Tunneling Regime Research Articles

Probing metal-molecule contact at the atomic scale via conductance jumps

Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic scale devices. Employing two isomers of bipyridine, $4, 4^\prime$ bipyridine and $2, 2^\prime$ bipyridine between gold electrodes, here, we investigate the formation of metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at room temperature. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for $4, 4^\prime$ bipyridine via a conductance jump from the tunneling regime, referred as `jump to molecular contact', being absent for $2, 2^\prime$ bipyridine. Through statistical analysis of the data, along with, molecular dynamics and first-principles calculations, we establish that contact formation is strongly connected with the molecular structure of the electrodes as well as how the junction is broken during breaking process, providing important insights for using a single-molecule in an electronic device.

Statistical detection of Josephson, Andreev, and single quasiparticle currents in scanning tunneling microscopy

We present a method to identify distinct tunneling modes in a tunable superconducting tunnel junction composed of a superconducting tip and a sample in a scanning tunneling microscope. Combining the relative decay constant of tunneling current extracted from $I\ensuremath{-}V\ensuremath{-}z$ spectroscopy with its statistical analysis over the atomic disorders in the sample surface, we identified the crossover of dominant tunneling modes between single charge tunneling, Andreev reflection (AR), and Josephson tunneling with respect to the bias voltage at a measurement temperature nearly half of the critical temperature. The method enables one to determine the specific tunneling regime independently of the spectral shapes and to reveal intrinsic modulation of AR and Josephson current by disorder that will be crucial for superconducting quantum information processing.

Quasiclassical theory of non-adiabatic tunneling in nanocontacts induced by phase-controlled ultrashort light pulses

We theoretically investigate tunneling through free-space or dielectric nanogaps between metallic nanocontacts driven by ultrashort ultrabroadband light pulses. For this purpose we develop a time-dependent quasiclassical theory being especially suitable to describe the tunneling process in the non-adiabatic regime, when tunneling can be significantly influenced by photon absorption as the electron moves in the classically forbidden region. Firstly, the case of driving by an ideal half-cycle pulse is studied. For different distances between the contacts, we analyze the main solutions having the form of a quasiclassical wave packet of the tunneling electron and an evanescent wave of the electron density. For each of these solutions the resulting tunneling probability is determined with the exponential accuracy inherent to the method. We identify a crossover between two tunneling regimes corresponding to both solutions in dependence on the field strength and intercontact distance that can be observed in the corresponding behaviour of the tunneling probability. Secondly, considering realistic temporal profiles of few-femtosecond pulses, we demonstrate that the preferred direction of the electron transport through the nanogap can be controlled by changing the carrier-envelope phase of the pulse, in agreement with recent experimental findings and numerical simulations. We find analytical expressions for the tunneling probability, determining the resulting charge transfer in dependence on the pulse parameters. Further, we determine temporal shifts of the outgoing electron trajectories with respect to the peaks of the laser field as a function of the pulse phase and illustrate when the non-adiabatical character of the tunneling process is particularly important.

Effect of a ferromagnetic STM cobalt tip on a single Co-phthalocyanine molecule adsorbed on a ferromagnetic substrate

The effect of a ferromagnetic scanning tunneling microscope (STM) cobalt tip on the electronic, magnetic and electronic transport properties of a Co-phthalocyanine (CoPc)/Co(111) junction has been investigated in the framework of density functional theory in conjunction with Landauer transport and the non-equilibrium Green function formalism. It is shown that the spin magnetic moment of the CoPc molecule can be flipped by varying the distance between the STM tip and the CoPc molecule when passing from the tunneling regime to the contact regime. Our calculations show that such spin flip of the CoPc molecule leads to a change of the sign of the Tunneling Magneto-Resistance ratio (TMR). The change from the tunneling to contact regime also leads to large changes in the total and spin-polarized I–V characteristics.

