Resonant Tunneling Process Research Articles

Manipulating the optimal transport orbital and virtual states in a supramolecular transistor

Supramolecular electronics bridges the gap between single-molecule electronics and bulk, plastic electronics, and is embodied as one of the biggest promises of nanometer electronics made by supramolecular functional materials. Within this field, the way in which electrons propagate through the supramolecule plays a profound role both in designing supramolecular devices and in understanding the structure–property relationships at the molecular level. Herein, we consider a supramolecular transistor, which contains bipartite molecular monomers, with one molecular orbital is non-interacting, whereas the other one is strongly correlated. Modeling this device using the two-impurity Anderson model, we unravel the orbital selections and the virtual states in the electronic transport process with the aid of the celebrated numerical renormalization group method. We demonstrate that if both orbitals are in their particle-hole symmetric regimes, electrons favor to transmit through the non-interacting molecular orbital via the resonant tunneling process, rather than through the strongly correlated molecular orbital via the Kondo effect, despite the energy of the strongly correlated orbital is lower. This phenomenon indicates that if the Kondo effect occurs, it experiences a temporarily doubly occupied virtual state. Interestingly, the situation reverses if the inter-molecule hopping integral is applied, where the strongly correlated orbital becomes optimal and the virtual state turns to be an empty state. To understand these behaviors, both the static and the dynamical properties, as well as those thermodynamic quantities are studied in detail, and necessary physical arguments are given. Our findings afford deep insights into the manipulations of the optimal transport orbital and the Kondo behaviors within quantum impurity devices.

Multiple Fano Resonances in Dynamic Resonant Tunneling Processes

The existence of Fano resonances in dynamic resonant tunneling (RT) systems has been investigated. Fano resonances are characterized by the appearance of a 100% reflection coefficient in proximity to a high transmission coefficient. For a Fano resonance to appear, a bound state must exist. On the other hand, a resonant tunneling process is characterized by a high transmission and the existence of a quasi-bound state (QBS) instead of a bound one. It has been shown that, by narrowing the width of the barrier, the resonance energy of the QBS gradually decreases and eventually turns into a bound state. Consequently, in a dynamic RT process, there are two scenarios: either a bound state exists, in which case, Fano resonances exist for any barrier width, or a QBS exists, and the barrier should be narrow enough for the Fano resonance to appear. In both cases, the incoming particle’s frequency must be lower than the oscillating well’s frequency. In this work, these resonances are investigated in detail, and both exactly numerically and approximated analytical expressions are derived for both the weak and strong oscillating amplitude regimes. One of the conclusions is that, when the oscillating frequency is low enough, multiple Fano resonances can appear by varying the barrier’s width. Since these resonances are very sharp and zero transmission can easily be detected, this property can be used as a very accurate method for measuring the barrier’s width, even when the particle’s de-Broglie wavelength is much larger than the barrier’s width.

Formation of Spontaneous Density-Wave Patterns in dc Driven Lattices

Driving a many-body system out of equilibrium induces phenomena such as the emergence and decay of transient states, which can manifest itself as pattern and domain formation. The understanding of these phenomena expands the scope of established thermodynamics into the out-of-equilibrium domain. Here, we experimentally and theoretically study the out-of-equilibrium dynamics of a bosonic lattice model subjected to a strong DC field, realized as ultracold atoms in a strongly tilted triangular optical lattice. We observe the emergence of pronounced density wave patterns -- which spontaneously break the underlying lattice symmetry -- using a novel single-shot imaging technique with two-dimensional single-site resolution in three-dimensional systems, which also resolves the domain structure. Our study suggests that the short-time dynamics arises from resonant pair tunneling processes within an effective description of the tilted Hubbard model. More broadly, we establish the far out-of-equilibrium regime of lattice models subjected to a strong DC field, as an exemplary and paradigmatic scenario for transient pattern formation.

Analytical Modeling and Band-Structure Engineering of Resonant Tunneling Diode for Biased Terahertz Detector

A new strategy to significantly enhance the nonlinearity of double-barrier resonant tunneling diode (RTD) in the positive differential resistance region is theoretically proposed and experimentally demonstrated. An analytical model containing physical insight into the resonant tunneling process is established to bridge the nonlinearity of the current–voltage curve and the band-structure information of RTD. It intuitively shows that the intrinsic current responsivity of RTD as well as its nonlinearity can be greatly improved via band-structure engineering, as enhancing the resonant energy level of the well and reducing the current before the resonant tunneling process. The validation of the proposed model and the associated band-structure engineering strategy are verified by both device simulation and experiments. A RTD device with intrinsic current responsivity of 6.74 A/W is realized, while the peak-to-valley current ratio (PVCR) is also as large as 8.92. This method provides a new pathway to design high-performance terahertz (THz) detectors using double-barrier RTD and may share new perspective on the potential of RTD-based THz transceivers.

