Research
Interfaces are natural places to look for interesting phenomena: life itself is found near the interface between Earth's surface and its atmosphere. In condensed matter systems, lattices with different electronic properties and lattice periodicities merge at an interface. In the most interesting systems, new interfacial properties foreign to both parent compounds can emerge.
Some of the most interesting electronic states of matter are found in interfaces that naturally occur in layered single crystals. For example, the copper-, iron- and nickel-based high-temperature superconductors are all comprised of stacks of repeating atomic planes. In the copper-oxide high-temperature superconductors in particular, a rich collection of correlated electronic phases arise on a sheet of copper and oxygen atoms arranged in a square lattice. This layer, in turn, is chemically stabilized by other crystalline layers which surround it, whose contribution of oxygen dopant atoms sets the charge density of the copper-oxide layer and determine its macroscopic electronic properties.
A subset of layered crystals can be mechanically cleaved into atomically thin sheets, thanks to extremely weak bonding between neighboring crystal planes. Such van der Waals (vdW) materials can be deterministically stacked, like a deck of cards, into new, thermodynamically out-of-equilibrium structures of arbitrary complexity. Chemically inert vdW materials such as graphene and hexagonal boron nitride (hBN) are now routinely stacked into functional nanoscale devices exhibiting emergent physics entirely foreign to the parent material systems.
Air-Sensitive Device Fabrication
A key focus of our lab is to bring these device fabrication ideas to new vdW material systems with new electronic properties. These materials, such as the vdW high-temperature superconductor Bi2Sr2CaCu2O8+x, can be extremely sensitive to moisture and heat. This renders traditional vdW assembly and nanofabrication techniques ineffective.
We have invented a set of techniques to bring the entire vdW assembly and nanofabrication pipeline into argon or vacuum environments at or below room temperature. We freeze out chemical degradation pathways on temporarily exposed surfaces during stacking by using cryogenic exfoliation and stacking techniques. We also shift the photolithography to an optically transparent silicon nitride mask, whose pattern can be transferred to the air-sensitive device without any solvent or air exposure. Finally, we wirebond the device inside the glovebox and seal up the package in argon for subsequent measurement.
Single-Crystal Synthesis
The techniques above were created for the cuprate high temperature superconductor Bi2Sr2CaCu2O8+x, which is extremely sensitive to moisture, but it can be used for any vdW crystal. In our lab, we grow new single-crystals to find quantum materials which are interesting in their own right, as well as for integration into vdW heterostructures. Single crystal growth is highly complementary to vdW device fabrication. While each layer in a vdW heterostructure can be deterministically controlled, the lattice structure within each layer must be controlled via crystal growth techniques. By combining the expertise to do both in the same group, we aim to control as much of the microscopic structures of crystals as possible in order to create new emergent quantum properties at atomically clean interfaces.
