Oleg P. Sushkov

Coulomb interactions among graphene's massless Dirac fermions are unusually strong, making graphene a unique testbed for correlated quantum phases and interacting field theories. Among the predicted phases is an excitonic insulator, in which strong interactions drive spontaneous electron-hole pairing and condensation into a superfluid ground state. This transition has evaded experimental observation because unscreened long-range Coulomb interactions renormalize and enhance the Dirac velocity, keeping graphene in a weakly coupled semimetallic regime. Here, we overcome this constraint by decorating graphene with nanoscale metals that screen long-range interactions and suppress velocity renormalization. Scanning tunnelling spectroscopy down to 150 mK reveals a hard spectral gap (2Δ ≈ 14 meV) that emerges strictly at the Dirac point and is strongly susceptible to doping, temperature, and magnetic field --- signatures of a many-body rather than single-particle gap. The gap closes at an anomalously low critical temperature (Δ/ k _B T _c ≥ 100), consistent with a Berezinskii--Kosterlitz--Thouless transition, and coincides with a charge-density wave at q = K' - K that reflects intervalley excitonic pairing. Together, these results provide strong evidence for spontaneous excitonic mass generation in graphene, a correlated insulating phase long predicted but never before observed.

The electronic properties of solids are determined by their crystal structure and electron interactions, giving rise to phenomena such as superconductivity, strange metals and correlated insulators. Many of these effects remain poorly understood, motivating efforts to create artificial crystals that mimic real materials while allowing controlled tuning of key parameters. Cold atoms in optical lattices offer flexibility but cannot reproduce the long-range Coulomb interactions and hopping present in solids. Solid-state systems naturally support these features, although they suffer from tunability and flexibility issues. Here we demonstrate a highly tunable artificial crystal formed by superimposing a periodic electrostatic potential onto a two-dimensional electron gas in a shallow GaAs quantum well. This engineered lattice exhibits a band structure characteristic of the artificial triangular lattice, distinct from that of the underlying cubic crystal. Electronic transport measurements show a sign change in the Hall coefficient as the chemical potential sweeps through the artificial bands. The band structure can be continuously tuned to realize linear graphene-like and flat kagome-like bands within a single device. A strong insulating state emerges at half filling of the kagome flat band, consistent with interaction-driven behaviour. This tunability provides an opportunity to explore correlated quantum states in a controlled setting.

In sufficiently clean materials where electron-electron interactions are strong compared to momentum-relaxing scattering processes, electron transport resembles the flow of a viscous fluid. We study hydrodynamic electron transport across density interfaces (n-n junctions) in a 2DEG in the Corbino geometry. From numerical simulations in comsol using realistic parameters, we show that we can produce tunable viscous layers at the density interface by varying the density ratio of charge carriers. We quantitatively explain this observation with simple analytic expressions together with boundary conditions at the interface. We also show signatures of these viscous layers in the magnetoresistance. Breaking down viscous and Ohmic contributions, we find that when the outer radial region of the Corbino has higher charge density compared to the inner region, the viscous layers at the interface serve to suppress the magnetoresistance produced by momentum-relaxing scattering. Conversely, the magnetoresistance is enhanced when the inner region has higher density than the outer. Our results add to the repertoire of techniques for engineering viscous electron flows, which hold a promise for applications in future electronic devices.