Anisotropy and Field-Dependence of the Spin-Density-Wave Dynamics in the Quasi One-Dimensional Conductor (TMTSF)_2PF_6

TL;DR

This study investigates the anisotropic and non-linear spin-density-wave dynamics in (TMTSF)_2PF_6, revealing collective transport in multiple directions, the presence of pinned modes, and new excitations at low temperatures.

Contribution

It provides new insights into the anisotropic collective transport and non-linear behavior of spin-density waves in (TMTSF)_2PF_6, including the discovery of a long relaxation time mode below 4 K.

Findings

Collective transport occurs in all three axes, including the perpendicular b' direction.

Pinned mode resonance is observed in the a and b' axes but not along c*.

A new long relaxation time mode appears below 4 K, linked to discommensurations.

Abstract

The anisotropic and non-linear transport properties of the quasi one-dimensional organic conductor (TMTSF)_2PF_6 have been studied by dc, radiofrequency, and microwave methods. Microwave experiments along all three axes reveal that collective transport, which is considered to be the fingerprint of the spin-density-wave condensate, also occurs in the perpendicular b' direction. The pinned mode resonance is present in the $a$ and b'-axes response, but not along the least conducting c* direction. The ac-field threshold, above which the spin-density-wave response is non-linear, strongly decreases as the temperature drops below 4 K. With increasing strength of the microwave electric field and of the radiofrequency signal, the pinned mode and the screened phason loss-peak shift to lower frequencies. In the non-linear regime, in addition to the phason relaxation mode with Arrhenius-like resistive decay, an additional mode with very long and temperature-independent relaxation time appears below 4 K. We attribute the new process to short-wavelength spin-density-wave excitations associated with discommensurations in a random commensurate N=4 domain structure.