Fractional quantum anomalous Hall effect in multilayer graphene.

The fractional quantum anomalous Hall effect (FQAHE), the analogue of the fractional quantum Hall effect1 at zero magnetic field, is predicted to exist in topological flat bands under spontaneous time-reversal-symmetry breaking2-6. The demonstration of FQAHE could lead to non-Abelian anyons that form the basis of topological quantum computation7-9. So far, FQAHE has been observed only in twisted MoTe2 at a moiré filling factor v > 1/2 (refs. 10-13). Graphene-based moiré superlattices are believed to host FQAHE with the potential advantage of superior material quality and higher electron mobility. Here we report the observation of integer and fractional QAH effects in a rhombohedral pentalayer graphene-hBN moiré superlattice. At zero magnetic field, we observed plateaus of quantized Hall resistance [Formula: see text] at v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5 of the moiré superlattice, respectively, accompanied by clear dips in the longitudinal resistance Rxx. Rxy equals [Formula: see text] at v = 1/2 and varies linearly with v, similar to the composite Fermi liquid in the half-filled lowest Landau level at high magnetic fields14-16. By tuning the gate-displacement field D and v, we observed phase transitions from composite Fermi liquid and FQAH states to other correlated electron states. Our system provides an ideal platform for exploring charge fractionalization and (non-Abelian) anyonic braiding at zero magnetic field7-9,17-19, especially considering a lateral junction between FQAHE and superconducting regions in the same device20-22.

Large quantum anomalous Hall effect in spin-orbit proximitized rhombohedral graphene.

The quantum anomalous Hall effect (QAHE) is a robust topological phenomenon that features quantized Hall resistance at zero magnetic field. We report the QAHE in a rhombohedral pentalayer graphene-monolayer tungsten disulfide (WS2) heterostructure. Distinct from other experimentally confirmed QAHE systems, this system has neither magnetic element nor moiré superlattice effect. The QAH states emerge at charge neutrality and feature Chern numbers C = ±5 at temperatures of up to about 1.5 kelvin. This large QAHE arises from the synergy of the electron correlation in intrinsic flat bands of pentalayer graphene, the gate-tuning effect, and the proximity-induced Ising spin-orbit coupling. Our experiment demonstrates the potential of crystalline two-dimensional materials for intertwined electron correlation and band topology physics and may enable a route for engineering chiral Majorana edge states.

Channel-Length-Modulated Avalanche Multiplication in Ambipolar WSe2 Field-Effect Transistors.

Recently there has been growing interest in avalanche multiplication in two-dimensional (2D) materials and device applications such as avalanche photodetectors and transistors. Previous studies have mainly utilized unipolar semiconductors as the active material and focused on developing high-performance devices. However, fundamental analysis of the multiplication process, particularly in ambipolar materials, is required to establish high-performance electronic devices and emerging architectures. Although ambipolar 2D materials have the advantage of facile carrier-type tuning through electrostatic gating, simultaneously allowing both carrier types in a single channel poses an inherent difficulty in analyzing their individual contributions to avalanche multiplication. In ambipolar field-effect transistors (FETs), two phenomena of ambipolar transport and avalanche multiplication can occur, and both exhibit secondary rise of output current at high lateral voltage. We distinguished these two competing phenomena using the method of channel length modulation and successfully analyzed the properties of electron- and hole-initiated multiplication in ambipolar WSe2 FETs. Our study provides a simple and robust method to examine carrier multiplication in ambipolar materials and will foster the development of high-performance atomically thin electronic devices utilizing avalanche multiplication.

Molecular Dopant-Dependent Charge Transport in Surface-Charge-Transfer-Doped Tungsten Diselenide Field Effect Transistors.

The controllability of carrier density and major carrier type of transition metal dichalcogenides(TMDCs) is critical for electronic and optoelectronic device applications. To utilize doping in TMDC devices, it is important to understand the role of dopants in charge transport properties of TMDCs. Here, the effects of molecular doping on the charge transport properties of tungsten diselenide (WSe2 ) are investigated using three p-type molecular dopants, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 -TCNQ), tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic blue), and molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd-COCF3 )3 ). The temperature-dependent transport measurements show that the dopant counterions on WSe2 surface can induce Coulomb scattering in WSe2 channel and the degree of scattering is significantly dependent on the dopant. Furthermore, the quantitative analysis revealed that the amount of charge transfer between WSe2 and dopants is related to not only doping density, but also the contribution of each dopant ion toward Coulomb scattering. The first-principles density functional theory calculations show that the amount of charge transfer is mainly determined by intrinsic properties of the dopant molecules such as relative frontier orbital positions and their spin configurations. The authors' systematic investigation of the charge transport of doped TMDCs will be directly relevant for pursuing molecular routes for efficient and controllable doping in TMDC nanoelectronic devices.

Ultrasensitive Photodetection in MoS2 Avalanche Phototransistors.

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high-gain route toward photodetection in the form of single-photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS2 synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS2 channel. By utilizing the photo-initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 107 A W-1 and the detectivity of ≈4.3 × 1016 Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.

Effect of Facile p-Doping on Electrical and Optoelectronic Characteristics of Ambipolar WSe2 Field-Effect Transistors.

We investigated the electrical and optoelectronic characteristics of ambipolar WSe2 field-effect transistors (FETs) via facile p-doping process during the thermal annealing in ambient. Through this annealing, the oxygen molecules were successfully doped into the WSe2 surface, which ensured higher p-type conductivity and the shift of the transfer curve to the positive gate voltage direction. Besides, considerably improved photoswitching response characteristics of ambipolar WSe2 FETs were achieved by the annealing in ambient. To explore the origin of the changes in electrical and optoelectronic properties, the analyses via X-ray photoelectron, Raman, and photoluminescence spectroscopies were performed. From these analyses, it turned out that WO3 layers formed by the annealing in ambient introduced p-doping to ambipolar WSe2 FETs, and disorders originated from the WO3/WSe2 interfaces acted as non-radiative recombination sites, leading to significantly improved photoswitching response time characteristics.