The importance of shear on the collective charge transport in CDWs revealed by an XFEL source

TL;DR

This study uses XFEL to reveal how shear influences collective charge transport in sliding charge density waves, showing shear precedes longitudinal deformation and surface strain accumulation, highlighting the role of structural changes in transport.

Contribution

It demonstrates the use of XFEL and genetic algorithms to analyze structural dynamics in CDWs, revealing the sequence of shear and longitudinal deformations during sliding.

Findings

Shear deformation occurs before longitudinal deformation.

Surface steps accumulate strain, indicating pinning effects.

Sliding involves large-scale spatial coherence of CDWs.

Abstract

Charge transport in materials has an impact on a wide range of devices based on semiconductor, battery or superconductor technology. Charge transport in sliding Charge Density Waves (CDW) differs from all others in that the atomic lattice is directly involved in the transport process. To obtain an overall picture of the structural changes associated to the collective transport, the large coherent X-ray beam generated by an X-ray free-electron laser (XFEL) source was used. The CDW phase can be retrieved over the entire sample from diffracted intensities using a genetic algorithm. For currents below threshold, increasing shear deformation is observed in the central part of the sample while longitudinal deformation appears above threshold when shear relaxes. Shear thus precedes longitudinal deformation, with relaxation of one leading to the appearance of the other. Moreover, strain accumulates on surface steps in the sliding regime, demonstrating the strong pinning character of these surface discontinuities. The sliding process of nanometric CDW is based on an impressive spatial coherence involving the macroscopic sample dimensions.