Coherent spin qubit transport in silicon

A fault-tolerant quantum processor may be configured using stationary qubits interacting only with their nearest neighbours, but at the cost of significant overheads in physical qubits per logical qubit. Such overheads could be reduced by coherently transporting qubits across the chip, allowing connectivity beyond immediate neighbours. Here we demonstrate high-fidelity coherent transport of an electron spin qubit between quantum dots in isotopically-enriched silicon. We observe qubit precession in the inter-site tunnelling regime and assess the impact of qubit transport using Ramsey interferometry and quantum state tomography techniques. We report a polarization transfer fidelity of 99.97% and an average coherent transfer fidelity of 99.4%. Our results provide key elements for high-fidelity, on-chip quantum information distribution, as long envisaged, reinforcing the scaling prospects of silicon-based spin qubits.

Gas sensing properties of a two-dimensional graphene/h-BN multi-heterostructure toward H2O, NH3 and NO2: A first principles study

Two dimensional nanomaterials are promising for gas sensing applications due to their large surface to volume ratio. Recent studies show that the sensitivity of pristine graphene, the most prominent two-dimensional material, can be improved by several methods such as doping, decoration and combination with other two-dimensional materials. In the present work a two-dimensional graphene/h-BN multi-heterostructure, in the form of G/h-BN/G/h-BN/G, is proposed for gas sensing applications, which is expected to outperform pristine graphene devices. The adsorption energies and charge transfer for H2O, NH3 and NO2 molecules are investigated at the density functional level of theory. The presence of two insulating h-BN layers induces potential barriers for charge carriers and changes the current mechanism to the quantum tunneling regime, which is highly sensitive to the modulation of the potential barrier due to the adsorbed molecules. A first principles method based on non-equilibrium Green’s function formalism is employed for the calculation of the device current in various environments. The proposed device shows little sensitivity toward H2O but it is very sensitive toward NH3 and NO2. Moreover, NO2 adsorption increases the device current, while NH3 adsorption reduces the current, a property which can result in a selective sensing of these two gas molecules.

Exploration of interference structure of a hydrogen atom with different wavelengths in a linearly polarized laser field

We investigate the ionization process of an atom with different wavelengths in a linearly polarized laser field. With the change of wavelength, the interference structure in photoelectron momentum distributions (PMDs) changes obviously. The temporal double-slit fringes can be clearly observed in the relatively high-energy region below the energy of 2U p for the laser wavelength , but we only observe the temporal double-slit fringes in the relatively low-energy region below the energy of 2U p for the laser wavelength . By analyzing the number of relative contribution of directly ionized electrons, we find a sensitive dependence of the direct electron on the laser wavelength. In addition, the temporal double-slit fringes can be also observed in the photoelectron momentum distributions in the deep tunneling regime ().

Femtosecond Field‐Driven On‐Chip Unidirectional Electronic Currents in Nonadiabatic Tunneling Regime

AbstractRecently, asymmetric plasmonic nanojunctions have shown promise as on‐chip electronic devices to convert femtosecond optical pulses to current bursts, with a bandwidth of multi‐terahertz scale, although yet at low temperatures and pressures. Such nanoscale devices are of great interest for novel ultrafast electronics and opto‐electronic applications. Here, the device is operated in air and at room temperature, revealing the mechanisms of photoemission from plasmonic nanojunctions, and the fundamental limitations on the speed of optical‐to‐electronic conversion. Inter‐cycle interference of coherent electronic wavepackets results in a complex energy electron distribution and birth of multiphoton effects. This energy structure, as well as reshaping of the wavepackets during their propagation from one tip to the other, determine the ultrafast dynamics of the current. It is shown that, up to some level of approximation, the electron flight time is well‐determined by the mean ponderomotive velocity in the driving field.

Highly Efficient Experimental Approach to Evaluate Metal to 2D Semiconductor Interfaces in Vertical Diodes with Asymmetric Metal Contacts.

