Quantum Dot Tunnel Research Articles

Spin-resolved counting statistics as a sensitive probe of spin correlation in transport through a quantum dot spin valve

We investigate the noise in spin transport through a single quantum dot (QD) tunnel coupled to ferromagnetic (FM) electrodes with noncollinear magnetizations. Based on a spin-resolved quantum master equation, auto- and cross-correlations of spin-resolved currents are analyzed to reveal the underlying spin transport dynamics and characteristics for various polarizations. We find the currents of majority and minority spins could be strongly autocorrelated despite uncorrelated charge transfer. The interplay between tunnel coupling and the Coulomb interaction gives rise to an exchange magnetic field, leading to the precession of the accumulated spin in the QD. It strongly suppresses the bunching of spin tunneling events and results in a unique double-peak structure in the noise of the net spin current. The spin autocorrelation is found to be susceptible to magnetization alignments, which may serve as a sensitive tool to measure the magnetization directions between the FM electrodes.

Transition from optical bistability to multistability in a tunneling quantum dot via structure hybrid light

In this paper, we proposed a new model based on the electron tunneling effect in a four-level quantum dot molecule (QDM) for studying the optical bistability (OB) and optical multistability (OM). The QDM interacts with a probe and two coupling and Laguerre–Gaussian (LG) fields. We found that by adjusting the electron tunneling effect and the parametric controlling of LG light, the transition from OB to OM or vice versa is possible. Moreover, due to the simultaneous interaction between coupling and LG lights with the same optical transition adjusting the threshold of OB and OM by orbital angular momentum (OAM) of the LG light becomes achievable. Our results show that by adjusting the simultaneous effect of electron tunneling and OAM state of the vortex light, the favorable OB and OM patterns with adjustable intensity thresholds are achievable. Our proposed model may have potential application in quantum information science based on quantum dot (QD) devices.

Efficient Room-Temperature Operation of a Quantum Dot Spin-Polarized Light-Emitting Diode under High-Bias Conditions

Spin-polarized light-emitting diodes (spin LEDs) are essential for the optical transfer of carrier-spin information in spin-based electronic circuits. Efficient operation under high-bias conditions is required for high-speed transfer of spin information and strong light emission. However, achieving high-electroluminescence (EL) circular polarization is hampered by electric-field-induced spin relaxation during drift transport in undoped $\mathrm{Ga}\mathrm{As}$ barriers. Herein, we demonstrate the efficient room-temperature operation of a spin LED using ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ quantum dots (QDs) tunnel coupled with a 5-nm-thick $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ quantum well (QW). A high-EL circular polarization of approximately 7% is achieved, even under high-bias conditions, in comparison with the lower value of approximately 3% for the conventional QD spin LED without $\mathrm{Ga}(\mathrm{N},\mathrm{As})$. This is realized by an increase of spin polarization after injection into the QDs by utilizing remote spin filtering of QD electrons via an adjacent tunnel-coupled $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ QW. These results demonstrate that the tunnel-coupled structure of ($\mathrm{In},\mathrm{Ga})\mathrm{As}$ QDs and $\mathrm{Ga}(\mathrm{N},\mathrm{As})$ QW is a promising active layer of spin LEDs for achieving both strong EL emission and high-EL circular polarization at room temperature.

Kondo resonance effects in emergent flat band materials

Macroscopic degrees of freedom that are involved in the transport of carriers through mesoscopic electronic devices are susceptible to the effects of strong many-body correlations. The presence of magnetic impurities in dilute magnetic alloys typically allow for insights into Kondo effect from the scattering of free carriers by localized electron states of the magnetic impurities but this effect is not well understood when there are no d-band electron states. Herein, the signatures of Kondo resonance effect are elucidated in quantum dots derived from a carbon-nanoline embedded monolayer hexagonal boron nitride whose electron states host flat band ferromagnetism as distinct broken symmetry states. Quantum transport state of mesoscopic devices modelled as quantum dots tunnel coupled to metallic leads is computed by direct diagonalization of the Hamiltonian. The possibility of realizing quantum dots with highly tunable electron states in energy interconversion devices is discussed to show the importance of screening effects on single-electron energy levels. The quantum master equation is solved within different formalisms to determine the stationary-state particle and energy currents. Stability diagrams are calculated to show the dependence of the conductance on experimental control variables of the quantum dot device. The computed responses of the stationary-state transport signatures are used to characterize Kondo resonance effects from flat band states of embedded carbon nanoline-based quantum dots. It is found that the local network structure of the hexagonal ring carbon cluster-based quantum dot has a broken particle-hole symmetry in the transport state. This signals the formation of the quasiparticle states expected in second order scattering when the macroscopic “charge” pseudospin symmetry of the tunnelling electron state is broken dynamically due to charging. The results are discussed to show the implications of a vanishing particle-hole symmetry in the carrier transport state of quantum dots for energy conversion applications.

