Dot Level Research Articles

Polarization switch in an elliptical micropillar – quantum dot system induced by a magnetic field in Faraday configuration

In this paper we study the effect of applying a magnetic field on an elliptical microcavity pillar with quantum dots embedded, in the presence of external laser excitation. To obtain the system dynamics we use the matrix density formalism, taking into account realistic parameters and including losses. Our results show that it is possible to use the magnetic field strength to control the polarization of the photons inside the cavity, making our system behave like a photon polarization switch. We also report the best set of parameters where this is possible. Our results also indicate that we can use the polarization of the cavity photons to look into the fine structure of the energy levels of quantum dots.

Effects of Majorana bound states on dissipation and charging in a quantum resistor-capacitor circuit

We investigate the effects of Majorana bound states on the ac response of a quantum resistor-capacitor circuit which is composed of a topological superconducting wire whose two ends are tunnel-coupled to a lead and a spinless quantum dot, respectively. The Majorana states formed at the two ends of the wire are found to suppress completely or enhance greatly the dissipation, depending on the strength of the overlap between two Majorana modes and/or the dot level. We compare the relaxation resistance and the quantum capacitance of the system with those of non-Majorana counterparts to find that the effects of the Majorana state on the ac response are genuine and cannot be reproduced in ordinary fermionic systems.

Universal Ratio of Coulomb Interaction to Geometric Quantization in (In, Ga)As/GaAs Quantum Dots

We study the photoluminescence of self-assembled (In,Ga)As/GaAs quantum dot ensembles with varying confinement potential height. The low energy shift of the s-shell emission with increasing excitation power gives a measure of the Coulomb interaction in these structures as it results from carrier–carrier interactions between the optically injected exciton complexes. When dividing this shift by the dot level splitting, determined by the geometric confinement, we obtain a universal function of the number of involved excitons that is independent of the confinement potential height. This shows an identical scaling of Coulomb interaction and geometric quantization with varying confinement.

Suppression of the 0−π transition in a Josephson junction with parallel double-quantum-dot barriers

With the help of the numerical renormalization group method, we theoretically investigate the Josephson phase transition in a parallel junction with one quantum dot embedded in each arm. It is found that in the cases of uniform dot levels and dot-superconductor couplings, the Josephson phase transition will be suppressed. This is manifested as the fact that with the enhancement of the electron correlation, the supercurrent only arrives at its $\pi'$ phase but cannot enter its $\pi$ phase. Moreover, when the dot levels are detuned, one $\pi'$-phase island appears in the phase diagram. Such a result is attributed to the nonlocal motion of the Cooper pair in this structure. We believe that this work can be helpful in understanding the Josephson phase transition modified by the electron correlation and quantum interference.

Quenched Fano effect due to one Majorana zero mode coupled to the Fano interferometer

We investigate the change of the Fano effect by considering one Majorana zero mode to couple laterally to the single-dot Fano interferometer. It is found that the Majorana zero mode quenches the Fano effect thoroughly and causes the conductance to be independent of the dot level, the dot-lead coupling, and the increase of the Majorana-dot coupling. As a result, the linear conductance becomes only related to the interlead coupling and the magnetic-flux phase factor. These results can be helpful for the detection of Majorana zero mode.

Quantum phase transition in a realistic double-quantum-dot system

Observing quantum phase transitions in mesoscopic systems is a daunting task, thwarted by the difficulty of experimentally varying the magnetic interactions, the typical driving force behind these phase transitions. Here we demonstrate that in realistic coupled double-dot systems, the level energy difference between the two dots, which can be easily tuned experimentally, can drive the system through a phase transition, when its value crosses the difference between the intra- and inter-dot Coulomb repulsion. Using the numerical renormalization group and the semi-analytic slave-boson mean-field theory, we study the nature of this phase transition, and demonstrate, by mapping the Hamiltonian into an even-odd basis, that indeed the competition between the dot level energy difference and the difference in repulsion energies governs the sign and magnitude of the effective magnetic interaction. The observational consequences of this transition are discussed.

Universal ratio of Coulomb interaction to geometric quantization in (In, Ga)As/GaAs quantum dots

AbstractWe study the photoluminescence of self-assembled (In,Ga)As/GaAs quantum dot ensembles with varying confinement potential height. The low energy shift of the s -shell emission with increasing excitation power gives a measure of the Coulomb interaction in these structures as it results from carrier–carrier interactions between the optically injected exciton complexes. When dividing this shift by the dot level splitting, determined by the geometric confinement, we obtain a universal function of the number of involved excitons that is independent of the confinement potential height. This shows an identical scaling of Coulomb interaction and geometric quantization with varying confinement.

