Topological Andreev Rectification

in Physical Review. B (2023), 107

We develop the theory of an Andreev junction, which provides a method to probe the intrinsic topology of the Fermi sea of a two-dimensional electron gas (2DEG). An Andreev junction is a Josephson π ... [more ▼]

We develop the theory of an Andreev junction, which provides a method to probe the intrinsic topology of the Fermi sea of a two-dimensional electron gas (2DEG). An Andreev junction is a Josephson π junction proximitizing a ballistic 2DEG, and exhibits low-energy Andreev bound states that propagate along the junction. It has been shown that measuring the nonlocal Landauer conductance due to these Andreev modes in a narrow linear junction leads to a topological Andreev rectification (TAR) effect characterized by a quantized conductance that is sensitive to the Euler characteristic χF of the 2DEG Fermi sea. Here we expand on that analysis and consider more realistic device geometries that go beyond the narrow linear junction and fully adiabatic limits considered earlier. Wider junctions exhibit additional Andreev modes that contribute to the transport and degrade the quantization of the conductance. Nonetheless, we show that an appropriately defined rectified conductance remains robustly quantized provided large momentum scattering is suppressed. We verify and demonstrate these predictions by performing extensive numerical simulations of realistic device geometries. We introduce a simple model system that demonstrates the robustness of the rectified conductance for wide linear junctions as well as point contacts, even when the nonlocal conductance is not quantized. Motivated by recent experimental advances, we model devices in specific materials, including InAs quantum wells, as well as monolayer and bilayer graphene. These studies indicate that for sufficiently ballistic samples observation of the TAR effect should be within experimental reach. [less ▲]

Network model for periodically strained graphene

in Physical Review. B (2023), 107

The long-wavelength physics of monolayer graphene in the presence of periodic strain fields has a natural chiral scattering network description. When the strain field varies slowly compared to the ... [more ▼]

The long-wavelength physics of monolayer graphene in the presence of periodic strain fields has a natural chiral scattering network description. When the strain field varies slowly compared to the graphene lattice and the effective magnetic length of the induced valley pseudomagnetic field, the low-energy physics can be understood in terms of valley-polarized percolating domain-wall modes. Inspired by a recent experiment, we consider a strain field with threefold rotation and mirror sym- metries but without twofold rotation symmetry, resulting in a system with the connectivity of the oriented kagome network. Scattering processes in this network are captured by a symmetry- constrained phenomenological S matrix. We analyze the phase diagram of the kagome network, and show that the bulk physics of the strained graphene can be qualitatively captured by the network when we account for a percolation transition at charge neutrality. We also discuss the limitations of this approach to properly account for boundary physics. [less ▲]

Berry Curvature Spectroscopy from Bloch Oscillations

E-print/Working paper (2023)

We demonstrate that the Berry curvature of an isolated Bloch miniband in two-dimensional superlattices can be probed by the dressed linear optical response when a uniform static field is applied to the ... [more ▼]

We demonstrate that the Berry curvature of an isolated Bloch miniband in two-dimensional superlattices can be probed by the dressed linear optical response when a uniform static field is applied to the system. In particular, when the static field is sufficiently strong such that full Bloch oscillations occur before the crystal momentum relaxes to equilibrium, the optical response of the dressed system becomes resonant at the Bloch frequencies. The latter are in the THz regime when the superlattice periodicity is of the order of 10 nm. Using a band-projected semiclassical theory, we define a dressed optical conductivity and find that the height of the resonances in the dressed Hall conductivity are proportional to the Fourier components of the Berry curvature. We illustrate our results with a low-energy model on an effective honeycomb lattice. [less ▲]

Network model and four-terminal transport in minimally twisted bilayer graphene

in Physical Review. B (2021), 104