
- Journal of Semiconductors
- Vol. 42, Issue 6, 060401 (2021)
Abstract
The Shubnikov–de Haas (SdH) oscillation, as evidenced by the oscillating resistivity as a function of magnetic field at sufficiently low temperature, is not uncommon in metals, semimetals and narrow gap semiconductors at their conducting state. The origin of SdH oscillation is associated with Landau quantization, at which the cyclotron orbits of electrons are quantized. In other words, electrons can only occupy discrete energy levels, which are called the Landau levels. As the magnetic field increases, the split, quantized Landau levels move across the Fermi level and hence the material’s transport properties start to oscillate periodically.
Writing in Nature (
https://doi.org/10.1038/s41586-020-03084-9), Wang et al. shows the oscillatory magnetoresistance in an insulating monolayer[
In order to probe the intrinsic resistance (or conductance) from the bulk in the monolayer, the authors have designed a special and creative detection scheme to avoid the conducting edge. As shown in Fig. 1(a), a thin insulating boron nitride (BN) layer is inserted between the monolayer and the palladium electrodes. The metal electrodes are deposited after BN, but they completely avoid touching the edge of the sample and are only in contact with the monolayer at small, selected regions, where BN layer is pre-etched by reactive ion etching. With this device geometry, Wang et al. measured the low temperature resistance of 120 MΩ, which strongly indicates that the monolayer WTe2 is at insulating state and contribution from the edge state is eliminated. By applying an out-of-plane magnetic field, the authors observed the oscillating magnetoresistance which starts to develop at a small magnetic field (Fig. 1(b)), which mimics the SdH oscillation though the sample is an insulator. The lowest onset magnetic field of quantum oscillation observed by Wang et al. is 0.5 T at 500 mK. Such small onset field implies the existence of highly mobile fermions. These fermions cannot be electrons or holes, because the monolayer device is at insulating state with a large resistance. As shown in Fig. 1(b), the measured resistance, from zero to 9 T, is least ~100 MΩ. Quantum oscillation at insulating state is consistently observed in three devices with similar device geometry. Wang et al. even observed the quantized regime from another device, where quantum oscillation develops into discrete peaks. The discrete Landau levels are reminiscent of the behavior of high mobility two-dimensional electron gases, where the conductance peak is related to Landau levels at the Fermi level. As monolayer WTe2 exhibits insulating behavior at all fields, existing theory based on charge carriers could not completely explain the observed phenomena.
Figure 1.(Color online) (a) Device schematic showing electrodes in contact with WTe2 in small selected areas without touching the edge of the sample. (b) A magnetoresistance curve taken at low temperature. Zoom-in view of data at low field is shown in the inset. Figures are adapted from Ref. [
It is challenging and non-trivial to demonstrate quantum oscillation in a monolayer WTe2 device. The low carrier mobility reported in previous reports[
While the existing model and theory could not provide a complete understanding on the observed quantum oscillation in the insulating monolayer, the authors have suggested charge-neutral fermions, which arise from electron fractionalization, as an alternative explanation for this emerging phenomenon. It is important to note that the theory for neutral fermion in monolayer WTe2 is still under-developed. Yet the experimental demonstration of quantum oscillation in a monolayer insulator is very encouraging, and it will inspire theoretical[
With the rapid development of “twistronics” in twisted homo- and hetero-bilayer system[
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