Resonant Quantum Tunneling Research Articles

Quantum transport in double tilted Dirac cone junction

We investigate quantum transport in tilt-inhomogeneous Dirac cone system, which partially mimic the behaviours of a double-pseudomagnetic-barrier structure. Exploring the effects of Dirac cone tilt helps us understand electron transmission probabilities through the barriers. Our results demonstrate significant enhancement of tunnelling transport properties by adjusting barrier parameters and the Dirac cone’s tilt angle. This study has important implications for optimizing resonant-tunnelling quantum devices based on the heterostructure, paving the way for novel quantum device development.

Phase slip from weak links formed in artificially-stacked NbSe2

The rich nature of van der Waals interactions between artificially-stacked atomic layers has been demonstrated by various quantum states and resonant tunneling transport in low-dimensional materials. However, the role of topological fluctuations in quantum transport through artificially-stacked junctions of 2D superconducting materials, and the resulting energy dissipation, remain elusive. In this research, unique phase-slip centers are designed in artificially-stacked junction areas, where nonequilibrium quasiparticles are formed and relaxed with energy dissipation. The phase slips are observed as voltage steps (peaks or valleys) in transport measurements across a junction between two exfoliated NbSe2 flakes, and at a distance of 4 μm from the junction using local and nonlocal chemical potential probes. Accordingly, two types of energy dissipation modes are newly identified in the artificially-stacked NbSe2 when subjected to an in-plane magnetic field, which implies distinct vortex formation and current flow in the superconducting junction under magnetic fields.

Energy Dissipation from Confined States in Nanoporous Molecular Networks.

Crystalline nanoporous molecular networks are assembled on the Ag(111) surface, where the pores confine electrons originating from the surface state of the metal. Depending on the pore sizes and their coupling, an antibonding level is shifted upward by 0.1-0.3 eV as measured by scanning tunneling microscopy. On molecular sites, a downshifted bonding state is observed, which is occupied under equilibrium conditions. Low-temperature force spectroscopy reveals energy dissipation peaks and jumps of frequency shifts at bias voltages, which are related to the confined states. The dissipation maps show delocalization on the supramolecular assembly and a weak distance dependence of the dissipation peaks. These observations indicate that two-dimensional arrays of coupled quantum dots are formed, which are quantitatively characterized by their quantum capacitances and resonant tunneling rates. Our work provides a method for studying the capacitive and dissipative response of quantum materials with nanomechanical oscillators.

Kinetics of Photostimulated Transport in Resonant Tunneling Quantum Well Structures in the Electric Field of a p–i–n Photovoltaic Cell

Kinetics of Photostimulated Transport in Resonant Tunneling Quantum Well Structures in the Electric Field of a p–i–n Photovoltaic Cell

Resonant tunneling in natural photosynthetic systems

The high internal quantum efficiency observed in higher plants remains an outstanding problem in understanding photosynthesis. Several approaches such as quantum entanglement and quantum coherence have been explored. However, none has yet drawn an analogy between superlattices and the geometrical structure of granal thylakoids in leaves. In this paper, we calculate the transmission coefficients and perform numerical simulations using the parameters relevant to a stack of thylakoid discs. We then show that quantum resonant tunneling can occur at low effective mass of particles for 680 nm and 700 nm incident wavelengths corresponding to energies at which photosynthesis occurs.

Thermal control across a chain of electronic nanocavities

We study a chain of alternating hot and cold electronic nanocavities -- connected to one another via resonant-tunneling quantum dots -- with the intent of achieving precise thermal control across the chain. This is accomplished by positioning the dots' energy levels such that a predetermined distribution of heat currents is realized across the chain in the steady state. The number of electrons in each cavity is conserved in the steady state which constrains the cavities' chemical potentials. We determine these chemical potentials analytically in the linear response regime where the energy differences between the dots' resonant levels and the neighboring chemical potentials are much smaller than the thermal energy. In this regime, the thermal control problem can be solved exactly, while, in the general case, thermal control can only be achieved in a relative sense, that is, when one only preassigns the ratios between different heat currents. We apply our results to two different cases: We first demonstrate that a "heat switch" can be easily realized with three coupled cavities, then we show that our linear response results can provide accurate results in situations with a large number of cavities.