Longitudinal and transversal resonant tunneling of interacting bosons in a two-dimensional Josephson junction

We unravel the out-of-equilibrium quantum dynamics of a few interacting bosonic clouds in a two-dimensional asymmetric double-well potential at the resonant tunneling scenario. At the single-particle level of resonant tunneling, particles tunnel under the barrier from, typically, the ground-state in the left well to an excited state in the right well, i.e., states of different shapes and properties are coupled when their one-particle energies coincide. In two spatial dimensions, two types of resonant tunneling processes are possible, to which we refer to as longitudinal and transversal resonant tunneling. Longitudinal resonant tunneling implies that the state in the right well is longitudinally-excited with respect to the state in the left well, whereas transversal resonant tunneling implies that the former is transversely-excited with respect to the latter. We show that interaction between bosons makes resonant tunneling phenomena in two spatial dimensions profoundly rich, and analyze these phenomena in terms of the loss of coherence of the junction and development of fragmentation, and coupling between transverse and longitudinal degrees-of-freedom and excitations. To this end, a detailed analysis of the tunneling dynamics is performed by exploring the time evolution of a few physical quantities, namely, the survival probability, occupation numbers of the reduced one-particle density matrix, and the many-particle position, momentum, and angular-momentum variances. To accurately calculate these physical quantities from the time-dependent many-boson wavefunction, we apply a well-established many-body method, the multiconfigurational time-dependent Hartree for bosons (MCTDHB), which incorporates quantum correlations exhaustively. By comparing the survival probabilities and variances at the mean-field and many-body levels of theory and investigating the development of fragmentation, we identify the detailed mechanisms of many-body longitudinal and transversal resonant tunneling in two dimensional asymmetric double-wells. In particular, we find that the position and momentum variances along the transversal direction are almost negligible at the longitudinal resonant tunneling, whereas they are substantial at the transversal resonant tunneling which is caused by the combination of the density and breathing mode oscillations. We show that the width of the interparticle interaction potential does not affect the qualitative physics of resonant tunneling dynamics, both at the mean-field and many-body levels. In general, we characterize the impact of the transversal and longitudinal degrees-of-freedom in the many-boson tunneling dynamics at the resonant tunneling scenarios.

Manifestations of Resonant-Tunneling Processes and Random Potential Fluctuations with the Participation of Quantum-Dot Levels in the Photocurrent Relaxation of p–i–n GaAs/AlAs Heterostructures

Manifestations of Resonant-Tunneling Processes and Random Potential Fluctuations with the Participation of Quantum-Dot Levels in the Photocurrent Relaxation of p–i–n GaAs/AlAs Heterostructures

Numerically "exact" approach to open quantum dynamics: The hierarchical equations of motion (HEOM).

An open quantum system refers to a system that is further coupled to a bath system consisting of surrounding radiation fields, atoms, molecules, or proteins. The bath system is typically modeled by an infinite number of harmonic oscillators. This system-bath model can describe the time-irreversible dynamics through which the system evolves toward a thermal equilibrium state at finite temperature. In nuclear magnetic resonance and atomic spectroscopy, dynamics can be studied easily by using simple quantum master equations under the assumption that the system-bath interaction is weak (perturbative approximation) and the bath fluctuations are very fast (Markovian approximation). However, such approximations cannot be applied in chemical physics and biochemical physics problems, where environmental materials are complex and strongly coupled with environments. The hierarchical equations of motion (HEOM) can describe the numerically "exact" dynamics of a reduced system under nonperturbative and non-Markovian system-bath interactions, which has been verified on the basis of exact analytical solutions (non-Markovian tests) with any desired numerical accuracy. The HEOM theory has been used to treat systems of practical interest, in particular, to account for various linear and nonlinear spectra in molecular and solid state materials, to evaluate charge and exciton transfer rates in biological systems, to simulate resonant tunneling and quantum ratchet processes in nanodevices, and to explore quantum entanglement states in quantum information theories. This article presents an overview of the HEOM theory, focusing on its theoretical background and applications, to help further the development of the study of open quantum dynamics.

Design and characterization of a quantum heat pump in a driven quantum gas.

We propose the implementation of a quantum heat pump with ultracold atoms. It is based on two periodically driven coherently coupled quantum dots using ultracold atoms. Each dot possesses two relevant quantum states and is coupled to a fermionic reservoir. The working principle is based on energy-selective driving-induced resonant tunneling processes, where a particle that tunnels from one dot to the other either absorbs or emits the energy quantum ℏω associated with the driving frequency, depending on its energy. We characterize the device using Floquet theory and compare simple analytical estimates to numerical simulations based on the Floquet-Born-Markov formalism. In particular, we show that driving-induced heating is directly linked to the micromotion of the Floquet states of the system.

