We present a comprehensive numerical investigation of the gate-induced insulator-to-metal transition in the charge-density-wave (CDW) phase of the Holstein model. Large-scale Brownian dynamics simulations are performed, in which the forces acting on the lattice degrees of freedom are evaluated using the nonequilibrium Green's function formalism. We demonstrate that the onset of CDW instability requires a threshold bias voltage set by the energy of in-gap edge modes. At sufficiently large voltages, the system undergoes an abrupt transition to a metallic state, reminiscent of dielectric breakdown. In the intermediate-voltage regime, our simulations reveal that the transition to a low-resistance state is initiated by the nucleation of a thin conducting layer at the gated electrode. The resulting metal-insulator interface subsequently propagates across the system under the applied bias, leading to the growth of a metallic domain. We further analyze the voltage- and temperature-dependent dynamics of the associated domain walls.
Metal contacts have been identified to be a key technological bottleneck for the realization of viable graphene electronics. Recently, it was observed that for structures that possess both a top and a bottom gate, the electron-hole conductance asymmetry can be modulated by the bottom gate. In this letter, we explain this observation by postulating the presence of an effective thin interfacial dielectric layer between the metal contact and the underlying graphene. Electrical results from quantum transport calculations accounting for this modified electrostatics corroborate well with the experim…
The thermal boundary resistance (Kapitza resistance) of (001) twist grain boundaries in silicon nanowires depends on the mismatch angle. This dependence is systematically investigated by means of nonequilibrium molecular dynamics simulations. Grain boundary systems with and without coincidence site lattice are compared. The Kapitza resistance increases with twist angle up to 40°. For larger angles, it varies only little around 1.56 $\pm$ 0.05 K m^2/GW, except for a drop by 30% near the 90° Σ 1 grain boundary. Finite size effects due to the fixed outer boundary conditions of the nanowire are ne…
For metallic point contacts with Be and Al the magnetoquantum oscillations in the contact resistance have been investigated as a function of the applied voltage over the contact. For one set of point contacts the oscillation amplitude is found to vary nonmonotonously with the applied voltage with similarities to the point-contact spectrum of the electron-phonon interaction. The other part of the investigated point contacts shows a decrease of the oscillation amplitude with increasing bias voltage. For the understanding of the voltage dependence of the amplitude of the point-contact magnetoresi…
The amplitude of magnetoresistance quantum oscillations of Al and Be point contacts in a magnetic field parallel to the contact axis has been studied as a function of voltage applied over the contact. It was found that for one group of contacts the oscillation amplitude nonmonotonously increases with the bias voltage increase, while for the other part of the contacts a decrease of amplitude was observed. The scattering of electrons with nonequilibrium phonons and also phonon-phonon collisions will be discussed as the possible reasons of the observed effects.
Using the Schwinger-Keldysh approach and the drone (Majorana) fermion representation, we evaluate current distribution in a single-electron transistor in a regime where a total tunnel resistance is small. Nonequilibrium quantum fluctuations of charge induce a lifetime broadening of charge-state level, as well as renormalization of system parameters. We find that the lifetime broadening effect may suppress the probability of relatively large current.