Thermal Spin Torque in Double-Barrier Tunnel Junctions with Magnetic Insulators

The thermal spin torque induced by the spin-dependent Seebeck effect in double-barrier tunnel junctions is derived considering free-electron and tight-binding calculations. We show that in systems comprising ferromagnetic electrodes and nonmagnetic barriers, the in-plane component of the thermal spin torque is the dominant term, whereas in junctions comprising nonmagnetic electrodes and ferromagnetic barriers, both components, the in-plane and the out-of-plane, are comparable in magnitude. Moreover, larger torque amplitudes up to 3 orders of magnitude are obtained in the second system as a result of the spin-filtering effect; consequently, double-barrier tunnel junctions in the presence of magnetic insulators offer an enhanced thermal spin-torque mechanism for reliable applications. We propose taking advantage of quantum resonant tunneling through resonance states below the Fermi level in these structures that can pave a route toward achieving larger spin-torque efficiencies, even when considering smaller values of the exchange splitting. Furthermore, we identify the parameters needed to tune efficiently these resonant states.

ULTRARAM: Toward the Development of a III–V Semiconductor, Nonvolatile, Random Access Memory

ULTRARAM is a III-V compound semiconductor memory concept that exploits quantum resonant tunneling to achieve nonvolatility at extremely low switching energy per unit area. Prototype devices are fabricated in a 2×2 memory array formation on GaAs substrates. The devices show 0/1 state contrast from program/erase (P/E) cycles with 2.5 V pulses of 500- μs duration, a remarkable switching speed for a 20 μm gate length. Memory retention is tested for 8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s, whereby the 0/1 states show adequate contrast throughout, whilst performing 8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> readout operations. Further reliability is demonstrated via program-read-erase-read endurance cycling for $10^{6}$ cycles with 0/1 contrast. A half-voltage array architecture proposed in our previous work is experimentally realized, with an outstandingly small disturb rate over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> half-voltage cycles.

Effect of quantum barrier width and quantum resonant tunneling through InGaN/GaN parabolic quantum well-LED structure on LED efficiency

The effect of quantum barrier width and electric field on the resonant tunneling through the double barrier In0.2Ga0.8N/GaN parabolic-quantum well light-emitting diode (LED) structure and LED efficiency was investigated analytically. LEDs are based on resonant tunneling diode (RTD) quantum well (QW) structures, which exhibit the characteristic property of negative differential resistance (NDR) that is crucial for LED operation. The predictions of the current density–voltage (J–V) characteristics of the RTDs made analytically in this study are in good agreement with previous experimental data. Moreover, the parabolic quantum well-LED with thicker quantum barriers (from 1nm to 12 nm) exhibits a smaller efficiency droop (from 5.2 to 11%), and the NDR increases significantly from 0.023 to 1 Ω leading to low power dissipation.

Resonant Tunneling and Negative Differential Resistance in Black Phosphorus Vertical Heterostructures

AbstractResonant tunneling diodes with negative differential resistance (NDR) have attracted significant attention due to their unique quantum resonant tunneling phenomena and potential applications in terahertz emission/detection and high‐density logic/memory. In this paper, resonant tunneling devices, where the carriers tunnel through a hexagonal boron nitride (hBN) barrier sandwiched by two black phosphorus (BP) layers, are explored. The resonance occurs when the energy bands of the two black phosphorus layers are aligned. The conductive atomic force microscopy (CAFM) measurements reveal prominent NDR peaks with large peak‐to‐valley ratios at room temperature. It is found that the positions of the NDR peaks are very sensitive to the amplitude and the shape of the voltage waveform used in CAFM, which can be explained by the charge trapping effect. Furthermore, resonant tunneling transistors are demonstrated based on BP/hBN/BP stacks in which the locations of the NDR peaks are tunable by the electrostatic gating. As compared to the traditional tunneling diodes based on bulk materials, the tunneling devices based on thin boron nitride tunneling barrier and high mobility black phosphorus offer ultra‐high‐speed response. This feature, together with the NDR characteristics, provides the potential for applications in THz oscillators and multi‐value logic devices.

Complex Density of Continuum States in Resonant Quantum Tunneling

We introduce a complex-extended continuum level density and apply it to one-dimensional scattering problems involving tunneling through finite-range potentials. We show that the real part of the density is proportional to a real "time shift" of the transmitted particle, while the imaginary part reflects the imaginary time of an instantonlike tunneling trajectory. We confirm these assumptions for several potentials using the complex scaling method. In particular, we show that stationary points of the potentials give rise to specific singularities of both real and imaginary densities which represent close analogues of excited-state quantum phase transitions in bound systems.

