Understanding the normal-metal state transport in twisted bilayer graphene near magic angle is of fundamental importance as it provides insights into the mechanisms responsible for the observed strongly correlated insulating and superconducting phase
s. Here we provide a rigorous theory for phonon-dominated transport in twisted bilayer graphene describing its unusual signatures in the resistivity (including the variation with electron density, temperature, and twist angle) showing good quantitative agreement with recent experiments. We contrast this with the alternative Planckian dissipation mechanism that we show is incompatible with available experimental data. An accurate treatment of the electron-phonon scattering requires us to go well beyond the usual treatment, including both interband and intraband processes, considering the finite-temperature dynamical screening of the electron-phonon matrix element, and going beyond the linear Dirac dispersion. In addition to explaining the observations in currently available experimental data, we make concrete predictions that can be tested in ongoing experiments.
Bilayer MoS2 is a centrosymmetric semiconductor with degenerate spin states in the six valleys at the corners of the Brillouin zone. It has been proposed that breaking of this inversion symmetry by an out-of-plane electric field breaks this degenerac
y, allowing for spin and valley lifetimes to be manipulated electrically in bilayer MoS2 with an electric field. In this work, we report phase-coherent transport properties of double-gated mono-, bi-, and tri-layer MoS2. We observe a similar crossover from weak localization to weak anti-localization, from which we extract the spin relaxation time as a function of both electric field and temperature. We find that the spin relaxation time is inversely proportional to momentum relaxation time, indicating that Dyakonov-Perel mechanism is dominant in all devices despite its centrosymmetry. Further, we found no evidence of electric-field induced changes in spin-orbit coupling strength. This suggests that the interlayer coupling is sufficiently weak and that electron-doped dichalcogenide multilayers behave electrically as decoupled monolayers.
We present low-temperature transport measurements of a gate-tunable thin film topological insulator system that features high mobility and low carrier density. Upon gate tuning to a regime around the charge neutrality point, we infer an absence of st
rong localization even at conductivities well below $e^2/h$, where two dimensional electron systems should conventionally scale to an insulating state. Oddly, in this regime the localization coherence peak lacks conventional temperature broadening, though its tails do change dramatically with temperature. Using a model with electron-impurity scattering, we extract values for the disorder potential and the hybridization of the top and bottom surface states.
Electrons behave like a classical fluid with a momentum distribution function that varies slowly in space and time when the quantum mechanical carrier-carrier scattering dominates over all other scattering processes. Recent experiments in monolayer a
nd bilayer graphene have reported signatures of such hydrodynamic electron behavior in ultra-clean devices. In this theoretical work, starting from a microscopic treatment of electron-electron, electron-phonon and electron-impurity interactions within the Random Phase Approximation, we demonstrate that monolayer and bilayer graphene both host two different hydrodynamic regimes. We predict that the hydrodynamic window in bilayer graphene is stronger than in monolayer graphene, and has a characteristic `v-shape as opposed to a `lung-shape. Finally, we collapse experimental data onto a universal disorder-limited theory, thereby proving that the observed violation of Wiedemann-Franz law in monolayers occurs in a regime dominated by impurity-induced electron-hole puddles.
Large-area thin films of topological Dirac semimetal Na$_3$Bi are grown on amorphous SiO$_2$:Si substrates to realise a field-effect transistor with the doped Si acting as back gate. As-grown films show charge carrier mobilities exceeding 7,000 cm$^2
$/Vs and carrier densities below 3 $times $10$^{18}$ cm$^{-3}$, comparable to the best thin-film Na$_3$Bi. An ambipolar field effect and minimum conductivity are observed, characteristic of Dirac electronic systems. The results are quantitatively understood within a model of disorder-induced charge inhomogeneity in topological Dirac semimetals. Due to the inverted band structure, the hole mobility is significantly larger than the electron mobility in Na$_3$Bi, and when present, these holes dominate the transport properties.
Coulomb drag between parallel two-dimensional electronic layers is an excellent tool for the study of electron-electron interactions. In actual experiments, the layers display spatial charge density fluctuations due to imperfections such as external
charged impurities. However, at present a systematic way of taking these inhomogeneities into account in drag calculations has been lacking, making the interpretation of experimental data problematic. On the other hand, there exists a highly successful and widely accepted formalism describing transport within single inhomogeneous layers known as effective medium theory. In this work, we generalize the standard effective medium theory to the case of Coulomb drag between two inhomogeneous sheets and demonstrate that inhomogeneity in the layers has a strong impact on drag transport. In the case of exciton condensation between the layers, we show that drag resistivity takes on a value determined by the amplitude of density fluctuation. Next we consider drag between graphene sheets, in which the existence of spatial charge density fluctuations is well-known. We show that these inhomogeneities play a crucial role in explaining existing experimental data. In particular, the temperature dependence of the experimentally observed peaks in drag resistivity can only be explained by taking the layer density fluctuations into account. We also propose a method of extracting information on the correlations between the inhomogeneities of the layers. The effective medium theory of Coulomb drag derived here is general and applies to all two-dimensional materials.
A clear gate voltage tunable weak antilocalization and a giant magnetoresistance of 400 percent are observed at 1.9 K in single layer graphene with an out-of-plane field. A large magnetoresistance value of 275 percent is obtained even at room tempera
ture implying potential applications of graphene in magnetic sensors. Both the weak antilocalization and giant magnetoresistance persists far away from the charge neutrality point in contrast to previous reports, and both effects are originated from charged impurities. Interestingly, the signatures of Shubnikov-de Haas oscillations and the quantum Hall effect are also observed for the same sample.
