The effect of external electric field on the exchange interaction has been studied by an exact diagonalization method for two electrons in laterally coupled quantum dots (QD's). We have performed a systematic study of several nanodevices that contain two gate-defined QD's with different shapes and sizes located between source and drain contacts. The confinement potential is modelled by two potential wells with a variable range and softness. In all the considered nanodevices, the overall dependence of exchange energy $J$ on electric field $F$ is similar, i.e., for low fields $J$ increases with increasing $F$, for intermediate fields $J$ reaches a maximum, and rapidly falls down to zero if $F$ exceeds a certain critical value. However, the $J(F)$ dependence shows characteristic properties that depend on the nanodevice geometry. We have found that the low- and intermediate-field behaviour can be accurately parametrized by linear function $J(F) = \αF + \β$, where $ alpha$ is independent of the nanodevice geometry and softness of the confinement potential. We have shown that the linear $J(F)$ dependence appears only if the tunnel coupling between the QD's is weak, i.e., the interdot separation is sufficiently large. The $J(F)$ dependence becomes non-linear for the strong interdot tunnel coupling. If the QD located near the contact, to which the higher voltage is applied, possesses the elliptic shape and is larger than the other QD, the $J(F)$ dependence shows a plateau in a broad electric-field regime. The linearity and rapid jumps of $J(F)$ as well as the existence of the plateau can be applied to tune the exchange interaction by changing the external electric field.
Free standing beams containing a two-dimensional electron system are shaped from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum dots are laterally defined using metal top gates. We investigate the electronic properties of these nanostructures by transport spectroscopy. Tunable localized electron states in freely suspended nanostructures are a promising tool to investigate the electron-phonon-interaction.
We calculate the emission spectrum of neutral and charged excitons in a pair of laterally coupled InGaAs quantum dots with nearly degenerate energy levels. As the interdot distance decreases, a number of changes take place in the emission spectrum which can be used as indications of molecular coupling. These signatures ensue from the stronger tunnel-coupling of trions as compared to that of neutral excitons.
Phonon-induced spin relaxation in coupled lateral quantum dots in the presence of spin-orbit coupling is calculated. The calculation for single dots is consistent with experiment. Spin relaxation in double dots at useful interdot couplings is dominated by spin hot spots that are strongly anisotropic. Spin hot spots are ineffective for a diagonal crystallographic orientation of the dots with a transverse in-plane field. This geometry is proposed for spin-based quantum information processing.
We theoretically investigate electron transport through an Aharonov-Bohm interferometer containing laterally coupled double quantum dots. We introduce the indirect coupling parameter $α$, which characterizes the strength of the coupling via the reservoirs between two quantum dots. $|α|=1$ indicates the strongest coupling, where only a single mode contributes to the transport in the system. Two conduction modes exist in a system where $|α|\neq 1$. The interference effects such as the Fano resonance and the Aharonov-Bohm oscillation are suppressed as the absolute value of the parameter $α$ decre…
We consider electronic transport through laterally parallel double open quantum dots embedded in a quantum wire in a perpendicular magnetic field. The coupling modes of the dots are tunable by adjusting the strength of a central barrier and the applied magnetic field. Probability density and electron current density are calculated to demonstrate transport effects including magnetic blocking, magnetic turbulence, and a hole-like quasibound state feature. Fano to dip line-shape crossover in the conductance is found by varying the magnetic field.
The effects of spin-orbit coupling on the two-electron spectra in lateral coupled quantum dots are investigated analytically and numerically. It is demonstrated that in the absence of magnetic field the exchange interaction is practically unaffected by spin-orbit coupling, for any interdot coupling, boosting prospects for spin-based quantum computing. The anisotropic exchange appears at finite magnetic fields. A numerically accurate effective spin Hamiltonian for modeling spin-orbit-induced two-electron spin dynamics in the presence of magnetic field is proposed.
