Interdot Tunnel Coupling Research Articles

Quantum Phase Transition and Ferromagnetic Spin Correlation inParallel Double Quantum Dots

We investigate electronic transport through a parallel double quantumdot (DQD) system with strong on-site Coulomb interaction, as well asthe interdot tunnelling. By applying numerical renormalizationgroup method, the ground state of the system and thetransmission probability at zero temperature are obtained.For a system of quantum dots with degenerate energy levels andsmall interdot tunnel coupling, the spin correlations between theDQDs is ferromagnetic, and the ground state of the system is aspin-1 triplet state. The linear conductance will reach theunitary limit (2e2/h) due to the Kondo effect at lowtemperature. As the interdot tunnel coupling increases, there is aquantum phase transition from ferromagnetic to anti-ferromagneticspin correlation in DQDs and the linear conductance is stronglysuppressed.

Observation of anti-bonding excited state in charging diagram of a few-electron double dot

We fabricated a unique hybrid vertical–lateral double dot device with two dots coupled in parallel to study the excitation energy spectra with the number of electrons in each dot and inter-dot level detuning as parameters. We measured the charging diagram for a finite bias voltage and observed the tunnel-coupled bonding state and anti-bonding state. The separation between the two states is a direct measure of the inter-dot tunnel coupling energy. We use this energy and other characteristic energies derived from the charging diagram to characterize the exchange interaction energy.

Interplay between electrostatic and tunnel couplings in an independently contacted double quantum dot–quantum wire coupled device

We have studied low temperature transport characteristics of an independently contacted double quantum dot–quantum wire coupled system. Each quantum dot is connected to three leads; source, drain and wire. Fano resonances associated with one dot appear in the conductance of the other dot, which are gradually suppressed as the inter-dot tunnel coupling via the wire is reduced. When the dot-wire tunnel coupling is lost, conductance modulation similar to the Fano resonances reappear, which is ascribed to purely inter-dot electrostatic coupling effect.

Sensing an electromagnetic field with photon-assisted Fano resonance in a two-branch carbon nanotube junction

A photon-assisted Fano resonance in a two-branch single wall carbon nanotube junction (NJ) is considered. The NJ branches are misaligned by a finite angle $\ensuremath{\vartheta}$ and represent two coupled one-dimensional quantum dots where quantized electron states are formed. The inter-dot tunneling coupling causes a Fano resonance. When an external electromagnetic field (EF) is applied, the interdot tunneling becomes photon assisted. The resulting dc conductivity of each NJ branch shows series of satellite singularities arising in addition to the steady state Fano peaks. Magnitudes and positions of the singularities depend on the frequency, amplitude, and polarization of the EF, which may be used for sensing purposes.

Coherent single-electron transport between coupled quantum dots

We propose a method for a fully coherent transport of single electrons between the ground states of two anharmonic coupled quantum dots. The transition is achieved as a result of an intrinsic interplay between an external applied radio-frequency field and the interdot tunnel coupling between the single dot states. We have developed a four-level model for the system, and full quantitative agreement with the exact solution of the time-dependent Schr\"odinger equation is obtained for a wide range of field parameters. The model can be solved analytically for the symmetric case and has the advantage that it can be used as guidance in the actual design of such systems.

Molecular states in carbon nanotube double quantum dots

We report electrical transport measurements through a semiconducting single-walled carbon nanotube (SWNT) with three additional top-gates. At low temperatures the system acts as a double quantum dot with large inter-dot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.

Phase coherence in the cotunneling regime of a coupled double quantum dot

Low-temperature transport measurements were performed on a two-terminal Aharonov–Bohm interferometer with one Coulomb-blockaded quantum dot (QD) embedded in each of the two interfering paths. Layered local anodic oxidation of the Ga[Al]As heterostructure and a thin titanium top gate give full tunability of the inter-dot tunnel coupling from purely capacitive to the QD molecule regime and of the tunnel coupling of the dots to the Aharonov–Bohm ring leads. Aharonov–Bohm oscillations are observed in the cotunneling regime far away from conductance resonances. This indicates that transport in the cotunneling regime is at least partially phase coherent. The phase of the Aharonov–Bohm oscillations is monitored as one crosses conductance resonances around a degeneracy point in the charge stability diagram. The phase in this two-terminal device is constant in the cotunneling regime between conductance resonances.