NanoLEDs for energy-efficient and gigahertz-speed spike-based sub-λ neuromorphic nanophotonic computing

AbstractEvent-activated biological-inspired subwavelength (sub-λ) photonic neural networks are of key importance for future energy-efficient and high-bandwidth artificial intelligence systems. However, a miniaturized light-emitting nanosource for spike-based operation of interest for neuromorphic optical computing is still lacking. In this work, we propose and theoretically analyze a novel nanoscale nanophotonic neuron circuit. It is formed by a quantum resonant tunneling (QRT) nanostructure monolithic integrated into a sub-λ metal-cavity nanolight-emitting diode (nanoLED). The resulting optical nanosource displays a negative differential conductance which controls the all-or-nothing optical spiking response of the nanoLED. Here we demonstrate efficient activation of the spiking response via high-speed nonlinear electrical modulation of the nanoLED. A model that combines the dynamical equations of the circuit which considers the nonlinear voltage-controlled current characteristic, and rate equations that takes into account the Purcell enhancement of the spontaneous emission, is used to provide a theoretical framework to investigate the optical spiking dynamic properties of the neuromorphic nanoLED. We show inhibitory- and excitatory-like optical spikes at multi-gigahertz speeds can be achieved upon receiving exceptionally low (sub-10 mV) synaptic-like electrical activation signals, lower than biological voltages of 100 mV, and with remarkably low energy consumption, in the range of 10–100 fJ per emitted spike. Importantly, the energy per spike is roughly constant and almost independent of the incoming modulating frequency signal, which is markedly different from conventional current modulation schemes. This method of spike generation in neuromorphic nanoLED devices paves the way for sub-λ incoherent neural elements for fast and efficient asynchronous neural computation in photonic spiking neural networks.

Quantum dynamics of a single molecule magnet on superconducting Pb(111).

Magnetic materials interfaced with superconductors may reveal new physical phenomena with potential for quantum technologies. The use of molecules as magnetic components has already shown great promise, but the diversity of properties offered by the molecular realm remains largely unexplored. Here we investigate a submonolayer of tetrairon(III) propeller-shaped single molecule magnets deposited on a superconducting lead surface. This material combination reveals a strong influence of the superconductor on the spin dynamics of the single molecule magnet. It is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime. Our results open perspectives to control single molecule magnetism via superconductors and to use single molecule magnets as local probes of the superconducting state.

Three-terminal refrigerator based on resonant-tunneling quantum wells.

A three-terminal refrigerator based on resonant-tunneling quantum wells is proposed. With the help of the Landauer formula, the expressions for the cooling rate and the coefficient of performance (COP) are derived. The working regions of the refrigerator are determined and the three-dimensional projection graphs of the cooling rate and the COP varying with the positions of the two energy levels are plotted. Moreover, the influence of the bias voltage, the asymmetric factor, and the temperature difference on the optimal performance parameters is analyzed in detail. Finally, the performance characteristics of the refrigerator in the case of negative temperature difference are discussed.

Experimental Realization of a Quantum Dot Energy Harvester.

We demonstrate experimentally an autonomous nanoscale energy harvester that utilizes the physics of resonant tunneling quantum dots. Gate-defined quantum dots on GaAs/AlGaAs high-electron-mobility transistors are placed on either side of a hot-electron reservoir. The discrete energy levels of the quantum dots are tuned to be aligned with low energy electrons on one side and high energy electrons on the other side of the hot reservoir. The quantum dots thus act as energy filters and allow for the conversion of heat from the cavity into electrical power. Our energy harvester, measured at an estimated base temperature of 75mK in a He^{3}/He^{4} dilution refrigerator, can generate a thermal power of 0.13fW for a temperature difference across each dot of about 67mK.

