Electric Field Effect on State Energies and Transition Frequency of a Strong-Coupling Polaron in an Asymmetric Quantum Dot

We study the ground and the first excited states’ energies and the corresponding transition frequency of a strong-coupling polaron in an asymmetric quantum dot (AQD). The effects of the electric field, the transverse and the longitudinal effective confinement lengths and the electron-phonon coupling strength are taken into account by using a variational method of the Pekar type. It is found that the ground and the first excited states’ energies and the transition frequency are decreasing functions of the electric field. They will increase rapidly with decreasing the transverse and longitudinal effective confinement lengths. The transition frequency is an increasing function of the electron-phonon coupling strength, whereas the energies are decreasing ones of it.     

Effects of Magnetic Field on the Coherence Time of a Parabolic Quantum Dot Qubit

We obtain the eigenenergies and eigenfunctions (EE) of the ground and first excited states of an electron strongly coupled to LO-phonon in a parabolic quantum dot. The effect of an applied magnetic field is considered by using variational method of Pekar type. This system may be regarded as a two-level qubit. Spontaneous phonon emission arouses the qubit’s decoherence. Relations between the coherence time (CT) and the magnetic field, the effective confinement length (ECL) and the polaron radius (PR) are numerically calculated. It is found that the CT is an increasing function of the ECL, whereas it is a decreasing one of the cyclotron frequency and PR. We can extend the CT by changing these parameters in the correlated quantum functional devices.     

Switching of Current Spin Polarization by Electron-Phonon Interaction in a Quantum Dot Device

We study spin-polarized electron transport through a quantum dot coupled to one normal metal lead and one ferromagnetic lead. Both the intradot Coulomb correlation and the electron-phonon interaction are taken into account in the framework of nonequilibrium Green’s function theory. We find that due to the interplay of the Coulomb blockade effect and the phonon-induced extra electron transport channels, the spin polarization of the electron current driven by external bias voltage is enhanced in a range of negative biases in which the current is flowing from the ferromagnetic lead to the normal metal one. While for the corresponding positive biases, the current polarization is suppressed to negative values where the current is flowing from the normal metal lead to the ferromagnetic one. The device thus operates as a current polarization switcher without the need of a magnetic field or spin-orbit interaction, and may find use in low-power spintronic devices with the help of phonon engineering techniques.     

Influence of the LO Phonon Effect on the Transition Rate and the Spontaneous Emission Rate in Donor-Center Quantum Dot with Spherical Gaussian Confinement Potential

Based on Lee-Low-Pines transformation, the longitudinal optical (LO) phonon effect in a donor-center quantum dot with a spherical Gaussian confinement potential is studied. The energy expressions of the ground state and the first excited state are derived by using a Pekar-type variational method, and then, a superposition state of the two-level system is constructed. On the basis of Fermi Golden Rule, two kinds of the decoherence of superposition states caused by LO phonon effects are discussed, which are the spontaneous emission of LO phonon and the quantum transition from the ground state to the excited state by absorbing a LO phonon, respectively. The numerical results show that for the former, the superposition state can be suppressed by increasing the electron–phonon coupling constant, the dielectric constant ratio, or the dispersion coefficient. For the latter, it can be used to suppress the decoherence of the superposition state by increasing the dielectric constant ratio or decreasing the electron–phonon coupling constant, or using the low-temperature environment. This work enriches and improves the theoretical scheme to suppress the decoherence of a semiconductor quantum dot qubit caused by LO phonon-related effects.

     

Properties of a Strong Coupling Bipolaron in an Asymmetric Quantum Dot

We study a strong coupling bipolaron’s vibrational frequency, self-trapping energy and potential induced by the electron–longitudinal optical (LO) phonon interaction in an asymmetric quantum dot (AQD). The effects of the electron–phonon coupling strength, the transverse and longitudinal effective confinement lengths are taken into account by using linear combination operator and unitary transformation methods. It is found that the vibrational frequency is an increasing function of the electron–phonon coupling strength, whereas it is a decreasing one of the transverse and longitudinal effective confinement lengths. The absolute values of the self-trapping energy and the potential induced by the electron–LO phonon interaction will increase with increasing coupling strength or decreasing effective confinement lengths.     

