Tunable quantum dot resonator embedded in a quantum wire

Combined quantum wire and quantum dot system is theoretically predicted to show unique conductance properties associated with Coulomb interactions. We use a split gate technique to fabricate a quantum wire containing a quantum dot with two tunable potential barriers in a two-dimensional electron gas. We observe the effects of the quantum dot cavity on the electron transport through the quantum wire, such as Coulomb oscillations near the pinch-off voltage and periodic conductance oscillations on the first conductance plateau.     

Nonequilibrium electron and spin properties in a parallel double quantum dot Fano interference device

Nonequilibrium electron and spin transport properties in a parallel double quantum dot (QD) Fano interferometer are theoretically studied. With the shift of gate voltage around the chemical potential of either lead, we find the Fano lineshapes in the differential conductance spectra, which is sensitively determined by the bias voltage strength and appropriate QD level distributions. The intradot Coulomb interactions modulate the Fano interference in a substantial way and can induce the emergence of negative differential conductance, because of its nontrivial role in splitting the QD levels. In the presence of a local Rashba spin-orbit coupling, the interplay between the magnetic and Rashba fields induces the occurrence of the nonequilibrium spin-related Fano interference, different from the linear-transport results. Furthermore, the striking Coulomb-driven spin accumulation in the ‘resonant-channel’ QD appears.     

Selecting wave function states in open quantum dots

We study how wave function scarring in an open quantum dot is influenced as the strength of its environmental coupling is varied and show evidence for groups of wave function scars that recur periodically with gate voltage. The precise form of these scars is found to evolve with gate voltage, which we discuss in terms of the properties of the semi-classical orbits that give rise to the scars. We also provide convincing experimental evidence for a correlation between the scars and the oscillations observed in the conductance when the gate voltage is varied.     

Observation of the Fano-Kondo antiresonance in a quantum wire with a side-coupled quantum dot

We have observed the Fano-Kondo antiresonance in a quantum wire with a side-coupled quantum dot. In a weak coupling regime, dips due to the Fano effect appeared. As the coupling strength increased, conductance in the regions between the dips decreased alternately. From the temperature dependence and the response to the magnetic field, we conclude that the conductance reduction is due to the Fano-Kondo antiresonance. At a Kondo valley with the Fano parameter q approximately 0, the phase shift is locked to pi/2 against the gate voltage when the system is close to the unitary limit in agreement with theoretical predictions by Gerland et al. [Phys. Rev. Lett. 84, 3710 (2000)].     

Thermoelectric transport through a quantum dot with a magnetic impurity

We study the thermoelectric effect in a small quantum dot with a magnetic impurity in the Coulomb blockade regime.The electrical conductance,thermal conductance,thermopower,and the thermoelectrical figure of merit(FOM)are calculated by using Green’s function method.It is found that the peaks in the electrical conductance are split by the exchange coupling between the electron entering into the dot and the magnetic impurity inside the dot,accompanied by the decrease in the height of peaks.As a result,the resonances in the thermoelectric quantities,such as the thermal conductance,thermopower,and the FOM,are all split,opening some effective new working regions.Despite of the significant reduction in the height of the electrical conductance peaks induced by the exchange coupling,the values of the FOM and the thermopower can be as large as those in the case of zero exchange coupling.We also find that the thermoelectric efficiency,characterized by the magnitude of the FOM,can be enhanced by adjusting the left–right asymmetry of the electrode–dot coupling or by optimizing the system’s temperature.     

Even-odd effect in Andreev transport through a carbon nanotube quantum dot

We have measured the current (I)-voltage (V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero-bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the dI/dV subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around V=Delta/e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a "hidden" Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean-field approach, yields good agreement with the experiment.     