Nanophotonic energy storage in upconversion nanoparticles

In nanophotonic energy storage, an energy conversion model is established for intrinsic nanophotonic energy storage (NPES) effects. Here we realize that the charge inhomogeneous distribution on the surface of upconversion nanoparticles (UCNPs) would persistently exist as well as the formation and migration of surface defects states despite of the compound component ratio, even following the stringent stoichiometric ratio. Our preliminary efforts on NPES effect has recognized from the recent published work by Chen et al. (2018) [4], in which the surface quantum confinement arose because of a recently found surface vacancy induced Coulomb states (SVIC) states. Further in-depth excavation on surface charge density distributions and defect orbitals of surface localized electrons and holes have affirmatively repeated the theory by Guerra et al. (2018) [32], and reflected the existence of surface defect states in both stoichiometric and non-stoichiometric compounds. Therefore, beyond the experimental trail-based multi-doping chemical modifications, we proposed that the surface electronic process for efficient NPES effect can be modulated by an intrinsic level-matching induced surface resonant quantum tunneling (LM-SRQT) in this work. The UCNP size-effect can be confirmed that simply might be not an influencing factor of dominating NPES effect while the surface degree of non-crystallizations indeed matters.

Quantum resonant tunneling in semiconductor double-barrier structure

Using Schrodinger equation, we have theoretically studied the quantum transport in double-barrier potential resonant tunneling diodes. To obtain higher peak in transmission coefficient in twofold barrier quantum well system require to reducing the barrier height and increasing the width of the quantum well which causes the electrons to turn unbounded. In addition, the potential energies should be below the height of the barrier. This occurs because on resonance with unity transmission requires exact cancellation in amplitude and phase of a coherent superposition of all contributions to the back-scattered particle-waves. At a certain height, width and potential energy, the resonances sit-down between barriers giving rise to peaks in transmission. Our model can be extended for several barriers to increasing the resonance peaks of transmission.

Smoothed finite element method for time dependent analysis of quantum resonance devices

In this paper, a new finite element method (FEM) is introduced to study the time-dependent wave nature of the electron in quantum resonance devices. Unlike the well-known FEM, the new method smooths the wave function derivatives over the edges. In this sense, the new method is termed “smoothed FEM” where an “inter-element” matrix is formed to smooth the derivatives over the edges. For the electron’s wave function propagation in time, the presented method exploits the time domain beam propagation method (TD-BPM). Based only on first order elements, our suggested SFETD-BPM has high accuracy levels comparable to second-order conventional FEM elements; thanks to the element smoothing. The proposed method numerical performance is tested through the analysis of a quantum resonance cavity and a quantum resonant tunneling device. It is clearly demonstrated that the presented method is not only accurate but also more time efficient than the conventional FEM approach.

Photoelectronic behaviors of self-assembled ZnSe/ZnS/L-Cys quantum dots synthesized at low temperature

The photogenerated carriers’ transport and microstructure of self-assembled core–shell ZnSe/ZnS/L-Cys quantum dots (QDs), which was synthesized at room temperature, are studied via the surface photovoltaic and transient photovoltaic techniques, X-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy and ultraviolet–visible absorption spectra. The results suggest that the ZnSe nanocrystals prepared at room temperature prefer to nucleate at (111) and (220) faces, and grow a shell–ZnS at (220) face rather than at (311) face. The quantum well depth in some interface space charge region (SCR) of the QDs prepared at room temperature is smaller than that prepared at 90 °C. The evolution of the band bending from a depletion layer to an accumulation layer may occur in the graded-band-gap and at the side of the interface SCR, as compared the QDs with p-type photovoltaic characteristic synthesized at room temperature to that at 90 °C. This electron structural shift may be ascribed to the reduced quantum well depth and then an obvious resonance quantum tunneling of the QDs synthesized at room temperature.

Dirac point induced ultralow-threshold laser and giant optoelectronic quantum oscillations in graphene-based heterojunctions

The occurrence of zero effective mass of electrons at the vicinity of the Dirac point is expected to create new paradigms for scientific research and technological applications, but the related discoveries are rather limited. Here, we demonstrate that a simple architecture composed of graphene quantum dots sandwiched by graphene layers can exhibit several intriguing features, including the Dirac point induced ultralow-threshold laser, giant peak-to-valley ratio (PVR) with ultra-narrow spectra of negative differential resistance and quantum oscillations of current as well as light emission intensity. In particular, the threshold of only 12.4 nA cm−2 is the lowest value ever reported on electrically driven lasers, and the PVR value of more than 100 also sets the highest record compared with all available reports on graphene-based devices. We show that all these intriguing phenomena can be interpreted based on the unique band structures of graphene quantum dots and graphene as well as resonant quantum tunneling.