Transport through a quantum-dot-ring with one dot connected to two electron reservoirs

Making use of the equation of motion method and Keldysh Green function technique, we obtain a general current formula for a quantum-dot-ring with one of the dots connected to two electron reservoirs under a DC bias voltage. The transmission probability T(ω) as a function of electron ω curve is presented, which displays oscillation structure. The resonant states of the system are found to distribute within an interval centered at ε0 (the dot energy level) with a width of two times the tunnel coupling amplitude between two neighboring dots, the number of the resonant peaks agrees with N (the number of the dots) when inter-dot tunnel coupling is sufficiently large and an odd–even behavior appears at the Fermi energy.

Large magnetoresistance induced by quantum charge fluctuations in magnetic double dots

We study electron tunnelling through two small ferromagnetic dots. Quantum charge fluctuations and interdot coupling cause each Coulomb peak of conductance at zero interdot coupling to split. The interdot tunnel coupling depends on the relative orientation of magnetizations of the two dots, leading to different splitting energies of the Coulomb peaks in parallel and antiparallel magnetization alignments. As a result, a very large tunnelling magnetoresistance occurs near the Coulomb peaks, and its sign may be either positive or negative.

Spin Polarized Transport through the Double-Dot System

Spin-polarized transport in nanoscopic systems was mainly investigated for a single quantum dot (QD) attached to ferromagnetic electrodes (see e.g. [1]). It was shown that such a set could act as a spin valve. The current flowing through QD strongly depends on the configuration of magnetic moments in the leads. Usually, it is maximal for configuration in which magnetic moments are parallel in both electrodes. Spin-based devices are very important due to possibility of their application in spintronics. Especially, double quantum dot (DQD) structures could probably work in quantum computer hardware [2]. Electron transport through a double dot system received much experimental and theoretical attention during the past decade [3, 4], however, systems with non-magnetic electrodes were mainly investigated. Recently, spin-dependent transport has been studied for a set of two quantum dots capacitively coupled [5]. The linear conductance in the Kondo regime has been mainly investigated. Here, we analyze spin-dependent transport through a system of two quantum dots in series. The inter-dot tunnel coupling can be easily tuned in such systems [6]. It strongly influences the current flowing through the system. Using the non-equilibrium Green function formalism based on the equation of motion we show that the DQD set can act as an effective spin valve in which not only tunnel magnetoresistance (TMR) value but also its sign can be easily changed with inter-dot coupling.

Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field

We observed mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing was suppressed when we applied a small magnetic field or increased the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarized the nuclei, resulting in marked bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 nanoseconds. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.

Single-Electron Delocalization in Hybrid Vertical-Lateral Double Quantum Dots

We used a hybrid vertical-lateral double-dot device, consisting of laterally coupled vertical quantum dots, to measure the interdot tunnel coupling. By using nonlinear transport measurements of "Coulomb diamonds," we showed that an inherent asymmetry in the capacitances of the component dots influences the diamond slopes, thereby allowing for the determination of the dot through which the electron has passed. We used this technique to prepare a delocalized one-electron state and Heitler-London (HL) two-electron state, and we showed that the interdot tunnel coupling, which determines whether HL is the ground state, is tunable. This implies that our device may be useful for implementing two-electron spin entanglement.

Shot noise in strongly correlated double quantum dots

We have investigated theoretically the shot noise of strongly correlated double quantum dot systems in the regime where the Kondo effect and the interdot antiferromagnetic exchange do compete. Our calculations are based on a generalization to nonequilibrium transport of the two-impurity Anderson Hamiltonian which is solved by the slave-boson mean-field theory. This mean-field approach together with nonequilibrium Green's-function techniques allow us to obtain closed formulas for the shot noise in the strong-coupling regime. Two configurations are considered: series and parallel. When the two quantum dots are in series, we obtain at low voltages, a nonmonotonic behavior of the Fano factor, as a function of the interdot tunneling coupling. When the competition between the Kondo effect and the interdot antiferromagnetic exchange starts to play a role this picture is strongly modified. In a parallel configuration, the functional form of the Fano factor as a function of the applied voltage ${V}_{\text{dc}}$ changes abruptly, depending on whether the ratio between the antiferromagnetic exchange constant $J$ and the Kondo temperature ${T}_{K}$ is below or above ${(J∕{T}_{K})}_{c}\ensuremath{\simeq}2.5$. Below ${(J∕{T}_{K})}_{c}$, the Fano factor behaves as in the single quantum dot case (monotonic dependence which saturates at large voltages). Above ${(J∕{T}_{K})}_{c}$, the Fano factor exhibits a nonmonotonic dependence on voltage, with a kink at $e{V}_{\text{dc}}=J$.

