The magnetoresistance of the MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state, and after brief illumination by a red-light diode at low temperature, T is approximately 0.3 K. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also the coupling among 2D electron layers in neighboring quantum wells has been reduced.
The influence of illumination on a high mobility two-dimensional electron gas with high concentration of charge carriers is studied in selectively doped single GaAs quantum wells with short-period AlAs/GaAs superlattice barriers at a temperature T = 4.2 K in magnetic fields B < 2 T. It is shown that illumination at low temperatures in the studied heterostructures leads to an increase in the concentration, mobility, and quantum lifetime of electrons. An increase in the quantum lifetime due to illumination of single GaAs quantum wells with modulated superlattice doping is explained by a decrease…
We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhance…
Flat bands emerging in buckled monolayer graphene superlattices have been recently shown to realize correlated states analogous to those observed in twisted graphene multilayers. Here, we demonstrate the emergence of valley topology driven by competing electronic correlations in buckled graphene superlattices. We show, both by means of atomistic models and a low-energy description, that the existence of long-range electronic correlations leads to a competition between antiferromagnetic and charge density wave instabilities, that can be controlled by means of screening engineering. Interestingl…
We propose Landau levels as a probe for the topological character of electronic bands in two-dimensional moiré superlattices. We consider two configurations of twisted double bilayer graphene (TDBG) that have very similar band structures, but show different valley Chern numbers of the flat bands. These differences between the AB-AB and AB-BA configurations of TDBG clearly manifest as different Landau level sequences in the Hofstadter butterfly spectra calculated using the tight-binding model. The Landau level sequences are explained from the point of view of the distribution of orbital magneti…
We outline an approach to endow a plain vanilla material with topological properties by creating topological bands in stacks of manifestly nontopological atomically thin materials. The approach is illustrated with a model system comprised of graphene stacked atop hexagonal-boron-nitride. In this case, the Berry curvature of the electron Bloch bands is highly sensitive to the stacking configuration. As a result, electron topology can be controlled by crystal axes alignment, granting a practical route to designer topological materials. Berry curvature manifests itself in transport via the valley…
The amplitude V_0 of unidirectional periodic potential modulation introduced by a surface grating into a two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs heterointerface is measured as a function of electron density n_e by analyzing commensurability oscillation of the magnetoresistance. The electron density is varied either by applying a bias to a metallic back gate or by illumination. The amplitude decreases with increasing density, with the rate |dV_0/dn_e| roughly an order of magnitude larger for the former method. The result is interpreted in terms of the rate, dE_1/d(delta E_c), …
'Magic'-angle twisted bilayer graphene has received a lot of interest due to its flat bands with potentially non-trivial topology that lead to intricate correlated phases. A spectrum with flat bands, however, does not require a twist between multiple sheets of van der Waals materials, but rather can be realized with the application of an appropriate periodic potential. Here, we propose the imposition of a tailored periodic potential onto a single graphene layer through local perturbations that could be created via lithography or adatom manipulation, which also results in an energy spectrum fea…
Twisted moiré superlattices (TMSs) are fascinating materials with exotic physical properties. Despite tremendous studies on electronic, photonic and phononic TMSs, it has never been witnessed that TMSs can exhibit higher-order band topology. Here, we report on the experimental observation of higher-order topological states in acoustic TMSs. By introducing moiré twisting in bilayer honeycomb lattices of coupled acoustic resonators, we find a regime with designed interlayer couplings where a sizable band gap with higher-order topology emerges. This higher-order topological phase host unique topo…
Twisting van der Waals heterostructures to induce correlated many-body states provides a novel tuning mechanism in solid-state physics. In this work, we theoretically investigate the fate of the surface Dirac cone of a three-dimensional topological insulator subject to a superlattice potential. Using a combination of diagrammatic perturbation theory, lattice model simulations, and ab initio calculations we elucidate the unique aspects of twisting a single Dirac cone with an induced moiré potential and the role of the bulk topology on the reconstructed surface band structure. We report a dramat…
In this work we discuss particular effects that take place in systems of quantum emitters coupled to two-dimensional bosonic topological insulators. For a single emitter coupled to the Haldane model, we find a "fragile" quasibound state that makes the emitter dynamics very sensitive to the model's parameters, and gives rise to effective long-range interactions that break time-reversal symmetry. We then discuss one-dimensional arrangements of emitters, emitter line defects, and how the topology of the bath affects the effective polariton models that appear in the weak-coupling regime when the e…
Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, correlated-electron physics hosted by flat bands in small-angle moiré systems has been at the foreground. However, the implications of moiré band topology within the single-particle framework are little explored experimentally. An outstanding problem is understanding the effect of band topology on Hofstadter physics, which does not require electron correlations. Our work experimentally studies Chern state switching in the Hofstadter regime using twisted double bilaye…
