Theory of charge density wave depinning by electromechanical effect

Abstract

We discuss the first theory for the depinning of low dimensional, incommensurate, charge density waves (CDWs) in the strong electron-phonon (e-p) regime. Arguing that most real CDWs systems invariably develop a gigantic dielectric constant (GDC) at very low frequencies, we propose an electromechanical mechanism which is based on a local field effect. At zero electric field and large enough e-p coupling the structures are naturally pinned by the lattice due to its discreteness, and develop modulation functions which are characterized by discontinuities. When the electric field is turned on, we show that it exists a finite threshold value for the electric field above which the discontinuities of the modulation functions vanish due to CDW deformation. The CDW is then free to move. The signature of this pinning/depinning transition as a function of the increasing electric field can be directly observed in the phonon spectrum by using inelastic neutrons or X-rays experiments.

On the intrinsic pinning and shape of charge-density waves in 1D Peierls systems

“…There is an Aubry transition from a high-pressure unpinned phase to a low-pressure pinned phase. This is similar to that occurring in the SSH model as function of pressure [39,42] or electric field via an electromechanical effect [43]. The corresponding band structures are shown in Fig.…”

“…15 (bottom) that the envelope function is no longer continuous but has gaps at several places. This is the breaking of analyticity of Aubry, which occurs in other similar models [39,42,43]. This result is independent on the fractional approximant of c as can be seen in Fig.…”

“…The BDK approach, through these equations, fixes the entire optimal band structure. This is different from the Peierls approach which introduces the Ansatz (43) and minimizes the energy with respect to the single small parameter u or, equivalently, the Peierls gap. The equations ( 45) replace the standard gap equation ( 44).…”

“…It modifies the physics of CDW systems [42]. In particular, the CDW electrodynamics can hardly be described in the stochastic regime by continuous models, owing to the intrinsic pinning of the CDW and the presence of many metastable states separated by local energy barriers [43]. Thanks to the similarity between the problem of the CDW energy minimization and the time evolution of a discrete dynamical system introduced by Aubry [38], the pinning is associated with the destruction of the KAM tori of the dynamical system, when the nonlinearities become strong enough.…”

“…They are characterized by the existence of ordered local atomic structures, which are dimers or oligomers. The transition between the two phases appears not only by breaking the integrability, but can also be triggered by an external pressure, making it experimentally possibly relevant, either by applying a real mechanical pressure or an external electric field [43]. These phases, appearing in various models and in a wide range of parameters, are ubiquitous.…”