Abstract:We demonstrate that a field effect transistor ( Main Text:Few-layer black phosphorus (BP) has received in recent years much attention due to its unique properties making this layered material attractive for technological applications(1-3). This twodimensional crystal has an anisotropic structure (Fig.1a) and is characterized by a BP thickness dependent direct band gap(4). In contrast to graphene, the presence of a band gap in BP permits for a selective depletion of charge carriers by electrostatic gating, which is an essential feature in field effect transistors (FETs). A high charge carrier mobility reaching 1000 cm 2 /Vs at room temperature accentuates this material for applications at room temperature(5). However, the exposure of BP crystals to ambient conditions causes the oxidation of BP and significantly degrades the quality of BP channels. Nevertheless, the encapsulation of BP layers by hexagonal boron nitride (h-BN) sheets in air or in an inert gas environment is found to be very effective for preventing BP oxidation(6-8). Surface impurity effects are largely reduced, and high charge carrier mobility up to several 10 3 cm 2 /Vs has been obtained in BP FETs at cryogenic temperature(6-8). The charge carrier scattering at the impurities encapsulated along with the BP layers hinders further mobility increase. Figure 1a shows Fig.1c). The mobility values are more than four times larger compared with that in previous studies, which indicates the improved quality of h-BN/BP interfaces(7). In spite of using the advanced fabrication technique,  FET and  H saturate at T<20 K, which implies that the disorder scattering dominates over the phonon scattering in this temperature regime, which limits the hole mobility at cryogenic temperature(9). The increase of  H with the carrier density p (Fig. 1c) suggests that the disorder potential is likely created by residual impurities and can be screened by the mobile carriers (7,10,11). The scattering behavior changes at high temperatures (T>100 K).  FET and  H decrease with increasing T and follow the dependence T    , where  =1.9 and 2.0 characterize the dependence for  H and  FET , respectively (black line in Fig. 1c). The large  values imply that the acoustic phonon rather than the optical phonon scattering dominates over the scattering by the residual impurities in this temperature regime. It is very noticeable that the room temperature hole mobility  H = 5200 cm 2 /Vs closely approaches the theoretically predicted hole mobility for a clean five-layer BP sheets, which lies in the range between 4,800 cm 2 V -1 s -1 and 6,400 cm 2 V -1 s -1 (9). The realization 4 of the predicated mobility value, which is solely limited by the phonon scattering at room temperature, is another demonstration of the improved BP heterostructure quality. Quantum Hall Effect (QHE) in BP 2DHGFigure 2a shows Hall resistance R yx and magnetoresistance R xx as a function of the magnetic field, which is measured in a clean heterostructure at the base temperature of the experim...

Low carrier mobility and high electrical contact resistance are two major obstacles prohibiting explorations of quantum transport in TMDCs. Here, we demonstrate an effective method to establish low-temperature Ohmic contacts in boron nitride encapsulated TMDC devices based on selective etching and conventional electron-beam evaporation of metal electrodes. This method works for most extensively studied TMDCs in recent years, including MoS2, MoSe2, WSe2, WS2, and 2H-MoTe2. Low electrical contact resistance is achieved at 2 K. All of the few-layer TMDC devices studied show excellent performance with remarkably 2 improved field-effect mobilities ranging from 2300 cm 2 /V s to 16000 cm 2 /V s, as verified by the high carrier mobilities extracted from Hall effect measurements. Moreover, both highmobility n-type and p-type TMDC channels can be realized by simply using appropriate contact metals. Prominent Shubnikov-de Haas oscillations have been observed and investigated in these high-quality TMDC devices.

This work reports an experimental study on an antiferromagnetic honeycomb lattice of MnPS that couples the valley degree of freedom to a macroscopic antiferromagnetic order. The crystal structure of MnPS is identified by high-resolution scanning transmission electron microscopy. Layer-dependent angle-resolved polarized Raman fingerprints of the MnPS crystal are obtained, and the Raman peak at 383 cm exhibits 100% polarity. Temperature dependences of anisotropic magnetic susceptibility of the MnPS crystal are measured in a superconducting quantum interference device. Anisotropic behaviors of the magnetic moment are explored on the basis of the mean field approximation model. Ambipolar electronic conducting channels in MnPS are realized by the liquid gating technique. The conducting channel of MnPS offers a platform for exploring the spin/valleytronics and magnetic orders in 2D limitation.