Phase winding effects in hollow semiconductor nanowires with superconducting shells have been proposed as a route to engineer topological superconducting states. We investigate GaAs/InAs core/shell nanowires with half-shells of epitaxial aluminium as a potential platform for such devices, where the thin InAs shell confines the electron wave function around the GaAs core. With normal contacts we observed pronounced h/e flux periodic oscillations in the magnetoconductance, indicating the presence of a tubular conductive channel in the InAs shell. Conversely, the switching current in Josephson junctions oscillates with approximately half that period, i.e. h/2e, indicating transport via Andreev transport processes in the junction enclosing threading magnetic flux. On these structures, we systematically studied the gate-, field-, and temperature-dependent evolution of the supercurrent. Results indicate that Andreev transport processes can occur about the wire circumference indicating full proximitization of the InAs shell from the half-shell superconducting contacts.

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.

The significant discrepancy observed between the predicted and experimental switching fields in correlated insulators under a DC electric field far-from-equilibrium necessitates a reevaluation of current microscopic understanding. Here we show that an electron avalanche can occur in the bulk limit of such insulators at arbitrarily small electric field by introducing a generic model of electrons coupled to an inelastic medium of phonons. The quantum avalanche arises by the generation of a ladder of in-gap states, created by a multi-phonon emission process. Hot-phonons in the avalanche trigger a premature and partial collapse of the correlated gap. The phonon spectrum dictates the existence of two-stage versus single-stage switching events which we associate with charge-density-wave and Mott resistive phase transitions, respectively. The behavior of electron and phonon temperatures, as well as the temperature dependence of the threshold fields, demonstrates how a crossover between the thermal and quantum switching scenarios emerges within a unified framework of the quantum avalanche.

Stacking of graphene with hexagonal boron nitride (h-BN) can dramatically modify its bands from their usual linear form, opening a series of narrow minigaps that are separated by wider minibands. While the resulting spectrum offers strong potential for use in functional (opto)electronic devices, a proper understanding of the dynamics of hot carriers in these bands is a prerequisite for such applications. In this work, we therefore apply a strategy of rapid electrical pulsing to drive carriers in graphene/h-BN heterostructures deep into the dissipative limit of strong electron-phonon coupling. By using electrical gating to move the chemical potential through the "Moiré bands", we demonstrate a cyclical evolution between metallic and semiconducting states. This behavior is captured in a self-consistent model of non-equilibrium transport that considers the competition of electrically driven inter-band tunneling and hot-carrier scattering by strongly non-equilibrium phonons. Overall, our results demonstrate how a treatment of the dynamics of both hot carriers and hot phonons is essential to understanding the properties of functional graphene superlattices.

Systems combining superconductors with topological insulators offer a platform for the study of Majorana bound states and a possible route to realize fault tolerant topological quantum computation. Among the systems being considered in this field, monolayers of tungsten ditelluride (WTe2 ) have a rare combination of properties. Notably, it has been demonstrated to be a quantum spin Hall insulator (QSHI) and can easily be gated into a superconducting state. Measurements on gate-defined Josephson weak-link devices fabricated using monolayer WTe2 are reported. It is found that consideration of the 2D superconducting leads are critical in the interpretation of magnetic interference in the resulting junctions. The reported fabrication procedures suggest a facile way to produce further devices from this technically challenging material and the results mark the first step toward realizing versatile all-in-one topological Josephson weak-links using monolayer WTe2 .

We report on the fabrication of Josephson junction devices with weak links utilizing the Weyl and higher-order topological semimetal $\text{WTe}_2$. We show that $\text{WTe}_2\text{/Pd}$ contact annealed at a low temperature of 80{\deg}C did not exhibit superconducting properties because neither $\text{WTe}_2$ nor Pd are superconductors in the ground state. Upon 180{\deg}C annealing, spontaneous formation of superconducting $\text{PdTe}$ due to Pd diffusion enabled us to obtain the interface between $\text{WTe}_2$ and superconductor suitable for the Josephson junction. This result is a facile technique to make a Josephson junction and induce Cooper pairs into topological telluride semimetals.

