Ballistic PbTe Nanowire Devices.

Disorder is the primary obstacle in the current Majorana nanowire experiments. Reducing disorder or achieving ballistic transport is thus of paramount importance. In clean and ballistic nanowire devices, quantized conductance is expected, with plateau quality serving as a benchmark for disorder assessment. Here, we introduce ballistic PbTe nanowire devices grown by using the selective-area-growth (SAG) technique. Quantized conductance plateaus in units of 2e2/h are observed at zero magnetic field. This observation represents an advancement in diminishing disorder within SAG nanowires as most of the previously studied SAG nanowires (InSb or InAs) have not exhibited zero-field ballistic transport. Notably, the plateau values indicate that the ubiquitous valley degeneracy in PbTe is lifted in nanowire devices. This degeneracy lifting addresses an additional concern in the pursuit of Majorana realization. Moreover, these ballistic PbTe nanowires may enable the search for clean signatures of the spin-orbit helical gap in future devices.

Plateau Regions for Zero-Bias Peaks within 5% of the Quantized Conductance Value 2e

Probing an isolated Majorana zero mode is predicted to reveal a tunneling conductance quantized at 2e^{2}/h at zero temperature. Experimentally, a zero-bias peak (ZBP) is expected and its height should remain robust against relevant parameter tuning, forming a quantized plateau. Here, we report the observation of large ZBPs in a thin InAs-Al hybrid nanowire device. The ZBP height can stick close to 2e^{2}/h, mostly within 5% tolerance, by sweeping gate voltages and magnetic field. We further map out the phase diagram and identify two plateau regions in the phase space. Despite the presence of disorder and quantum dots, our result constitutes a step forward toward establishing Majorana zero modes.

Asymmetric Modulation on Exchange Field in a Graphene/BiFeO3 Heterostructure by External Magnetic Field.

Graphene, having all atoms on its surface, is favorable to extend the functions by introducing the spin-orbit coupling and magnetism through proximity effect. Here, we report the tunable interfacial exchange field produced by proximity coupling in graphene/BiFeO3 heterostructures. The exchange field has a notable dependence with external magnetic field, and it is much larger under negative magnetic field than that under positive magnetic field. For negative external magnetic field, interfacial exchange coupling gives rise to evident spin splitting for N ≠ 0 Landau levels and a quantum Hall metal state for N = 0 Landau level. Our findings suggest graphene/BiFeO3 heterostructures are promising for spintronics.

MoS2 Memtransistors Fabricated by Localized Helium Ion Beam Irradiation.

Two-dimensional (2D) layered semiconductors have recently emerged as attractive building blocks for next-generation low-power nonvolatile memories. However, challenges remain in the controllable fabrication of bipolar resistive switching circuit components from these materials. Here, the experimental realization of lateral memtransistors from monolayer single-crystal molybdenum disulfide (MoS2) utilizing a focused helium ion beam is reported. Site-specific irradiation with the focused probe of a helium ion microscope creates a nanometer-scale defect-rich region, bisecting the MoS2 lattice. The reversible drift of these defects in the applied electric field modulates the resistance of the channel, enabling versatile memristive functionality. The device can reliably retain its resistance ratios and set/reset biases for 1180 switching cycles. Long-term potentiation and depression with sharp habituation are demonstrated. This work establishes the feasibility of ion irradiation for controllable fabrication of 2D memristive devices with promising key performance parameters, such as low power consumption. The applicability of these devices for synaptic emulation may address the demands of future neuromorphic architectures.

Ultrafast growth of single-crystal graphene assisted by a continuous oxygen supply.

Graphene has a range of unique physical properties and could be of use in the development of a variety of electronic, photonic and photovoltaic devices. For most applications, large-area high-quality graphene films are required and chemical vapour deposition (CVD) synthesis of graphene on copper surfaces has been of particular interest due to its simplicity and cost effectiveness. However, the rates of growth for graphene by CVD on copper are less than 0.4 µm s-1, and therefore the synthesis of large, single-crystal graphene domains takes at least a few hours. Here, we show that single-crystal graphene can be grown on copper foils with a growth rate of 60 µm s-1. Our high growth rate is achieved by placing the copper foil above an oxide substrate with a gap of ∼15 µm between them. The oxide substrate provides a continuous supply of oxygen to the surface of the copper catalyst during the CVD growth, which significantly lowers the energy barrier to the decomposition of the carbon feedstock and increases the growth rate. With this approach, we are able to grow single-crystal graphene domains with a lateral size of 0.3 mm in just 5 s.

Lateral graphene p-n junctions formed by the graphene/MoS2 hybrid interface.

Graphene/two-dimensional (2D) semiconductor heterostructures have been demonstrated to possess many advantages for electronic and optoelectronic devices. However, there are few reports about the utilization of a 2D semiconductor monolayer to tune the properties of graphene. Here, we report the fabrication and characterization of graphene p-n junctions based on graphene/MoS2 hybrid interfaces. Monolayered graphene across the monolayered MoS2 boundary is divided into n-type regions on the MoS2 and p-type regions on the SiO2 substrate. Such van der Waals heterostructure based graphene p-n junctions show good photoelectric properties. The photocurrent modulation of such devices by a single back gate is also demonstrated for the first time, which shows that the graphene on and off MoS2 regions have different responses to the gate voltage. Our results suggest that the atomic thin hybrid structure can remarkably extend the device applications.