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Register a new userQuantum Hall Transitions in (TMTSF)$_2$PF$_6$
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S. Valfells
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We have studied the temperature dependence of the integer quantum Hall transitions in the molecular crystal (TMTSF)$_2$PF$_6$. We find that the transition width between the quantum Hall plateaus does not exhibit the universal power-law scaling behavior of the integer quantum Hall effect observed in semiconducting devices. Instead, the slope of the $rho_{xy}$ risers, $drho_{xy}/dB$, and the (inverse) width of the $rho_{xx}$ peaks, $(Delta B)^{-1}$, show a BCS-like energy gap temperature dependence. We discuss these results in terms of the field-induced spin-density wave gap and order parameter of the system.
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We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$AsF$_6$ (TMTSF : tetramethyl-tetraselenafulvalene). The 4 K neutron scattering structure refinement of the fully deuterated (TMTSF)$_2$PF$_6$-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)$_2$PF$_6$-H12 salt previously determined at the same temperature. Surprisingly it is found that deuteration corresponds to the application of a negative pressure of 5 x 10$^2$ MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF$_6$ and AsF$_6$ salts. Two different thermal behaviors have been distinguished. Low-Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies {theta}$_E$ = 8.3 K and {theta}$_E$ = 6.7 K for the PF$_6$-D12 and AsF$_6$-D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large-Bragg-angle measurements evidence an unexpected structural change around 55 K which probably corresponds to the linkage of the anions to the methyl groups via the formation of F...D-CD2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF)$_2$PF$_6$ is dominated by the librational motion of the PF$_6$ units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF6 librations: {theta}$_E$ = 50 K and {theta}$_E$ = 76 K for the PF$_6$-D12 and PF$_6$-H12 salts, respectively.
The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$ is a very exotic material with a superconducting order parameter whose ground state symmetry has remained ill-defined. Here we present a pulsed NMR Knight shift (K) study of $^{77}$Se measured simultaneously with transport in pressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the electron spin susceptibility $chi_s$, and is therefore a direct measure of the spin polarization in the superconducting state. For a singlet superconductor, the spin contribution to the Knight shift, K$_s$, falls rapidly on cooling through the transition. The present experiments indicate no observable change in K between the metallic and superconducting states, and thus strongly support the hypothesis of triplet p-wave superconductivity in (TMTSF)$_2$PF$_6$.
We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)$_2$PF$_6$. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge ordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened in the deuterated one where the transition temperature has increased by more than thirty percent. Anomalies in the dielectric function are also observed in the spin-Peierls ground state revealing some intricate lattice effects in a temperature range where both phases coexist. The variation of the spin-Peierls ordering temperature under magnetic field appears to follow a mean-field prediction despite the presence of spin-Peierls fluctuations over a very wide temperature range in the charge ordered state of these salts.
Using a combination of Density Functional Theory, mean-field analysis and exact diagonalization calculations we reveal the emergence of a dimerized charge ordered state in TMTTF$_2$-PF$_6$ organic crystal. The interplay between charge and spin order leads to a rich phase diagram. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase, which has been observed experimentally. The tendency to the dimerization is magnetically driven revealing TMTTF$_2$-PF$_6$ as a multiferroic material.
We report on a study of Seebeck coefficient and resistivity in the quasi-one-dimensional conductor (TMTSF)$_{2}$PF$_{6}$ extended deep into the Spin-Density-Wave(SDW) state. The metal-insulator transition at $T_{SDW}$ = 12 K leads to a reduction in carrier concentration by seven orders of magnitude. Below 1 K, charge transport displays the behavior known as Variable Range Hopping (VRH). Until now, the Seebeck response of electrons in this regime has been barely explored and even less understood. We find that in this system, residual carriers, hopping from one trap to another, generate a Seebeck coefficient as large as 400 $k_{B}$/$e$. The results provide the first solid evidence for a long-standing prediction according to which hopping electrons in presence of Coulomb interaction can generate a sizeable Seebeck coefficient in the zero-temperature limit.
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