Evidence of robust spin-dependent transport in monolayer graphene, deposited on the (0001) surface of the antiferromagnetic (AFM)/magneto-electric oxide chromia (Cr2 O3 ), is provided. Measurements performed in the non-local spin-Hall geometry reveal a robust signal that is present at zero external magnetic field and which is significantly larger than any possible ohmic contribution. The spin-related signal persists well beyond the Néel temperature (≈307 K) that defines the transition between the AFM and paramagnetic states, remaining visible at the highest studied temperature of close to 450 K. This robust character is consistent with prior theoretical studies of the graphene/Cr2 O3 system, predicting that the lifting of sub-lattice symmetry in the graphene shall induce an effective spin-orbit term of ≈40 meV. Overall, the results indicate that graphene-on-chromia heterostructures are a highly promising framework for the implementation of spintronic devices, capable of operation well beyond room temperature.

We investigate the transient response of N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)-imide-based ionic liquid (IL) planar capacitors, studying this response over time scales ranging from as little as a few nanoseconds to as much as several days. Our measurements point to the existence of three distinct mechanisms for charging/discharging of the IL. The fastest of these is associated with the development of a standard polarization charge in the bulk of the liquid dielectric, which dominates at times less than similar to 10(-6) s. The second process is attributed to electric double layer formation, which is initiated after similar to 10(-6) but which takes as long as similar to 10(-2) s to reach completion. Finally, we also identify the presence of a pseudocapacitance that arises from electrochemical reactions; this process is only activated at voltages above similar to 2.5 V and is relatively slow. Indeed, we find evidence that full discharging of this pseudocapacitance can take as long as 10(5) s (i.e., days). Overall, our findings provide useful insights into the mechanisms for slow ion dynamics in ILs and highlight the constraints that these dynamics place on the potential operational speed of IL-based transistors.

We investigate the behavior of the metal-insulator transition (MIT) in TiS3 nanowire field-effect transistors, in the strongly nonequilibrium limit that has, thus far, largely been neglected. Under high electric fields within the TiS3 channel (≤115 kV/cm), we observe the emergence of a critical fixed point, separating insulating and metallic regions in the transfer curves of the device. The critical gate voltage that defines this fixed point evolves systematically with the drain bias (field), allowing us to map out a phase diagram that identifies the conditions for metallicity or for insulating behavior. Dependent upon the choice of the gate voltage used to tune the carrier concentration in the nanowire, the existence of the field-induced MIT allows the TiS3 to be either insulating or metallic over an extensive range of temperature. The possible connection of this strongly nonequilibrium state to some form of charge density wave is discussed.

Terahertz (THz) plasma oscillations represent a potential path to implement ultrafast electronic devices and circuits. Here, we present an approach to generate on-chip THz signals that relies on plasma-wave stabilization in nanoscale transistors with specific structural asymmetry. A hydrodynamic treatment shows how the transistor asymmetry supports plasma-wave amplification, giving rise to pronounced negative differential conductance (NDC). A demonstration of these behaviors is provided in InGaAs high-mobility transistors, which exhibit NDC in accordance with their designed asymmetry. The NDC onsets once the drift velocity in the channel reaches a threshold value, triggering the initial plasma instability. We also show how this feature can be made to persist beyond room temperature (to at least 75 °C), when the gating is configured to facilitate a transition between the hydrodynamic and ballistic regimes (of electron-electron transport). Our findings represent a significant step forward for efforts to develop active components for THz electronics.

Mesoscopic conductance fluctuations are a ubiquitous signature of phase-coherent transport in small conductors, exhibiting universal character independent of system details. In this Letter, however, we demonstrate a pronounced breakdown of this universality, due to the interplay of local and remote phenomena in transport. Our experiments are performed in a graphene-based interaction-detection geometry, in which an artificial magnetic texture is induced in the graphene layer by covering a portion of it with a micromagnet. When probing conduction at some distance from this region, the strong influence of remote factors is manifested through the appearance of giant conductance fluctuations, with amplitude much larger than e^{2}/h. This violation of one of the fundamental tenets of mesoscopic physics dramatically demonstrates how local considerations can be overwhelmed by remote signatures in phase-coherent conductors.