Isolation and manipulation of a single-donor detector in a silicon quantum dot

We demonstrate the isolation and electrostatic control of a single phosphorus donor in a silicon quantum dot by making use of source-drain bias during cooldown and biases applied to capacitively coupled gates. Characterization of the device at low temperatures and in magnetic fields shows single donors can be electrostatically isolated near one of the quantum dot's tunnel barriers with either single or double occupancy. This model is well supported by capacitance-based simulations. The ability to use the ${\mathrm{D}}^{0}$ state of such isolated donors as a charge detector is demonstrated by observing the charge stability diagram of a nearby and capacitively coupled semiconnected double quantum dot.

Efficient Room‐Temperature Voltage Control of Picosecond Optical Spin Orientation Using a III‐V Semiconductor Nanostructure

AbstractManipulation of the optical spin orientation in semiconductors is a key technology for realizing spin‐based photoelectric information processing. Application of magnetic field is a simple method to control the spin polarization degree through Zeeman splitting. However, this effect can only be achieved at cryogenic temperatures and in a strong magnetic field. Here, room‐temperature voltage control of optical polarization in the range of 3–15%, corresponding to a 12–60% change in relative spin polarization is demonstrated. For this, a III‐V semiconductor quantum dot tunnel coupled with a quantum well spin reservoir is used. The spin‐flip scattering rate within quantum dots is electric‐field‐controlled in the time domain of several tens of picoseconds. This is achieved by precise control of the tunnel injection efficiency of electrons and holes via coupled potential modification. The findings will pave the way for the generation of ultrafast spin‐modulated optical signals by the electric field effect.

Reduced Electron Temperature in Silicon Multi-Quantum-Dot Single-Electron Tunneling Devices.

The high-performance room-temperature-operating Si single-electron transistors (SETs) were devised in the form of the multiple quantum-dot (MQD) multiple tunnel junction (MTJ) system. The key device architecture of the Si MQD MTJ system was self-formed along the volumetrically undulated [110] Si nanowire that was fabricated by isotropic wet etching and subsequent oxidation of the e-beam-lithographically patterned [110] Si nanowire. The strong subband modulation in the volumetrically undulated [110] Si nanowire could create both the large quantum level spacings and the high tunnel barriers in the Si MQD MTJ system. Such a device scheme can not only decrease the cotunneling effect, but also reduce the effective electron temperature. These eventually led to the energetic stability for both the Coulomb blockade and the negative differential conductance characteristics at room temperature. The results suggest that the present device scheme (i.e., [110] Si MQD MTJ) holds great promise for the room-temperature demonstration of the high-performance Si SETs.

Electron transport in single colloidal quantum dots in an interelectrode nanogap

Based on the current-voltage characteristics of single colloidal QDs -InSb, -PbS, -HgSe, -CdSe quantum dots, on random samples statistically determined and investigated the mechanisms of barrier and ballistic tunneling, Coulomb confinement, quasi-periodic modulation of electron transport in the model of a deep extended potential well and depending on QDs size and de Broglie wavelength for the electron. The possibility of manifestation of Bloch oscillations has been experimentally confirmed. The parameters of all investigated processes were determined and tabulated. Keywords: Nanoparticle, quantum dot, dimensional quantization, quantum selection, barrier and ballistic tunneling, Coulomb confinement, Coulomb ladder, Bloch oscillations.

Investigation on characteristics of Monte Carlo model of single electron transistor using Orthodox theory

An improved Monte Carlo Model of the Single Electron Transistor using the Orthodox Theory and Master Equations is proposed in this paper. The modifications are done in the parasitic parameters to investigate the current–voltage characteristics of a Single Electron Transistor. This work is carried out for determining the parasitic parameters and their impacts on the characteristics of the Single Electron Transistor. The simulation results show that the Coulomb stair-case is highly visible at the gate capacitance of Cg=0.2x10-18F with the highest conductance peak of7.3x10-8Siemens(S). The consecutive conductance peaks are separated by a regular interval, i.e., voltage bias. This determines the Coulomb Blockade in Quantum Dot Island and tunneling of the electron in a controlled manner. The bias voltage is 0.4V and charging, or electrostatic energy is 0.190725eV.

Pure spin-current diode based on interacting quantum dot tunneling junction* *Project supported by the National Natural Science Foundation of China (Grant No. 11404322) and the Natural Science Foundation of Huai’an (Grant No. HAB202150).

