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...

SmB 6 , a well-known Kondo insulator, exhibits a transport anomaly at low temperature. This anomaly is usually attributed to states within the hybridization gap. Recent theoretical work and transport measurements suggest that these in-gap states could be ascribed to topological surface states, which would make SmB 6 the first realization of topological Kondo insulator. Here by performing angle-resolved photoemission spectroscopy experiments, we directly observe several dispersive states within the hybridization gap of SmB 6 . These states show negligible k z dependence, which indicates their surface origin. Furthermore, we perform photoemission circular dichroism experiments, which suggest that the in-gap states possess chirality of the orbital angular momentum. These states vanish simultaneously with the hybridization gap at around 150 K. Together, these observations suggest the possible topological origin of the in-gap states.

The superconducting gap is a pivotal character for a superconductor. While the cuprates and conventional phonon-mediated superconductors are characterized by distinct d-wave and s-wave pairing symmetry with nodal and nodeless gap distribution respectively, the superconducting gap distributions in iron-based superconductors are rather diversified. While nodeless gap distributions have been directly observed in Ba 1−x K x Fe 2 As 2 , BaFe 2−x Co x As 2 , K x Fe 2−y Se 2 , and FeTe 1−x Se x [1-4], the signatures of nodal superconducting gap have been reported in LaOFeP, LiFeP, KFe 2 As 2 , BaFe 2 (As 1−x P x ) 2 , BaFe 2−x Ru x As 2 and FeSe [5-12]. We here report the angle resolved photoemission spectroscopy (ARPES) measurements on the superconducting gap structure of BaFe 2 (As 1−x P x ) 2 in the momentum space, and particularly, the first direct observation of a circular line node on the largest hole Fermi surface around the Z point at the Brillouin zone boundary. Our data rules out the d-wave pairing origin of the nodal gap, and unify both the nodal and nodeless gaps in iron pnictides under the s ± pairing symmetry.The pairing symmetry of the Cooper pair in a superconductor is manifested in its gap structure. Particularly, nodes or nodal lines of the superconducting gap often imply unconventional (e.g. non-s-wave) pairing symmetries. For most ironbased superconductors, there are electron Fermi surfaces at the Brillouin zone corner and hole Fermi surfaces at the center. It has been proposed that the pairing interactions between the electron and hole Fermi surfaces will induce nodeless s-wave order parameter with opposite signs on them [13][14][15]. While this nodeless s ± -wave pairing symmetry has gained increasing experimental support [16][17][18], nodal gap has been reported in LaOFeP, LiFeP, KFe 2 As 2 , BaFe 2 (As 1−x P x ) 2 , BaFe 2−x Ru x As 2 , and FeSe by thermal conductivity, penetration depth, nuclear magnetic resonance, and scanning tunneling spectroscopy studies [5][6][7][8][9][10][11][12]. However, no direct measurement on any of these compounds has been reported regarding the gap structure so far, and especially the location of the nodes remains unknown. Since BaFe 2 (As 1−x P x ) 2 has relatively high superconducting transition temperature T c , it provides an opportunity for direct access of the detailed gap structure in the momentum space by angle resolved photoemission spectroscopy (ARPES).We have conducted ARPES measurements on BaFe 2 (As 0.7 P 0.3 ) 2 with a T c of 30 K (see Method section for details). As previous detailed polarization dependent studies have shown [19] and replicated here in Fig. 1a, there * Electronic address: dlfeng@fudan.edu.cn

The superconductivity discovered in iron-pnictides is intimately related to a nematic ground state, where the C4 rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with the polarization dependent angle-resolved photoemission spectroscopy. A uniaxial strain was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state, and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy-and dyz-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the dxz-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the development of orbitaldependent spin polarization is likely the dominant force to drive the nematicity, while the ferroorbital ordering between dxz and dyz orbitals can only play a minor role here.

We report the electronic structure of the iron-chalcogenide superconductor, Fe 1.04 ͑Te 0.66 Se 0.34 ͒, obtained with high-resolution angle-resolved photoemission spectroscopy and density-functional calculations. In photoemission measurements, various photon energies and polarizations are exploited to study the Fermi surface topology and symmetry properties of the bands. The measured band structure and their symmetry characters qualitatively agree with our density-functional theory calculations of Fe͑Te 0.66 Se 0.34 ͒, although the band structure is renormalized by about a factor of three. We find that the electronic structures of this iron chalcogenides and the iron pnictides have many aspects in common; however, significant differences exist near the ⌫ point. For Fe 1.04 Te 0.66 Se 0.34 , there are clearly separated three bands with distinct even or odd symmetry that cross the Fermi energy ͑E F ͒ near the zone center, which contribute to three holelike Fermi surfaces. Especially, both experiments and calculations show a holelike elliptical Fermi surface at the zone center. Moreover, no sign of spin density wave was observed in the electronic structure and susceptibility measurements of this compound.