We describe the electronic properties of a double dot for which the lateral coupling between the two vertical dots can be controlled in-situ with a center gate voltage (Vc) and the current flows through the two dots in series. When Vc is large and positive, the two dots merge. As Vc is made less positive, two dots are formed whose coupling is reduced. We measure charging diagrams for positive and negative source-drain voltages in the weak coupling regime and observe current rectification due to the Pauli spin blockade when the hyperfine interaction between the electrons and the nuclei is suppr…
We have fabricated single electron tunneling structures by forming nanogap metallic electrodes directly upon single self-assembled InAs quantum dots (QDs). The fabricated samples exhibited clear Coulomb blockade effects. Furthermore, a clear Kondo effect was observed when strong coupling between the electrodes and the QDs was realized using a large QD with a diameter of ~ 100 nm. From the temperature dependence of the linear conductance at the Kondo valley, the Kondo temperature TK was determined to be ~ 81 K. This is the highest TK ever reported for artificial quantum nanostructures.
We calculate the exchange energy in two dimensional laterally coupled quantum dots using Heitler-London, Hund-Mullikan and variational methods. We assess the quality of these approximations in zero and finite magnetic fields comparing against numerically exact results. We find that surprisingly, the Hund-Mullikan method does not offer any significant improvement over the much simpler Heitler-London method, whether at large or small interdot distances. Contrary to that, our variational ansatz proves substantially better. In a single dot at finite magnetic field, all approximate methods fail. Th…
We investigate the electronic and optical properties of InAs double quantum dots grown on GaAs (001) and laterally aligned along the [110] crystal direction. The emission spectrum has been investigated as a function of a lateral electric field applied along the quantum dot pair mutual axis. The number of confined electrons can be controlled with the external bias leading to sharp energy shifts which we use to identify the emission from neutral and charged exciton complexes. Quantum tunnelling of these electrons is proposed to explain the reversed ordering of the trion emission lines as compare…
We investigate electron charge decoherence in a laterally-coupled single-electron semiconductor double quantum dot through electron-phonon interaction. We analytically and numerically evaluate the relaxation and dephasing rates due to electron coupling to both acoustic and optical phonons, and explore the system parameter space in terms of interdot distance, strength of single-dot confinement, and inter-dot coupling strength. Our numerical results show that the electron scattering rates are strongly dependent on the strength of the electron confinement and the size of the system. In addition, …
We present results of atomistic empirical pseudopotential calculations and configuration interaction for excitons, positive and negative trions (X\pm), positive and negative quartons (X2\pm) and biexcitons. The structure investigated are laterally aligned InGaAs quantum dot molecules embedded in GaAs under a lateral electric field. The rather simple energetic of excitons becomes more complex in the case of charged quasiparticles but remains tractable. The negative trion spectrum shows four anticrossings in the presently available range of fields while the positive trion shows two. The magnitud…
We present a detailed investigation of different excitonic states weakly confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching method. For our analysis we make use of temperature-, polarization- and magnetic field-dependent $μ$-photoluminescence measurements, which allow us to identify different excited states of the quantum dot system. Besides that, we present a comprehensive analysis of g-factors and diamagnetic coefficients of charged and neutral excitonic states in Voigt and Faraday configuration. Supported by theoretical calculations by the Configuration interacti…
We study the transient phenomena appearing in a subgap region of the double quantum dot coupled in series between the superconducting and normal metallic leads, focusing on the development of the superconducting proximity effect. For the uncorrelated nanostructure we derive explicit expressions of the time-dependent occupancies in both quantum dots, charge currents, and electron pairing induced on individual dots and between them. We show that the initial configurations substantially affect the dynamical processes, in which the in-gap bound states emerge upon coupling the double quantum dot to…
We study the electronic and optical properties of laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric field. We find that electrons perceive the double-dot structure as a compound single object, while the holes discern two well separated dots. Through a combination of predictive atomistic modeling, detailed morphology studies, and single object micro-photoluminescence measurements, we show that this peculiar confinement results in an unusual heterogeneous behavior of electrons and holes with profound consequences on optical properties.
