van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.

The pentatellurides ZrTe 5 and HfTe 5 are layered compounds with one-dimensional transition-metal chains that show a not-yet-understood temperature-dependent transition in transport properties as well as recently discovered properties suggesting topological semimetallic behavior. Here, we report magnetotransport properties for two kinds of ZrTe 5 single crystals grown with the chemical vapor transport (CVT) and the flux method (Flux), respectively. They show distinct transport properties at zero field: The CVT crystal displays a metallic behavior with a pronounced resistance peak and a sudden sign reversal in thermopower at approximately 130 K, consistent with previous observations of the electronic transition; in striking contrast, the Flux crystal exhibits a semiconducting-like behavior at low temperatures and a positive thermopower over the whole temperature range. For both samples, strong effects on the transport properties are observed when the magnetic field is applied along the orthorhombic b and c axes, i.e., perpendicular to the chain direction. Refinements on the single-crystal x-ray diffraction and the measurements of energy dispersive spectroscopy reveal the presence of noticeable Te vacancies in the CVT samples, while the Flux samples are close to the stoichiometry. Analyses on the magnetotransport properties confirm that the carrier densities of the CVT sample are about two orders higher than those of the Flux sample. Our results thus indicate that the widely observed anomalous transport behaviors in pentatellurides actually take place in the Te-deficient samples. For the stoichiometric pentatellurides, our electronic structure calculations show narrow-gap semiconducting behavior, with different transport anisotropies for holes and electrons. For the degenerately doped n-type samples, our transport calculations can result in a resistivity peak and crossover in thermopower from negative to positive at temperatures close to those observed experimentally due to a combination of bipolar effects and different anisotropies of electrons and holes. Our present work resolves the long-standing puzzle regarding the anomalous transport behaviors of pentatellurides, as well as the electronic structure in favor of a semiconducting state.

Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust Dirac line-nodes (with non-trivial topology or near the critical point between topological phase transitions) have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and their dispersions. Interestingly, along directions other than the line-nodes in the 3D BZ, the bulk degeneracy is lifted by spinorbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac line-node semimetal family protected by nonsymmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds.

Recently, the extremely large magnetoresistance observed in transition metal telluride, like WTe2, attracted much attention because of the potential applications in magnetic sensor.Here we report the observation of extremely large magnetoresistance as 3.0×10 4 % measured at 2 K and 9 T magnetic field aligned along [001]-ZrSiS. The significant magnetoresistance change (～1.4×10 4 %) can be obtained when the magnetic field is titled from [001] to [011]-ZrSiS.These abnormal magnetoresistance behaviors in ZrSiS can be understood by electron-hole compensation and the open orbital of Fermi surface. Because of these superior MR properties, ZrSiS may be used in the novel magnetic sensors.Recently, materials scientists observed the extremely large magnetoresistance (MR) in a series of transition metal chalcogenides/telluride, rare-earth monopnictides, for example, WTe2, LaSb. [1][2][3] These studies inspire the hot research on exploring the similar compounds with superior MR properties. From the view point of application, extremely large and sensitive MR are the basic requirements for magnetic memory/sensor devices. [4][5][6] As far as the basic science is concerned, what is the physical origin of extremely large MR in these materials? In addition, there are some strange fermions in these materials, such as Weyl fermion or Dirac nodal line at the reciprocal space. 7-12 A natural question is whether the extremely large MR is related to these un-conventional fermions? Bearing these considerations, we pay our attention to a new material-ZrSiS. In accordance to the theoretical prediction, ZrSiS is a semimetal, and maybe evolve to a weak/strong topological insulator under the external stimuli. 13 A recent angle resolved photoemission spectroscopy study claims that there is a Dirac nodal line feature in ZrSiS. 14 Then we asked several questions. Such as, can we observe the extremely large MR in ZrSiS too? Is MR behavior in ZrSiS different from that in WTe2 that comes from the resonant electron-hole compensation, 1 or MR is only related to Fermi surface topology? 15 To answer these questions, we synthesized the ZrSiS single crystals and measured its magneto-transport property. As described as follows, we observe the unsaturated extremely large MR (3.0×10 4 % at 2 K and 9Tesla magnetic field aligned along [001]-ZrSiS) in ZrSiS, and significant MR change (～ 1.4×10 4 % at 2 K under 9 Tesla magnetic field) when the magnetic field is tilted from [001] to[011]-ZrSiS. The MR is still kept as 11% at the ambient condition. The unsaturated, extremely large and significantly anisotropic MR in ZrSiS can be attributed to the electron-hole compensation, as well as open orbitals in Fermi surface of ZrSiS. Because of these superior MR properties, ZrSiS may be used in the novel Hall/MR magnetic sensors.