It has been nearly a century since the original mechanism for charge density wave (CDW) formation was suggested by Peierls. Since then, the term has come to describe several related concepts in condensed matter physics, having their origin in either the electron–phonon or electron–electron interaction. The vast majority of CDW literature deals with systems that are metallic, where discussions of mechanisms related to the Fermi surface are valid. Recently, it has been suggested that semiconducting systems such as TiS3 and TiSe2 exhibit behavior related to CDWs. In such cases, the origin of the behavior is more subtle and intimately tied to electron–electron interactions. We introduce the different classifications of CDW systems that have been proposed and discuss work on the group IV transition metal trichalcogenides (TMTs) (ZrTe3, HfTe3, TiTe3, and TiS3), which are an exciting and emergent material system whose members exhibit quasi-one-dimensional properties. TMTs are van der Waals materials and can be readily studied in the few-layer limit, opening new avenues to manipulating collective states. We emphasize the semiconducting compound TiS3 and suggest how it can be classified based on available data. Although we can conjecture on the origin of the CDW in TiS3, further measurements are required to properly characterize it.

Drift velocity saturation (at some characteristic value, v(d)(sat)) is a critical process that limits the ultimate current-carrying capacity of semiconductors at high electric fields (similar to 10(4) V/cm). With the recent emergence of two-dimensional (2D) semiconductors, there is a need to understand the manner in which velocity saturation is impacted when materials are thinned to the monolayer scale. Efforts to determine v(d)(sat) are typically hampered, however, by self-heating effects that arise from undesirable energy loss from the active 2D layer to the dielectric substrate that supports it. In this work, we explore this problem for an important 2D semiconductor, namely monolayer molybdenum disulfide (MoS2). By applying a strategy of rapid (nanosecond duration), single-shot, pulsing, we are able to probe the true hot-carrier dynamics in this material, free of the influence of self-heating of its SiO2 substrate. Our approach allows us to realize high current densities (-mA/p.m) in the MoS2 layers, representing a significant enhancement over prior studies. We similarly infer values for the saturated drift velocity (v(d)(sat) similar to 5 - 7 x 10(6 )cms(-1) ) that are higher than those reported in earlier works, in which the influence of self-heating (and carrier injection into oxide traps) could not be excluded. In fact, our estimates for v(d)(sat) are somewhat close to the ideal velocity expected for normal (parabolic) semiconductors. Since a proper knowledge of this parameter is essential to the design of active electronic and optoelectronic devices, the insight into velocity saturation provided here should provide useful guidance for such efforts.

Al0.35In0.65As is a direct semiconductor whose multivalley conduction-band structure has been proposed for use in so-called valley photovoltaics. In such hot-carrier solar cells, energetic (hot) photocarriers are stored in satellite valleys away from the Gamma point, allowing them to be extracted prior to thermalization and to thereby increase power-conversion efficiency. While prior theoretical work has highlighted the potential of Al0.35In0.65As-a widely used barrier material in electronic and optoelectronic devices, for use in valley photovoltaics-surprisingly little is known about its electrical properties, especially how these are impacted by the application of high fields. In this work, we therefore undertake a detailed characterization of the electrical properties of Te-doped (n-type) Al0.35In0.65As, over wide ranges of temperature (3-400 K) and electric field (<50 kV/cm). Using pulsed measurements to suppress the influence of Joule heating, we reveal the presence of clear negative-differential conductance in the current-voltage characteristics of the films, suggestive of the intervalley transfer of hot electrons. This conclusion is supported by the results of ensemble Monte Carlo simulations of the hot-carrier action, which confirm the connection of the observed negative-differential conductance to hot-electron transfer from the conduction-band (Gamma) minimum, to the side valleys at the L point. The quantitative features of the experimentally determined velocity-field curves are found to be in good agreement with the results of these calculations, providing further confidence in the role of the implied intervalley transfer mechanism. Overall, these results confirm the excellent potential of Al0.35In0.65As for use as the absorber material in hot-carrier solar cell technology.

We study temperature dependent (200 – 400 K) dielectric current leakage in high-quality, epitaxial chromia films, synthesized on various conductive substrates (Pd, Pt and V2O3). We find that trap-assisted space-charge limited conduction is the dominant source of electrical leakage in the films, and that the density and distribution of charge traps within them is strongly dependent upon the choice of the underlying substrate. Pd-based chromia is found to exhibit leakage consistent with the presence of deep, discrete traps, a characteristic that is related to the known properties of twinning defects in the material. The Pt- and V2O3-based films, in contrast, show behavior typical of insulators with shallow, exponentially-distributed traps. The highest resistivity is obtained for chromia fabricated on V2O3 substrates, consistent with a lower total trap density in these films. Our studies suggest that chromia thin films formed on V2O3 substrates are a promising candidate for next-generation spintronics.