We define two laterally gated small quantum dots (~ 15 electrons) in an Aharonov-Bohm geometry in which the coupling between the two dots can be broadly changed. For weakly coupled quantum dots we find Aharonov-Bohm oscillations. In an intermediate coupling regime we concentrate on the molecular states of the double dot and extract the magnetic field dependence of the coherent coupling.
Systems of photonic crystal cavities coupled to quantum dots are a promising architecture for quantum networking and quantum simulators. The ability to independently tune the frequencies of laterally separated quantum dots is a crucial component of such a scheme. Here, we demonstrate independent tuning of laterally separated quantum dots in photonic crystal cavities coupled by in-plane waveguides by implanting lines of protons which serve to electrically isolate different sections of a diode structure.
We investigate the energy spectra of three electrons in SiGe/Si/SiGe equilateral triangular and symmetric linear triple quantum dots in the presence of magnetic (in either Faraday or Voigt configuration) and electric fields with single valley approximation by using the real-space configuration interaction method. The strong electron-electron Coulomb interaction, which is crucial to the energy spectra, is explicitly calculated whereas the weak spin-orbit coupling is treated perturbatively. In both equilateral triangular and symmetric linear triple quantum dots, we find doublet-quartet transitio…
We present a study of the electronic structure of two laterally coupled gaussian quantum dots filled with two particles. The exact diagonalization method has been used in order to inspect the spatial correlations and examine the particular spin singlet-triplet configurations for different coupling degrees between quantum dots. The outcome of our research shows this structure to have highly modifiable properties promoting it as an interesting quantum device, showing the possible use of this states as a quantum bit gate.
The spin-dependent localization of electrons in quadruple quantum dots (QD's) has been studied by the configuration interaction method. We have investigated two nanodevices that consist of laterally coupled quadruple QD's. We have shown that -- in both the nanodevices with suitably chosen parameters -- the exclusive OR (XOR) logic gate can be realized by all-electrical control with the readout of output via the spin-to-charge conversion. We have determined the nanodevice parameters that are optimal for the performance of the XOR logic gate.
The authors report that anisotropic confining potentials in laterally-coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of inter-shell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared inter-shell optical transitions …
We report on electron transport through an artificial molecule formed by two tunnel coupled quantum dots, which are laterally confined in a two-dimensional electron system of an Al$_x$Ga$_{1-x}$As/GaAs heterostructure. Coherent molecular states in the coupled dots are probed by photon-assisted tunneling (PAT). Above 10 GHz, we observe clear PAT as a result of the resonance between the microwave photons and the molecular states. Below 8 GHz, a pronounced superposition of phonon- and photon-assisted tunneling is observed. Coherent superposition of molecular states persists under excitation of ac…
The polarization splitting of the exciton ground state in two laterally coupled quantum dots under an in-plane electric field is investigated and its effective tuning is designed. It is found that there are significant Stark effect and anticrossing in energy levels. Due to coupling between inter- and intra-dot states, the absolute value of polarization splitting is significantly reduced, and it could be tuned to zero by the electric field for proper inter-dot separations. Our scheme is interesting for the research on the quantum dots-based entangled-photon source.