Controlling Hydrogen-Transfer Rate in Molecules on Graphene by Tunable Molecular Orbital Levels.

Molecular switches are building blocks of potential interest to store binary information, especially when they can be organized in periodic lattices. Among the variety of possible systems, switches based on hydrogen transfer are of special importance because they allow the switching operation to occur without severe conformational change that may interfere with neighboring molecular units. We have studied the excitation process of hydrogen transfer inside porphyrin molecules assembled on a graphene surface, using a low-temperature scanning tunneling microscope. We show that this hydrogen transfer is induced by an electronic resonant tunneling process through the molecular orbitals. Using nitrogen doping of graphene, we tune the rate of hydrogen transfer by shifting the molecular orbital energies owing to the charge transfer at nitrogen dopant sites in the graphene lattice. The control of the switching process allows the storage of information inside a molecular lattice, which is demonstrated by writing an artificial pattern inside a molecular island.

Iterative path-integral summations for the tunneling magnetoresistance in interacting quantum-dot spin valves

We report on the importance of resonant-tunneling processes on quantum transport through interacting quantum-dot spin valves. To include Coulomb interaction in the calculation of the tunneling magnetoresistance (TMR), we reformulate and generalize the recently-developed, numerically-exact method of iterative summation of path integrals (ISPI) to account for spin-dependent tunneling. The ISPI scheme allows us to investigate weak to intermediate Coulomb interaction in a wide range of gate and bias voltage and down to temperatures at which a perturbative treatment of tunneling severely fails.

Bond-Selected Photodissociation of Single Molecules Adsorbed on Metal Surfaces.

We report the photoassisted activation of selected C─H bonds in individual molecules adsorbed on metal surfaces within the junction of a scanning tunneling microscope. Photons can couple to the C─H bond activation of specific hydrocarbons through a resonant photoassisted tunneling process. The molecule to be activated can be selected by positioning the tip with subangstrom resolution. Furthermore, structural tomography of the molecule and its dissociation products are imaged at different heights by the inelastic tunneling probe. The demonstration of single bond dissociation induced by resonant photoassisted tunneling electrons implies the attainment of atomic scale spatial resolution for bond-selected photochemistry.

Low injection losses in InGaN/GaN LEDs: The correlation of photoluminescence, electroluminescence, and photocurrent measurements

Time-resolved photoluminescence spectroscopy and photocurrent measurements at quasi-resonant laser excitation are combined with electroluminescence studies to get access to low injection losses in high power InGaN/GaN LEDs. A direct relation between electroluminescence and photoluminescence efficiencies with photocurrent is found, indicating that tunneling losses play a key role in the low injection regime. This assertion is confirmed by comparing photoluminescence efficiencies under open and closed circuit conditions. Experiments under various excitation wavelengths hint at the role of resonant tunneling processes in the efficiency losses.

Resonant electron tunnelling assisted by charged domain walls in multiferroic tunnel junctions.

The peculiar features of domain walls observed in ferroelectrics make them promising active elements for next-generation non-volatile memories, logic gates and energy-harvesting devices. Although extensive research activity has been devoted recently to making full use of this technological potential, concrete realizations of working nanodevices exploiting these functional properties are yet to be demonstrated. Here, we fabricate a multiferroic tunnel junction based on ferromagnetic La0.7Sr0.3MnO3 electrodes separated by an ultrathin ferroelectric BaTiO3 tunnel barrier, where a head-to-head domain wall is constrained. An electron gas stabilized by oxygen vacancies is confined within the domain wall, displaying discrete quantum-well energy levels. These states assist resonant electron tunnelling processes across the barrier, leading to strong quantum oscillations of the electrical conductance.

Negative Differential Resistance Device from Organic/Inorganic Hybrid Nanostructures.

Negative differential resistance device (NDR) fabricated by spin coating of organic/inorganic hybrid nanostructures at room temperature, is reported in the present paper. The coated organic layer is MEH-PPV (poly-[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] and inorganic layer is ZnO nanoparticles. The device shows negative differential resistance at low voltage and I–V characteristics of the device show multiple peaks at low voltage values. A value of 13 and 4 for the peak- to-valley ratio of current are reported in bi-layer and single layer structures respectively. Depending on the observed NDR signature, operating mechanisms are explored based on carrier (resonant) tunneling process and donor like trap mechanisms. This results show that the MEH-PPV/ZnO thin films gives good performance and is relevant for applications in optoelectronic devices such as a negative differential resistance.