Developments in the physics of 2D electron systems during the last decade have revealed a new class of nonequilibrium phenomena in the presence of a moderately strong magnetic field. The hallmark of these phenomena is magnetoresistance oscillations generated by the external forces that drive the electron system out of equilibrium. The rich set of dramatic phenomena of this kind, discovered in high mobility semiconductor nanostructures, includes, in particular, microwave radiation-induced resistance oscillations and zero-resistance states, as well as Hall field-induced resistance oscillations a…
The ability to tune the optical response of a material via electrostatic gating is crucial for optoelectronic applications, such as electro-optic modulators, saturable absorbers, optical limiters, photodetectors and transparent electrodes. The band structure of single layer graphene (SLG), with zero-gap, linearly dispersive conduction and valence bands, enables an easy control of the Fermi energy E$_F$ and of the threshold for interband optical absorption. Here, we report the tunability of the SLG non-equilibrium optical response in the near-infrared (1000-1700nm/0.729-1.240eV), exploring a ra…
We consider a single molecule circuit embedded into solvent. The Born dielectric solvation model is combined with Keldysh nonequilibrium Green's functions to describe the electron transport properties of the system. Depending on the dielectric constant, the solvent induces multiple nonequilibrium steady states with corresponding hysteresis in molecular current-voltage characteristics as well as negative differential resistance. We identify the physical range of solvent and molecular parameters where the effects are present. The position of the negative differential resistance peak can be contr…
We investigate models of molecular junctions which constitute minimal Hamiltonians to account for zero-bias-anomaly and the satellite features of inelastic transport by molecular phonons. Through nonlinear transport calculations with the imaginary-time nonequilibrium formalism, a HOMO-LUMO model with Anderson-Holstein interaction is shown to produce co-tunneling conductance peak in the vicinity of Kondo resonance which is mediated by a re-emergent many-body resonance assisted by phonon excitations at bias equal to the phonon frequency. Destruction of the resonance leads to negative-differentia…
Difficulties in obtaining high-performance p-type transistors and gate insulator charge-trapping effects present two major challenges for III-V complementary metal-oxide semiconductor (CMOS) electronics. We report a p-GaAs nanowire metal-semiconductor field-effect transistor (MESFET) that eliminates the need for a gate insulator by exploiting the Schottky barrier at the metal-GaAs interface. Our device beats the best-performing p-GaSb nanowire metal-oxide-semiconductor field effect transistor (MOSFET), giving a typical sub-threshold swing of 62 mV/dec, within 4% of the thermal limit, on-off ra…
Doping is a common route to reducing nanowire transistor on-resistance but has limits. High doping level gives significant loss in gate performance and ultimately complete gate failure. We show that electrolyte gating remains effective even when the Be doping in our GaAs nanowires is so high that traditional metal-oxide gates fail. In this regime we obtain a combination of sub-threshold swing and contact resistance that surpasses the best existing p-type nanowire MOSFETs. Our sub-threshold swing of 75 mV/dec is within 25% of the room-temperature thermal limit and comparable with n-InP and n-Ga…
We report nonlocal spin injection and detection experiments on mesoscopic Co-Al2O3-Cu spin valves. We have observed a temperature dependent asymmetry in the nonlocal resistance between parallel and antiparallel configurations of the magnetic injector and detector. This strongly supports the existence of a nonequilibrium resistance that depends on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal providing evidence for increasing interface spin scattering with temperature.
We study phonons generated by current in a Pt nanowire, by measuring resistance of another nanowire separated from the first one by an insulating spacer. For thin spacers, the resistance varies almost linearly with current at cryogenic temperatures, while an additional quadratic contribution emerges for thicker spacers. These observations suggest a non-thermal distribution of current-generated phonons that relax via strongly nonlinear dynamical processes rather than few-phonon scattering. Our results provide insight into the nonequilibrium phonon dynamics at nanoscale, which may facilitate eff…
Magnetoquantum oscillations of resistance as functions of the potential difference applied across the contact are studied for metallic point contacts made of $Al$ and $Be$. The amplitude of resistance oscillations in a magnetic field increases with voltage and is identical to the EPI spectrum of the point contact for one group of contacts, and with the bias voltage across the contact for another. The increase in the oscillation amplitude as well as its decrease has a nonmonotonic dependence on energy. The scattering of electrons by nonequilibrium phonons and the Fermi-liquid effects in the non…
