Abstract Through intense research on Weyl semimetals during the past few years, we have come to appreciate that typical Weyl semimetals host many Weyl points. Nonetheless, the minimum nonzero number of Weyl points allowed in a time-reversal invariant Weyl semimetal is four. Realizing such a system is of fundamental interest and may simplify transport experiments. Recently, it was predicted that TaIrTe 4 realizes a minimal Weyl semimetal. However, the Weyl points and Fermi arcs live entirely above the Fermi level, making them inaccessible to conventional angle-resolved photoemission spectroscopy (ARPES). Here, we use pump-probe ARPES to directly access the band structure above the Fermi level in TaIrTe 4 . We observe signatures of Weyl points and topological Fermi arcs. Combined with ab initio calculation, our results show that TaIrTe 4 is a Weyl semimetal with the minimum number of four Weyl points. Our work provides a simpler platform for accessing exotic transport phenomena arising in Weyl semimetals.
Many novel physical phenomena and fascinating properties emerge when materials are thinned down to two dimension (2D) [1,2].Therefore, in the last decade of the 2D materials research, the search for new 2D materials with fascinating physical properties became the scientific mainstream.So far, all known 2D materials have essentially the same atomic structure as a single layer of the corresponding layered bulk form [3,4].
Abstract Due to their low‐symmetry lattice characteristics and intrinsic in‐plane anisotropy, 2D pentagonal materials, a new class of 2D materials composed entirely of pentagonal atomic rings, are attracting increasing research attention. However, the existence of these 2D materials has not been proven experimentally until the recent discovery of PdSe 2 . Herein, penta ‐PdPSe, a new 2D pentagonal material with a novel low‐symmetry puckered pentagonal structure, is introduced to the 2D family. Interestingly, a peculiar polyanion of [SePPSe] 4− is discovered in this material, which is the biggest polyanion in 2D materials yet discovered. Strong intrinsic in‐plane anisotropic behavior endows penta ‐PdPSe with highly anisotropic optical, electronic, and optoelectronic properties. Impressively, few‐layer penta ‐PdPSe‐based phototransistor not only achieves excellent electronic performances, a moderate electron mobility of 21.37 cm 2 V −1 s −1 and a high on/off ratio of up to 10 8 , but it also has a high photoresponsivity of ≈5.07 × 10 3 A W −1 at 635 nm, which is ascribed to the photogating effect. More importantly, penta ‐PdPSe also exhibits a large anisotropic conductance (σ max /σ max = 3.85) and responsivity ( R max / R min = 6.17 at 808 nm), superior to most 2D anisotropic materials. These findings make penta ‐PdPSe an ideal material for the design of next‐generation anisotropic devices.
As scaling down the size of metal oxide semiconductor field-effect transistors (FETs), power dissipation has become a major challenge. Lowering down the sub-threshold swing (SS) is known as an effective technique to decrease the operating voltage of FETs and hence lower down the power consumption. However, the Boltzmann distribution of electrons (so-called 'Boltzmann tyranny') implements a physical limit to the SS value. Use of negative capacitance (NC) effect has enabled a new path to achieve a low SS below the Boltzmann limit (60 mV dec-1at room temperature). In this work, we have demonstrated a NC-FET from an all two-dimensional (2D) metal ferroelectric semiconductor (MFS) vertical heterostructure: Graphene/CuInP2S6/MoS2. The negative capacitance from the ferroelectric CuInP2S6has enabled the breaking of the 'Boltzmann tyranny'. The heterostructure based device has shown steep slopes switching below 60 mV dec-1(lowest to < 10 mV dec-1) over 3 orders of source-drain current, which provides an avenue for all 2D material based steep slope FETs.
Two-dimensional (2D) materials are easily fabricated when their bulk form has a layered structure. The monolayer form in layered transition-metal dichalcogenides is typically the same as a single layer of the bulk material. However, ${\mathrm{PdSe}}_{2}$ presents a puzzle. Its monolayer form has been theoretically shown to be stable, but there have been no reports that monolayer ${\mathrm{PdSe}}_{2}$ has been fabricated. Here, combining atomic-scale imaging in a scanning transmission electron microscope and density functional theory, we demonstrate that the preferred monolayer form of this material amounts to a melding of two bulk monolayers accompanied by the emission of Se atoms so that the resulting stoichiometry is ${\mathrm{Pd}}_{2}{\mathrm{Se}}_{3}$. We further verify the interlayer melding mechanism by creating Se vacancies in situ in the layered ${\mathrm{PdSe}}_{2}$ matrix using electron irradiation. The discovery that strong interlayer interactions can be induced by defects and lead to the formation of new 2D materials opens a new venue for the exploration of defect engineering and novel 2D structures.