We present a theory and Coulomb and Spin Blockade spectroscopy experiments on quantum Hall droplets with controlled electron numbers (N1,N2) in laterally coupled gated quantum dots. The theory is based on the configuration interaction method (CI) coupled with the unrestricted Hartree-Fock (URHF) basis. It allows us to calculate the magnetic field evolution of ground and excited states of coupled quantum dots with large electron numbers. The method is applied to the spin transitions in the (5,5) droplet. Preliminary experimental results demonstrate the creation of the (5,5) droplet and its Spin…
The creation, coherent manipulation, and measurement of spins in nanostructures open up completely new possibilities for electronics and information processing, among them quantum computing and quantum communication. We review our theoretical proposal for using electron spins in quantum dots as quantum bits. We present single- and two qubit gate mechanisms in laterally as well as vertically coupled quantum dots and discuss the possibility to couple spins in quantum dots via superexchange. We further present the recently proposed schemes for using a single quantum dot as spin-filter and spin re…
We theoretically investigate the energy of the ground state exciton confined to two-dimensional (2D) monolayers with circular shape. Within an effective mass approach employing a nonlocal screening effect on the Coulomb potential energy, we demonstrate how the exciton energy is correlated with the radius of the circle, electron-hole reduced mass, and 2D susceptibility. In addition, we show that a dead layer around the circle edge, into which the electron-hole pair cannot penetrate, is necessary for understanding the energy shift recently observed in monolayer WSe2 quantum dots.
Theoretical and numerical calculations of two-phonon processes on gated lateral semiconductor quantum dots (QDs) are outlined. A heterostructure made with two laterally coupled QDs, in the presence of an external magnetic field, has been employed in order to study the electron scattering rate due to two-phonon processes. The formalism is based on the acoustic phonon modes via the unscreened deformation potential and the piezoelectric interaction whenever the crystal lattice lacks a center of inversion symmetry. The rates are calculated by using second order perturbation theory. The strong depe…
Quantum dot arrays are a promising media for transferring quantum information between two distant points without resorting to mobile qubits. Here we study two most common disorders namely, hyperfine interaction and exchange coupling fluctuations, in quantum dot arrays and their effects on quantum communication through these chains. Our results show that the hyperfine interaction is more destructive than the exchange coupling fluctuations. The average optimal time for communication is not affected by any disorder in the system and our simulations show that anti-ferromagnetic chains are much mor…
We perform time-dependent simulations of spin exchange for an electron pair in laterally coupled quantum dots. The calculation is based on configuration interaction scheme accounting for spin-orbit (SO) coupling and electron-electron interaction in a numerically exact way. Noninteracting electrons exchange orientations of their spins in a manner that can be understood by interdot tunneling associated with spin precession in an effective SO magnetic field that results in anisotropy of the spin swap. The Coulomb interaction blocks the electron transfer between the dots but the spin transfer and …
We present a theory of laterally coupled quantum Hall droplets with electron numbers (N1,N2) at filling factor $ν=2$. We show that the edge states of each droplet are tunnel coupled and form a two-level artificial molecule. By populating the edge states with one electron each a two electron molecule is formed. We predict the singlet-triplet transitions of the effective two-electron molecule as a function of the magnetic field, the number of electrons, and confining potential using the configuration interaction method (CI) coupled with the unrestricted Hartree-Fock (URHF) basis. In addition to …
In this work the intersubband electronic properties of two laterally coupled dome-shaped InAs/GaAs quantum dots were investigated. The envelope functions and eigenenergies were calculated as function of distance between the dots. The coupling between the dots was studied using transition lifetime between the dots. The results showed that in close distances (smaller than 3 nm) the quantum dots are coupled and by increasing the distance transition lifetime fall down drastically and the dots become uncoupled.
