Umklapp electron-electron scattering in bilayer graphene moir\'e superlattice

Recent experimental advances have been marked by the observations of ballistic electron transport in moir\'e superlattices in highly aligned heterostructures of graphene and hexagonal boron nitride (hBN). Here, we predict that a high-quality graphene bilayer aligned with an hBN substrate features $T^2$-dependent resistivity caused by umklapp electron-electron (Uee) scattering from the moir\'e superlattice, that is, a momentum kick by Bragg scattering experienced by a pair of electrons. Substantial Uee scattering appears upon $p$-doping of the bilayer above a threshold density, which depends on the twist angle between graphene and hBN, and its contribution towards the resistivity grows rapidly with hole density until it reaches a peak value, whose amplitude changes non-monotonically with the superlattice period. We also analyse the influence of an electrostatically induced bandgap in the bilayer and trigonal warping it enhances in the electron dispersion on the electron-electron umklapp scattering.Comment: 5 pages, 3 figures, supplementary materia

Engineering of the topological magnetic moment of electrons in bilayer graphene using strain and electrical bias

Topological properties of electronic states in multivalley two-dimensional materials, such as mono- and bilayer graphene, or thin films of rhombohedral graphite, give rise to various unusual magneto-transport regimes. Here, we investigate the tunability of the topological magnetic moment (related to the Berry curvature) of electronic states in bilayer graphene using strain and vertical bias. We show how one can controllably vary the valley $g$-factor of the band-edge electrons, $g_v^*$, across the range $10 < |g_v^*| < 200$, and we discuss the manifestations of the topological magnetic moment in the anomalous contribution towards the Hall conductivity and in the Landau level spectrum.Comment: 6 pages, 5 figure

Kagome network of miniband-edge states in double-aligned graphene–hexagonal boron nitride structures

Twistronic heterostructures have recently emerged as a new class of quantum electronic materials with properties determined by the twist angle between the adjacent two-dimensional materials. Here we study moir\'e superlattice minibands in graphene (G) encapsulated in hexagonal boron nitride (hBN) with an almost perfect alignment with both the top and bottom hBN crystals. We show that, for such an orientation of the unit cells of the hBN layers that locally breaks inversion symmetry of the graphene lattice, the hBN/G/hBN structure features a Kagom\'e network of topologically protected chiral states with energies near the miniband edge, propagating along the lines separating the areas with different miniband Chern numbers.Comment: 6 pages, 3 figures (Supplemental Material: 7 pages, 5 figures, 1 table

Semimetallic and semiconducting graphene-hBN multilayers with parallel or reverse stacking

We theoretically investigate 3D layered crystals of alternating graphene and hBN layers with different symmetries. Depending on the hopping parameters between the graphene layers, we find that these synthetic 3D materials can feature semimetallic, gapped, or Weyl semimetal phases. Our results demonstrate that 3D crystals stacked from individual 2D materials represent a new materials class with emergent properties different from their constituents

Semimetallic and semiconducting graphene-h\mathrmBN multilayers with parallel or reverse stacking

We theoretically investigate three-dimensional (3D) layered crystals of alternating graphene and hBN layers with different symmetries. Depending on the hopping parameters between the graphene layers, we find that these synthetic 3D materials can feature semimetallic, gapped, or Weyl semimetal phases. Using first-principles calculations to parametrize the low-energy Hamiltonians we establish the most likely electronic phases. Our results demonstrate that 3D crystals stacked from individual 2D materials represent a synthetic materials class with emergent properties different from their constituents

One-dimensional proximity superconductivity in the quantum Hall regime

Extensive efforts have been undertaken to combine superconductivity and the quantum Hall effect so that Cooper-pair transport between superconducting electrodes in Josephson junctions is mediated by one-dimensional (1D) edge states. This interest has been motivated by prospects of finding new physics, including topologically-protected quasiparticles, but also extends into metrology and device applications. So far it has proven challenging to achieve detectable supercurrents through quantum Hall conductors. Here we show that domain walls in minimally twisted bilayer graphene support exceptionally robust proximity superconductivity in the quantum Hall regime, allowing Josephson junctions operational in fields close to the upper critical field of superconducting electrodes. The critical current is found to be non-oscillatory, practically unchanging over the entire range of quantizing fields, with its value being limited by the quantum conductance of ballistic strictly-1D electronic channels residing within the domain walls. The described system is unique in its ability to support Andreev bound states in high fields and offers many interesting directions for further exploration

Unconventional Thermal Magnon Hall Effect in a Ferromagnetic Topological Insulator

We present theoretically the thermal Hall effect of magnons in a ferromagnetic lattice with a Kekul\'e-O coupling (KOC) modulation and a Dzyaloshinskii-Moriya interaction (DMI). Through a strain-based mechanism for inducing the KOC modulation, we identify four topological phases in terms of the KOC parameter and DMI strength. We calculate the thermal magnon Hall conductivity ${\kappa^{xy}}$ at low temperature in each of these phases. We predict an unconventional conductivity due to a non-zero Berry curvature emerging from band proximity effects in the topologically trivial phase. We find sign changes of ${\kappa^{xy}}$ as a function of the model parameters, associated with the local Berry curvature and occupation probability of the bulk bands. Throughout, ${\kappa^{xy}}$ can be easily tuned with external parameters such as the magnetic field and temperature.Comment: 9 pages, 7 figure