Using the nonequilibrium Green function formalism combined with density functional theory, we study finite-bias quantum transport in Ni/Gr_n/Ni vertical heterostructures where $n$ graphene layers are sandwiched between two semi-infinite Ni(111) electrodes. We find that recently predicted "pessimistic" magnetoresistance of 100% for $n \ge 5$ junctions at zero bias voltage $V_b \rightarrow 0$, persists up to $V_b \simeq 0.4$ V, which makes such devices promising for spin-torque-based device applications. In addition, for parallel orientations of the Ni magnetizations, the $n=5$ junction exhibits…
Due to the lack of simulation tools that take into account the actual geometry of complicated quantum Hall samples there are lots of experiments that are not yet fully understood. Already some years ago R. G. Mani recorded a shift of the Hall resistance transitions to lower magnetic fields in samples of a Hall bar with embedded anti-Hall bar by using partial gating. We use a Nonequilibrium Network Model (NNM) to simulate this geometry and find qualitative agreement. Fitting the simulated resistance curves to the experimental results we can not only determine the carrier concentration but also …
By using an electrochemical gating technique with a new combination of polymer and electrolyte, we were able to inject surface charge densities n_2D as high as 3.5 \times 10^15 e/cm^2 in gold films and to observe large relative variations in the film resistance, DeltaR/R', up to 10% at low temperature. DeltaR/R' is a linear function of n_2D - as expected within a free-electron model - if the film is thick enough (> 25 nm), otherwise a tendency to saturation due to size effects is observed. The application of this technique to 2D materials will allow extending the field-effect experiments to a …
We show scalable and complete suppression of the recently reported terahertz-pulse-induced switching between different resistance states of antiferromagnetic CuMnAs thin films by ultrafast gating. The gating functionality is achieved by an optically generated transiently conductive parallel channel in the semiconducting substrate underneath the metallic layer. The photocarrier lifetime determines the time scale of the suppression. As we do not observe a direct impact of the optical pulse on the state of CuMnAs, all observed effects are primarily mediated by the substrate. The sample region of …
The field-effect technique, popular thanks to its application in common field-effect transistors, is here applied to metallic thin films by using as a dielectric a novel polymer electrolyte solution. The maximum injected surface charge, determined by a suitable modification of a classic method of electrochemistry called double-step chronocoulometry, reached some units in 10^15 charges/cm^2. At room temperature, relative variations of resistance up to 8%, 1.9% and 1.6% were observed in the case of gold, silver and copper, respectively and, if the films are thick enough (> 25 nm), results can be…
We present multi-scale dynamical simulations of voltage-induced insulator-to-metal transition in the double exchange model, a canonical example of itinerant magnet and correlated electron systems. By combining nonequilibrium Green's function method with large-scale Landau-Lifshitz-Gilbert dynamics, we show that the transition from an antiferromagnetic insulator to the low-resistance state is initiated by the nucleation of a thin ferromagnetic conducting layer at the anode. The metal-insulator interface separating the two phases is then driven toward the opposite electrode by the voltage stress…
We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe$_2$ thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we propose that soliton-antisoliton pairs in the charge density wave of NbSe$_2$ modulated by the SAW serve as a time-dependent capacitance in the superconducting state, leading to the dc negative resistance. The present experimental result would provi…
The resistance of dual-gated bilayer graphene is measured as a function of temperature and gating electric fields in the Corbino geometry which precludes edge transport. The temperature-dependent resistance is quantitatively described by a two channel conductance model including parallel thermal activation and variable range hopping channels, which gives the electric-field-dependent band gap whose magnitude is found to be in good agreement with infrared absorption experiments. Low temperature transport is similar to previous studies of dual-gated bilayer graphene with edges, suggesting that ed…
