Experimental Studies of Correlated Electron States in Van der Waals Materials

The power of materials to produce transformative technologies has fueled an age-old search for new compounds and for techniques to endow existing ones with desirable properties. Traditionally, materials discovery has been the result of serendipity or painstaking exploration of a large phase space of chemically synthesized compounds. A new era started in 2004 with the breakthrough isolation of the first free standing two-dimensional (2D) material, graphene, and the discovery of a slew of novel physical properties which are otherwise absent in its bulk counterpart. Dozens new 2D materials were realized since and many more predicted theoretically. Strategies to assemble heterostructures by stacking various 2D layers, have made it possible to stabilize van der Waals materials characterized by strong intra-plane bonds and weak inter-plane interactions, that could not have been synthesize by standard chemical means. One of their distinctive characteristics, which is due to all the atoms residing at the surface, is that it is possible to manipulate the electronic properties with non-chemical 'knobs' such as electrostatic gating or substrate engineering.Thus far, the most successful technique to engineer electronic properties in van der Waals materials has been to introduce a twist between the crystallographic axes of overlayed 2D crystals. This creates a moiré superstructure that radically alters the electronic properties of the material, leading to exotic properties such as topological insulators, orbital magnetism, and nematic superconductivity. However, contrary to expectations, the correlated phases in these 2D moiré materials are quite fragile and cannot survive beyond single digit temperatures above absolute zero.We will explore alternative strategies to achieve robust correlated states. The approach is based on our recent findings that a spatially periodic compressive strain induced by a buckling transition in a graphene membrane, produces flat electronic bands which are much less susceptible to sample inhomogeneity than the moiré materials. The periodic strain patterns will be induced either by substrate engineering, by controlled buckling transitions, or by designing pillar arrays on which the 2D membranes will be stretched to create periodic strain patterns with prescribed geometries. Van der Waals materials to be explored include graphene, MoS2, NbSe2, WSe2, TaS2 and Bi-221. We will employ electrostatic gating to induce the correlated electronic states by aligning the Fermi energy with the flat bands. The emergent correlated states will be characterized by scanning tunneling microscopy (STM) scanning tunneling spectroscopy (STS), Landau level spectroscopy, electrical and thermal transport.