Person:

Kim, Philip

Interferometers probe the wave-nature and exchange statistics of indistinguishable particles, for example electrons in the chiral one-dimensional edge channels of the quantum Hall effect (QHE). Quantum point contacts can split and recombine these channels, enabling interference of charged particles. Such quantum Hall interferometers (QHIs) can unveil the exchange statistics of anyonic quasiparticles in the fractional quantum Hall effect (FQHE). Here, we present a fabrication technique for QHIs in van der Waals (vdW) materials and realize a tunable, graphene-based Fabry-Pérot (FP) QHI. The graphite encapsulated architecture allows observation of FQHE at 3T magnetic field and precise partitioning of integer and fractional edge modes. We measure pure Aharonov-Bohm interference in the integer QHE, a major technical challenge in small FP interferometers, and find that edge modes exhibit high visibility interference due to large velocities. Our results establish vdW heterostructures as a versatile alternative to GaAs-based interferometers for future experiments targeting anyonic quasiparticles.

Reducing the energy bandwidth of electrons in a lattice below the long-range Coulomb interaction energy promotes correlation effects. Created by stacking van der Waals (vdW) heterostructures with a controlled twist angle1–3, moire ́ superlattices enable the engineering of electron band structure. In an engineered moire ́ flat band, exotic quantum phases can emerge. The correlated insulator, superconductivity, and quantum anomalous Hall ef- fect found in the flat band of the magic angle twisted bilayer graphene (MA-TBG) 4–8 have sparkled exploration of correlated electron states in other moire ́ systems 9–11. The electronic properties of vdW moire ́ superlattices can further be tuned by adjusting the interlayer cou- pling 6 or the band structure of constituent layers 9. Here, employing vdW heterostructures of twisted double bilayer graphene (TDBG), we demonstrate a flat electron band that is tun- able by perpendicular electric fields in a range of twist angles. Similar to the MA-TBG, TDBG exhibits energy gaps at the half- and quarter-filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the half-filled insulator, a sudden drop of resistivity is observed with lowering temperature. This critical behavior is confined in a small area in the density-electric field plane, and is attributed to a phase transition from a normal metal to a spin-polarized correlated state. Spin-polarized correlated states discovered in the electric field tunable TDBG provide a new route to engineering interaction-driven quantum phases.

Sensitive microwave detectors are critical instruments in radioastronomy, dark matter, axion searches , and superconducting quantum information science. The conventional strategy towards higher-sensitivity bolometry is to nanofabricate an ever-smaller device to augment the thermal response. However, this direction is increasingly more difficult to obtain efficient photon and maintain the material properties in a device with a large surface-to-volume ratio. Here we advance this concept to an ultimately thin bolometric sensor based on monolayer graphene. To utilize its minute electronic specific heat and thermal conductivity, we develop a superconductor-graphene-superconductor (SGS) Josephson junction 8–13 bolometer embedded in a microwave resonator of resonant frequency 7.9 GHz with over 99% coupling efficiency. From the dependence of the Josephson switching current on the operating temperature, charge density, input power, and frequency, we demonstrate a noise equivalent power (NEP) of 7 ×10−19 W/Hz1/2, corresponding to an energy resolution of one single photon at 32 GHz 14 and reaching the fundamental limit imposed by intrinsic thermal fluctuation at 0.19 K.

Van der Waals heterostructures obtained via stacking and twisting have been used to create moiré superlattices, enabling new optical and electronic properties in solid-state systems. Moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping, host Mott insulating and superconducting states, and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation. However, due to the nanoscale sizes of moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties could not be systematically investigated. Here, we use near 0o twist angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains to directly probe excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Γ-K interlayer excitons can be flipped with electric fields, while higher-energy K-K interlayer excitons undergo field-asymmetric hybridization with intralayer K-K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically nontrivial systems, exotic metasurfaces, collective excitonic phases, and quantum emitter arrays via domain-pattern engineering.