Biaxial Strain Control of Electronic Properties of Quantum Materials – BISCEPS-QM
Tuning Charge Density Waves with biaxial strain at all temperatures
Developing a unique cryogenic biaxial tensile stress device to explore strained 2D materials in an unprecedented way
Strain Engineering to Reveal Novel Electronic Phases
The electronic properties of crystalline solids have driven major technological advances and are now central to societal challenges including green energy, batteries, healthcare, nanotechnology, and quantum electronics and information. Addressing these demands requires precise control of electronic band structures, particularly band gaps, at the core of gap engineering. Near the Fermi energy, both gaps and singularities (such as Dirac points in 2D systems) can emerge, while room-temperature superconductivity remains a key objective for lossless energy transport. Exploring these phenomena requires investigating broad regions of materials phase diagrams under controlled external parameters. Among fundamental collective states, charge density waves (CDWs) are directly linked to Fermi surface topology. Below a critical temperature, some metals undergo a periodic lattice distortion that opens an electronic gap and lowers the total energy. Although known for decades, CDWs have regained strong interest due to their proximity to superconducting domes in many materials, raising questions about coexistence or competition between ordered phases. CDWs originate from the pairing of low-energy electrons and holes separated by a nesting vector Q, often through a phonon-assisted mechanism, leading to a real-space modulation of the electronic density (Q = 2kF). Because the Fermi surface is determined by crystal structure, any structural distortion modifies nesting conditions and therefore CDW properties (transition temperature, wavevector, amplitude). Commensurability with the underlying lattice is crucial: incommensurate CDWs can slide and display non-linear transport, whereas commensurate CDWs are pinned and exhibit ohmic behavior. This project aimed to access novel electronic states by directly tuning crystal structure through controlled biaxial mechanical strain over a wide temperature range, from cryogenic to room temperature. The study focused on quasi-two-dimensional RTe₃ compounds (R = lanthanide), van der Waals materials hosting CDWs within tellurium planes. Depending on the rare-earth element, these systems exhibit slightly distorted rectangular structures and widely varying CDW transition temperatures (differences exceeding 100 K), with some compounds displaying two orthogonal in-plane CDWs at low temperature. The objective was to elucidate the relationship between atomic structure and stabilized CDW states, and to determine whether controlled strain could tune—or even induce—new electronic phases. The project led to the development of a novel cryogenic biaxial strain device, combined with structural probes (laboratory and synchrotron X-ray diffraction), electronic transport measurements, and theoretical modeling, enabling exploration of a three-dimensional phase diagram defined by temperature and strain along two orthogonal directions.
The project was based on the realization of a new device to perform biaxial mechanical deformation of quasi-2D systems at cryogenic temperatures, that should be compatible with several experimental probes: x-ray diffraction, electrical transport measurements and optical measurements.
We successfully designed, assembled and characterized this device, that is made of a cross-shaped chamber in which a helium flow cryostat is inserted, equipped with 4 motors that pull on a deformable kapton cross-shaped substrate sitting on the cold finger of tha cryostat and on which a 2D sample can be glued. We demonstrated that this device works perfectly, and characterized the mechanical deformation performances both by optical ways (digital image correlation) and x-ray diffraction on polycrystalline and crystalline samples. XRD measurements were both performed in the laboratory and at SOLEIL synchrotron, as the device is fully compatible with diffraction beamlines equipped with large diffractometers. Besides, we also characterized the performances at cryogenic temperatures, and proved that the deformed samples can be cooled down to 15K.
The strength of our device, compared to other existing device, is that it can deform samples biaxially, and that the deformation state can be directly measured on the studied samples by XRD looking at at least 3 non-colinear Bragg reflections. This approach was used in the project to precisely determine the evolution of structural parameters and link them to the observed changes in electronic properties. The latter were mainly adressed by electrical transport, following both the temperature dependence of the resistivities, and the V(I) curves to get the non-linear behaviours of our systems.
The project aimed at studying CDW structural properties under biaxial strain in RTe3 systems from cryogenic to room temperatures particularly adressing the question of whether the properties of the two orthogonal CDWs can be controlled with biaxial strain ?
