Numerical renormalization group (NRG) is formulated for nonequilibrium steady state by converting finite-lattice many-body eigenstates into scattering states. Extension of the full-density-matrix NRG for a biased Anderson impurity model, simplified by using the original orbital basis of the Hamiltonian, enables detailed studies of the sub-Kondo spectral evolution in the zero-temperature limit, confirming the double-resonance structure emerging at bias V equal to the Kondo energy scale T-K. The distribution shows distinct multiscale spectral features of population inversion at energy omega below the Kondo scale (omega less than or similar to T-K) and very strong nonequilibrium excitations at energy beyond the bias (omega greater than or similar to V ). The strong departure from the Fermi-Dirac distribution leads to the local nonequilibrium temperature Tloc scaling as k(B)T(loc) approximate to V for V >> T-K. The current-voltage relation at the low temperatures (T << T-K) in the strong Kondo regime evolves from the unitary limit to the current saturation behavior as the bias increases from zero to beyond the Kondo scale.

The metal-insulator transition (MIT) in VO2 nanowire devices, driven by electric field-induced nonequilibrium effects, is observed at liquid helium temperature. This unusually low transition temperature is achieved by surface doping VO2 nanowires with gold nanoparticles (NPs). The presence of gold NPs may promote percolative conduction pathways, enhance electronic inhomogeneity, or alter lattice-electron coupling. These effects collectively enable a smoother MIT at a lower electric field compared to pristine VO2 nanowires. An empirical model is used to distinguish Joule heating from electric field contributions, confirming the dominance of nonequilibrium electric field effects. Repeated voltage sweeps further reveal hysteresis and current variations, providing insights into carrier dynamics and correlated electron behavior introduced by NP decoration. These findings advance understanding of the MIT mechanism in VO2 and emphasize the role of surface modifications in tailoring phase transitions at cryogenic temperatures.

Measurements of resistive breakdown in electronic systems under a DC electric field have shown that the threshold fields for the insulator-to-metal transition are significantly lower than predicted by single-electron excitation scenarios, such as the Landau-Zener theory. In this work, we propose an alternate mechanism of destabilizing ordered insulators under a DC electric field by fluctuations of the order parameters through the Goldstone mode excitation. The low-energy bosonic excitations receive energy from accelerated electrons and thus destroy the spontaneous symmetry breaking. Using the Keldysh Gree's function formalism, we numerically confirm that the Goldstone mode remains well-defined in the nonequilibrium steady state, while its nonequilibrium excitations are sensitive to the electric field. The effective temperature of the Goldstone mode increases much more rapidly than the electronic effective temperature, with the bosonic threshold field significantly smaller than the electronic one, which suggests that collective phase dynamics may further reduce the transition field to the experimental range via a purely electronic mechanism.

Negative differential resistance (NDR) behavior observed in several transition metal oxides is crucial for developing next-generation memory devices and neuromorphic computing systems. NbO2-based memristors exhibit two regions of NDR at room temperature, making them promising candidates for such applications. Despite this potential, the physical mechanisms behind the onset and the ability to engineer these NDR regions remain unclear, hindering further development of these devices for applications. This study employed electrical transport and ultra-low frequency noise spectroscopy measurements to investigate two distinct NDR phenomena in nanoscale thin films of NbO2. By analyzing the residual current fluctuations as a function of time, spatially inhomogeneous and non-linear conduction are found near NDR-1 and a two-state switching near NDR-2, leading to an insulator-to-metal (IMT) transition. The power spectral density of the residual fluctuations exhibits significantly elevated noise magnitudes around both NDR regions, providing insights into physical mechanisms and device size scaling for electronic applications. A simple theoretical model, based on the dimerization of correlated insulators, offers a comprehensive explanation of observed transport and noise behaviors near NDRs, affirming the presence of non-linear conduction followed by an IMT connecting macroscopic device response to transport signatures at the atomic level.

Quenched disorder in a solid state system can result in Anderson localization, where electrons are exponentially localized and the system behaves like an insulator. By exactly solving a disordered electronic lattice model out of equilibrium, we investigate the effect of a DC electric field on Anderson localization in an open system and provide a minimal platform to study disorder-nonequilibrium interplay in electronic lattice systems. We perform steady-state Keldysh-Green's function calculations on an infinite lattice with a finite range of disorder-active regions that are coupled to fermion reservoirs. Our solutions out of a fully electronic model verify Mott's temperature scaling of the variable-range-hopping transport and the Lifshitz tail, well corroborated by the coherent potential approximation. We further reveal that a nonequilibrium electronic lattice creates a statistical evolution that shows a counterintuitive shift of the distribution edge in the opposite direction of the band edge. The rich evolution of nonthermal statistics highlights the importance of an explicit band structure and the impurity correlations in strong nonequilibrium theories.