Differential charge sensing and charge delocalization in a tunable double quantum dot.

We report measurements of a tunable double quantum dot, operating in the quantum regime, with integrated local charge sensors. The spatial resolution of the sensors allows the charge distribution within the double dot system to be resolved at fixed total charge. We use this readout scheme to investigate charge delocalization as a function of temperature and strength of tunnel coupling, demonstrating that local charge sensing can be used to accurately determine the interdot coupling in the absence of transport.

Gate-voltage dependence of inter dot coupling and Aharanov–Bohm oscillation in laterally coupled vertical double dot

We fabricated laterally coupled vertical double-dot devices and measured their electrical transport properties. In these devices, two dots are laterally coupled in parallel and connected to a common source, and drain contacts placed above, and below the two dots. The number of electrons in each dot and the inter-dot tunnel coupling are all tunable. The inter-dot tunnel coupling was changed between the weak and strong coupling regimes, as a function of gate voltage placed between two dots. When a magnetic field was applied parallel to the plane of the two dots and source/drain contacts, we observed oscillations of the current (Aharanov–Bohm oscillation) in the weak coupling regime (for a different device).

Electron transport through double quantum dots

Electron transport experiments on two lateral quantum dots coupled in series are reviewed. An introduction to the charge stability diagram is given in terms of the electrochemical potentials of both dots. Resonant tunneling experiments show that the double dot geometry allows for an accurate determination of the intrinsic lifetime of discrete energy states in quantum dots. The evolution of discrete energy levels in magnetic field is studied. The resolution allows to resolve avoided crossings in the spectrum of a quantum dot. With microwave spectroscopy it is possible to probe the transition from ionic bonding (for weak inter-dot tunnel coupling) to covalent bonding (for strong inter-dot tunnel coupling) in a double dot artificial molecule. This review on the present experimental status of double quantum dot studies is motivated by their relevance for realizing solid state quantum bits.

Probing spin states of coupled quantum dots by a dc Josephson current

We propose an idea for probing spin states of two coupled quantum dots (CQD), by the dc Josephson current flowing through them. This theory requires weak coupling between CQD and electrodes, but allows arbitrary inter-dot tunnel coupling, intra- and inter- dot Coulomb interactions. We find that the Coulomb blockade peaks exhibit a non-monotonous dependence on the Zeeman splitting of CQD, which can be understood in terms of the Andreev bound states. More importantly, the supercurrent in the Coulomb blockade valleys may provide the information of the spin states of CQD: for CQD with total electron number N=1,3 (odd), the supercurrent will reverse its sign if CQD becomes a magnetic molecule; for CQD with N=2 (even), the supercurrent will decrease sharply around the transition between the spin singlet and triplet ground states of CQD.

Coulomb effects in artificial molecules

We study the capacitance spectra of artificial molecules consisting of two and three coupled quantum dots from an extended Hubbard Hamiltonian model that takes into account quantum confinement, intra- and inter-dot Coulomb interaction and tunneling coupling between all single particle states in nearest neighbor dots. We find that, for weak coupling, the interdot Coulomb interaction dominates the formation of a collective molecular state. We also calculate the effects of correlations on the tunneling probability through the evaluation of the spectral weights, and corroborate the importance of selection rules for understanding experimental conductance spectra.

Single electron charging in parallel coupled quantum dots

We report low-temperature conductance measurements in the Coulomb blockade regime on two nominally identical tunnel-coupled quantum dots in parallel defined electrostatically in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure. At low interdot tunnel coupling we find that the conductance measured through one dot is sensitive to the charge state of the neighboring dot. At larger interdot coupling the conductance data reflect the role of quantum charge fluctuations between the dots. As the interdot conductance approaches 2 e 2/ h, the coupled dots behave as a single large dot.