A magnetic field-controlled spin-current diode is theoretically proposed, which consists of a junction with an interacting quantum dot sandwiched between a pair of nonmagnetic electrodes. By applying a spin bias V S across the junction, a pure spin current can be obtained in a certain gate voltage regime,regardless of whether the Coulomb repulsion energy exists. More interestingly, if we applied an external magnetic field on the quantum dot, we observed a clear asymmetry in the spectrum of spin current I S as a function of spin bias, while the charge current always decays to zero in the Coulomb blockade regime. Such asymmetry in the current profile suggests a spin diode-like behavior with respect to the spin bias, while the net charge through the device is almost zero. Different from the traditional charge current diode, this design can change the polarity direction and rectifying ability by adjusting the external magnetic field, which is very convenient. This device scheme can be compatible with current technologies and has potential applications in spintronics or quantum processing.

Lasing in a coupled hybrid double quantum dot-resonator system

We theoretically investigate the possibility of lasing in an electromagnetic resonator coupled to a voltage-biased hybrid double quantum dot comprised of a double quantum dot tunnel coupled simultaneously to a normal metal and a superconducting lead. Using a unitary transformation, we derive a resonator-double quantum dot interaction Hamiltonian in the rotating-wave approximation which reveals the fact that lasing in this system is mediated by electron transitions between Andreev energy levels in the system's density of states. Moreover, by employing a Markovian master equation incorporating dissipation effects for the electronic and photonic degrees of freedom, we numerically calculate the steady-state reduced density matrix of the system from which we determine the average photon number and its statistics in the resonator in various parameter regimes. We find that at some appropriate parameter configurations, lasing can be considerably enhanced due to the possibility of electron transitions between multiple Andreev levels in the system.

Many-body tunneling and nonequilibrium dynamics in double quantum dots with capacitive coupling

Double quantum dots (DQDs) systems may be the minimal setups for realization of QD-based qubits and quantum computation. Pauli spin blockade (PSB) and a kind of novel many-body tunneling (MBT) are identified to play important roles in these systems, and dominate the quantum tunneling at moderate and weak interdot coupling t, respectively. On the other hand, inter-dot Coulomb interaction U′ and related inter-dot Coulomb blockade (IDCB) is inevitable in DQDs. However, what would happen on the effect of U′ in DQDs has not been touched, in particular for PSB and MBT. Here, we study the tunneling processes and transport properties with various U′ in series-coupled DQDs, and find MBT process is rather robust against U′ within U′/U < 0.1, where U is the intra-dot Coulomb interaction. Meanwhile, the linearity relationship between the carrier doublon number and MBT current remains valid. These findings enrich the understanding of the many-body tunneling in the DQDs and may shed light on the manipulation of the QD-based qubits.

Berry phase in superconducting multiterminal quantum dots

We report on the study of the non-trivial Berry phase in superconducting multiterminal quantum dots biased at commensurate voltages. Starting with the time-periodic Bogoliubov-de Gennes equations, we obtain a tight binding model in the Floquet space, and we solve these equations in the semiclassical limit. We observe that the parameter space defined by the contact transparencies and quartet phase splits into two components with a non-trivial Berry phase. We use the Bohr-Sommerfeld quantization to calculate the Berry phase. We find that if the quantum dot level sits at zero energy, then the Berry phase takes the values $\varphi_B=0$ or $\varphi_B=\pi$. We demonstrate that this non-trivial Berry phase can be observed by tunneling spectroscopy in the Floquet spectra. Consequently, the Floquet-Wannier-Stark ladder spectra of superconducting multiterminal quantum dots are shifted by half-a-period if $\varphi_B=\pi$. Our numerical calculations based on Keldysh Green's functions show that this Berry phase spectral shift can be observed from the quantum dot tunneling density of states.

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.

Enhancing Third- and Fifth-Order Nonlinearity via Tunneling in Multiple Quantum Dots.

The nonlinearity of semiconductor quantum dots under the condition of low light levels has many important applications. In this study, linear absorption, self-Kerr nonlinearity, fifth-order nonlinearity and cross-Kerr nonlinearity of multiple quantum dots, which are coupled by multiple tunneling, are investigated by using the probability amplitude method. It is found that the linear and nonlinear properties of multiple quantum dots can be modified by the tunneling intensity and energy splitting of the system. Most importantly, it is possible to realize enhanced self-Kerr nonlinearity, fifth-order nonlinearity and cross-Kerr nonlinearity with low linear absorption by choosing suitable parameters for the multiple quantum dots. These results have many potential applications in nonlinear optics and quantum information devices using semiconductor quantum dots.