Electrically tunable spin polarization in a carbon nanotube spin diode

We have studied the current through a carbon-nanotube quantum dot with one ferromagnetic and one normal-metal lead. For the values of gate voltage at which the normal lead is resonant with the single available nondegenerate energy level on the dot, we observe a pronounced decrease in the current for one bias direction. We show that this rectification is spin dependent, and that it stems from the interplay between the spin accumulation and the Coulomb blockade on the quantum dot. The degree of resulting spin polarization is fully and precisely tunable using the gate and bias voltages.     

Conductance oscillations and transport spectroscopy of a quantum dot

Results are reported for low temperature measurements of the conductance through small regions of a two-dimensional electron gas (2 DEG). An unconventional GaAs heterostructure is used to form a 2 DEG whose density can be tuned by the gate voltage applied to its conductive substrate. Electron beam lithography is used to pattern a narrow channel in the 2 DEG interrupted by two constrictions, defining a small 2 DEG island between them. The conductance is found to oscillate periodically with the gate voltage, namely with electron density. Calculations of the capacitance between the substrate and the island show that the period of oscillation corresponds to adding one electron to the island. The oscillatory behavior results primarily from the discreteness of charge and the Coulomb interaction between electrons. However, the observed temperature dependence of these oscillations requires a more sophisticated treatment which includes the quantized electron energy levels as well. The magnetic field dependence of the oscillations allows us to extract the discrete energy spectrum of the quantum dot in the quantum-Hall regime.     

Single-electron charging of triangular quantum dots in a ring interferometer

Small-size semiconductor ring interferometers operating in the Coulomb blockade regime have been experimentally and theoretically studied. The conductance as a function of the gate voltage exhibits narrow quasiperiodic peaks, which are further split into doublets. Based on the experimental structural data, a three-dimensional electrostatic potential, the energy spectrum, and the single-electron transport in the interferometer were modeled. The electron system can be divided into two triangular quantum dots connected by single-mode microcontacts to each other and to the reservoirs. A model of quantum dot charging in this system is proposed that explains the appearance of doublets in the conductance-gate voltage characteristics.     

Transport in split-gate silicon quantum dots

We report on the transport properties of novel Si quantum dot structures with controllable electron number through both top and side gates. Quantum dots were fabricated by a split-gate technique within a standard MOSFET process. Four-terminal dc electrical measurements were performed at 4.2 K in a liquid helium cryostat. Strong oscillations in the conductance through the dot are observed as a function of both the top gate bias and of the plunger bias. An overall monotonic and quasi-periodic movement of the peak conductance is observed which is believed to be associated with the bare level structure of the electronic states in the dot coupled with the Coulomb charging energy. Crossing behavior is observed as well, suggestive of either many-body effects or symmetry breaking of the dot states by the applied bias.     

High magnetic field investigations of quantum dots

We have performed systematic investigations of the Coulomb blockade oscillations observed in a single quantum dot defined in the plane of a two-dimensional electron gas. At high magnetic fields these oscillations reflect the inner electronic structure of the dot, showing both a significant periodic amplitude modulation as well as a systematic variation of the conductance oscillation period. The former results from the modulation of the coupling of the electronic states in the dot with the leads, and can be readily explained within an activated transport model. The latter effect reflects the detailed electronic structure of the quantum dot and permits a comparison with the structure calculated within a simple capacitance model. The experimental results are in excellent qualitative agreement with the theoretical model, however a detailed quantitative comparison must include both the additional coupling of the dot to its environment as well as the gate voltage dependence of the dot structure itself.PACS: 73.20.Dx; 72.20. My.     

Quantum light emission of two lateral tunnel-coupled (In,Ga)As/GaAs quantum dots controlled by a tunable static electric field

Lateral quantum coupling between two self-assembled (In,Ga)As quantum dots has been observed. Photon statistics measurements between the various excitonic and biexcitonic transitions of these lateral quantum dot molecules display strong antibunching confirming the presence of coupling. Furthermore, we observe an anomalous exciton Stark shift with respect to static electric field. A simple model indicates that the lateral coupling is due to electron tunneling between the dots when the ground states are in resonance. The electron probability can then be shifted to either dot and the system can be used to create a wavelength-tunable single-photon emitter by simply applying a voltage.     