Spin Effects on Heat Current Through a Quantum Dot Attached to Ferromagnetic Leads

A heat current originating from electron–phonon coupling in a quantum dot (QD) molecule connected to ferromagnetic leads is studied by the non-equilibrium Green’s function technique. The system is driven out of equilibrium by a temperature gradient (thermal bias) applied across the two terminals of the structure. We find that when the magnetic moments of the two leads are arranged in parallel configuration, the heat current is not sensitive to the leads’ ferromagnetism, whereas in the case of antiparallel configuration, the magnitude of the heat current increases with increasing spin polarization of the leads, with the reduction of the electric current’s intensity. We also find that the ferromagnetism on the leads can amplify the heat rectification effect occurring for some particular dot levels, i.e., the strength of the heat flowing between the QD and the phonon bath can be very small for one direction of the temperature gradient, while it becomes quite large when the corresponding direction of the temperature gradient is reversed.     

Influences of the Temperature on the Parabolic Quantum Dot Qubit in the Magnetic Field

Using the variational method of the Pekar type, we study the influences of the temperature on the parabolic quantum dot qubit in the magnetic field under the condition of electric–LO-phonon strong coupling. Then we derive the numerical results and formulate the derivative relationships of the oscillation period of the electron in the superposition state of the ground state and the first-excited state with the magnetic field, the electron–LO-phonon coupling constant and the confinement length at different temperatures, respectively.     

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.     

Temperature Effects on the Self-trapping Energy of a Polaron in a GaAs Parabolic Quantum Dot

The temperature and the size dependences of the self-trapping energy of a polaron in a GaAs parabolic quantum dot are investigated by the second order Rayleigh-Schrodinger perturbation method using the framework of the effective mass approximation. The numerical results show that the self-trapping energies of polaron in GaAs parabolic quantum dots shrink with the enhancement of temperature and the size of the quantum dot. The results also indicate that the temperature effect becomes obvious in small quantum dots     

Spin Degree of Freedom and Tunneling through a Quantum Dot

A review is given on theoretical studies of roles of the spindegree of freedom in transport through a quantum dot with thetotal spin S for N electrons and for N+1electrons in the ground state. The conductance and the phasecoherency are expressed in terms of universal functions ofS, , and the spin splitting at small tunnelingrates in the absence of spin-orbit interactions and are shownto exhibit the correlation between tunnelings of electronswith opposite spins and the dephasing due to spin flips.     

The Effects of Electric Field on a Triangular Bound Potential Quantum Dot Qubit

On the condition of electron-LO-phonon strong coupling in a triangular bound potential quantum dot, we obtain the eigenenergy and eigenfuctions of the ground state and the first-excited state by using the Pekar type of variational method. This two-level system in a quantum dot can be employed as a qubit, which is a basic unit for quantum information operation and storage. Our numerical results indicate that the oscillation period of this qubit is an increasing function of the confinement length and the electric field. The influence of electric field on the period of oscillation becomes greater when the confinement length is increased. The electron probability density of the qubit is an increasing function of the electron-LO-phonon coupling constant. On the contrary, it is a decreasing function of the electric field. Meanwhile, the electron probability density varies periodically with the polar angle.     

Temperature Dependence of Polaronic Correction to the Ground State Energy in a GaAs Parabolic Quantum Dot

Within the framework of second-order Rayleigh-Schrodinger perturbation theory (RSPT), the polaronic corrections to the ground state (GS) energy of an electron in both two-dimensional (2D) and three-dimensional (3D) parabolic quantum dots (QDs) are presented at finite temperature. We apply our calculations to GaAs. It is found that the polaronic corrections to the GS energy of an electron in both 2D and 3D QDs increase with temperature increasing and size of the QD decreasing. Furthermore, this trend is much more pronounced with dimensionality decreasing.     