The electronic property of NiFe$_2$O$_4$ nanowire device is investigated through nonequilibrium Green's functions (NEGF) in combination with density functional theory (DFT). The electronic transport properties of NiFe$_2$O$_4$ nanowire are studied in terms of density of states, transmission spectrum and $I{-}V$ characteristics. The density of states gets modified with the applied bias voltage across NiFe$_2$O$_4$ nanowire device, the density of charge is observed both in the valence band and in the conduction band on increasing the bias voltage. The transmission spectrum of NiFe$_2$O$_4$ nanow…
The temperature dependence of electric transport properties of single-layer and few-layer graphene at large charge doping is of great interest both for the study of the scattering processes dominating the conductivity at different temperatures and in view of the theoretically predicted possibility to reach the superconducting state in such extreme conditions. Here we present the results obtained in 3-, 4- and 5-layer graphene devices down to 3.5 K, where a large surface charge density up to about 6.8x10^14 cm^(-2) has been reached by employing a novel polymer electrolyte solution for the elect…
A brief overview is presented of recent work which investigates the time-dependent relaxation of charge and its spontaneous fluctuations on mesoscopic conductors in the proximity of gates. The leading terms of the low frequency conductance are determined by a capacitive or inductive emittance and a dissipative charge relaxation resistance. The charge relaxation resistance is determined by the ratio of the mean square dwell time of the carriers in the conductor and the square of the mean dwell time. The contribution of each scattering channel is proportional to half a resistance quantum. We dis…
We study heat transport across individual grain boundaries in suspended monolayer graphene using extensive classical molecular dynamics (MD) simulations. We construct bicrystalline graphene samples containing grain boundaries with symmetric tilt angles using the two-dimensional phase field crystal method and then relax the samples with MD. The corresponding Kapitza resistances are then computed using nonequilibrium MD simulations. We find that the Kapitza resistance depends strongly on the tilt angle and shows a clear correlation with the average density of defects in a given grain boundary, b…
To study the effect of spin-orbit coupling (SOC) on spin-transfer torque in magnetic materials, we have implemented two theoretical formalisms that can accommodate SOC. Using the "charge-pumping" formalism, we find two contributions to the out-of-plane spin-transfer torque parameter $β$ in ballistic Ni domain walls (DWs). For short DWs, the nonadiabatic reflection of conduction electrons caused by the rapid spatial variation of the exchange potential results in an out-of-plane torque that increases rapidly with decreasing DW length. For long DWs, the Fermi level conduction channel anisotropy t…
We consider spin polarized transport in a ferromagnet-insulator/semiconductor/insulator-ferromagnet (F1-I-S-I-F2) junction. We find that the spin current is strongly dependent on the spin configurations, the doping and space charge distribution in the semiconductor. When the ferromagnet-semiconductor interface resistance is comparable to the semiconductor resistance, the magnetoresistance ratio of this junction can be greatly enhanced under appropriate doping when the space charge effect in the nonequilibrium transport processes is taken into consideration.
We investigate the 1/f noise properties of epitaxial graphene devices at low temperatures as a function of temperature, current and magnetic flux density. At low currents, an exponential decay of the 1/f noise power spectral density with increasing temperature is observed that indicates mesoscopic conductance fluctuations as the origin of 1/f noise at temperatures below 50 K. At higher currents, deviations from the typical quadratic current dependence and the exponential temperature dependence occur as a result of nonequilibrium conditions due to current heating. By applying the theory of Kuba…
The interplay between interference effects and electron-electron interactions in electron transport through an interacting double quantum dot system is investigated using a hierarchical quantum master equation approach which becomes exact if carried to infinite order and converges well if the temperature is not too low. Decoherence due to electron-electron interactions is found to give rise to pronounced negative differential resistance, enhanced broadening of structures in current-voltage characteristics and an inversion of the electronic population. Dependence on gate voltage is shown to be …
We study the nonequilibrium energy transport across a topological insulator/superconductor junction, by deriving an interfacial heat current formula through scattering wave approach. Several anomalous thermal properties are uncovered, such as thermal energy's Klein tunneling, asymmetric Kapitza resistance and negative differential thermal resistance. We expect these findings could have potential applications for the energy control in various hybridized mesoscopic systems.