The energy band alignments and associated material properties at the contacts between metal and two-dimensional (2D) semiconducting transition metal dichalcogenide (SCTMD) films determine the important traits in 2D SCTMD-based electronic and optical device applications. In this work, we realize 2D vertical diodes with asymmetric metal-SCTMD contact areas where currents are dominated by the contact-limited charge flows in the transport regimes of Fowler-Nordheim tunneling and Schottky emission. With straightforward current-voltage characteristics, we can accurately evaluate the interface parameters such as Schottky barrier heights and the vertical effective masses of tunneling charges. In particular, the differing contact areas and resultant current rectifications allow us to address specific Schottky barrier locations with respect to the conduction and valence band edges of 2D semiconducting WSe2, WS2, MoSe2, and MoS2, thereby determining whether p-type holes or n-type electrons become the majority charge carriers in the SCTMD devices. We demonstrate that our experimental and analytical approaches can be utilized as a simple but powerful material metrology to qualitatively and quantitatively evaluate various metal-SCTMD contacts.

Dynamical Coulomb blockade under a temperature bias

We observe and comprehend the dynamical Coulomb blockade suppression of the electrical conductance across an electronic quantum channel submitted to a temperature difference. A broadly tunable, spin-polarized Ga(Al)As quantum channel is connected on-chip, through a micron-scale metallic node, to a linear $RC$ circuit. The latter is made up of the node's geometrical capacitance $C$ in parallel with an adjustable resistance $R\in \{1/2,1/3,1/4\}\times h/e^2$ formed by 2--4 quantum Hall channels. The system is characterized by three temperatures: a temperature of the electrons in the large electrodes ($T$) and in the node ($T_\mathrm{node}$), and a temperature of the electromagnetic modes of the $RC$ circuit ($T_\mathrm{env}$). The temperature in the node is selectively increased by local Joule dissipation, and characterized from current fluctuations. For a quantum channel in the tunnel regime, a close match is found between conductance measurements and tunnel dynamical Coulomb blockade theory. In the opposite near ballistic regime, we develop a theory that accounts for different electronic and electromagnetic bath temperatures, again in very good agreement with experimental data. Beyond these regimes, for an arbitrary quantum channel set in the far out-of-equilibrium situation where the temperature in the node significantly exceeds the one in the large electrodes, the equilibrium (uniform temperature) prediction for the conductance is recovered, albeit at a rescaled temperature $\alpha T_\mathrm{node}$.

Gap solitons in Bose–Einstein condensate loaded in a honeycomb optical lattice: Nonlinear dynamical stability, tunneling, and self-trapping

We investigate the gap solitons of Bose–Einstein condensate in honeycomb optical lattices. It is found that the two-dimensional honeycomb optical lattices admit multipole gap solitons. These multipoles can have their bright solitary structures be in-phase or out-of-phase. The nonlinear dynamical stabilities of these solitons are investigated using direct simulations of the Gross–Pitaevskii equation. For the unipole gap solitons, the nonlinear evolution shows dynamical stability or instability, which depends on the properties of atomic interactions and the dependence of soliton power. A fascinating property of dipole gap solitons is that they can present self-trapping or tunneling instabilities under atomic nonlinearity. The in-phase and out-of-phase of multipole gap solitons support different tunneling or self-trapping regimes. These results have an application to investigations of localized structures in nonlinear optics and Bose–Einstein condensate.

Simulating twistronics in acoustic metamaterials

Twisted van der Waals heterostructures have recently emerged as a tunable platform for studying correlated electrons. However, these materials require laborious and expensive effort for both theoretical and experimental exploration. Here we numerically simulate twistronic behavior in acoustic metamaterials composed of interconnected air cavities in two stacked steel plates. Our classical analog of twisted bilayer graphene perfectly replicates the band structures of its quantum counterpart, including mode localization at a magic angle of 1.12∘. By tuning the thickness of the interlayer membrane, we reach a regime of strong interlayer tunneling where the acoustic magic angle appears as high as 6.01∘, equivalent to applying 130 GPa to twisted bilayer graphene. In this regime, the localized modes are over five times closer together than at 1.12∘, increasing the strength of any emergent non-linear acoustic couplings.

