Gate-Tunable Negative Longitudinal Magnetoresistance in the Predicted Type-II Weyl Semimetal WTe2 (1608.05003v1)

Abstract: The progress in exploiting new electronic materials and devices has been a major driving force in solid-state physics. As a new state of matter, a Weyl semimetal (WSM), particularly a type-II WSM, hosts Weyl fermions as emergent quasiparticles and may harbor novel electrical transport properties because of the exotic Fermi surface. Nevertheless, such a type-II WSM material has not been experimentally observed in nature. In this work, by performing systematic magneto-transport studies on thin films of a predicted material candidate WTe2, we observe notable angle-sensitive (between the electric and magnetic fields) negative longitudinal magnetoresistance (MR), which can likely be attributed to the chiral anomaly in WSM. This phenomenon also exhibits strong planar orientation dependence with the absence of negative longitudinal MR along the tungsten chains (a axis), which is consistent with the distinctive feature of a type-II WSM. By applying a gate voltage, we demonstrate that the Fermi energy can be tuned through the Weyl points via the electric field effect; this is the first report of controlling the unique transport properties in situ in a WSM system. Our results have important implications for investigating simulated quantum field theory in solid-state systems and may open opportunities for implementing new types of electronic applications, such as field-effect chiral electronic devices.

• The paper reports experimental observation of gate-tunable negative longitudinal magnetoresistance attributed to the chiral anomaly in thin-film WTe2.
• The research employs magneto-transport measurements in 7–15 nm films to isolate Weyl point behavior while ruling out effects like current jetting.
• The findings pave the way for designing chiral electronic devices and further exploration of type-II Weyl semimetal topological properties.

Gate-Tunable Negative Longitudinal Magnetoresistance in Type-II Weyl Semimetal WTe$_2$

The paper presents a detailed investigation into the electronic properties of tungsten ditelluride (WTe$_2$), a candidate material for a type-II Weyl semimetal (WSM), known for its unique topological properties. Weyl semimetals, a recently recognized class of topological materials, exhibit linear dispersion relations at Weyl points where conduction and valence bands intersect. Type-II WSMs, distinguished by their open Fermi surfaces and electron-hole pocket boundaries, have eluded direct observation until this paper. This work demonstrates the experimental observation of negative longitudinal magnetoresistance (MR) attributable to the chiral anomaly in WTe$_2$ thin film samples, representing a step forward in the practical realization of type-II WSMs.

The researchers focused on investigating the magneto-transport phenomena in WTe$_2$ by employing thin-film samples with thicknesses in the range of 7-15 nm. This thickness allows the suppression of positive longitudinal MR while maintaining band structure characteristics similar to bulk materials, thus supporting the presence of Weyl points. The key experimental results highlight an angle-sensitive negative longitudinal MR, particularly prominent when the electric and magnetic fields are parallel, consistent with the chiral anomaly behavior predicted in WSMs. For type-II WSMs specifically, a noteworthy planar orientation dependence was detected; a negative longitudinal MR was absent along the tungsten chain direction (a-axis) but present along the b-axis, affirming the theoretical predictions of type-II Weyl fermion behavior.

A pioneering aspect of this paper lies in the demonstration of gate-tunability of electronic properties in WSMs. Using a gate voltage, the researchers managed to effectively tune the Fermi level across the Weyl points in the WTe$_2$ samples. This gate-tunable negative longitudinal MR, observed under varying carrier densities, confirms the potential to manipulate electronic properties within WSMs using an electric field, paving the way for innovative "chiral" electronic devices.

Experimental observations were complemented by a semi-classical analysis that dismissed alternative explanations for the negative longitudinal MR, such as magnetic effects or current jetting, strengthening the case for the chiral anomaly as the underlying mechanism. Additionally, this gate-tunability distinguishes thin-film type-II WSMs from bulk counterparts, where electronic doping fixes the Weyl points irreversibly during sample preparation.

The implications of these findings are profound. Practically, they suggest that WTe$_2$, and potentially other layered type-II WSMs, could form the basis for next-generation electronic devices that exploit their topological properties. Theoretically, the results provide a tangible platform to simulate quantum field theories in solid-state systems and further explore "chiral" physics. Future research could explore other layered type-II WSM candidates to uncover more about their transport properties, potentially broadening the scope of electronic applications leveraging topological protections.

Overall, this work represents a significant contribution to the understanding of type-II Weyl semimetals, through systematic transport studies, providing a framework upon which future research can build to harness these materials' unique properties in technological applications.