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...

Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. 3) and Na3Bi (refs 4, 5), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena.

The record superconducting transition temperature (T(c)) for the iron-based high-temperature superconductors (Fe-HTS) has long been 56 K. Recently, in single-layer FeSe films grown on SrTiO3 substrates, indications of a new record of 65 K have been reported. Using in situ photoemission measurements, we substantiate the presence of spin density waves (SDWs) in FeSe films--a key ingredient of Fe-HTS that was missed in FeSe before--and we find that this weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise pronounced SDWs in single-layer FeSe. Beyond providing a comprehensive understanding of FeSe films and directions to further enhance its T(c), we map out the phase diagram of FeSe as a function of lattice constant, which contains all the essential physics of Fe-HTS. With the simplest structure, cleanest composition and single tuning parameter, monolayer FeSe is an ideal system for testing theories of Fe-HTS.

. However, as a neutron is neutral, it does not detect charge but rather its associated lattice distortion 7 , so it is not known whether the stripes involve ordering of the doped holes. Here we present a study of the charge order in LBCO with resonant soft X-ray scattering (RSXS). We observe giant resonances near the Fermi level as well as near the correlated gap 8,9 , demonstrating significant modulation in both the doped-hole density and the 'Mottness', or the degree to which the system resembles a Mott insulator 10 . The peak-to-trough amplitude of the valence modulation is estimated to be 0.063 holes, which suggests 11 an integrated area of 0.59 holes under a single stripe, close to the expected 0.5 for half-filled stripes.The charge/spin superstructure in LNSCO 5 and LBCO 6 appears only in the low-temperature tetragonal (LTT) phase, is most stable at x = 1/8 and coincides with an anomalous suppression of the critical temperature T c (ref. 12). This phase is frequently interpreted as (quasi) static stripes that have been pinned by the LTT distortion. The charge reflections observed with neutron scattering are weak (∼6 times less intense than the magnetic reflections) as neutrons only detect the lattice distortion, which was estimated to be only about 0.004Å (ref. 7). However, one assumes that the hole modulation itself is significant. We point out, however, that the spin-density wave in elemental Cr also exhibits half-wavelength charge reflections that are weaker by about a factor of 4.1 and represent a distortion of similar size 13 . So, in the neutron Bragg peaks alone there is no clear difference between the phenomenon in LNSCO and a simple spin-density wave. To determine whether the doped holes are actually involved we have studied LBCO with RSXS near the O K (1s → 2p) and Cu L 3/2 (2p 3/2 → 3d x2−y2 ) edges, which provide direct sensitivity to valence electron ordering [14][15][16][17][18][19][20] .Single crystals of La 2−x Ba x CuO 4 with x = 1/8 were grown by the floating-zone method 21 . The sample used in this study had T c = 2.5 K indicating suppressed superconductivity and stabilized spin/charge order. The sample was cleaved in air revealing a surface with (0,0,1) orientation. RSXS measurements were performed on beam line X1B at the National Synchrotron Light Source, Brookhaven, using a 10-axis, ultrahigh-vacuumcompatible diffractometer. The sample was cooled with a He flow cryostat connected through Cu braids, providing a base temperature of 18 K. X-ray absorption spectra (XAS) were measured in situ in fluorescence yield mode at the O K and Cu L 3/2 edges and found to be consistent with previous studies 8 (see Figs 1, 3a). We will denote reciprocal space with Miller indices (H,K,L), which represent a momentum transfer Q = (2π/a H,2π/b K,2π/c L) where a = b = 3.788Å, c = 13.23Å. The incident X-ray polarization depends on Q but was approximately 60 • from the Cu-O bond for measurements at both edges.The O K XAS in the cuprates exhibits a mobile carrier peak (MCP) at 528.6 eV, corresponding to tran...

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.