Double quantum dots are one of the promising two-state quantum systems for realizing qubits. In the quest of successfully manipulating and reading information in qubit systems, it is of prime interest to control the charge response of the system to a gate voltage, as filled in by the dynamical charge susceptibility. We theoretically study this quantity for a nonequilibrium double quantum dot by using the functional integral approach and derive its general analytical expression. One highlights the existence of two lines of maxima as a function of the dot level energies, each of them being split…
We consider the effect of the Rashba spin-orbital interaction and space charge in a ferromagnet-insulator/semiconductor/insulator-ferromagnet junction where the spin current is severely affected by the doping, band structure and charge screening in the semiconductor. In diffusion region, if the the resistance of the tunneling barriers is comparable to the semiconductor resistance, the magnetoresistance of this junction can be greatly enhanced under appropriate doping by the co-ordination between the Rashba effect and screened Coulomb interaction in the nonequilibrium transport processes within…
We report on nonequilibrium transport measurements in a high-mobility two-dimensional electron system subject to weak magnetic field and dc excitation. Detailed study of dc-induced magneto-oscillations, first observed by Yang {\em et al}., reveals a resonant condition that is qualitatively different from that reported earlier. In addition, we observe dramatic reduction of resistance induced by a weak dc field in the regime of separated Landau levels. These results demonstrate similarity of transport phenomena in dc-driven and microwave-driven systems and have important implications for ongoing…
By combining transport and photo emission measurements on (Bi1-xInx)2Se3 thin films, we report that this system transforms from a topologically non-trivial metal into a topologically trivial band insulator through three quantum phase transitions. At x = 3-7%, there is a transition from a topologically non-trivial metal to a trivial metal. At x = 15%, the metal becomes a variable-range-hopping insulator. Finally, above x = 25%, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically…
The poor performance of copper interconnects at the nanometer scale calls for new material solutions for continued scaling of integrated circuits. We propose the use of three dimensional time-reversal-invariant topological insulators (TIs), which host backscattering-protected surface states, for this purpose. Using semiclassical methods, we demonstrate that nanoscale TI interconnects have a resistance 1-3 orders of magnitude lower than copper interconnects and graphene nanoribbons at the nanometer scale. We use the nonequilibrium Green function (NEGF) formalism to measure the change in conduct…
We simulate quantum transport between a graphene nanoribbon (GNR) and a single-walled carbon nanotube (CNT) where electrons traverse vacuum gap between them. The GNR covers CNT over a nanoscale region while their relative rotation is 90 degrees, thereby forming a four-terminal crossbar where the bias voltage is applied between CNT and GNR terminals. The CNT and GNR are chosen as either semiconducting (s) or metallic (m) based on whether their two-terminal conductance exhibits a gap as a function of the Fermi energy or not, respectively. We find nonlinear current-voltage (I-V) characteristics i…
Metal-based electronics is attractive for fast and radiation-hard electronic circuits and remains one of the longstanding goals for researchers. The emergence of 1T-TaS$_2$, a layered material exhibiting strong charge density wave (CDW) driven resistivity switching that can be controlled by an external stimulus such as electric field and optical pulses, has triggered a renewed interest in metal-electronics. Here we demonstrate a negative differential resistor (NDR) using electrically driven CDW phase transition in an asymmetrically designed T-junction made up of 1T-TaS$_2$/2H-MoS$_2$ van der W…
Although extensive experimental and theoretical works have been conducted to understand the ballistic and diffusive phonon transport in nanomaterials recently, direct observation of temperature and thermal nonequilibrium of different phonon modes has not been realized. Herein, we have developed a method within the framework of molecular dynamics to calculate the temperatures of phonon in both real and phase spaces. Taking silicon thin film and graphene as examples, we directly obtained the spectral phonon temperature (SPT) and observed the local thermal nonequilibrium between the ballistic and…
Based on a microcanonical picture that maps the steady-state quantum transport process to a drain-to-source excitation, we develop a constrained-search density functional formalism for finite-bias quantum transport calculations. By variationally minimizing the total energy of an electrode-channel-electrode system without introducing separate bulk electrode information, ambiguities in identifying its nonequilibrium electronic structure under a bias is reduced and finite electrode cases can be naturally treated. We apply the approach to vertically stacked van der Waals heterostructures made of a…