The Temperature Effects on the Parabolic Quantum Dot Qubit in the Electric Field

The temperature effects on the parabolic quantum dot qubit in the electric field have been studied under the condition of electric-LO-phonon strong coupling using the variational method of Pekar type. The numerical results lead us to formulate the derivative relationships of the oscillation period of the electron in the superposition state of the ground state and the first-excited state with the electric field, the electron-LO-phonon coupling constant and the confinement length at different temperatures, respectively.     

Cost of Energy Quantum Correlation in a Spin Model

Identification of quantum phase transitions has been a long standing issue in quantum systems. In this work, we study the renormalization of Wigner–Yanase skew information in XY spin chain and compare its relation with cost of energy quantum correlation near the critical point. This study is presented by implementing the quantum renormalization group (RG) technique. We apply the (RG) method to examine existing phase transition in XY spin chain by using the cost of energy quantum correlations and Wigner–Yanase skew information. We demonstrate that cost of energy quantum can provide crucial information about quantum phase transitions as well as quantum correlations.

     

The Effect of Magnetic on the Properties of a Parabolic Quantum Dot Qubit

On the condition of electric-LO phonon strong coupling in a parabolic quantum dot, we obtained the eigen energies and the eigenfunctions of the ground state and the first excited state by using the variational method of Pekar type. This system in a quantum dot may be employed as a two-level quantum system qubit. When the electron is in the superposition state of the ground state and the first excited state, we obtained the space-time evolution of the electron density. The relations on the cyclotron frequency with the electron probability density and the period of oscillation are derived in a parabolic quantum dot.     

Magnetic Field Effects on Transport Through a Strongly Correlated Dot Coupled to Luttinger Leads

The effects of the magnetic field and intralead electron interaction on the transport have been investigated in the Kondo regime by means of the nonequilibrium Green functions with the equation of motion method. The numerical results show that for weak intralead interaction, in addition to the Kondo dip zero bias, the differential conductance shows two peaks separated from the origin by the Zeeman energy. When the intralead interaction becomes moderately strong, the Zeeman splitting peaks turn into Zeeman splitting dips. With the further increase in the intralead interaction, all the dips disappear and the differential conductance is characterized by a power law scaling in bias voltage in the limit of the strong intralead interaction. Our results are valuable for analyzing transport in carbon nanotubes.     

The Polar Optical Phonon Confinement Effect on the Binding Energy of a Hydrogenic Impurity in Quantum Wires Under Applied Electric and Magnetic Fields

The impurity bound polaron in a cylindrical quantum wire with a parabolic confining potential was studied by the variational approach. The polaron effects on the ground-state binding energy in electric and magnetic fields are investigated by means of Pekar-Landau variation technique by taking into account optical phonon confinement within the wire region and localization at its boundaries. It is shown that not only electron confinement, but also polar optical phonon confinement leads to a considerable enhancement of the polaron effect. The results for the binding energy as well as polaronic correction are obtained as a function of the applied fields.     

The Properties of the Polaron Ground State in a Parabolic Quantum Dot and Ring

The polaron ground state energy is obtained by using variational method of Pekar type on the condition of electric-LO phonon strong coupling in a quantum dot and ring. The relations of the polaron ground state energy on the inner confinement strength, the outer confinement strength, the inner and outer radius of quantum dot and ring are derived.     

Spin Polarization by Tilted Magnetic Field in Wide Ga1?xAlxAs Parabolic Quantum Wells

Wide parabolic wells can be created by properly controlling the Al content during the growth of successive Ga_1–xAl_xAs thin layers. Under a tilted magnetic field these systems present interesting transport properties, which are associated to their composition dependent g-factor. We present an exact solution for the eigenstates of an electron gas inside such a quantum well. We calculate the renormalized cyclotron frequencies as functions of the angle of tilt, as well as the density of states, and the Fermi level. We discuss the conditions for the existence of spin-polarized charge.