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The quantum anomalous Hall (QAH) effect is a consequence of non-zero Berry curvature in momentum space. The QAH insulator harbours dissipation-free chiral edge states in the absence of an external magnetic field. However, the topological Hall (TH) effect, a hallmark of chiral spin textures, is a consequence of real-space Berry curvature. Here, by inserting a topological insulator (TI) layer between two magnetic TI layers, we realized the concurrence of the TH effect and the QAH effect through electric-field gating. The TH effect is probed by bulk carriers, whereas the QAH effect is characterized by chiral edge states. The appearance of the TH effect in the QAH insulating regime is a consequence of chiral magnetic domain walls that result from the gate-induced Dzyaloshinskii–Moriya interaction and occurs during the magnetization reversal process in the magnetic TI sandwich samples. The coexistence of chiral edge states and chiral spin textures provides a platform for proof-of-concept dissipationless spin-textured spintronic applications. The coexistence of chiral edge states and chiral spin textures in magnetic topological insulator sandwiches provides a platform for proof-of-concept dissipationless spin-textured spintronic applications.

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.

Three-dimensional topological insulators (TIs) in proximity with superconductors are expected to exhibit exotic phenomena, such as topological superconductivity (TSC) and Majorana-bound states (MBS), which may have applications in topological quantum computation. In superconductor–TI–superconductor Josephson junctions, the supercurrent versus the phase difference between the superconductors, referred to as the current–phase relation (CPR), reveals important information including the nature of the superconducting transport. Here, we study the induced superconductivity in gate-tunable Josephson junctions (JJs) made from topological insulator BiSbTeSe2 with superconducting Nb electrodes. We observe highly skewed (non-sinusoidal) CPR in these junctions. The critical current, or the magnitude of the CPR, increases with decreasing temperature down to the lowest accessible temperature (T ~ 20 mK), revealing the existence of low-energy modes in our junctions. The gate dependence shows that close to the Dirac point the CPR becomes less skewed, indicating the transport is more diffusive, most likely due to the presence of electron/hole puddles and charge inhomogeneity. Our experiments provide strong evidence that superconductivity is induced in the highly ballistic topological surface states (TSS) in our gate-tunable TI-based JJs. Furthermore, the measured CPR is in good agreement with the prediction of a model which calculates the phase-dependent eigenstate energies in our system, considering the finite width of the electrodes, as well as the TSS wave functions extending over the entire circumference of the TI.

The transition metal carbide (TMC) family has been previously studied for various catalytic, mechanical, and electronic applications, and recently TMCs have been isolated into the ultrathin limit. A bottom-up approach has been developed to synthesize non-layered, ultrathin TMCs (UThTMCs). In this work, liquid metal assisted chemical vapor deposition is used to control the growth of different phases of tungsten carbide. In particular, WC and W 2 C single crystal nanoplates are synthesized when using copper and gallium, respectively, as the tungsten diffusion barrier. First principles calculations confirm the stability found experimentally in the two synthesized carbide phases. We also report the first low temperature measurements of electronic transport in WC below 300 mK and ultrathin W 2 C down to 1.8 K. We find that WC does not enter a superconducting state, while UThTMCs of W 2 C enter a quasi-2D superconducting state below 2.8 K. Our results provide new ground on the synthesis of other UThTMCs with controlled crystal phase. Given the richness of phases among metal carbides and the related transition metal nitride family, further research will be motivated by this work to isolate novel carbide and nitride phases in this ultrathin limit. Finally, it is important to note that the electronic transport of UThTMCs follows a quasi-2D regime and further systems should be evaluated and compared, especially the superconducting phases. These UThTMCs and their heterointerfaces could find important applications in electrocatalysis, carbide plasmonics, transparent conducting films, and materials for effective radiation shielding due to their high density and ability to absorb neutrons and gamma rays.

A quantum anomalous Hall (QAH) insulator is a topological phase in which the interior is insulating but electrical current flows along the edges of the sample in either a clockwise or counterclockwise direction, as dictated by the spontaneous magnetization orientation. Such a chiral edge current eliminates any backscattering, giving rise to quantized Hall resistance and zero longitudinal resistance. Here we fabricate mesoscopic QAH sandwich Hall bar devices and succeed in switching the edge current chirality through thermally assisted spin–orbit torque (SOT). The well-quantized QAH states before and after SOT switching with opposite edge current chiralities are demonstrated through four- and three-terminal measurements. We show that the SOT responsible for magnetization switching can be generated by both surface and bulk carriers. Our results further our understanding of the interplay between magnetism and topological states and usher in an easy and instantaneous method to manipulate the QAH state. Thermally assisted spin–orbit torque is used to switch the edge current chirality in mesoscopic quantum anomalous Hall devices.

