Grain boundaries can markedly affect the electronic, thermal, mechanical and optical properties of a polycrystalline graphene. While in many applications the presence of grain boundaries in graphene is undesired, here we show that they have an ideal structure for the detection of chemical analytes. We observe that an isolated graphene grain boundary has B300 times higher sensitivity to the adsorbed gas molecules than a single-crystalline graphene grain. Our electronic structure and transport modelling reveal that the ultrasensitivity in grain boundaries is caused by a synergetic combination of gas molecules accumulation at the grain boundary, together with the existence of a sharp onset energy in the transmission spectrum of its conduction channels. The discovered sensing platform opens up new pathways for the design of nanometre-scale highly sensitive chemical detectors.
The quantum anomalous Hall (QAH) effect is a quintessential consequence of non-zero Berry curvature in momentum-space. The QAH insulator harbors dissipation-free chiral edge states in the absence of an external magnetic field. On the other hand, the topological Hall (TH) effect, a transport hallmark of the chiral spin textures, is a consequence of realspace Berry curvature. While both the QAH and TH effects have been reported separately, their coexistence, a manifestation of entangled chiral edge states and chiral spin textures, has not been reported. Here, by inserting a TI layer between two magnetic TI layers to form a sandwich heterostructure, we realized a concurrence of the TH effect and the QAH effect through electric field gating. The TH effect is probed by bulk carriers, while the QAH effect is characterized by chiral edge states. The appearance of TH effect in the QAH insulating regime is the consequence of chiral magnetic domain walls that result from the gate-induced Dzyaloshinskii-Moriya interaction and occur during the magnetization reversal process in the magnetic TI sandwich samples. The coexistence of chiral edge states and chiral spin textures potentially provides a unique platform for proof-of-concept dissipationless spintextured spintronic applications. Electronic band structures of nontrivial topology in momentum-space and magnetic chiral spin textures in real-space have attracted enormous attention in the past decade since they harbor elegant Berry curvature physics 1, 2, 3 . The intrinsic anomalous Hall (AH) effect is such an example: it is induced by the Berry curvature in momentum-space in ferromagnetic (FM) materials 4 and can even be quantized under certain circumstances, leading to the quantum anomalous Hall (QAH)effect. The QAH effect has been theoretically proposed 5, 6, 7, 8 and experimentally realized 9,10,11,12,13 in magnetically doped topological insulator (TI) films. On the other hand, chiral spin textures (e.g. skyrmions) provide another example of nontrivial topology, but in real-space. It has been shown that chiral spin textures can also induce a Hall current: this is known as the topological Hall (TH) effect and is generally regarded as the transport signature of non-zero spin chirality 3 . The TH effect has been experimentally observed in many metallic systems, such as MnSi 14, 15 , MnGe 16 , FeGe 17 , and SrIrO3/SrRuO3 interface 18,19 as well as magnetically doped TI films and heterostructures 20, 21 . The TH effect in these systems is usually observed accompanied by the AH effect. However, there is no conclusive evidence to date that the AH effect found in these metallic systems to be intrinsic, i.e., exclusively induced by the momentum-space Berry curvature 4 .The QAH and TH effects have been separately observed in magnetically doped TI 9, 10, 20, 21 , with distinctly different sample geometries. The QAH effect can be realized only in the insulating regime of a magnetic TI 9,10,11,12,13 while the TH effect is usually seen in metallic systems 20, 21 .In this Article, we re...
A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface.
Over the past few years, there has been a growing interest in layered transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS 2 ). Most studies so far have focused on the electronic and optoelectronic properties of single-layer MoS 2 , whose band structure features a direct bandgap, in sharp contrast to the indirect bandgap of thicker MoS 2 . In this paper, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of few-layer MoS 2 . We observe that the electrical conductivity ( ) increases as we reduce the thickness of MoS 2 and peaks at about two layers, with six-time larger conductivity than our thickest sample (23-layer MoS 2 ). Using a back-gate voltage, we modulate the Fermi energy ( # ) of the sample where an increase in the Seebeck coefficient ( ) is observed with decreasing gate voltage ( # ) towards the subthreshold (OFF state) of the device, reaching as large as 500 µV/K in a four-layer MoS 2 .While previous reports have focused on a single-layer MoS 2 and measured Seebeck coefficient in the OFF state, which has vanishing electrical conductivity and thermoelectric power factor ( = / ), we show that MoS 2 -based devices in their ON state can have as large as > 50 12 345 6 in the two-layer sample. The increases with decreasing thickness then drops abruptly from double-layer to single-layer MoS 2 , a feature we suggest as due to a change in the energy dependence of the electron mean-free-path according to our theoretical calculation. Moreover, we show that care must be taken in thermoelectric measurements in the OFF state to avoid obtaining erroneously large Seebeck coefficients when the channel resistance is very high. Our study paves the way towards a more comprehensive examination of the thermoelectric performance of two-dimensional (2D)semiconductors.3
We have investigated the low frequency (f) flicker (also called 1/f) noise of single-layer graphene devices on h-BN (placed on SiO 2 /Si) along with those on SiO 2 /Si. We observe that the devices fabricated on h-BN have on average one order of magnitude lower noise amplitude compared with devices fabricated on SiO 2 /Si. We associate this noise reduction to the lower densities of impurities and trap sites in h-BN than in SiO 2 .Furthermore, the gate voltage dependent noise amplitude shows a broad maximum at Dirac point for devices on h-BN, in contrast to the M-shaped behavior showing a minimum at Dirac point for devices on SiO 2 , consistent with the reduced charge inhomogeneity (puddles) for graphene on h-BN. This study demonstrates that the use of h-BN as a substrate or dielectric can be a simple and efficient noise reduction technique valuable for electronic applications of graphene and other nanomaterials.3
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