Effect of PT symmetry in a parallel double-quantum-dot structure

We present a comprehensive analysis about the effect of $\mathcal{PT}$-symmetric complex potentials on quantum transport through one parallel coupled double-quantum-dot structure. It is found that such potentials are able to eliminate the decoupling phenomenon while inducing the Fano effect, even under the situation of uniform dot-lead coupling. Moreover, the Fano line shape in the transmission function spectrum is opposite to that that arises from the detuning of the dot levels. We ascertain that this work can be helpful in understanding the quantum transport properties in the quantum systems with $\mathcal{PT}$ symmetry.

Transport through capacitively coupled embedded and T-shape quantum dots in the Kondo range

Strong electron correlations resulting from spin and charge pseudospin fluctuations together with interference effects are discussed in asymmetric set of capacitively coupled dots: embedded and side attached (EDTD). Finite - U mean field slave boson approach is used. The many body resonance exhibits SU(4) Kondo character at the embedded dot and SU(4) Kondo-Fano in the T-shaped arm. For the deep dot levels characteristic temperatures of these resonances coincide with the temperatures of corresponding resonances in symmetric double dot systems, embedded or side attached respectively. Linear transport properties through embedded or side attached dots are identical with linear transport characteristics of corresponding symmetric dot systems. Breaking of spin symmetry in one of the arms or in both allows a crossover to resonances of lower symmetries: SU(3) or SU(2). We also show the possibility of gate control of spin filtering in EDTD with polarized electrodes.

Photon-Selective Spin-Dependent Transport Through a Quantum Dot Driven by Electrical and Thermal Biases

Impacts of both the electrical and thermal biases on the spin-dependent transport through a quantum dot are studied by using the nonequilibrium Green’s function technique. Due to the Zeeman splitting of the dot level, the spin polarization of the current approaches to ± 1 at some photon-induced transport channels when the spin-split dot levels locate in between the conduction window opened by the electrical bias. While under zero electrical bias but finite thermal bias, a totally 100% spin-polarized current emerges at some photon frequencies, where spin-up and spin-down electrons are selected with the help of the photon field. Moreover, for several photon frequencies, the currents of different spin components may tunnel through the dot in opposite directions, resulting in a pure spin current. We find that in both cases the intensity and the position of the resonant peaks of the spin-polarized currents are sensitive to the intradot Coulomb interaction. When the two biases coexist in the system, the intensity and the spin-polarization of the currents can be effectively adjusted by changing their relative strengthes.

Cotunneling spin blockade observed in a three-terminal triple quantum dot

We prepare a triple quantum dot with a separate contact lead to each dot to study Pauli spin blockade in the tunnel-coupled three dots in a row. We measure the tunneling current flowing between the center dot and either the left or right dot with the left and right leads as a common source and the center lead as a drain. In the biased stability diagram, we establish Pauli spin blockade in the respective neighboring dots, with features similarly obtained in double quantum dot systems. We further realize Pauli spin blockade with two different conditions by tuning the inter-dot coupling gates: strong and weak inter-dot tunnel coupling regimes. In the strong-coupling regime we observe significant suppression of co-tunneling through the respective double dots due to Pauli spin blockade. We reveal the influence from the third dot in the triple dot device on this co-tunneling Pauli spin blockade and clarify that the co-tunneling Pauli spin blockade is lifted by the resonant coupling of excited states to the third dot level as well as spin exchange of the left and right dots with the adjacent reservoir.

