Magnetic proximity and nonreciprocal current switching in a monolayer WTe2 helical edge

Abstract

The integration of diverse electronic phenomena, such as magnetism and nontrivial topology, into a single system is normally studied either by seeking materials that contain both ingredients, or by layered growth of contrasting materials 1-9 . The ability to simply stack very different twodimensional (2D) van der Waals materials in intimate contact permits a different approach 10,11 . Here we use this approach to couple the helical edges states in a 2D topological insulator, monolayer WTe2 12-16 , to a 2D layered antiferromagnet, CrI3 17 . We find that the edge conductance is sensitive to the magnetization state of the CrI3, and the coupling can be understood in terms of an exchange field from the nearest and next-nearest CrI3 layers that produces a gap in the helical edge. We also find that the nonlinear edge conductance depends on the magnetization of the nearest CrI3 layer relative to the current direction. At low temperatures this produces an extraordinarily large nonreciprocal current that is switched by changing the antiferromagnetic state of the CrI3. Main Text:The introduction of magnetic order into topological band structure gives rise to new phenomena such as the quantum anomalous Hall effect 1,8,9 and nonreciprocal magnetoelectric effects [5][6][7]18 . In the case of a two-dimensional topological insulator (2D TI), topology guarantees the existence of helical edge modes in which the spin is locked to momentum, causing current at the edge to be fully spin-polarized (the quantum spin Hall effect) 19 . Combining 2D TIs with magnets should therefore directly yield magnetoelectric coupling 20,21 . For example, a magnetic proximity effect may modify the spin polarization and hence the current, or the flow of current in the edge may produce a torque on the magnetization 22,23 . Since backscattering in the edge modes is suppressed by time reversal symmetry, the edge conduction should be affected by magnetic order, which will mix the two opposite-spin branches and so modify backscattering. The expected gapping of the helical edge modes by proximity with a ferromagnet is an important way to control them which, combined with induced superconductivity 24,25 , plays a role in schemes to produce Majorana modes 26 .Stacking van der Waals materials offers a simple, flexible and low-disorder approach to combining magnets with other materials 10,11 . In this work we measure transport through a 2D TI, monolayer (1L) WTe2 13-16 , stacked under the layered magnetic insulator CrI3 17,27-30 . We find that the magnetism of the CrI3 suppresses the edge conduction in the WTe2, in a manner consistent with

Controlling bimerons as skyrmion analogues by ferroelectric polarization in 2D van der Waals multiferroic heterostructures

“…Note that the method by increasing the layer thickness for better effect does not apply to all 2D HS system. For example, in the WTe 2 /CrI 3 HS, the conductance of WTe 2 depends on the nearest CrI 3 layer and is not affected by the CrI 3 thickness 62 . The impact of the polarization intensity in In 2 Se 3 on the magnetism in LaCl is shown in Fig.…”

Robust ferromagnetism in wafer-scale monolayer and multilayer Fe3GeTe2

“…A recent study using immense pressures revealed that the antiferromagnet-ferromagnet switching is not a result of the interlayer coupling, but due to stacking order differences in CrBr 3 41 . Looking forward, a recent publication shows the coupling of ferromagnetism and topological states introduces controllable currents in helical edge states in CrI 3 and WTe 2 heterostructures 42 , suggesting an interesting future in the heterostucture design of 2D magnetic materials.…”

Atomically Thin Quantum Spin Hall Insulators

“…Combined with topological superconductivity, such a 1D system may carry Majorana bound states at the topological phase-boundary between trivial and helical parts of the edge, [219,220] as discussed in the following section. With relevance to atomically thin QSH insulators in particular, magnetic fields may be locally imposed by ferromagnetic materials [59,221] proximal to the QSH edge, such as ferromagnetic clusters [59] or even layered ferromagnets and ferromagnetic insulators, [221][222][223][224][225][226][227][228] the latter of which are fields that have burgeoned over the last three years. Layered ferromagnets exhibit intrinsic ferromagnetism in which each layer has a uniform magnetic polarization, even down to the monolayer limit, some of which are stable up to room temperature [223] and can be switched [223] between ferromagnetic and paramagnetic states with an applied gate voltage.…”

“…Layered ferromagnets exhibit intrinsic ferromagnetism in which each layer has a uniform magnetic polarization, even down to the monolayer limit, some of which are stable up to room temperature [ 223 ] and can be switched [ 223 ] between ferromagnetic and paramagnetic states with an applied gate voltage. A layered ferromagnet recently demonstrated [ 221 ] strong, polarization‐dependent modulation of the edge conductance in the QSH material WTe 2 , holding great promise as a “topological switch.”…”