Quantum well resonant tunneling FET based on topological insulator

In this work, we propose a scheme of the resonant tunneling field effect transistor based on a double ferromagnetic barrier on the surface of a topological insulator. In the proposed structure a quantum well will be created in separation between the barriers and therefore the confined states appear in the quantum well region. By tuning of the confined states with a gate voltage exerted on the well region, the carriers will be transferred through this structure with the resonant tunneling process. Our theoretical analysis illustrates that the transfer characteristics of the proposed resonant tunneling device display an oscillatory quantum switching on-off behavior with Ion/Ioff which exceeds from 104. It is also shown that the device output characteristic displays a step-like behavior. We also investigate the influence of the well and barrier thicknesses on the transport properties of the device.

Rate equation modelling and investigation of quantum cascade detector characteristics

A simple precise transport model has been proposed using rate equation approach for the characterization of a quantum cascade detector. The resonant tunneling transport is incorporated in the rate equation model through a resonant tunneling current density term. All the major scattering processes are included in the rate equation model. The effect of temperature on the quantum cascade detector characteristics has been examined considering the temperature dependent band parameters and the carrier scattering processes. Incorporation of the resonant tunneling process in the rate equation model improves the detector performance appreciably and reproduces the detector characteristics within experimental accuracy.

Contrasting influence of charged impurities on transport and gain in terahertz quantum cascade lasers

Transport and gain properties of a resonant-phonon terahertz quantum cascade laser are calculated using nonequilibrium Green's functions. Impurity scattering is shown to be responsible for contrasting nonlinear effects in the transport and the gain properties. For typical doping concentrations, the current density is found to be weakly sensitive to the impurity scattering strength. In contrast, the calculated gain is found to be very sensitive to the impurity scattering strength. This difference is attributed to the strong momentum dependence of the long-range coupling to charged impurities. Small-momentum impurity scattering is shown to be responsible for an incoherent regime of resonant tunneling processes. These new insights into the crucial role of impurity scattering open a new route of improvement of terahertz quantum cascade lasers by engineering of the doping profile.

Room-Temperature Organic Negative Differential Resistance Device Using CdSe Quantum Dots as the ITO Modification Layer**Supported by the National Natural Science Foundation of China under Grant Nos 61106123 and 61275034, and the National Basic Research Program of China under Grant No 2013CB328705.

Room-temperature negative differential resistance (NDR) has been observed in different types of organic materials. However, detailed study on the influence of the organic material on NDR performance is still scarce. In this work, room-temperature NDR is observed when CdSe quantum dot (QD) modified ITO is used as the electrode. Furthermore, material dependence of the NDR performance is observed by selecting materials with different charge transporting properties as the active layer, respectively. A peak-to-valley current ratio up to 9 is observed. It is demonstrated that the injection barrier between ITO and the organic active layer plays a decisive role for the device NDR performance. The influence of the aggregation state of CdSe QDs on the NDR performance is also studied, which indicates that the NDR is caused by the resonant tunneling process in the ITO/CdSe QD/organic active layer structure.

Universal Properties of Many-Body Delocalization Transitions

We study the dynamical melting of "hot" one-dimensional many-body localized systems. As disorder is weakened below a critical value these non-thermal quantum glasses melt via a continuous dynamical phase transition into classical thermal liquids. By accounting for collective resonant tunneling processes, we derive and numerically solve an effective model for such quantum-to-classical transitions and compute their universal critical properties. Notably, the classical thermal liquid exhibits a broad regime of anomalously slow sub-diffusive equilibration dynamics and energy transport. The subdiffusive regime is characterized by a continuously evolving dynamical critical exponent that diverges with a universal power at the transition. Our approach elucidates the universal long-distance, low-energy scaling structure of many-body delocalization transitions in one dimension, in a way that is transparently connected to the underlying microscopic physics.

Eigenstate suppressed activation

It is well known that varying quasi-eigenstate can increase the probability for barrier penetration substantially. This process was named Eigenstate Assisted Activation [1]. In this paper the dynamics of a temporally varying resonant tunneling process was thoroughly investigated. It was found that there are three time scale regimes. In the very long adiabatic time scales the process is not qualitatively different than any Quantum Tunneling one, and the average energy of the tunneling particle remains unchanged. However, below a certain time scale activation occurs as was anticipated by Azbel [1]. In this process that particle, temporally trapped in the quasi eigenstate, is elevated energetically due to the changes in the perturbation. Nevertheless, in this paper a third regime is explored for the first time. Below even a shorter time scale, a destructive interference can occur inside the well for specific discrete incoming particles energies (or specific time scales), for which activation is totally suppressed. Each one of these energetic, i.e. time scales, regimes was investigated both numerically and analytically with excellent agreement between them.