Nonlinear electric and thermoelectric Andreev transport through a hybrid quantum dot coupled to ferromagnetic and superconducting leads

We discuss the nonlinear Andreev current of an interacting quantum dot coupled to spin-polarized and superconducting reservoirs when voltage and temperature biases are applied across the nanostructure. Due to the particle-hole symmetry introduced by the superconducting (S) lead, the subgap spin current vanishes identically. Nevertheless, the Andreev charge current depends on the degree of polarization in the ferromagnetic (F) contact since the shift of electrostatic internal potential of the conductor depends on spin orientation of the charge carrier. This spin-dependent potential shift characterizes nonlinear responses in our device. We show how the subgap current versus the bias voltage or temperature difference depends on the lead polarization in two cases, namely (i) S-dominant case, when the dot-superconductor tunneling rate ($\Gamma_R$) is much higher than the ferromagnet-dot tunnel coupling ($\Gamma_L$), and (ii) F-dominant case, when $\Gamma_L\gg \Gamma_R$. For the ferromagnetic dominant case the spin-dependent potential shows a nonmonotonic behavior as the dot level is detuned. Thus the subgap current can also exhibit interesting behaviors such as current rectification and the maximization of thermocurrents with smaller thermal biases when the lead polarization and the quantum dot level are adjusted.

Dynamic Rabi oscillations in a quantum dot embedded in a nanobridge in the presence of surface acoustic waves

A quantum dot is created within a suspended nanobridge containing a two-dimensional electron gas. The electron current through this dot exhibits well-pronounced Coulomb blockade oscillations. When surface acoustic waves (SAW) are driven through the nanobridge, Coulomb blockade peaks are shifted. To explain this feature, we derive the expressions for the quantum dot level populations and electron currents through these levels and show that SAW-induced Rabi oscillations lead to the observed phenomenology.

Effects of the Spin Heat Accumulation on the Heat Generation in a Quantum Dot Coupled to Leads

Heat generation by a spin-polarized current in a single-level quantum dot (QD) subjected to spin heat accumulation (SHA), which denotes the spin-dependent electron temperature, is studied by using the nonequilibrium Green’s function technique. The heat generation originates from the energy exchange between the conduction electrons and the phonon reservoir coupled to the QD. Due to the SHA, the spin-up and spin-down heat generations are opposite in sign, and each has a maximum when the QD level is aligned to the chemical potentials of the leads, where the electric current is zero. Under a magnetic field, the maxima of the spin-up and spin-down heat generations are shifted to different dot level regimes. Now total negative heat generation emerges, indicating that the electron absorbs heat from the phonon reservoir to the dot. By tuning the dot levels and the system temperature, the magnitude of the negative heat generation can be enhanced accompanied by weakened electric current, an ideal condition for the realization of nanorefrigerator.

Charge-vibration interaction effects in normal-superconductor quantum dots

We study the quantum transport and the nonequilibrium vibrational states of a quantum dot embedded between a normal and a superconducting lead with the charge on the quantum dot linearly coupled to a harmonic oscillator of frequency $\omega$. To the leading order in the charge-vibration interaction, we calculate the current and the nonequilibrium phonon occupation by the Keldsyh Green's function technique. We analyze the inelastic, vibration-assisted tunneling processes in the regime $\omega <\Delta$, with the superconducting energy gap $\Delta$, and for sharp resonant transmission through the dot. When the energy $\varepsilon_0$ of the dot's level is close to the Fermi energy $\mu$, i.e. $|\varepsilon_0-\mu|\ll \Delta$, inelastic Andreev reflections dominate up to voltage $eV\gtrsim\Delta$. The inelastic quasiparticle tunneling becomes the leading process when the dot's level is close to the gap $|\varepsilon_0-\mu|\sim \Delta \pm \omega$. In both cases, the inelastic tunneling processes appear as sharp and prominent peaks - not broadened by temperature - in the $I$-$V$ characteristic and pave the way for inelastic spectroscopy of vibrational modes even at temperatures $T \gg \omega$. We also found that inelastic Andreev reflections as well as quasiparticle tunneling induce a strong nonequilibrium state of the oscillator. In different ranges on the dot's level, we found that the current produces: (i) ground-state cooling of the oscillator with phonon occupation $n\ll 1$, (ii) accumulation of energy in the oscillator with $n\gg 1$ and (iii) a mechanical instability which is a precursor of self-sustained oscillations. We show that ground-state cooling is achieved simultaneously for several modes of different frequencies. Finally, we discuss how the nonequilibrium vibrational state can be detected by the asymmetric behavior of the inelastic current peaks respect to the gate voltage.

Spin-polarized transport through a quantum ring with an embedded protein-like single-helical molecule.