We evaluate the Lande g factor of electrons in quantum dots (QDs) fabricated from GaAs quantum well (QW) structures of different well width. We first determine the Lande electron g factor of the QWs through resistive detection of electron spin resonance and compare it to the enhanced electron g factor determined from analysis of the magneto-transport. Next, we form laterally defined quantum dots using these quantum wells and extract the electron g factor from analysis of the cotunneling and Kondo effect within the quantum dots. We conclude that the Lande electron g factor of the quantum dot is…
Laterally coupled charge sensing of quantum dots is highly desirable, because it enables measurement even when conduction through the quantum dot itself is suppressed. In this work, we demonstrate such charge sensing in a double top gated MOS system. The current through a point contact constriction integrated near a quantum dot shows sharp 2% changes corresponding to charge transitions between the dot and a nearby lead. We extract the coupling capacitance between the charge sensor and the quantum dot, and we show that it agrees well with a 3D capacitance model of the integrated sensor and quan…
In bilayer graphene, electrostatic confinement can be realized by a suitable design of top and back gate electrodes. We measure electronic transport through a bilayer graphene quantum dot, which is laterally confined by gapped regions and connected to the leads via p-n junctions. Single electron and hole occupancy is realized and charge carriers $n = 1, 2,\dots 50$ can be filled successively into the quantum system with charging energies exceeding $10 \ \mathrm{meV}$. For the lowest quantum states, we can clearly observe valley and Zeeman splittings with a spin g-factor of $g_{s}\approx 2$. In…
We propose a fast optically induced two-qubit \textsc{c-phase} gate between two resident spins in a pair of coupled quantum dots. An excited bound state which extends over the two dots provides an effective electron-electron exchange interaction. The gate is made possible by the electron-hole exchange interaction, which isolates a single transition in the system. When combined with appropriate single qubit rotations, this gate generates an entangled state of the two spins.
We present a comprehensive photoluminescence study of exchange interaction in self-assembled CdTe/ZnTe quantum dots. We exploit the presence of multiple charge states in the photoluminescence spectra of single quantum dots to analyze simultaneously fine structure of different excitonic transitions, including recombination of neutral exciton/biexciton, doubly charged negative exciton and negatively charged biexciton. We demonstrate that the fine structure results from electron-hole exchange interaction and that spin Hamiltonians with effective exchange constants $δ_i$ can provide a good descrip…
We study the effect of the exchange interaction on the Coulomb blockade peak height statistics in chaotic quantum dots. Because exchange reduces the level repulsion in the many body spectrum, it strongly affects the fluctuations of the peak conductance at finite temperature. We find that including exchange substantially improves the description of the experimental data. Moreover, it provides further evidence of the presence of high spin states (S>1) in such systems.
Quantum computing and quantum communication are remarkable examples of new information processing technologies that arise from the coherent manipulation of spins in nanostructures. We review our theoretical proposal for using electron spins in quantum-confined nanostructures as qubits. We present single- and two-qubit gate mechanisms in laterally as well as vertically coupled quantum dots and discuss the possibility to couple spins in quantum dots via exchange or superexchange. In addition, we propose a new stationary wave switch, which allows to perform quantum operations with quantum dots or…
The influence of the spin-orbit interactions on the energy spectrum of two-electron laterally coupled quantum dots is investigated. The effective Hamiltonian for a spin qubit pair proposed in F. Baruffa et al., Phys. Rev. Lett. 104, 126401 (2010) is confronted with exact numerical results in single and double quantum dots in zero and finite magnetic field. The anisotropic exchange Hamiltonian is found quantitatively reliable in double dots in general. There are two findings of particular practical importance: i) The model stays valid even for maximal possible interdot coupling (a single dot), …
Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots were studied by circularly polarized magneto-photoluminescence. Mn substitutional doping leads to paramagnetic behavior down to 5 K. While undoped quantum dots show negative circular polarization, Mn doping changes its sign to positive. A circular polarization value of 40% was achieved at T=7 K and B=7 tesla. The results are interpreted in terms of Zeeman splitting of the band edge states in the presence of carrier-dopant exchange interactions that are qualitatively different from the s,p-d exchange interactions in II-…
An external magnetic field has been applied in laterally coupled dots (QDs) and we have studied the QD properties related to charge decoherence. The significance of the applied magnetic field to the suppression of electron-phonon relaxation and dephasing rates has been explored. The coupled QDs have been studied by varing the magnetic field and the interdot distance as other system parameters. Our numerical results show that the electron scattering rates are strongly dependent on the applied external magnetic field and the details of the double QD configuration.