Tunneling processes between Yu-Shiba-Rusinov bound states

Very recent experiments have reported the tunneling between Yu-Shiba-Rusinov (YSR) bound states at the atomic scale. These experiments have been realized with the help of a scanning tunneling microscope where a superconducting tip is functionalized with a magnetic impurity and is used to probe another magnetic impurity deposited on a superconducting substrate. In this way it has become possible to study for the first time the spin-dependent transport between individual superconducting bound states. Motivated by these experiments, we present here a comprehensive theoretical study of the tunneling processes between YSR bound states in a system in which two magnetic impurities are coupled to superconducting leads. Our theory is based on a combination of an Anderson model with broken spin degeneracy to describe the impurities and nonequilibrium Green's function techniques to compute the current-voltage characteristics. This combination allows us to describe the spin-dependent transport for an arbitrary strength of the tunnel coupling between the impurities. We first focus on the tunnel regime and show that our theory naturally explains the experimental observations of the appearance of current peaks in the subgap region due to both the direct and thermal tunneling between the YSR states in both impurities. Then, we study in detail the case of junctions with increasing transparency, which has not been experimentally explored yet, and predict the occurrence of a large variety of (multiple) Andreev reflections mediated by YSR states that give rise to a very rich structure in the subgap current. In particular, we predict the occurrence of multiple Andreev reflections that involve YSR states in different impurities. These processes have no analogue in single-impurity junctions and they are manifested as current peaks with negative differential conductance for subgap voltages.

Tunneling Spectroscopy for Electronic Bands in Multi-Walled Carbon Nanotubes with Van Der Waals Gap.

Various intriguing quantum transport measurements for carbon nanotubes (CNTs) based on their unique electronic band structures have been performed adopting a field-effect transistor (FET), where the contact resistance represents the interaction between the one-dimensional and three-dimensional systems. Recently, van der Waals (vdW) gap tunneling spectroscopy for single-walled CNTs with indium–metal contacts was performed adopting an FET device, providing the direct assignment of the subband location in terms of the current–voltage characteristic. Here, we extend the vdW gap tunneling spectroscopy to multi-walled CNTs, which provides transport spectroscopy in a tunneling regime of ~1 eV, directly reflecting the electronic density of states. This new quantum transport regime may allow the development of novel quantum devices by selective electron (or hole) injection to specific subbands.

Frustration effect on escape rate in Josephson junctions between single-band and three-band superconductors in the macroscopic quantum tunneling regime

The escape rate of S → R switching (from superconducting S state to resistive R state) in Josephson junction formed by s-wave single-band (SB) and three-band (ThB) superconductors (SB/ThB junctions) in macroscopic quantum tunneling regime is investigated. We use the effective critical current approximation in SB/ThB junctions. It was shown that escape rate can exhibit qualitative features in the case of frustration effects in ThB superconductors. Inclusion of Leggett modes in ThB superconductors leads to enhancement of escape rate in three-channel Josephson junction in comparison with single-channel junctions.

Interface-engineered electron and hole tunneling.

Although the phenomenon of tunneling has been known since the advent of quantum mechanics, it continues to enrich our understanding of many fields of science. Commonly, this effect is described in terms of electrons traversing the potential barrier that exceeds their kinetic energy due to the wave nature of electrons. This picture of electron tunneling fails, however, for tunnel junctions, where the Fermi energy lies sufficiently close to the insulator valence band, in which case, hole tunneling dominates. We demonstrate the deterministic control of electron and hole tunneling in interface-engineered Pt/BaTiO3/La0.7Sr0.3MnO3 ferroelectric tunnel junctions by reversal of tunneling electroresistance. Our electrical measurements, electron microscopy and spectroscopy characterization, and theoretical modeling unambiguously point out to electron or hole tunneling regimes depending on interface termination. The interface control of the tunneling regime offers designed functionalities of electronic devices.

Modulation dynamics of atomic Rydberg excitation in strong-field tunneling ionization

We theoretically investigate the modulation structures of atomic Rydberg excitation in tunneling ionization and show its various features with diversity of population amplitude as a function of laser peak intensity. Simulating by the time-dependent Schrödinger equation, we find that the distribution of bound states with low-lying principal quantum number n tends to be dominant on total excitation probability with opposite phase compared with other higher n states. At the same time, a well-defined parity angular momentum state makes the main contribution under one certain laser intensity. This Rydberg residual population effect of a rare-gas atom surviving from the laser field is elaborated by the repopulation of Rydberg states via Λ -type Raman transitions joining with a low-energy free electron, which confirms that multiphoton resonance exists in the tunneling regime. It helps in understanding the peak structures of the excited state population quantitatively as well as the atomic stabilization established in the tunneling regime. A physical scenario is also established for the role of Rydberg atom radial size in the ionization.

