Jonathan Paul Bird

Electrostatic control of the band alignment in two-dimensional van der Waals (vdW) heterostructures is a fundamental requirement for realizing low-power tunnelling electronics, multi-valued logic and next-generation functional devices. Yet, its experimental demonstration in scalable chemical vapour deposition (CVD)-grown heterostructures remains largely unexplored. Here, we demonstrate a simple heterostructure field-effect transistor based on deterministically transferred CVD-grown monolayers of WSe2 and MoS2. These materials are stacked using a contact-first, transfer-later fabrication strategy that enables localized electrostatic modulation of the heterointerface. The fabricated device exhibits two distinct drain-current minima in its transfer characteristics, with the first minimum occurring at a fixed back-gate voltage, while the second minimum shifts with drain voltage. Analysis around the first current minimum reveals a pronounced negative differential transconductance (NDT), with a peak-to-valley current ratio (PVCR) of 16 at room temperature, providing a clear electrical signature of band-to-band tunnelling (BTBT). The observed transport behaviour originates from strong electrostatic doping induced by the back gate, which drives the WSe2/MoS2 heterointerface from a staggered (type-II) to a broken (type-III) bandgap alignment, enabling interlayer BTBT under large negative gate bias. As the gate voltage is increased, the loss of the broken-gap alignment terminates the tunnelling process, producing a gate-voltage-pinned current minimum and the associated NDT response. These findings establish that CVD-grown vdW heterostructures can achieve controllable electrostatic band engineering and room-temperature tunnelling phenomena using a simple single-back-gate architecture, providing an experimentally practical pathway towards future low-power tunnelling transistors, multi-valued logic circuits, and electrically reconfigurable two-dimensional electronic devices.

We demonstrate negative differential conductance (NDC), consistent with extreme band filling, in titanium trisulfide (TiS3) nanowire field-effect transistors (FETs) subject to ionic gating. Electronic-structure calculations connect the NDC to the narrow and nonparabolic nature of the conduction band states in TiS3. These exhibit inflection points at relatively low energies, which become accessible when ionic gating induces extreme carrier concentration (∼1020 cm–3). In contrast with the Gunn effect, which requires electron–phonon scattering to transfer electrons between conduction-band valleys to initiate NDC, the mechanism here derives solely from single-carrier, intraband dynamics. As such, the potential should exist to exploit its associated NDC in ultrafast (i.e., terahertz) electronic devices. Since the NDC ultimately results from the heavily d-orbital character of the conduction-band states of TiS3, this phenomenon need not be unique to this material. This should motivate future studies of other materials with similarly strong nonparabolicity.

In this work, we explore the potential of MoSe₂-based multilayer field-effect transistors (FETs) for application in on-chip thermal sensing. By implementing these devices with Schottky-barrier (SB) contacts, we demonstrate a strong (exponential) temperature dependence of their current, across both the thermionic (subthreshold) and tunneling (nominally on) regimes. The quantitative features of this behavior are captured by a Landauer-formula-based treatment of tunneling transmission at the SB contacts. Building on this understanding, we use a resistive line, fabricated in the proximity of the FET channel, to mimic the generation of heat in an actively biased integrated circuit. In this way, we are able to show that a measurement of the transistor drain current in the subthreshold regime should enable effective thermal sensing. Overall, our results emphasize the strong potential of SB FETs, based on thin transition-metal dichalcogenide channels, for such sensing. In addition to the desirable sensitivity of their transistor current to variations in temperature, the capacity of these devices for heterogeneous (back-end-of-line) integration should make them useful in a variety of technology scenarios.

We explore the manifestations of spin rotation in graphene in proximity with two different types of high-spin-orbit-coupling (SOC) materials (ferromagnetic Co and nominally diamagnetic WSe₂). Using weak antilocalization (WAL) as a probe of the induced rotation, we demonstrate that spin interference exhibits a highly stochastic (nonself-averaging) character in the mesoscopic limit. At low temperatures (2 mT). The ZBA moreover exhibits a stochastic variation when a gate voltage is used to sweep the Fermi level through the graphene bands, with ranges for which the antilocalization is either prominent or strongly suppressed. This mesoscopic character is exhibited by both of the studied systems, whose ZBA is also damped in similar fashion with increasing temperature. We thus provide fundamental insight into the nonensemble-averaged (nonself-averaged) character of spin interference in mesoscopic systems with strong SOC and, more specifically, into how the details of spin rotation are impacted by external gating. This understanding may ultimately enable the efficient modulation of spin currents in future spintronic devices.

