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How does an incommensurate system slide ? Role of dimensionality.


A Charge Density Wave (CDW) system is a system in which a periodic lattice distortion and a metal-insulator transition appear concomitantly. It is actually an electronic transition involving a modification of electrons close to the Fermi level, accompanied by a change of lattice structure.
When the periodic modulation is incommensurate, CDW systems display original dynamical properties. When they are submitted to a weak current, an additional current appears in the sample and a temporal study shows that it is made of spatially-correlated charges that reach the cathode periodically [1]. This phenomenon was first observed more than 35 years ago [2], but the nature of the involved charge carriers and their propagation mode are still not well known.
The use of coherent x-ray diffraction brought a lot in the comprehension of this phenomenon. First, these systems stabilize CDW dislocations that only affect the periodic modulation and not the atomic lattice host [3]. In addition, those dislocations move when the additional current appears, which suggests that these phase defects are involved in the charge carriage [4].


2kF satellite reflection associated to the CDW, measured by coherent x-ray diffraction a) in TbTe3 (bidimensional system made of tellurium square planes) and b) in the quasi-one dimensional system NbSe3, below and above the threshold current IS. For I>Is, the incommensurate CDW slides on the atomic lattice inducing a current made of spatially-correlated charges. c) Schematics of the CDW behaviour in real space (dots represents atoms, and blue lines the CDW wavefronts).

The other problematics concerns the origin of the additional current, especially how does an incommensurate wave pinned by defects can depin from the host lattice ? The modulation actually behaves like an elastic object in a disordered medium, similar to many other fields of solid state physics. It is also clear that this sliding motion is highly dependent on the system dimensionality. In a quasi-one dimensional system, made of weakly interacting atomic chains, the CDW gets strongly disordered for very weak currents by stabilizing very localized defects. This regime which is well-known in magnetic or plastic systems is called creep [5]. Above a threshold current IS, the CDW slides and recovers a long range order. In two-dimensional systems, like TbTe3, the CDW behavior is different. The CDW-lattice coupling is stronger and the CDW structure is unchanged below the threshold current. Only when I=IS does the CDW change abruptly, with a wave vector rotation. In that case, the CDW depins thanks to a shear [6] around defects.

[1] For a recent review : P. Monceau, Advances in Physics, 61, 325 (2012).
[2] R.M. Fleming and C.C. Grimes, Phys. Rev. Lett., 42, 1423 (1979).
[3] D. Le Bolloc’h, S. Ravy, J. Dumas, J. Marcus, F. Livet, Phys. Rev. Lett. 95, 116401 (2005).
[4] E. Pinsolle, N. Kirova, V.L.R. Jacques, A. Sinchenko, D.Le Bolloc’h, Phys. Rev. Lett. 109, 256402 (2012).
[5] D. Le Bolloc’h, A.A. Sinchenko, V.L.R. Jacques, L. Ortega, E. Lorenzo, G. Chahine, P. Lejay, P. Monceau, PRB accepted.
[6] D. Feinberg and J. Friedel, J. Physique (Paris) 49, 485 (1988)

Référence :

Effect of dimensionality on sliding charge density waves. The case of the quasi-two dimensional TbTe3 system probed by coherent x-ray diffraction.
D. Le Bolloc’h, A.A. Sinchenko, V.L.R. Jacques, L. Ortega, E. Lorenzo, G. Chahine, P. Lejay, P. Monceau
Phys. Rev. B 93, 165124 (2016).

Contacts :


David Le Bolloc’h
Vincent Jacques
Luc Ortega