arXiv subjects

Wenjin Zhao

Publications and source records attributed to Wenjin Zhao.

22 records · Page 2Linked to original sources

arXiv2020

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

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. The ability to simply stack very different two dimensional (2D) van der Waals materials in intimate contact permits a different approach. Here we use this approach to couple the helical edges states in a 2D topological insulator, monolayer WTe2, to a 2D layered antiferromagnet, CrI3. 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.

cond-mat.mes-hall↗

arXiv2018

Ferroelectric switching of a two-dimensional metal

A ferroelectric is a material with a polar structure whose polarity can be reversed by applying an electric field. In metals, the itinerant electrons tend to screen electrostatic forces between ions, helping to explain why polar metals are very rare. Screening also excludes external electric fields, apparently ruling out the possibility of polarity reversal and thus ferroelectric switching. In principle, however, a thin enough polar metal could be penetrated by an electric field sufficiently to be switched. Here we show that the layered topological semimetal WTe2 provides the first embodiment of this principle. Although monolayer WTe2 is centrosymmetric and thus nonpolar, the stacked bulk structure is polar. We find that two- or three-layer WTe2 exhibits a spontaneous out-of-plane electric polarization which can be switched using gate electrodes. We directly detect and quantify the polarization using graphene as an electric field sensor. Moreover, the polarization states can be differentiated by conductivity, and the carrier density can be varied to modify the properties. The critical temperature is above 350 K, and even when WTe2 is sandwiched in graphene it retains its switching capability at room temperature, demonstrating a robustness suitable for applications in combination with other two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2018

Gate-induced superconductivity in a monolayer topological insulator

The layered semimetal WTe_2 has recently been found to be a two-dimensional topological insulator (2D TI) when thinned down to a single monolayer, with conducting helical edge channels. We report here that intrinsic superconductivity can be induced in this monolayer 2D TI by mild electrostatic doping, at temperatures below 1 K. The 2D TI-superconductor transition can be easily driven by applying a just a small gate voltage. This discovery offers new possibilities for gate-controlled devices combining superconductivity and topology, and could provide a basis for quantum information schemes based on topological protection.

cond-mat.mes-hall↗

arXiv2016

Edge conduction in monolayer WTe2

A two-dimensional topological insulator (2DTI) is guaranteed to have a helical 1D edge mode in which spin is locked to momentum, producing the quantum spin Hall effect and prohibiting elastic backscattering at zero magnetic field. No monolayer material has yet been shown to be a 2DTI, but recently the Weyl semimetal WTe2 was predicted to become a 2DTI in monolayer form if a bulk gap opens. Here, we report that at temperatures below about 100 K monolayer WTe2 does become insulating in its interior, while the edges still conduct. The edge conduction is strongly suppressed by in-plane magnetic field and is independent of gate voltage, save for mesoscopic fluctuations that grow on cooling due to a zero-bias anomaly which reduces the linear-response conductance. Bilayer WTe2 also becomes insulating at low temperatures but does not show edge conduction. Many of these observations are consistent with monolayer WTe2 being a 2DTI. However, the low temperature edge conductance, for contacts spacings down to 150 nm, is below the quantized value,

cond-mat.mes-hall↗