Quantized current in a quantum dot turnstile

We have performed RF experiments on a lateral quantum dot defined in the two dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The small capacitance of the quantum dot gives rise to single-electron charging effects, which we employed to realize a quantum dot turnstile device. By modulating the tunnel barriers between the quantum dot and the 2DEG leads with two phase-shifted RF signals, we pass an integer number of electrons through the quantum dot per RF cycle. This is demonstrated by the observation of quantized current plateaus at multiples ofef in current-voltage characteristics, wheref is the frequency of the RF signals. When an asymmetry is induced by applying unequal RF voltages, our quantum dot turnstile operates as a single-electron pump producing a quantized current at zero bias voltage.     

Bias-dependent bandwidth of the conductance in the presence of electron–phonon interaction

We study the electronic transport in the presence of electron–phonon interaction (EPI) for a molecular electronic device. Instead of mean field approximation (MFA), the related phonon correlation function is conducted with the Langreth theorem (LT). We present formal expressions for the bandwidth of the electron’s spectral function in the central region of the devices, such as quantum dot (QD), or single molecular transistor (SMT). Our results show that the out-tunneling rate depends on the energy, bias voltage and the phonon field. Besides, the predicted conductance map, behaving as a function of bias voltage and the gate voltage, gets blurred at the high bias voltage region. These EPI effects are consistent with the experimental observations in the EPI transport experiment.     

Quantum-dot transport in carbon nanotubes

Transport measurements on a bundle of single-walled carbon nanotubes have been made below 4.2 K as a function of side gate and source–drain bias voltage. The transport of an individual nanotube is described by the Coulomb blockade effect. The zero-dimensional quantum states of the nanotube become clear for measurements of large bias voltage. In addition, we present preliminary results of microwave application to the SWNT dot, and the results can be qualitatively explained by classical coupling to the dot.     

Quantum interference effects in semiconductor–superconductor microjunctions

We investigate the quantum interference effects in quantum dots of a two-dimensional electron gas attached to a superconductor. When the dot size is comparable to the Fermi wavelength of an electron, transmission resonance shows up in the conductance as distinct peaks and dips. The coupling of electron-like and hole-like excitations by the Andreev reflection leads to a rich variety of behavior of the resonance, in particular, against the bias voltage. Enlarging the dot size, the transmission resonance evolves into conductance fluctuations. The low-magnetic-field conductance fluctuations are shown to be remarkably geometry-specific in comparison to those in the normal counterparts.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Interference Behaviors of Resonant Transport Through a Mesoscopic System with ac Responses

Time-dependent interference behaviors on currents transporting through a mesoscopic system are investigated by using the Keldysh nonequilibrium Green function technique. The system is composed of a quantum dot coupled with two electron reservoirs. The electrons in the quantum dot are perturbed by two microwave fields (MWFs) through gate. The MWFs cause the energy level splitting in the quantum dot to form multi-channel for the tunneling current, and these branches of current interfere to produce stable oscillation. The resulting oscillation of current is strongly associated with frequency relations between MWFs. The timedependent current is the consequence of resonant effects for electrons resonating with quantum dot state and with MWFs. We present numerical calculations for the cases where the Coulomb interaction U = 0. Negative temporal current and differential conductance are observed even if the dc bias is not small. We compare the results with corresponding quantities in the system perturbed by single MWF.     

Transport through a quantum dot subject to spin and charge bias

Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron-phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.     

Electrical control of spin relaxation in a quantum dot

We demonstrate electrical control of the spin relaxation time T1 between Zeeman-split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W identical withT1(-1) by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions, and from these data we extract the spin-orbit length. We also measure the dependence of W on the magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1 T, where T1 exceeds 1 s.