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Stoichiometric Cr2Se3 single crystals are particular layer-structured antiferromagnets which possess noncolinear spin configuration, weak ferromagnetic moments, moderate magnetoresistance (MR ~ 14.3%), and bad metallic conductivity below the antiferromagnetic phase transition temperature. Here, we report an interesting >16000% colossal magnetoresistance (CMR) effect in Ti (1.5 atomic percent) lightly-doped Cr2Se3 single crystals. Such a CMR is approximately 1143 times larger than that of the stoichiometric Cr2Se3 crystals and is rarely observed in layered antiferromagnets and is attributed to the frustrated spin configuration. Moreover, the Ti doping not only dramatically changes the electronic conductivity of the Cr2Se3 crystal from a bad metal to a semiconductor with a gap of ~ 15 meV, but also induces a change of the magnetic anisotropy of the Cr2Se3 crystal from strong out-of-plane to weak in plane. Further, magnetotransport measurements reveal that the low-field MR scales with the square of the reduced magnetization, which is a signature of CMR materials. The layered Ti:Cr2Se3 with CMR effect could be used as 2D heterostructure building blocks to provide colossal negative MR in spintronic devices.

Spin-orbit interaction (SOI) offers a nonferromagnetic scheme to realize spin polarization through utilizing an electric field. Electrically tunable SOI through electrostatic gates have been investigated, however, the relatively weak and volatile tunability limit its practical applications in spintronics. Here, we demonstrate the nonvolatile electric-field control of SOI via constructing ferroelectric Rashba architectures, i.e., 2D Bi2O2Se/PMN-PT ferroelectric field effect transistors. The experimentally observed weak antilocalization (WAL) cusp in Bi2O2Se films implies the Rashba-type SOI that arises from asymmetric confinement potential. Significantly, taking advantage of the switchable ferroelectric polarization, the WAL-to-weak localization (WL) transition trend reveals the competition between spin relaxation and dephasing process, and the variation of carrier density leads to a reversible and nonvolatile modulation of spin relaxation time and spin splitting energy of Bi2O2Se films by this ferroelectric gating. Our work provides a scheme to achieve nonvolatile control of Rashba SOI with the utilization of ferroelectric remanent polarization.

We report the synthesis of transition-metal-doped ferromagnetic elemental single-crystal semiconductors with quantum oscillations using the physical vapor transport method. The 7.7 atom% Cr-doped Te crystals (Cr_Te) show ferromagnetism, butterfly-like negative magnetoresistance in the low temperature (< 3.8 K) and low field (< 0.15 T) region, and high Hall mobility, e.g., 1320 cm2 V-1 s-1 at 30 K and 350 cm2 V-1 s-1 at 300 K, implying that Cr_Te crystals are ferromagnetic elemental semiconductors. When B // c // I, the maximum negative MR is -27% at T = 20 K and B = 8 T. In the low temperature semiconducting region, Cr_Te crystals show strong discrete scale invariance dominated logarithmic quantum oscillations when the direction of the magnetic field B is parallel to the [100] crystallographic direction and show Landau quantization dominated Shubnikov-de Haas (SdH) oscillations for B // [210] direction, which suggests the broken rotation symmetry of the Fermi pockets in the Cr_Te crystals. The findings of coexistence of multiple quantum oscillations and ferromagnetism in such an elemental quantum material may inspire more study of narrow bandgap semiconductors with ferromagnetism and quantum phenomena., Comment: 25 pages, 25 figures

Charge-based field-effect transistors (FETs) greatly suffer from unavoidable carrier scattering and heat dissipation. In analogy to valley degree of freedom in semiconductors, chiral anomaly current in Weyl/Dirac semimetals is theoretically predicted to be nearly non-dissipative over long distances, but still lacks experimental ways to efficiently control its transport. Here we demonstrate field-effect chirality devices with Dirac semimetal PtSe2, in which its Fermi level is close to the Dirac point in conduction band owing to intrinsic defects. The chiral anomaly is further corroborated with nonlocal valley transport measurement, which can also be effectively modulated by external fields, showing robust nonlocal valley transport with micrometer diffusion length. Similar to charge-based FETs, the chiral conductivity in PtSe2 devices can be modulated by electrostatic gating with an ON/OFF ratio more than 103. We also demonstrate basic logic functions in the devices with electric and magnetic fields as input signals.