No abstract

We report an atomic-scale characterization of ZrTe_{5} by using scanning tunneling microscopy. We observe a bulk band gap of ∼80  meV with topological edge states at the step edge and, thus, demonstrate that ZrTe_{5} is a two-dimensional topological insulator. We also find that an applied magnetic field induces an energetic splitting of the topological edge states, which can be attributed to a strong link between the topological edge states and bulk topology. The relatively large band gap makes ZrTe_{5} a potential candidate for future fundamental studies and device applications.

We have performed a systematic high-momentum-resolution photoemission study on ZrTe5 using 6 eV photon energy. We have measured the band structure near the Γ point, and quantified the gap between the conduction and valence band as 18 ≤ ∆ ≤ 29 meV. We have also observed photonenergy-dependent behavior attributed to final-state effects and the 3D nature of the material's band structure. Our interpretation indicates the gap is intrinsic and reconciles discrepancies on the existence of a topological surface state reported by different studies. The existence of a gap suggests that ZrTe5 is not a 3D strong topological insulator nor a 3D Dirac semimetal. Therefore, our experiment is consistent with ZrTe5 being a 3D weak topological insulator.

single-layered materials have been predicted and realized, such as Si, [21][22][23] Ge, [24,25] Sn, [26] B, [27,28] Hf, [29] and Te, [30] but few of them share the same crystal structure as BP.It is believed that BP-structured monolayer (α-allotrope) can be formed in other group V elements, such as Bi (bismuthene), Sb (antimonene), or As (arsenene), and many theoretical efforts have been made to predict their structures and properties. [31][32][33][34][35][36][37] Comparing to their β-allotrope of hexagonal honeycomb structure that has been widely studied experimentally, [38][39][40][41][42][43][44] it still remains challenging to fabricate the large-scale and highquality monolayer α-allotrope of these group V monoelements, [36] even though small patches of the α-allotrope has been observed in some mixed structures. [45] In this study, we successfully synthesize the large-scale and high-quality α-antimonene with puckered BP structure on the T d -WTe 2 substrate, by using molecular beam epitaxy (MBE). In our experiment, the thickness of BP-structured antimonene can be well controlled in a layer-by-layer fashion. Owing to the high quality and large scale of the Sb monolayer, it becomes possible to map the electronic band structure via quasiparticle interference (QPI) with scanning tunneling microscopy (STM). The α-antimonene exhibits a hole-doped nature with a linearly dispersed band crossing the Fermi level and a high electrical Atomically thin 2D crystals have gained tremendous attention owing to their potential impact on future electronics technologies, as well as the exotic phenomena emerging in these materials. Monolayers of α-phase Sb (α-antimonene), which shares the same puckered structure as black phosphorous, are predicted to be stable with precious properties. However, the experimental realization still remains challenging. Here, high-quality monolayerα-antimonene is successfully grown, with the thickness finely controlled. The α-antimonene exhibits great stability upon exposure to air. Combining scanning tunneling microscopy, density functional theory calculations, and transport measurements, it is found that the electron band crossing the Fermi level exhibits a linear dispersion with a fairly small effective mass, and thus a good electrical conductivity. All of these properties make the α-antimonene promising for future electronic applications. AntimoneneSpurred by their prospect in electronic technologies, 2D crystals have been attracting increasing attentions. As the thickness is decreased down to the single-layer limit, 2D crystals usually exhibit different electronic properties from their bulk counterparts. [1] Exotic phenomena are also expected in single-layered materials, such as the quantum spin Hall effect, [2][3][4][5][6] 2D superconductivity, [7,8] charge density wave, [9][10][11][12] or magnetism. [13,14] Following the discovery of graphene, [15,16] black phosphorus (BP) has been revived as a potential candidate for optoelectronics and field-effect transistor (FET) applications, [17][18][19][...