Cooling by heating in nonequilibrium nanosystems

We demonstrate the possiblity to cool nanoelectronic systems in nonequilibrium situations by increasing the temperature of the environment. Such cooling by heating is possible for a variety of experimental conditions where the relevant transport-induced excitation processes become quenched and deexcitation processes are enhanced upon an increase of temperature. The phenomenon turns out to be robust with respect to all relevant parameters. It is especially pronounced for higher bias voltages and weak to moderate coupling. Our findings have implications for open quantum systems in general, where electron transport is coupled to mechanical (phononic) or photonic degrees of freedom. In particular, molecular junctions with rigid tunneling pathways or quantum dot circuit QED systems meet the required conditions.

Yu\u2013Shiba\u2013Rusinov screening of spins in double quantum dots

A magnetic impurity coupled to a superconductor gives rise to a Yu–Shiba–Rusinov (YSR) state inside the superconducting energy gap. With increasing exchange coupling the excitation energy of this state eventually crosses zero and the system switches to a YSR ground state with bound quasiparticles screening the impurity spin by ħ/2. Here we explore indium arsenide (InAs) nanowire double quantum dots tunnel coupled to a superconductor and demonstrate YSR screening of spin-1/2 and spin-1 states. Gating the double dot through nine different charge states, we show that the honeycomb pattern of zero-bias conductance peaks, archetypal of double dots coupled to normal leads, is replaced by lines of zero-energy YSR states. These enclose regions of YSR-screened dot spins displaying distinctive spectral features, and their characteristic shape and topology change markedly with tunnel coupling strengths. We find excellent agreement with a simple zero-bandwidth approximation, and with numerical renormalization group calculations for the two-orbital Anderson model.

QmeQ 1.0: An open-source Python package for calculations of transport through quantum dot devices

QmeQ is an open-source Python package for numerical modeling of transport through quantum dot devices with strong electron–electron interactions using various approximate master equation approaches. The package provides a framework for calculating stationary particle or energy currents driven by differences in chemical potentials or temperatures between the leads which are tunnel coupled to the quantum dots. The electronic structures of the quantum dots are described by their single-particle states and the Coulomb matrix elements between the states. When transport is treated perturbatively to lowest order in the tunneling couplings, the possible approaches are Pauli (classical), first-order Redfield, and first-order von Neumann master equations, and a particular form of the Lindblad equation. When all processes involving two-particle excitations in the leads are of interest, the second-order von Neumann approach can be applied. All these approaches are implemented in QmeQ. We here give an overview of the basic structure of the package, give examples of transport calculations, and outline the range of applicability of the different approximate approaches. Program summaryProgram Title:QmeQProgram Files doi:http://dx.doi.org/10.17632/8687mrhgg9.1Licensing provisions: BSD 2-ClauseProgramming language:PythonExternal libraries:NumPy, SciPy, CythonNature of problem: Calculation of stationary state currents through quantum dots tunnel coupled to leads.Solution method: Exact diagonalization of the quantum dot Hamiltonian for a given set of single particle states and Coulomb matrix elements. Numerical solution of the stationary-state master equation for a given approximate approach.Restrictions: Depending on the approximate approach the temperature needs to be sufficiently large compared to the coupling strength for the approach to be valid.

Bipolar spin blockade, many-body tunneling and long-range resonance in triple quantum dots

We investigate the transport properties in triple quantum dots (TQDs) by accurately solving the Anderson three-impurity model under nonequilibrium condition with the hierarchical equations of motion (HEOM) approach. Firstly, we reproduce the experimental features of bipolar Pauli spin blockade (PSB) in doubly-singly-doubly occupied TQDs, where the current is strongly suppressed in both bias directions. Then, we predict similar features in singly-doubly-singly occupied TQDs and find that the many-body tunneling dominates the transport in the strongly correlated regime in both structures. Finally, we demonstrate the long-range resonance between two edge dots characterized by the current enhancement, which is in agreement with experiments. It indicates that the long-range quantum coherence can play an important role near the PSB region, which provides a possible way to achieve distance quantum simulation and computation.

Spin-valley resolved photon-assisted tunneling in carbon nanotube double quantum dots

We consider the photon-assisted tunneling (PAT) and the Landau-Zener-Stueckelberg (LZS) interference for double quantum dots induced electrostatically along a semiconducting carbon nanotube. An atomistic tight-binding approach and the time-dependent configuration interaction method are employed for description of the systems of a few confined electrons and holes. We reproduce the patterns of the LZS interference recently observed for the quantum double dots describing transport across hole-localized states. Moreover, we indicate that for charge configurations for which the ground-state is Pauli blocked PAT can be used for resolution of the transitions that involve spin-flip or intervalley transitions without the spin-valley conserving background signal.