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The precise arrangement and nature of atoms drive electronic phase transitions in condensed matter. To explore this tenuous link, we developed a true biaxial mechanical deformation device working at cryogenic temperatures, compatible with x-ray diffraction and transport measurements, well adapted to layered samples. Here we show that a slight deformation of TbTe 3 can have a dramatic influence on its Charge Density Wave (CDW), with an orientational transition from c to a driven by the a / c parameter, a tiny coexistence region near a  =  c , and without space group change. The CDW transition temperature T c displays a linear dependence with a / c − 1 while the gap saturates out of the coexistence region. This behaviour is well accounted for within a tight-binding model. Our results question the relationship between gap and T c in RTe 3 systems. This method opens a new route towards the study of coexisting or competing electronic orders in condensed matter. Previous studies of the effects of strain on charge density waves have mostly focused on uniaxial strain. Here the authors use a biaxial-strain device to demonstrate switching of the charge density wave orientation, as well as a strong linear increase of the transition temperature while the gap seems to saturate.

As part of the ongoing search for new material properties that could present fundamental and application interest, efforts are constantly being made to create new materials and study them under new experimental conditions. To do so, phase diagrams need to be studied as exhaustively as possible. Several parameters are commonly modified to access the physical parameters of materials: temperature, pressure, magnetic and electric fields, doping or chemical substitution, etc. In this work, we will follow a new, original and promising route to access the new electronic properties of crystalline materials, by carrying out biaxial deformation at cryogenic and at room temperature, and by going beyond current achievements in this field. Indeed, the electronic properties are intimately linked to the atomic structure of crystals, and any deviation from this can lead to radically different properties, such as a shift in phase transition temperatures, or even lead to new atomic or electronic phases. We will apply this methodology to materials with electronic vacancies, as they exhibit fantastic and diverse electronic properties relevant to applications: semiconductors, superconductors and metal-insulator systems in general. More specifically, we will study new phases in quasi-2D systems, the RTe3 (R = rare earth) which already exhibit charge density waves (CDW) and superconductivity (SC) in the (T,P) phase diagram, and we will study the evolution of transition temperatures as a function of biaxial tensile stress. To this end, a biaxial stress device operating at low temperature and compatible with X-ray diffraction, in-situ transport measurements and optical measurements has been developed at the Laboratoire de Physique des Solides. Initially, this new device will be assembled and characterised, and initial biaxial deformation tests will be carried out, following the deformation of samples by X-ray diffraction. Next, the deformations will be induced in the samples, where the structural parameters will be measured, as well as the parameters related to CDW such as the transition temperature. Finally, changes in resistivity will be followed using in-situ transport measurements to obtain information on changes related to the geometry of the Fermi surface.

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.