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 ~300 times higher sensitivity to the adsorbed gas molecules than a single-crystalline graphene grain. Our electronic structure and transport modelling reveal that the ultra-sensitivity 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. Grain boundaries in graphene are present between misaligned crystalline areas, and influence the resulting properties, often in a negative fashion. Here, the authors use these boundaries for chemical detection, observing markedly higher sensitivities as compared with single-crystalline domains.

Over the past few years there has been a growing interest in layered two dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and three dimensional topological insulators (TIs). In this thesis, we experimentally study electrical, thermoelectric, and phase coherent transport in these 2D materials and work on three main projects. First, we investigate the low frequency (f) flicker (also called 1/f) noise of single-layer graphene devices on h-BN along with those on SiO2/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 SiO2/Si despite having comparable mobility at room temperature, a result that can be associated with the lower densities of impurities and trap sites in h-BN. Our 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 2D materials.Secondly, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of single- and few-layer MoS2 . We observe that the electrical conductivity (sigma) increases as we reduce the thickness of MoS2 and peaks at about two layers, with six times larger conductivity than the bulk. We also show that the thermoelectric power factor (PF) increases with decreasing thickness then drops abruptly from double-layer to single-layer MoS2 , a feature, which according to our theoretical modeling, is due to a change in the energy dependence of the electron mean-free-path.Lastly, we focus on Josephson effects and phase coherent transport in bulk-insulating topological insulator BiSbTeSe2 flakes and nanoribbons (TINRs) with superconducting Nb contacts. We observe an ambipolar field effect critical current (IC) and multiple Andreev reflections (MAR), indicating high quality of the junctions including the TI-superconductor interfaces. We also study the nature of the induced superconductivity in such junctions, where we observe (i) an anomalous low-temperature enhancement of IC, (ii) Aharonov-Bohm oscillations of the normal-state resistance and IC in TINR-based Josephson junctions, and (iii) highly skewed (non-sinusoidal) current-phase relation in TI-based junctions, revealing the induced superconductivity is carried by ballistic topological surface states (TSS) of the TI/TINR. Such TSS in TI-based junctions are predicted to support topological superconductivity and host Majorana fermions, particles that are their own anti-particles and hence are of paramount importance in topological quantum computing applications.

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.

A Josephson junction, formed between two phase-biased superconductors and a normal metal, hosts a discrete spectrum of Andreev bound states (ABS). In this paper, we develop a theory for long ballistic Andreev interferometers in two-dimensional metals. We consider three frameworks in our theoretical analysis: (i) perturbation theory in the tunneling amplitudes; (ii) non-perturbative transport theory; and (iii) physically motivated approximations to visualize the conductance maps in the (flux, voltage) plane. We find a non-standard phase-sensitive Andreev reflection process in ballistic interferometers that couples the supercurrent to the non-equilibrium populations of the ABS in the normal region. Furthermore, our model shows that conductance spectroscopy follows the spectrum of the ABS in long junctions. We also discuss our results in terms of the semiclassical theory, the classical orbits being the one-dimensional Andreev tubes. Our theoretical analysis captures the results of recent experiments by the Penn State and Harvard groups.

Motivated by recent experiments on multiterminal Josephson junctions (MJJs) that probe different ranges of the size and bias voltage parameters, we explore the regime of increasing bias voltage in large-scale devices, where the electrochemical potential becomes comparable to the 1D energy level spacing. We find that the relative number of quantum-correlated pairs formed by colliding Floquet--Kulik quartet levels is equal to the inverse of the number of channels. This observation motivates a model for the intermediate regime in which the ballistic central two-dimensional normal metal is treated as a continuum under the adiabatic approximation, while Andreev modes propagate in a background of voltage- and flux-tunable nonequilibrium electronic populations. The model predicts characteristic voltage scales that govern the mesoscopic oscillations of the critical current, and these scales are at the crossroads of interpreting experiments in all sectors of the MJJs: quartets, topology, and Floquet theory. Our model is specifically inspired by the recent Harvard and Penn State group experiments.