Comparative study of photoionization of atomic hydrogen by solving the one- and three-dimensional time-dependent Schrödinger equations* *Project supported by the National Natural Science Foundation of China (Gant Nos. 12074265, 11804233, and 11575118), the National Key Research and Development Project of China (Grant No. 2017YFF0106500), the Natural Science Foundation of Guangdong, China (Grant Nos. 2018A0303130311 and 2021A1515010082), and the Shenzhen Fundamental

We develop a numerical scheme for solving the one-dimensional (1D) time-dependent Schrödinger equation (TDSE), and use it to study the strong-field photoionization of the atomic hydrogen. The photoelectron energy spectra obtained for pulses ranging from XUV to near infrared are compared in detail to the spectra calculated with our well-developed code for accurately solving the three-dimensional (3D) TDSE. For XUV pulses, our discussions cover intensities at which the ionization is in the perturbative and nonperturbative regimes. For pulses of 400 nm or longer wavelengths, we distinguish the multiphoton and tunneling regimes. Similarities and discrepancies between the 1D and 3D calculations in each regime are discussed. The observed discrepancies mainly originate from the differences in the transition matrix elements and the energy level structures created in the 1D and 3D calculations.

Power spectrum of density perturbations in chain inflation

Chain Inflation is an alternative to slow roll inflation in which the universe undergoes a series of transitions between different vacua. The density perturbations (studied in this paper) are seeded by the probabilistic nature of tunneling rather than quantum fluctuations of the inflaton. We find the scalar power spectrum of chain inflation and show that it is fully consistent with a $\Lambda$CDM cosmology. In agreement with some of the previous literature (and disagreement with others), we show that $10^4$ phase transitions per e-fold are required in order to agree with the amplitude of Cosmic Microwave Background anisotropies within the observed range of scales. Interestingly, the amplitude of perturbations constrains chain inflation to a regime of highly unstable de Sitter spaces, which may be favorable from a quantum gravity perspective since the Swampland Conjecture on Trans-Planckian Censorship is automatically satisfied. We provide new analytic estimates for the bounce action and the tunneling rate in periodic potentials which replace the thin-wall approximation in the regime of fast tunneling. Finally, we study model implications and derive an upper limit of $\sim 10^{10}$ GeV on the axion decay constant in viable chain inflation with axions.

InSitu Generation of High-Energy Spin-Polarized Electrons in a Beam-Driven Plasma Wakefield Accelerator.

Insitu generation of a high-energy, high-current, spin-polarized electron beam is an outstanding scientific challenge to the development of plasma-based accelerators for high-energy colliders. In this Letter, we show how such a spin-polarized relativistic beam can be produced by ionization injection of electrons of certain atoms with a circularly polarized laser field into a beam-driven plasma wakefield accelerator, providing a much desired one-step solution to this challenge. Using time-dependent Schrödinger equation (TDSE) simulations, we show the propensity rule of spin-dependent ionization of xenon atoms can be reversed in the strong-field multiphoton regime compared with the non-adiabatic tunneling regime, leading to high total spin polarization. Furthermore, three-dimensional particle-in-cell simulations are incorporated with TDSE simulations, providing start-to-end simulations of spin-dependent strong-field ionization of xenon atoms and subsequent trapping, acceleration, and preservation of electron spin polarization in lithium plasma. We show the generation of a high-current (0.8kA), ultralow-normalized-emittance (∼37 nm), and high-energy (2.7GeV) electron beam within just 11cm distance, with up to ∼31% net spin polarization. Higher current, energy, and net spin-polarization beams are possible by optimizing this concept, thus solving a long-standing problem facing the development of plasma accelerators.