The pumping intensity (I) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear I dependence of the PL intensity. At ap…
It has been experimentally shown that negative exchange interactions can arise in a linear three-dot system when a two-electron double quantum dot is exchange coupled to a larger quantum dot containing on the order of one hundred electrons. The origin of this negative exchange can be traced to the larger quantum dot exhibiting a spin triplet-like rather than singlet-like ground state. Here, we show using a microscopic model based on the configuration interaction (CI) method that both triplet-like and singlet-like ground states are realized depending on the number of electrons. In the case of o…
We report on the fabrication and characterization of quantum dot devices in a Schottky-gated silicon/silicon-germanium two-dimensional electron gas (2DEG). The dots are confined laterally inside an etch-defined channel, while their potential is modulated by an etch-defined 2DEG gate in the plane of the dot. For the first time in this material, Schottky top gates are used to define and tune the tunnel barriers of the dot. The leakage current from the gates is reduced by minimizing their active area. Further suppression of the leakage is achieved by increasing the etch depth of the channel. The …
We report simple expressions for the exchange coupling in double quantum dots calculated within the Heitler-London and the Hund-Mulliken approximations using four different confining potentials. At large interdot distances and at large magnetic fields the exchange coupling does not depend significantly on the details of the potentials. In contrast, at low fields and short distances different behaviors of the exchange coupling can be attributed to particular features of the potentials. Our results may be useful as guidelines in numerical studies and in the modeling of experiments.
Coupled elliptic quantum dots with different aspect ratios containing up to two electrons are studied using a model confinement potential in the presence of magnetic fields. Single and two particle Schroedinger equations are solved using numerical exact diagonolization to obtain the exchange energy and chemical potentials. As the ratio between the confinement strengths in directions perpendicular and parallel to the coupling direction of the double dots increases, the exchange energy at zero magnetic field increases, while the magnetic field of the singlet-triplet transition decreases. By inve…
The spin and orbital Kondo effects and the related shot noise for a laterally coupled double quantum dot are studied taking account of coherent indirect coupling via a reservoir. We calculate the linear conductance and shot noise for various charge states to distinguish between the spin and orbital Kondo effects. We find that a novel antiferromagnetic exchange coupling can be generated by the coherent indirect coupling, and it works to suppress the spin Kondo effect when each quantum dot holds just one electron. We also show that we can capture the feature of the pseudospin Kondo effect from t…
We calculate spin relaxation rates in lateral quantum dot systems due to electron exchange between dots and leads. Using rate equations, we develop a theoretical description of the experimentally observed electric current in the spin blockade regime of double quantum dots. Single expression fits the entire current profile and describes the structure of both the conduction peaks and of the suppressed (`valley') region. Extrinsic rates calculated here have to be taken into account for accurate extraction of intrinsic relaxation rates due to the spin-orbit and hyperfine spin scattering mechanisms…
We study exchange coupling in Si double quantum dots, which have been proposed as suitable candidates for spin qubits due to their long spin coherence times. We discuss in detail two alternative schemes which have been proposed for implementing spin qubits in quantum dots. One scheme uses spin states in a single dot and the interdot exchange coupling controls interactions between unbiased dots. The other scheme employs the singlet and triplet states of a biased double dot as the two-level system making up the qubit and exchange controls the energy splitting of the levels. Exchange in these two…
We show theoretically that arbitrary coherent rotations can be performed quickly (with a gating time ~1 ns) and with high fidelity on the spin of a single confined electron using control of exchange only, without the need for spin-orbit coupling or ac fields. We expect that implementations of this scheme would achieve gate error rates on the order of η~ 10^{-3} in GaAs quantum dots, within reach of several known error-correction protocols
