap rise under pressure at the same rate of ∼ 6% kbar-1. It has been found that the pressure dependence of the metal-insulator phase transition is qualitatively consistent with a mean field formulation of the Peierls transition. A Gruneisen constant of 0.64 for TSeF-TCNQ has been derived from this pressure study together with recent optical and compressibility investigations. The pressure dependence of the conductivity anisotropies at room temperature in TSeF-TCNQ and TTF-TCNQ are reported. The magnitude of the anisotropies in the two compounds are found to be essentially the same. The striking result, however, is that the anisotropies in both compounds are found to be independent of pressure up to 9 kbar.

Differential thermal analysis of the field induced phase transitions of (TMTSF)2ClO4 above 1.2 K

Abstract We present some results of a differential thermal analysis of the magnetic field induced phase transitions in the organic conductor (TMTSF) 2 ClO 4 above 1.2 K. This study shows that transitions between different spin density wave states are first order and that the total entropy change involved in the two detected transitions (in the temperature range 1.2–2 K) is close to that of the quasi-one-dimensional electron gas. Above 2 and 4.2 K, only a single transition has been detected in our measurements. The entropy of that transition decreases and extrapolates to zero near 5 K. We present some arguments suggesting that if longitudinal nesting (2 k F , 0, 0) is to take place in the semi-metallic SDW phase at high fields it exists only above 2 K or so.

The Electrical Conductivity of TTF-TCNQ Under Pressure

Abstract We have measured the b-axis conductivity of three single crystals of the organic charge transfer salt TTF-TCNQ from 20-300 K under hydrostatic (gas) pressures of up to 6 kilobars in an attempt to clarify some results reported a year ago1 whch indicated that there were two transitions below 60 K in this compound and what is more important that one of them was strongly pressure (p) dependent. We will also briefly mention the effect of pressure in the metallic region at higher temperatures.

CDW Transport in TTF-TCNQ : Impurity, Coulombic and Commensurability Pinning

We review the salient results of a recently performed investigation of TTF-TCNQ showing the existence of non-linear conductivity related to CDW transport below the Peierls phase transition.

The evolution of nonlinear electrical transport effects in TTF-TCNQ as driven through CDW commensurability

Abstract The b-axis conductivity of the one-dimensional two-chain organic conductor TTF-TCNQ was measured as a function of electric field E for temperatures T ≲ 30 K and pressures P ≳ 24 kbars. At P = 1 bar, TTF-TCNQ undergoes three successive phase transitions : the high temperature T H (incommensurate Peierls) transition involving the electron-like TCNQ l stacks, the lower two transitions (T M ,T L ) involving the hole-like TTF stacks. We have found at P = 1 bar deviations from Ohms law, with a threshold field E T , in the neighbourhood and below T H . E T reaches a minimum just below T H before rapidly increasing below T M . At 4 and 9 kbars T M = T L , and hence, the range of T where there is a single incommensurate CDW is large. In this P range, nonlinear effects are observed and the T dependence of E T is less weak below its minimum value than at 1 bar. As the CDW is driven through third-order commensurability at ∼ 19 kbars, we find at constant T/T H a peak in E T and a minimum in the excess current at 30 V/cm. These results are discussed in terms of the depinning of the CDW by the electric field, taking into account the interactions between the two types of chains and the possibility of discommensuration transport near commensurability.