Wide-range T ² resistivity and umklapp scattering in moiré graphene

Abstract

We argue that the unusually strong electron-electron interactions in the narrow bands in moiré superlattices originate from compact Wannier orbitals. Enhanced overlaps of electronic wavefunctions, enabled by such orbitals, resulting in a strong el-el superlattice umklapp scattering. We identify the umklapp scattering processes as a source of the strong temperature-dependent resistivity observed in these systems. In a simple model, the umklapp scattering predicts a T-dependent resistivity that grows as T 2 with a numerical prefactor that grows as the Wannier orbital radius decreases. We quantify the enhancement in el-el scattering by the Kadowaki-Woods (KW) ratio, a quantity that is sensitive to umklapp scattering but, helpfully, insensitive to the effects due to the high density of electronic states. Our analysis predicts anomalously large KW ratio values that clearly indicate the importance of the umklapp el-el processes and their impact on the T-dependent resistivity.

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene

“…This electron temperature is established via electron-electron interactions that take place on a timescale of tens of femtoseconds for nontwisted bilayer graphene ( 53 ). The electron-electron interactions are even stronger in twisted bilayer graphene, leading to even faster carrier thermalization rates ( 54 ).…”

“…For metals, electron-electron Umklapp scattering gives rise to finite electrical resistance at low temperatures. In graphene/hBN superlattices and MATBG, this effect dominates transport at temperatures up to 10 K or higher, leading to excess resistivity and degradation of charge carrier mobility ( 50 , 54 , 61 ). In MATBG, electron-phonon Umklapp scattering could explain some of the open questions from electrical transport measurements, such as the strange metal phase or the role of phonons in superconductivity ( 25 , 60 ).…”

Ultrafast Umklapp-assisted electron-phonon cooling in magic-angle twisted bilayer graphene

“…In graphene/hBN superlattices and MATBG, this effect dominates transport at temperatures up to 10 K or higher, leading to excess resistivity and degradation of charge carrier mobility. [41][42][43] In MATBG, electron-phonon Umklapp scattering could explain some of the open questions from electrical transport measurements, such as the strange metal phase or the role of phonons in superconductivity. [16,40] Finally, the ultrafast Umklapp-assisted electron-phonon cooling, enhanced density of states, and rich phase diagram are appealing for single-photon detection in the highly sought after mid-IR wavelength range.…”

Controlling Umklapp Scattering in a Bilayer Graphene Moiré Superlattice

“…A realistic Fermi liquid is, however, not Galilean-invariant; a finite coupling to an underlying lattice provides a mechanism for the momentum relaxation of the quasiparticles via the Umklapp process . Umklapp electron–electron (Uee) scattering is the fundamental mechanism that allows the transfer of momentum from electrons to the lattice and imparts electrical resistance to the metal. − In this process, the crystal lattice gives a momentum kick to a pair of interacting electrons, backscattering them on the other side of the Fermi surface. Their quasimomentum is conserved, modulo a reciprocal lattice vector G boldk bold1 + boldk bold2 = boldk bold3 + boldk bold4 + boldG where ℏ k 1 and ℏ k 2 and ℏ k 3 and ℏ k 4 are the initial and final quasimomenta of the two electrons near the Fermi level, respectively, and G is a non-zero reciprocal lattice vector of the crystal.…”

“…Graphene-based moiré superlattices ,,− provide a system with precise tunability of reciprocal lattice vectors G (via the twist angle between the constituent layers) and Fermi wave vectors k F (by controlling carrier density n through electrostatic gating). It thus provides a vast phase space in which eq may be satisfied, and the scaling of f n versus E F can be verified.…”