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 ▲]

Origin of nonlinear magnetoelectric response in rare-earth orthoferrite perovskite oxides

in Physical Review. B (2022), 105

Origin of negative electrocaloric effect in Pnma-type antiferroelectric perovskites

in Physical Review. B (2022)

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

in Physical Review. B (2021), 104

Kerr effect in tilted nodal loop semimetals

in Physical Review. B (2021), 104(12), 125411

We investigate the optical activity of tilted nodal loop semimetals. We calculate the full conductivity matrix for a band structure containing a nodal loop with possible tilt in the x−y plane, which ... [more ▼]

We investigate the optical activity of tilted nodal loop semimetals. We calculate the full conductivity matrix for a band structure containing a nodal loop with possible tilt in the x−y plane, which allows us to study the Kerr rotation and ellipticity both for a thin film and a bulk material. We find signatures in the Kerr signal that give direct information about the tilt velocity and direction, the radius of the nodal loop and the internal chemical potential of the system. These findings should serve as guide to understanding optical measurements of nodal loop semimetals and as an additional tool to characterize them. [less ▲]

Electron hydrodynamics of anomalous Hall materials

in Physical Review. B (2021), 103(12), 125106

We study two-dimensional electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological ... [more ▼]

We study two-dimensional electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological materials. For small electric fields, the Hall current becomes negligible compared to the viscous longitudinal current. In this regime, we highlight an unconventional Poiseuille flow with an asymmetric profile and a deviation of the maximum of the current from the center of the system. In a two-dimensional infinite geometry, the Berry curvature leads to current whirlpools and an asymmetry of potential profile. This phenomenon can be probed by measuring the asymmetric non-local resistance profile. [less ▲]

Collective electronic excitations in charge density wave systems: The case of CuTe

in Physical Review. B (2021), 104(12), 125101

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW ... [more ▼]

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative. [less ▲]

First-principles description of the exciton-phonon interaction: A cumulant approach

in Physical Review. B (2020), 102

Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic ... [more ▼]

Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic equations of many-body perturbation theory, we derived a cumulant formulation for neutral excitation spectra that contains excitonic effects and the coupling between excitons and phonons. The cumulant approach allows us to include dynamical effects arising from the electron-phonon coupling in a simple and intuitive way. It can be implemented as a postprocessing of state-of-the-art GW-plus-Bethe-Salpeter calculation of excitonic states and a density functional perturbation theory calculation of phonons and electron-phonon coupling. We demonstrate that, in order to obtain a consistent treatment of exciton-phonon coupling, diagrams have to be taken into account that can be neglected when the effect of lattice vibrations is treated in a static or quasistatic approximation. From the application of this approach to a model system, we analyzed the main features of the exciton-phonon interaction and provided a general picture of their link with the properties of materials such as exciton mass and exciton Bohr radius. [less ▲]

Phonon coupling and shallow defects in CuInS2

in Physical Review. B (2020), 101(8), 085119-