We present an experimental study of electron-electron exchange interaction in self-assembled CdTe/ZnTe quantum dots based on the photoluminescence measurements. The character and strength of this interaction are obtained by simultaneous observation of various recombination channels of a doubly negatively charged exciton, including previously unrecognized emission lines related to the electron-singlet configuration in the final state. A typical value of the electron singlet-triplet splitting, which corresponds to the exchange integral of electron-electron interaction, has been determined as 20.…
We study the effect of Coulomb correlation on the emission properties of the ground state exciton in zincblende CdSe/ZnS core-shell and in wurtzite ZnO quantum dots. We validate our theory model by comparing results of computed exciton energies of CdSe/ZnS quantum dots to photoluminescence and scanning near-field optical microscopy measurements. We use that to estimate the diameter of the quantum dots using a simple model based on infinitely deep quantum well and compare the results with the statistics of the atomic force microscopy scans of CdSe/ZnS dots, obtaining excellent agreement. Therea…
The charge stability diagram for two coupled quantum dots containing up to two electrons is computed in magnetic fields. One- and two-particle Schroedinger equations are solved by exact diagonalization to obtain the chemical potentials and exchange energy in these systems. By analyzing the chemical potentials variation with external biases and magnetic fields, it is possible to distinguish between the weak and strong inter-dot couplings. The variation of the chemical potential curvatures and the double-triple point separations in the stability diagrams confirms the inter-dot coupling decrease …
A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations on spin qubits. We study different processes of swapping the electron spins by controlled switching on/off the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum information processing. We perform a direct time evolution simulations of the time-dependent Schroedinger equation. Our results show that -- in order to obtain the full interchange of spins -- the exchange interaction should change smoothly in time.…
We theoretically investigate the properties of a two-electron system confined in the three-dimensional potential of coupled quantum dots formed in a quantum wire. For this purpose, we implement a variational Heitler-London method that minimize the system energies with respect to variational parameters in electron trial wavefunctions. We find that tunneling and exchange couplings exponentially decay with increasing inter-dot distance and inter-dot barrier height. In the quasi-one-dimensional limit achieved by reducing the wire diameter, we find that the overlap between the dots decreases, which…
The presence of degenerate conduction band valleys and how they are mixed by interfaces play critical roles in determining electron interaction and spectrum in a silicon nanostructure. Here we investigate how the valley phases affect the exchange interaction in a symmetric two-electron silicon double quantum dot. Through a configuration interaction calculation, we find that exchange splitting is suppressed at a finite value of valley phase difference between the two dots, and reaches its minimum value ({\sim} 0) when the phase difference is π. Such a suppression can be explained using the Hubb…
We reexamine the tunneling density of states in quantum dots and nanoparticles within the model which is extension of the universal Hamiltonian to the case of uniaxial anisotropic exchange. We derive the exact analytical result for the tunneling density of states in the case of arbitrary single-particle energy spectrum. We find that, similar to the case of the isotropic exchange, the tunneling density of states as a function of energy has the maximum due to a finite value of the total spin of the ground state near the Stoner instability. We demonstrate that there are no additional extrema whic…
We have fabricated superconductor-quantum dot-superconductor (SC-QD-SC) junctions by using SC aluminum electrodes with narrow gaps laterally contacting a single self-assembled InAs QD. The fabricated junctions exhibited clear Coulomb staircases and Coulomb oscillations at 40 mK. Furthermore, clear suppression in conductance was observed for the source-drain voltage $|V_{\rm SD}| < 2Δ/e$, where $Δ$ is the SC energy gap of Al. The absence of Josephson current that flows through QDs is due to the strong Coulomb interaction and non-negligible thermal fluctuation in our measurement system.
We study electron-electron interaction effects in the thermodynamic properties of quantum-dot systems. We obtain the direct and exchange contributions to the specific heat C_v in the self-consistent Hartree-Fock approximation at finite temperatures. An exchange-induced phase transition is observed and the transition temperature is shown to be inversely proportional to the size of the system. The exchange contribution to C_v dominates over the direct and kinetic contributions in the intermediate regime of interaction strength (r_s ~ 1). Furthermore, the electron-electron interaction modifies bo…