By measuring the shot noise, a consequence of charge quantization, in super-conductor/insulator/ferromagnet (V/MgO/Fe) junctions, we discover a giant increase, orders of magnitude larger than expected. The origin of this giant noise is a peculiar realization of a superconducting proximity effect, where a simple superconductor influences its neighbors. Our measurements reveal largely unexplored implications of orbital-symmetry-controlled proximity effects. The importance of orbital symmetries and the accompanying spin-orbit coupling is manifested by an unexpected emergence of another superconducting region, strikingly different from the parent superconductor. Unlike vanadium's common spin-singlet superconductivity, the broken inversion symmetry in V/MgO/Fe junctions and the resulting interfacial spin-orbit coupling lead to the formation of spin-triplet superconductivity across the ferromagnetic iron. Here, we show that the enhanced shot noise, known from Josephson junctions with two superconductors, is measured even in a single superconductor. This discovery motivates revisiting how the spin-orbit coupling and superconducting proximity effects can transform many materials.

Transport in disordered systems often occurs via variable range hopping (VRH) in the dilute carrier density limit, where electrons hop between randomly distributed localized levels. We study the nonequilibrium transport by a uniform DC electric field on a one-dimensional (1D) insulating tight-binding chain with the on-site disorder, using a disordered-lattice calculation and the coherent potential approximation. We develop a theory of electric-field-assisted VRH as a mechanism for nonlinear transport in a disordered chain. Our disordered-lattice calculations of the electron propagation distance and the electron mobility determine the range of VRH as Delta

Planar Josephson junctions provide a platform to host topological superconductivity which, through manipulating Majorana bound states (MBS), could enable fault-tolerant quantum computing. However, what constitutes experimental signatures of topological superconductivity and how MBS can be detected remains strongly debated. In addition to spurious effects that mimic MBS, there is a challenge to discern the inherent topological signals in realistic systems with many topologically-trivial Andreev bound states, determining the transport properties of Josephson junctions. Guided by the advances in microwave spectroscopy, we theoretically study Al/InAs-based planar Josephson junction embedded into a radio-frequency superconducting quantum interference device to identify microwave signatures of topological superconductivity. Remarkably, by exploring the closing and reopening of a topological gap, we show that even in a wide planar Josephson junction with many Andreev bound states, such a topological signature is distinguishable in the resonance frequency shift of a microwave drive and the “half-slope” feature of the microwave absorption spectrum. Our findings provide an important step towards experimental detection of non-Abelian statistics and implementing scalable topological quantum computing.

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.

A fundamental instability in the nonequilibrium conduction band under a electric field bias is proposed via the spontaneous emission of coherent phonons. Analytic theory, supported by numerical calculations, establishes that the quantum avalanche, an abrupt nonequilibrium occupation of excited bands, results from the competition between the collapse of the band minimum via the phonon emission and the dephasing of the electron with the environment. The continuous avalanche transition is a quantum phase transition with the nonequilibrium phase diagram determined by the avalanche parameter $β$, with peculiar reentrant avalanche domes close to the phase boundary. We further confirm the nature of the quantum avalanche with the temperature dependence.

While spin-triplet pairing remains elusive in nature, there is a growing effort to realize proximity-induced equal-spin triplet superconductivity in junctions with magnetic regions or an applied magnetic field and common s-wave superconductors. To enhance such spin-triplet contribution, it is expected that junctions with a weak interfacial barrier and strong spin-orbit coupling are desirable. Intuitively, a weak interfacial barrier enables a robust proximity-induced superconductivity and strong spin-orbit coupling promotes spin mixing, converting spin-singlet into spin-triplet superconductivity. In contrast, we reveal a nonmonotonic spin-triplet contribution with the strength of the interfacial barrier and spin-orbit coupling. This picture is established by considering different signatures in conductance and superconducting correlations, as well as by performing self-consistent calculations. As a result, we identify a strongly enhanced spin-triplet superconductivity, realized for an intermediate strength of interfacial barrier and spin-orbit coupling. In junctions with magnetic regions, an enhanced spin-triplet superconductivity leads to a large magnetoanisotropy of conductance and superconducting correlations. This picture of an enhanced spin-triplet superconductivity is consistent with experiments demonstrating a huge increase in the conductance magnetoanisotropy, which we predict can be further enhanced at a finite bias.

