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Rotationally Controlled van der Waals Heterosturctures


The isolation of graphene, a two-dimensional (2D) layer of carbon in 2004, and subsequently of other materials such hexagonal boron-nitride and transition metal dichalcogenide have opened up an entire new world of possibilities to design and realize van der Waals heterostructures, to create quantum materials with entirely new properties. A key ingredient that can add a new dimension to the atomic layer heterostructures palette is the rotational control, and alignment of different 2D layers. We are interested in developing new techniques to realize rotationally controlled van der Waals heterostructures of 2D materials, and to explore quantum transport and device applications of these materials.  Below are two examples from our current research.


Controlled Moiré Patterns of 2D Materials


Moiré patterns created by stacking van der Waals materials have opened up new opportunities to band engineering and collective quantum states in two-dimensional materials. At certain twist angles between the two layers, electrons can move with very low velocity, which stabilizes collective states such as superconductivity. We are investigating the exciting new physics of interacting electrons in these systems.

(A) Illustration of the local stacking in twisted bilayer graphene. (B) Schematic of a twisted TMD bilayer. (C) The first Brillouin zone (BZ) of each graphene layer (red, blue) in twisted bilayer graphene, along with the BZ (black) of the moiré pattern. (D) Optical micrograph and schematic of the Hall bar used to probe transport in a moiré pattern realized in twisted double bilayer graphene. (E) Contour plot of the resistance vs. top and bottom gate bias in a dual-gated twisted double bilayer graphene. Resistance maxima are observed at integer, and fractional moiré band filling factor.

Resonant Tunneling and Exciton Condensates in Double Layers of 2D Materials


Collective quantum states can form when two layers of electrons or holes are brought in close proximity. One example consists of two graphene layers separated by a thin tunneling barrier.  When the two graphene layers are tuned to equal and opposite charge densities, electrons in one layer can bind with holes in the opposite layer to form indirect excitons, which may condense at low temperatures. We have developed exquisite techniques to fabricate twist-controlled double layers of graphene or transition metal dichalcogenides with independent contacts to each layer.  The tunneling current-voltage characteristics between the two layers can serve as a powerful tool to probe energy and momentum tunneling, as well as many-body enhanced tunneling and other signatures of exciton condensates.

A) Illustration showing pairing of electrons and holes in separate graphene bilayers to form indirect excitons. (B) Schematic of the tunneling between two graphene bilayers separated by a WSe2 barrier. (C) Optical micrograph showing a graphene – Wse2 – graphene tunneling heterostructure encapsulated in hBN. (D) Transmission electron microscope image showing the individual atomic layers in a tunneling heterostructure. (E) Tunneling characteristic showing negative differential resistance.

Growth of Germanium, Silicon Nanowires and Core-Shell Heterostructures


Semiconductor nanowires offer a natural, quasi-one dimensional test-bed for electron physics in reduced dimensions and as a platform for electronic devices. Core-shell nanowires represent the quasi one-dimensional counterpart to the two-dimensional quantum well. We are exploring the growth of band- and strain-engineered Ge-SiGe and Si-SiGe core-shell nanowires in order to tailor the structure’s electronic properties. We have also grown modulation doped Ge-SiGe nanowires as a method to enhance the carrier mobility of this system.

Electron microscope images of core-shell nanowires and cut-away schematic of modulation doped core-shell nanowire





Last update 1/17/2020