Atomically thin materials offer a transformative frontier in materials science and physics. Single atomic sheets can be stacked without restriction, forming heterostructures with unprecedented quantum properties, tunability, and functionality. For example, changing the relative twist between layers gives rise to a moiré pattern: a new length scale that hosts a new electronic structure. Within moiré crystals, flat electronic bands feature interactions that can be widely different from the original materials, enabling the emergence of new quantum states. Ultimately, 2D crystals and synthetic heterostructures open a pathway to explore collective particle interactions and dynamics of quantum matter not observable in naturally occurring materials.
We are addressing the following challenges:
1. Autonomous Fabrication of Designer 2D Quantum Materials and Devices: 2D heterostructure devices consisting of only a handful of individual atomic layers have led to extraordinary breakthroughs in photonics, electronics, and quantum materials. A tantalizing dream is to go beyond these few-layer systems and construct designer materials comprised of tens, hundreds, or even thousands of atomic layers, each precisely chosen and placed. Unfortunately, state-of-the-art layer-by-layer fabrication of such structures is tedious, low-yield, and not reproducible at the quantum level. We are developing a fully autonomous 2D Pilot Line to assemble heterostructures with full control of crystal stacking symmetries, interlayer interactions, and twist angle in a reproducible and deterministic fashion. Ultimately, we aim to realize scalable moiré materials, with reproducibility at the quantum level, such that programmable three-dimensional moiré solids can be realized.
2. Advanced Metrology of Designer 2D Quantum Materials: New metrology techniques are crucial to understanding the interplay between the structural, electronic, magnetic, and optical properties of new quantum materials. Before characterizing emergent states such as superconductivity or exotic magnetism, understanding atomic interfaces and moiré superlattice details is paramount: atomic arrangement encodes the local interlayer interactions, and lattice homogeneity is critical for robust emergent phases. Further, atomic relaxation into commensurate interface domains prevails in the limit of long moiré or under strain. We are establishing robust links between correlative atomic force microscopy (including lateral force, conductive, piezoelectric force, and Kelvin probe force), a slow but highly precise structural analysis tool, and much faster optical spectroscopy techniques (including Raman, second harmonic generation, absorption, photoluminescence, and ellipsometry) which can “remotely” and non-destructively probe atomic interfaces.
3. Probing quantum materials and emergent phase diagrams from mK to room temperature. Determining phase diagrams, in particular superconducting phases, is a significant challenge. Quantum transport (e.g. to show zero resistance) is highly valuable but challenging with 2D semiconductors due to ohmic contact difficulties. Bulk probes (specific heat or thermal conductivity) are complicated by sample inhomogeneity and dominant substrate contribution for atomically thin devices. Our approach is to combine our magneto-optical spectroscopy expertise with a suite of spin-based quantum sensing techniques.
With the world’s first commercial low-temperature scanning probe quantum sensor and a homemade widefield quantum sensor implemented in a dilution refrigerator, we would like to answer key questions in condensed matter physics in regards to superconductivity, charge density waves, spin waves, exotic magnetic textures, and even the elusive quantum spin liquid (by measuring spin bath correlations). For example, quantum sensing enables the detection of magnetic noise via spin relaxation and dephasing measurements which can help unravel the nature of different phases and phase transitions in quantum materials. We complement our quantum sensing tools with magneto-optical spectroscopy techniques, such as differential optical reflection measurements (ΔR/R) as a function of doping, reflective magnetic circular dichroism (RMCD), exciton-polaron spectroscopy, and ultrafast optical spectroscopy
4. In-situ tuning of moiré geometry, symmetry, and periodicity. Via partnership with Razorbill Instruments, we are developing cryogenic in-situ tuning tools to control the relative twist (for continuous tuning of moiré periodicity and thus electron-electron interactions) and heterostrain of 2D heterostructures, which enables continuous adjustment of the moiré symmetry (e.g. from triangular to asymmetric triangle to 1D stripe) and the precise alignment between vertically stacked layers which can lead to ferroelectricity.