Using first-principles calculations based on density functional theory and the nonequilibrium Green function formalism, we studied the spin transport through metal-phthalocyanine (MPc, M=Ni, Fe, Co, Mn, Cr) molecules connected to aurum nanowire electrodes. We found that the MnPc, FePc, and CrPc molecular devices exhibit a perfect spin filtering effect compared to CoPc and NiPc. Moreover, negative differential resistance appears in FePc molecular devices. The transmission coefficients at different bias voltages were further presented to understand this phenomenon. These results would be useful …
High-mobility 2D electron systems in a perpendicular magnetic field exhibit zero resistance states (ZRS) when driven with microwave radiation. We study the nonequilibrium phase transition into this ZRS using phenomenological equations of motion to describe the current and density fluctuations. We focus on two models for the transition into a time-independent steady state. Model-I assumes rotational invariance, density conservation, and symmetry under shifting the density globally by a constant. This model is argued to describe physics on small length scales where the density does not vary appr…
We study dynamic nonequilibrium electron charging phenomena in ballistic molecular devices at room temperature that compromise their response to bias and whose nature is evidently distinguishable from static Schottky-type potential barriers. Using various metallic/semiconducting carbon nanotubes and alkane dithiol molecules as active parts of a molecular bridge, we perform self-consistent quantum transport calculations under the nonequilibrium Green's function formalism coupled to a three-dimensional Poisson solver for a mutual description of chemistry and electrostatics. Our results sketch a …
The negative differential resistance (NDR) tunnel diodes are promising alternative devices for beyond-CMOS computing as they offer several potential applications when integrated with transistors. We propose a novel semiconductor-free NDR tunnel diode concept that exhibits an ultra-high peak-to-valley current ratio (PVCR) value. Our proposed NDR diode consists of two cold metal electrodes separated by a thin insulating tunnel barrier. The NDR effect stems from the unique electronic band structure of the cold metal electrodes, i.e., the width of the isolated metallic bands around the Fermi level…
When the electron density of highly crystalline thin films is tuned by chemical doping or ionic liq- uid gating, interesting effects appear including unconventional superconductivity, sizeable spin-orbit coupling, competition with charge-density waves, and a debated low-temperature metallic state that seems to avoid the superconducting or insulating fate of standard two-dimensional electron systems. Some experiments also find a marked tendency to a negative electronic compressibility. We suggest that this indicates an inclination for electronic phase separation resulting in a nanoscopic inhomo…
Graphene is believed to be an excellent candidate material for next-generation electronic devices. However, one needs to take into account the nontrivial effect of metal contacts in order to precisely control the charge injection and extraction processes. We have performed transport calculations for graphene junctions with wetting metal leads (metal leads that bind covalently to graphene) using nonequilibrium Green's functions and density functional theory. Quantitative information is provided on the increased resistance with respect to ideal contacts and on the statistics of current fluctuati…
Bulk and surface state contributions to the electrical resistance of single-crystal samples of the topological Kondo insulator compound SmB6 are investigated as a function of crystal thickness and surface charge density, the latter tuned by ionic liquid gating with electrodes patterned in a Corbino disk geometry on a single surface. By separately tuning bulk and surface conduction channels, we show conclusive evidence for a model with an insulating bulk and metallic surface states, with a crossover temperature that depends solely on the relative contributions of each conduction channel. The su…
We fabricate ion-gated field-effect transistors (iFET) on mechanically exfoliated multilayer MoS$_2$. We encapsulate the flake by Al$_2$O$_3$, leaving the device channel exposed at the edges only. A stable Li$^+$ intercalation in the MoS$_2$ lattice is induced by gating the samples with a Li-based polymeric electrolyte above $\sim$ 330 K and the doping state is fixed by quenching the device to $\sim$ 300 K. This intercalation process induces the emergence of anomalies in the temperature dependence of the sheet resistance and its first derivative, which are typically associated with structural/…