We fabricated graphene field-effect transistors (GFETs) with hybrid organic/inorganic gate dielectrics, in which parylene C is used as the organic component. The HOMO-LUMO gap of parylene is large enough to provide effective gate insulation, yet significantly smaller than that of the inorganic component (SiO2) of the dielectric. This allows this polymeric material to serve as an effective "floating node" that may be programmed by applying large voltage pulses to the GFET drain. We identify the role of two types of trapping in these devices: the first is mediated by short-lived interfacial states at the graphene parylene interface, while the second, which is responsible for the nonvolatile memory function, involves hot-carrier injection into long-lived trap states deep in the parylene layer. Retention measurements demonstrate that charge injected into the parylene interior may be retained over long decay times (months), thereby confirming the potential of graphene-on-parylene for nonvolatile memory implementations.

Here we describe some preliminary device results from field effect transistors made from metal trichalcogenides. Although not much investigated, is both promise and room for improvement. Improvements could come from better contacts and lower semiconductor channel defect densities, in metal trichalcogenides transistors. Devices with ohmic contacts have now been fabricated, and the surface termination of these materials modeled by density functional theory.

We use transient electrical measurements to investigate the details of self-heating and charge trapping in graphene transistors encapsulated in hexagonal boron nitride (h-BN) and operated under strongly nonequilibrium conditions. Relative to more standard devices fabricated on SiO2 substrates, encapsulation is shown to lead to an enhanced immunity to charge trapping, the influence of which is only apparent under the combined influence of strong gate and drain electric fields. Although the precise source of the trapping remains to be determined, one possibility is that the strong gate field may lower the barriers associated with native defects in the h-BN, allowing them to mediate the capture of energetic carriers from the graphene channel. Self-heating in these devices is identified through the observation of time-dependent variations of the current in graphene and is found to be described by a time constant consistent with expectations for nonequilibrium phonon conduction into the dielectric layers of the device. Overall, our results suggest that h-BN-encapsulated graphene devices provide an excellent system for implementations in which operation under strongly nonequilibrium conditions is desired.

ADVERTISEMENT RETURN TO ISSUEPREVLetter to the EditorNEXTReply to "Comment on 'Gate-Controlled Metal–Insulator Transition in TiS3 Nanowire Field-Effect Transistors'"Michael RandleMichael RandleDepartment of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, United StatesMore by Michael Randle, Alexey LipatovAlexey LipatovDepartment of Chemistry, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United StatesMore by Alexey Lipatovhttp://orcid.org/0000-0001-5043-1616, Avinash KumarAvinash KumarDepartment of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, United StatesMore by Avinash Kumar, Peter A. DowbenPeter A. DowbenDepartment of Physics & Astronomy, Theodore Jorgensen Hall, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United StatesMore by Peter A. Dowbenhttp://orcid.org/0000-0002-2198-4710, Alexander SinitskiiAlexander SinitskiiDepartment of Chemistry, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United StatesMore by Alexander Sinitskiihttp://orcid.org/0000-0002-8688-3451, Uttam SingisettiUttam SingisettiDepartment of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, United StatesMore by Uttam Singisetti, and Jonathan P. Bird*Jonathan P. BirdDepartment of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900, United States*Email: [email protected]More by Jonathan P. Birdhttp://orcid.org/0000-0002-6966-9007Cite this: ACS Nano 2019, 13, 8, 8498–8500Publication Date (Web):August 27, 2019Publication History Received31 July 2019Published online27 August 2019Published inissue 27 August 2019https://pubs.acs.org/doi/10.1021/acsnano.9b06062https://doi.org/10.1021/acsnano.9b06062letterACS PublicationsCopyright © 2019 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views1135Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (422 KB) Get e-AlertscloseSUBJECTS:Electrical conductivity,Materials,Phonons,Scattering,Semiconductors Get e-Alerts