We investigate the spin-polarized electron transport through a quantum ring whose arms are embedded by one protein-like single-helical molecule and one quantum dot, respectively. It is found that the inter-arm quantum interference leads to the enhancement of the spin polarization in this structure. Moreover, when local magnetic flux is applied through the ring, the spin polarization in the electron transport process, including the polarization strength and direction, can be further adjusted. Next in the finite-bias case, the spin polarization is also apparent and can be tuned by changing the magnetic flux or the dot level. This work provides a new scheme to manipulate the spin transport based on the single-helical molecule.

Electrostatically confined trilayer graphene quantum dots

Electrically gating of trilayer graphene (TLG) opens a band gap offering the possibility to electrically engineer TLG quantum dots. We study the energy levels of such quantum dots and investigate their dependence on a perpendicular magnetic field $B$ and different types of stacking of the graphene layers. The dots are modeled as circular and confined by a truncated parabolic potential which can be realized by nanostructured gates or position-dependent doping. The energy spectra exhibit the intervalley symmetry ${E}_{K}^{e}(m)=\ensuremath{-}{E}_{{K}^{\ensuremath{'}}}^{h}(m)$ for the electron $(e)$ and hole $(h)$ states, where $m$ is the angular momentum quantum number and $K$ and ${K}^{\ensuremath{'}}$ label the two valleys. The electron and hole spectra for $B=0$ are twofold degenerate due to the intervalley symmetry ${E}_{K}(m)={E}_{{K}^{\ensuremath{'}}}[\ensuremath{-}(m+1)]$. For both ABC $[\ensuremath{\alpha}=1.5$ (1.2) for large (small) $R]$ and ABA $(\ensuremath{\alpha}=1)$ stackings, the lowest-energy levels show approximately a ${R}^{\ensuremath{-}\ensuremath{\alpha}}$ dependence on the dot radius $R$ in contrast with the $1/{R}^{3}$ one for ABC-stacked dots with infinite-mass boundary. As functions of the field $B$, the oscillator strengths for dipole-allowed transitions differ drastically for the two types of stackings.

Transport properties of a hybrid Majorana wire-quantum dot system with ferromagnetic contacts

The transport properties of a quantum dot coupled to ferromagnetic contacts and attached to a topological superconductor wire hosting Majorana zero-energy modes at its ends are studied theoretically in the Kondo regime. By using the numerical renormalization group method, the temperature and dot level dependence of the spectral function, the conductance, and its spin polarization are studied for different coupling strengths to a topological superconductor. It is shown that the transport characteristics are generally determined by the interplay of three competing energy scales, resulting from Kondo correlations, a ferromagnetic-lead-induced exchange field, and topological-wire-induced splitting. These two splittings are found to have opposite signs. Moreover, they can compensate for each other in an appropriate parameter regime.

Reversible Electrochemistry of Mercury Chalcogenide Colloidal Quantum Dot Films.

The absolute positions of the energy levels of colloidal quantum dots of Hg(S, Se, Te), which are of interest as mid-infrared materials, are determined by electrochemistry. The bulk valence bands are at -5.85, -5.50, and -4.77 eV (±0.05 eV) for zinc-blend HgS, HgSe, HgTe, respectively, in the same order as the anions p-orbital energies. The conduction bands are conversely at -5.20, -5.50, and -4.77 eV. The stable ambient n-doping of Hg(S, Se) quantum dots compared to HgTe arises because the conduction band is sufficiently lower than the measured environment Fermi level of ∼ -4.7 eV to allow for n-doping for HgS and HgSe quantum dots even with significant electron confinement. The position of the Fermi level and the quantum dots states are reported for a specific surface treatment with ethanedithiol and electrolyte environment. The positions are however sensitive to different surface treatments, providing an avenue to control doping. Electrochemical gating is further used to determine the carrier mobility in the films of the three different systems as a function of CQD size. HgSe and HgS show increasing mobility with increasing particle sizes while HgTe shows a nonmonotonous behavior, which is attributed to some degree of aggregation of HgTe QDs.

Fano-Josephson effect in the junction with DIII-class topological and s-wave superconductors

We investigate the Josephson effects in the junction formed by the direct and indirect couplings between DIII-class topological and s-wave superconductors. As a result, the Josephson current is found to oscillate in period 2π. The presence of Majorana doublet in the DIII-class superconductor renders the current finite at the case of zero phase difference, with its sign determined by the fermion parity of such a junction. In addition to the dot level and intradot Coulomb interaction, the Fano interference is an important factor to adjust the Josephson current. It is believed that these results will be helpful in understanding the transport properties of the DIII-class superconductor.