All those questions were successfully answered in this project for TbTe3, using both x-ray diffraction (XRD) and transport measurements. This work led to a high-impact publication in which all the part are explained in details (A. Gallo–Frantz, V. L. R. Jacques, A. A. Sinchenko, D. Ghoneim, L. Ortega, P. Godard, P.-O. Renault, A. Hadj-Azzem, J. E. Lorenzo, P. Monceau, D. Thiaudière, P. D. Grigoriev, E. Bellec & D. Le Bolloc’h, Charge density waves tuned by biaxial tensile stress, Nature Communications 15, 3667 (2024), accessible online at www.nature.com/articles/s41467-024-47626-5).
We succeeded to follow the transition temperatures of TbTe3 and ErTe3 as a function of the in-plane asymmetry a/c measured by following 3 non-colinear Bragg reflections by XRD. We could also follow the satellite reflections appearing in reciprocal space and associated to the charge density wave along c (CDWc) and along a (CDWa) as a function of the in-plane asymmetry, and could directly conclude that if the system displays a CDWc only in the pristine state for which ac induces a reorientation of the CDW along a: CDWc disappears and CDWa appears. For a=c, we find a coexistence state. This also clearly appears in transport measurements, were we observe a clear inversion of the behaviour of the resistivities measured along a and c went changing the asymmetry parameter a/c. In addition to the inversion of transport signature along a and c as a function of in-plane structure asymmetry a/c, visible through the amplitude of the resistivity kink at the transition, we also observe that the transition temperatures evolve with the in-plane deformation (temperature at which the kink appears). The CDW reorientational transition takes place exactly when a=c. We can also have an idea of the evolution of the CDW gaps magnitude as they should be proportional to the square root of the measured intensity in XRD. From our measurements, we should have a saturation of the CDW gaps, but also a gap magnitude that is smaller for the strain-induced CDWa compared to the pristine CDWc. In addition, the Tc do not saturate, but diverge linearly as a function of a/c, with an impressive increase of more than 30K in the probed range.
We also studied the non-linear properties of TbTe3 as a function of deformation. By measuring the V(I) curves, we can get the dV/dI(I) curves, on which the sliding phenomenon appear as a clear drop of differential resistance. When forces are applied along a or c crystallographic directions, the in-plane crystal asymmetry a/c is changed, and the threshold voltages also evolve.
This work opens new questions and thus new experimental perspectives: is the gap vlaue saturating in the CDWa and CDWc phase of TbTe3? For this, we need to probe the gap in the strained state of these systems.
Similarly, we still do not know how the orthogonal CDW can coexist when a=c in TbTe3. We need to perform experiments, using for instance micro-XRD, to determine the spatial arrangement of both CDWs in this state.
Finally, the perspectives are very large in the exploration of other 2D systems of interest. For instance the mutli-q CDW transition metal dichalcogenides display a differetn symmetry, and determining whether the mechanism of CDW reorientational transition is the same would be highly interesting. The link to superconductivity and CDW would also be interesting to study as a function of deformation, especially in relation to the phonons.
The electronic properties of materials are intimately linked to their chemical composition and crystallographic structure. Understanding the close link between them is thus of prime importance, especially in systems displaying interesting phases both at the fundamental and applied levels. Quantum materials belong to such promising systems that display a variety of interesting phases and in which, in the last years, the application of uniaxial mechanical strain allowed to start modifying them. However, the best would be a biaxial control of the crystallographic structures at various temperatures to explore vast unexplored regions of phase diagrams. This ANR project aims at taking this challenge, and explore the temperature/biaxial strain phase diagram of quasi-2D quantum materials that display charge orders and superconductivity, to observe and control these phase transitions at will and eventually discover new phases.
Project coordination
Vincent Jacques (Laboratoire de Physique des Solides)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partnership
LPS Laboratoire de Physique des Solides
NEEL Institut Néel
IRE-RAS / Kotelnikov Institute of Radio-engineering and Electronics of RAS
Pprime Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique
Help of the ANR 301,622 euros
Beginning and duration of the scientific project:
January 2022
- 36 Months