This perspective outlines a selection of research directions that members of the JPCM editorial board anticipate may shape the frontier of condensed matter physics over the next decade. Rather than a comprehensive review or formal roadmap, this perspective reflects a set of informed views drawn from diverse areas of expertise. Our intention is to spark curiosity, provoke discussion, and encourage readers to imagine—and pursue—the exciting possibilities that lie ahead.

This perspective article explores the potential of two-dimensional electron and hole gases at oxide/oxide interfaces as promising conduction channels in energy-efficient spintronic devices for memory and logic applications. Magneto-electric and ferro-electric device architectures are considered, highlighting their advantages in integrating non-volatility with ultra-fast switching capabilities. We emphasize the significance of spin–orbit coupling in generating robust spin currents and examine how the Rashba and inverse Edelstein effects can be harnessed in device operations. Specific heterostructures, including HfO₂/SrTiO₃, Cr₂O₃/TiO_2-x, and ZrO_2-x-based systems, are reviewed in terms of their material properties, interface stability, and suitability for practical implementation.

We provide evidence of incipient edge-state formation in graphene-on-chromia heterostructures, resulting from the presence of substrate-induced spin-orbit coupling (SOC). Low-temperature spin-Hall measurements of the graphene exhibit large fluctuations of the nonlocal resistance (R_nl) as the Fermi level is swept through its bands. These features persist to much higher temperatures (>60 K) than is typical for mesoscopic effects in normal metals and semiconductors, suggesting that they are associated with a very different mechanism to normal charge-based transport. The fluctuations of the nonlocal resistance are moreover invariant to the application of large magnetic fields (as high as 7 T), a characteristic that speaks to a significant edge-like component to transport. To provide insight into these results, we have formulated a (Landauer-Büttiker) model of quantum transport in graphene in the presence of strong SOC. Our calculations capture the key features of experiments, connecting the large quantum fluctuations in R_nl to the impact of the substrate-induced SOC. More specifically, the spin-orbit interaction is found to imbue the spin diffusion with an edge-like character, displacing injected spins toward the edges of the graphene channel while retaining a coupling to disorder within its interior. It is this interplay of edge-like conduction, with more conventional transport within the interior of the two-dimensional channel, which gives rise to the observed fluctuations in R_nl as the Fermi level is varied.

Memory-augmented neural networks (MANNs) require large external memories to enable long-term memory storage and retrieval. Content-addressable memory (CAM) is a type of memory used for high-speed searching applications and is well-suited for MANNs. Recent advances in exploratory non-volatile devices have spurred the development of non-volatile CAMs. However, these devices suffer from poor on-off ratio, large write voltages, and long write times. This work proposes a non-volatile ternary CAM using magneto-electric field effect transistors (MEFETs). The energy and delay of the various operations are simulated using the ASAP 7-nm predictive technology for the transistors and a Verilog-A model of the MEFET. The proposed structure achieves orders of magnitude improvement in search energy and > 45× improvement in search energy-delay product compared to prior works. The write energy and delay are also improved by 8× and 12×, respectively, compared to CAMs designed with other non-volatile devices. A variability analysis is performed to study the effect of process variations on the CAM. The proposed CAM is then used to build a one-shot learning MANN and is benchmarked with the MNIST, EMNIST, and Labeled Faces in the Wild (LFW) datasets with binary embeddings, giving > 99% accuracy on MNIST, a top-3 accuracy of 97.11% on the EMNIST dataset, and > 97% accuracy on the LFW dataset, with embedding sizes of 16, 64, and 512, respectively. The proposed CAM is shown to be fast, energy-efficient, and scalable, making it suitable for MANNs.

Transient electrical pulsing is used to investigate the slowed charge density wave (CDW) kinetics of 1T‐TaS₂. These measurements distinguish a fast response of the material, consistent with the onset of self‐heating, from much slower transients that occur on timescales orders of magnitude longer than this. The latter variations appear consistent with slow configurational changes in the CDW, which, due to the thin nature of the 1T‐TaS₂, can be distinguished from the much faster dynamics of Joule heating. Experiments in which the cooling of the material is interrupted, demonstrate the possibility of “programming” it in different, strongly nonequilibrium, CDW phases. Collectively, the results point to the existence of a complex free‐energy space for the thinned material, whose multi‐valley structure and hidden metastable states govern the resulting thermal and field‐driven dynamics. Crucially, this work demonstrates that while the CDW dynamics in this material may have a thermal character, the timescales associated with these motions can be very different from those on which self‐heating occurs. This discovery will be important for efforts to implement active devices that utilize the CDW states of thinned 1T‐TaS₂.