Mid-infrared (MIR) photodetection is of significance in civil and military applications because it shows superiority in absorbing the vibration of various molecules and covering atmospheric transmission windows. Recently, the PtTe2, a typical type-II Dirac semimetal, has come under the spotlight due to its unique photodetection sensibility in the MIR region and robust stability in the atmosphere. Here, the high-quality and large-scale 1T-PtTe2 thin films with air stability were grown by molecular beam epitaxy. Broadband photoresponse of the photodetectors of PtTe2 from 420 nm to 10.7 μm shows high responsivity and detectivity of 0.2 mA W–1 and 2.6 × 107 Jones at 10.7 μm and 1.6 mA W–1 and 2.2 × 108 Jones at 4.7 μm under the atmosphere, respectively. Moreover, the photodetectors exhibit high sensitivity in visible and near-infrared regions (8.2 mA W–1 at 650 nm and 15.6 mA W–1 at 960 nm). The power- and polarization-dependent photoresponse measurements reveal the linear relationship of power photoresponse and obvious anisotropic photoresponse (the ratio of anisotropy ellipse is 8.3 at 10.7 μm), respectively. These results suggest that the PtTe2 could be expected to be an advanced photodetection material for polarization angle-sensitive detection, infrared imaging, and photodetection from the visible to MIR range.
An optical refractive index (RI) nanosensor with a high sensitivity and figure of merit (FOM), good stability, and biocompatibility is of great significance for biological detection and sensing in narrow spaces. However, the current optical RI nanosensors are mainly fabricated using metals, semiconductors, and quartz, which are not biocompatible and are even biotoxic, and often face a trade-off between a high sensitivity and a high FOM. Moreover, the sensors are mainly based on surface plasmon resonance, photonic crystals, fiber grating, etc., and, thus, most of them usually require a laser source with a specific optical wavelength or harsh excitation conditions, which are likely to cause photodamage and are unfavorable for biological applications. Hence, polylactic acid (PLA), a flexible dielectric material with good biocompatibility, is functioned by doping high refractive index quantum dots (QDs) and fabricated as a nanowire RI sensor. Doping the QDs into a PLA nanowire can improve the light confinement ability and then enhance Mie resonant scattering of the PLA nanowire, which is very beneficial to obtain a higher quality factor and then a higher-performance nanowire sensor. Under irradiation of a white light source, a high sensitivity with 833.78 nm/RIU (per refractive index unit) and the highest FOM of 9.64 RIU−1 are obtained. The good reliability and reproducibility of the sensors are further demonstrated. By choosing a proper diameter, the scattering peak of the nanosensor can be tuned into a biofriendly spectral range (600–900 nm), which predicts that the PLA nanowire RI sensors have a great potential in biological microenvironment monitoring, biosensing, and biomedical treatment.
The electromagnetic spectrum between microwave and infrared light is termed the "terahertz (THz) gap," of which there is an urgent lack of feasible and efficient room-temperature (RT) THz detectors. Type-II Weyl semimetals (WSMs) have been predicted to host significant RT topological photoresponses in low-frequency regions, especially in the THz gap, well addressing the shortcomings of THz detectors. However, such devices have not been experimentally realized yet. Herein, a type-II WSM (NbIrTe4 ) is selected to fabricate THz detector, which exhibits a photoresponsivity of 5.7 × 104 V W-1 and a one-year air stability at RT. Such excellent THz-detection performance can be attributed to the topological effect of type-II WSM in which the effective mass of photogenerated electrons can be reduced by the large tilting angle of Weyl nodes to further improve mobility and photoresponsivity. Impressively, this device shows a giant intrinsic anisotropic conductance (σmax /σmin = 339) and THz response (Iph-max /Iph-min = 40.9), both of which are record values known. The findings open a new avenue for the realization of uncooled and highly sensitive THz detectors by exploring type-II WSM-based devices.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='translate(288.889 -214.211)'/%3e%3cuse xlink:href='%23a' transform='translate(108 342.749)'/%3e%3cuse xlink:href='%23a' transform='translate(469.189 342.749)'/%3e%3c/svg%3e)