Coulomb drag in topological materials

Abstract

Dirac fermions are at the forefront of modern condensed matter physics research. They are known to occur in materials as diverse as graphene, topological insulators, and transition metal dichalcogenides, while closely related Weyl fermions have been discovered in other materials. They have been predicted to lend themselves to a variety of technological applications, while the recent prediction and discovery of the quantized anomalous Hall effect of massive Dirac fermions is regarded as a potential gateway towards low-energy electronics. Some materials hosting Dirac fermions are natural platforms for interlayer coherence effects such as Coulomb drag and exciton condensation. The top and bottom surfaces of a thin topological insulator film provide such a prototype system. Here we describe recent insights into Coulomb drag between two layers of Dirac fermions relying primarily on topological insulator films as a minimal model. We consider both non-magnetic topological insulators, hosting massless Dirac fermions, and magnetic topological insulators, in which the fermions are massive. We discuss in general terms the dynamics of the thin-film spin density matrix, outlining numerical results and approximate analytical expressions where appropriate for the drag resistivity ρ D at low temperatures and low electron densities. In magnetic topological insulators with out-of-plane magnetizations in both the active and passive layers we analyze the role of the anomalous Hall effect in Coulomb drag. Whereas the transverse response of the active layer is dominated by a topological term stemming from the Berry curvature, we show that neither the topological mechanism nor disorder renormalizations associated with it contribute to Coulomb drag. Nevertheless, the longitudinal drag force in the passive layer does give rise to a transverse drag current that is independent of the active-layer magnetization. It depends non-monotonically on the passive-layer magnetization, exhibiting a peak that becomes more pronounced at low densities. All of these observations can be verified in the laboratory. We compare results for topological insulators with results for graphene, identifying qualitative and quantitative differences, and discuss generalisations to multi-valley systems, ultra-thin films and electron-hole layers.

Frictional Drag Effect between Massless and Massive Fermions in Single-Layer/Bilayer Graphene Heterostructures

“…In particular, the unique carrier density dependent features are revealed as a fingerprint feature for massless–massive systems. These results may be extended to similar hybrid systems in which an alternative material serves as the massless fermion layer, such as topological insulators and Dirac semimetals . Our study deepens the understanding of the unique interlayer scattering process in a massless–massive system and promotes further investigations of interactions between different types of carriers, such as the ones with different band dispersions and chiralities.…”

“…These results may be extended to similar hybrid systems in which an alternative material serves as the massless fermion layer, such as topological insulators and Dirac semimetals. 45 Our study deepens the understanding of the unique interlayer scattering process in a massless−massive system and promotes further investigations of interactions between different types of carriers, such as the ones with different band dispersions and chiralities. Gaining a fundamental level understanding of electronic frictional effects may also contribute to the future design of novel hybrid devices.…”

Topology-Driven Coulomb Drag in van der Waals Heterostructure with Broken Inversion Symmetry

“…The intricate interplay between electron–electron interactions and the nontrivial topology of materials could have a significant influence on the behavior of condensed matter systems. − The Coulomb drag effect, which refers to the transfer of momentum/energy between two closely spaced electron layers, serves as an unambiguous probe to understand these interactions. − Coulomb drag at charge neutrality (CN) has garnered significant attention due to its complex nature. Drag events are mainly dominated by energy-driven mechanisms, , rather than momentum-mediated drag at CN. , The key requirement for energy-driven drag at CN is the coupling between energy (neutral mode) and charge currents which is generally achieved by applying a weak magnetic field ( B -field) .…”

Coulomb drag in metal monochalcogenides double-layer structures with Mexican-hat band dispersions

“…The transresistivity or the drag coefficient (ρ D ) is a measure of inter-layer interaction and can be determined by calculating the ratio of V drag to I drive [33,34]. This phenomenon has previously been studied in some nanostructures such as n-doped and p-doped double quantum wells [35][36][37][38], double quantum wires [39][40][41], mismatched subsystems [42], double layers of topological materials [43], double-layer and bilayer graphene [29,33,44], and doublelayer phosphorene [45]. For a double quantum wells system with a 2D electron density n and layer separation d, the drag transresistivity changes as T 2 /n 2 d 4 (1/Tn 3(4) d 3 ) at low (high) temperature (T ).…”