The inferior electrical contact to two-dimensional (2D) materials is a critical challenge for their application in post-silicon very large-scale integrated circuits. Electrical contacts were generally related to their resistive effect, quantified as contact resistance. With a systematic investigation, this work demonstrates a capacitive metal-insulator-semiconductor (MIS) field-effect at the electrical contacts to 2D materials: the field-effect depletes or accumulates charge carriers, redistributes the voltage potential, and give rise to abnormal current saturation and nonlinearity. On the one hand, the current saturation hinders the devices' driving ability, which can be eliminated with carefully engineered contact configurations. On the other hand, by introducing the nonlinearity to monolithic analog artificial neural network circuits, the circuits' perception ability can be significantly enhanced, as evidenced using a COVID-19 critical illness prediction model. This work provides a comprehension of the field-effect at the electrical contacts to 2D materials, which is fundamental to the design, simulation, and fabrication of electronics based on 2D material.

In this work, we demonstrate a dual-gated (DG) MoS2 field effect transistors (FETs) in which the degraded switching performance of multilayer MoS2 can be compensated by the DG structure. It produces large current density (>100 {\mu}A/{\mu}m for a monolayer), steep subthreshold swing (SS) (~100 mV/dec for 5 nm thickness), and high on/off current ratio (greater than 107 for 10 nm thickness). Such DG structure not only improves electrostatic control but also provides an extra degree of freedom for manipulating the threshold voltage (VTH) and SS by separately tuning the top and back gate voltages, which are demonstrated in a logic inverter. Dynamic random access memory (DRAM) has a short retention time because of large OFF-state current in the Si MOSFET. Based on our DG MoS2-FETs, and a DRAM unit cell with a long retention time of 1260 ms are realized. A large-scale isolated MoS2 DG-FETs based on CVD-synthesized continuous films is also demonstrated, which shows potential applications for future wafer-scale digital and low-power electronics.

At least four two- or quasi-one- dimensional allotropes and a mixture of them were theoretically predicted or experimentally observed for low-dimensional Te, namely the {\alpha}, \b{eta}, {\gamma}, {\delta} and chiral-{\alpha}+{\delta} phases. Among them the {\gamma} and {\alpha} phases were found the most stable phases for monolayer and thicker layers, respectively. Here, we found two novel low-dimensional phases, namely the {\epsilon} and {\zeta} phases. The {\zeta} phase is over 29 meV/Te more stable than and the {\epsilon} phase shows comparable stability with the most stable monolayer {\gamma} phase. The energetic difference between the {\zeta} and {\alpha} phases reduces with respect to the increased layer thickness and vanishes at the four-layer (12-sublayer) thickness, while this thickness increases under change doping. Both {\epsilon} and {\zeta} phases are metallic chains and layers, respectively. The {\zeta} phase, with very strong interlayer coupling, shows quantum well states in its layer-dependent bandstructures. These results provide significantly insight into the understanding of polytypism in Te few-layers and may boost tremendous studies on properties of various few-layer phases., Comment: 3 figures with 9 SI figures and 3 SI tables

Recently, a covalent-like quasi-bonding was unveiled for the inter-chain interaction in a promising semiconductor, few-layer Tellurium. Such quasi-bond offers comparable bond lengths and strengths with those of a typical Te-Te covalent bond, which may lead to much easier transformations between the quasi-bonds and the covalent bonds. Here, we show a few structural phase-transitions among four Te allotropes in few-layer Te under charge doping. In particular, the semiconducting {\alpha}-phase can transform into a smaller-bandgap {\beta}-phase, even-smaller-bandgap {\gamma}-phase and then metallic {\delta}-phase in a Te bilayer. In a tri-layer, a metallic chiral {\alpha}+{\delta} phase is more stable under initial electron doping, leading to the appearance of chirality. Variations of electronic structures aside, these transitions are accompanied by the emergence or elimination of inversion centers ({\alpha}-{\beta}, {\alpha}-{\gamma}, {\alpha}-{\alpha}+{\delta}), structural anisotropy ({\alpha}-{\gamma}, {\gamma}-{\delta}) and chirality ({\alpha}-{\alpha}+{\delta}), which could result in substantial changes in optical and other properties. In light of this, this work opens the possibility toward building mono-elemental electronic and optoelectronic heterostructures or devices. It also offers a platform for studying relations between charge doping and electric/optical properties.