Strain-Induced Proliferation of Electronic Orders in Quasi-2D Superconductors (Part II)

Quantum materials are model systems to study the interplay between competing orders, with many nearly-degenerate ground states in close proximity in their phase diagrams. Local perturbations such as defects and/or inhomogeneous strains can have a strong influence on the resulting phases, seeding or suppressing certain orders over others. Mapping such rich phase diagrams necessitates the use of external fields (electric, magnetic, strain) to perturb the system, allowing it to crystallize into other proximal states. One such method is strain, which has the unique ability to directly alter electron-phonon coupling and, in the limit of large deformations, create defects in the crystal lattice. 

In Part II of this talk, we demonstrate enormous (over ~10%) inhomogeneous local in-plane strains in quasi-2D van der Waals crystals and elucidate their influence on charge-ordered states via scanning tunneling microscopy/spectroscopy (STM/STS). We access the far reaches of the phase diagram, and uncover the behavior of these crystals near the limit of their yield strength. By uniaxially straining along particular crystal axes of NbSe2, we observe deterministic stabilization of specific charge-ordered phases (1x4 or 2x2 CDW), in agreement with previous work using differential thermal contraction [1]. Atomic-scale investigation of these states and the boundaries between them gives us insight into the nature and interplay of quantum phases in the limit of extreme perturbation. 

[1] Gao et al., PNAS 115, 6986 (2018)