A planar Josephson junction is a versatile platform to realize topological superconductivity over a large parameter space and host Majorana bound states. With a change in the Zeeman field, this system undergoes a transition from trivial to topological superconductivity accompanied by a jump in the superconducting phase difference between the two superconductors. A standard model of these Josephson junctions, which can be fabricated to have a nearly perfect interfacial transparency, predicts a simple universal behavior. In that model, at the same value of Zeeman field for the topological transition, there is a π phase jump and a minimum in the critical superconducting current, while applying a controllable phase difference yields a diamond-shaped topological region as a function of that phase difference and a Zeeman field. In contrast, even for a perfect interfacial transparency, we find a much richer and nonuniversal behavior as the width of the superconductor is varied or the Dresselhaus spin–orbit coupling is considered. The Zeeman field for the phase jump, not necessarily π, is different from the value for the minimum critical current, while there is a strong deviation from the diamond-like topological region. These Josephson junctions show a striking example of a nonreciprocal transport and superconducting diode effect, revealing the importance of our findings not only for topological superconductivity and fault-tolerant quantum computing but also for superconducting spintronics.

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.

We explain how Rashba spin-orbit coupling (SOC) in a two-dimensional electron gas (2DEG), or in a conventional s-wave superconductor, can lead to a large magnetoresistance even with one ferromagnet. However, such enhanced magnetoresistance is not generic and can be nonmonotonic and change its sign with Rashba SOC. For an in-plane rotation of magnetization, it is typically negligibly small for a 2DEG and depends on the perfect transmission which emerges from a spin-parity-time symmetry of the scattering states, while this symmetry is generally absent from the Hamiltonian of the system. The key difference from considering the normal-state magnetoresistance is the presence of the spin-dependent Andreev reflection at superconducting interfaces. In the fabricated junctions of quasi-2D van der Waals ferromagnets with conventional s-wave superconductors (Fe_0.29TaS_2/NbN) we find another example of enhanced magnetoresistance where the presence of Rashba SOC reduces the effective interfacial strength and is responsible for an equal-spin Andreev reflection. The observed nonmonotonic trend in the out-of-plane magnetoresistance with the interfacial barrier is an evidence for the proximity-induced equal-spin-triplet superconductivity.

Motivated by the resistive switchings in transition-metal oxides (TMOs) induced by a voltage bias, we study the far-from-equilibrium dynamics of an electric-field-driven strongly correlated model featuring a first-order insulator-to-metal transition at equilibrium, namely the dimer-Hubbard model. We use a nonequilibrium imple-mentation of the dynamical cluster approximation to access the steady-state spectral and transport properties. We show that the electric field can drive both metal-to-insulator and insulator-to-metal transitions. While they proceed by quite distinct mechanisms, specifically simple heating of the metal versus nonequilibrium effects in the correlated charge gap, we show that both of these nonequilibrium transitions can be unified in a single framework once the excitations are accounted for in terms of an effective temperature. This conceptual advance brings together the two sides of the long-lasting debate over the origins of the electrically driven resistive switching in TMOs.

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.

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.

A hallmark of topological superconductivity is the non-Abelian statistics of Majorana bound states (MBS), its chargeless zero-energy emergent quasiparticles. The resulting fractionalization of a single electron, stored nonlocally as a two spatially-separated MBS, provides a powerful platform for implementing fault-tolerant topological quantum computing. However, despite intensive efforts, experimental support for MBS remains indirect and does not probe their non-Abelian statistics. Here we propose how to overcome this obstacle in mini-gate controlled planar Josephson junctions (JJs) and demonstrate non-Abelian statistics through MBS fusion, detected by charge sensing using a quantum point contact, based on dynamical simulations. The feasibility of preparing, manipulating, and fusing MBS in two-dimensional (2D) systems is supported in our experiments which demonstrate the gate control of topological transition and superconducting properties with five mini gates in InAs/Al-based JJs. While we focus on this well-established platform, where the topological superconductivity was already experimentally detected, our proposal to identify elusive non-Abelian statistics motivates also further MBS studies in other gate-controlled 2D systems.

With non-Abelian statistics and nonlocal degrees of freedom, Majorana bound states (MBS) are suitable for implementing fault-tolerant topological quantum computing. While the main efforts to realize MBS have focused on one-dimensional systems, it requires delicate parameter tuning and its geometric constraints pose significant challenges for the demonstration of non-Abelian statistics. Building on recent experimental advances in planar Josephson junctions (JJs), we propose how to overcome this obstacle in topological JJs and demonstrate non-Abelian statistics with phase or mini-gate control, detected by charge sensing using quantum point contacts. Our proposals, supported by the experiments, would constitute an important milestone towards topological quantum computing.

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.