Black phosphorus is a layered material in which individual atomic layers are stacked together by Van der Waals interactions, much like bulk graphite 1 . Inside a single layer, each phosphorus atom is covalently bonded with three adjacent phosphorus atoms to form a puckered honeycomb structure [2][3][4] (Fig. 1a). The three bonds take up all three valence electrons of phosphorus, so unlike graphene 5,6 a monolayer black phosphorus (termed "phosphorene") is a semiconductor with a predicted direct band gap of ~ 2 eV at the Γ point of the first Brillouin zone 7 . For few-layer phosphorene, interlayer interactions reduce the band gap for each layer 3 added, and eventually reach ~ 0.3 eV (refs 8-11) for bulk black phosphorus. The direct gap also moves to the Z point as a consequence 7,12 . Such a band structure provides a much needed gap for the field-effect transistor (FET) application of two dimensional (2D) materials such as graphene, and the thickness-dependent direct band gap may lead to potential applications in optoelectronics, especially in the infrared regime. In addition, observations of phase transition from semiconductor to metal 13,14 and superconductor under high pressure 15,16 We next fabricate few-layer phosphorene FETs with a back-gate electrode (see Fig. 2a). A scotch tape based mechanical exfoliation method is employed to peel thin flakes from bulk crystal onto degenerately doped silicon wafer covered with a layer of thermally grown silicon dioxide. Optical microscopy and atomic force microscopy (AFM) are used to hunt thin flake samples and determine their thickness (Fig. 2a). The switching behaviour of our few-layer phosphorene transistor at room temperature is characterized in vacuum (~ 10 -6 mBar), in a configuration depicted in -30 V to 0 V, the channel switches from "on" state to "off" state, and a drop in drain current by a factor of ~ 10 5 is observed. The measured drain current modulation is 4 orders of magnitude larger than that in graphene (due to its lack of bandgap), and 5 approaches the value recently reported in MoS2 devices 17 . Such a high drain current modulation makes black phosphorus thin film a promising material for applications in digital electronics 22 . Similar switching behaviour (with varying drain current modulation) is observed on all black phosphorous thin film transistors with thicknesses up to 50 nm. We note that the "on" state current of our devices has not yet reached saturation, due to the fact that the doping level is limited by the break-down electric field of the SiO2 back-gate dielectric. It is therefore possible to achieve even higher drain current modulation by using high-k materials as gate dielectrics for higher doping. Meanwhile, a subthreshold swing (SS) of ~ 5 V/decade is observed, which is much larger than the SS in commercial Si-based devices (~ 70 mV/decade).We note that the SS in our devices varies from sample to sample (from ~ 3.7 V/decade to ~ 13.3 V/decade), and is on the same order of magnitude as reported in multilayerMoS2 devices with a simila...

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In a magnetic topological insulator, nontrivial band topology combines with magnetic order to produce exotic states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. In this work, we probe quantum transport in MnBi2Te4 thin flakes—a topological insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple antiparallel to each other; atomically thin MnBi2Te4, however, becomes ferromagnetic when the sample has an odd number of septuple layers. We observe a zero-field QAH effect in a five–septuple-layer specimen at 1.4 kelvin, and an external magnetic field further raises the quantization temperature to 6.5 kelvin by aligning all layers ferromagnetically. The results establish MnBi2Te4 as an ideal arena for further exploring various topological phenomena with a spontaneously broken time-reversal symmetry.