Optical Properties of Quantum Dots with a Core–Multishell Structure

JETP Letters - In the last decade, colloidal semiconductor nanocrystals (quantum dots) have been not only studied fundamentally but also applied in photovoltaics, optoelectronics, and biomedicine....     

Spin-Polarized Transport through the T-Shaped Double Quantum Dots with Fano-Kondo Interaction

We theoretically investigate the spin-polarized transport properties of the T-shaped double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. We calculate the density of states and the liner conductance in this system with both parallel and antiparallel lead-polarization alignments, and our results show that the transport properties of this system depend on both the tunnelling strength between the two dots and the spin-polarized strength p. This system is a possible candidate for spin valve transistors in the spintronics.     

Non-Equilibrium Quantum Transport of Bosons through a Quantum Dot

The quantum dot coupled to reservoirs is known as a typical mesoscopic setup to manifest the quantum characteristics of particles in transport. In analogue to many efforts made on the study of electronic quantum dots in the past decades, we study the transport of bosons through such a device. We first generalize the formula which relates the current to the local properties of dot in the bosonic situation. Then, as an illustrative example, we calculate the local density of state and lesser Green function of the localized boson with a bosonic Fano-Anderson model. The current-voltage （I - V） behaviour at zero temperature is presented, and in the bosonic dot it is the I - V curve, in contrast to the differential conductance in the electronic dot, which is found to be proportional to the spectral function.     

Transport Properties in a One—Dimensional Chain with Randomly Side—Coupled Impurities

We investigate the transport properties of a one-dimensional(1D) chain with randomly side-coupled impurities,By using the transfer matrix technique,we present numerical results of the transmission coefficient as a function of the electron energy,It is found that an extended state will be shown in such a random 1D system if the impurities are side-coupled to the chain with not only the nearest-neighbour bonds but also the next-nearestneighbour bonds.We present an analytical expression for the energy of this extended state,which is determined by the strength of the nearest and next-nearest couplings between the impurities and the chain.The obtained results can be used to explain the transport properties of DNA chains and other quasi-1D organic structures.     

Heat Conduction and Entropy Production in Anharmonic Crystals with Self-Consistent Stochastic Reservoirs

We investigate a class of anharmonic crystals in d dimensions, d≥1, coupled to both external and internal heat baths of the Ornstein-Uhlenbeck type. The external heat baths, applied at the boundaries in the 1-direction, are at specified, unequal, temperatures T _l and T _r . The temperatures of the internal baths are determined in a self-consistent way by the requirement that there be no net energy exchange with the system in the non-equilibrium stationary state (NESS). We prove the existence of such a stationary self-consistent profile of temperatures for a finite system and show that it minimizes the entropy production to leading order in (T _l ?T _r ). In the NESS the heat conductivity κ is defined as the heat flux per unit area divided by the length of the system and (T _l ?T _r ). In the limit when the temperatures of the external reservoirs go to the same temperature T, κ(T) is given by the Green-Kubo formula, evaluated in an equilibrium system coupled to reservoirs all having the temperature T. This κ(T) remains bounded as the size of the system goes to infinity. We also show that the corresponding infinite system Green-Kubo formula yields a finite result. Stronger results are obtained under the assumption that the self-consistent profile remains bounded.     

Modified Optical Properties of Semiconductor Quantum Dots by Photonic Structures

Specially designed photonic structures, such as photonic crystals, can prevent light from propagating in certain directions with specified frequencies.Such photonic stsuctures exhibit many unique features that are highly desirable for the manufacture of photonic integrated circuits.There has been great interest in controlling light-emitters via photonic structures, which can partially suppress and redirect spontaneous emission.Encapsulating an active material in a well-designed photonic structure can successively reduce the active volume.Because the dimensions of the active volume are reduced to a few micrometers, spontaneous emission control can be achieved, which can provide lasing with improved directional, modal control, and reduced noise.     

Geometric Phase of Quantum Dots in the Time-Dependent Isotropic Magnetic Field

By using of the invariant theory, we have studied the geometric phase of quantum dots in the time-dependent isotropic magnetic field, the dynamical and geometric phases are given, respectively.     

Spin Transport Properties of the One-Dimensional Heisenberg Chain： Bethe-Ansatz Solution with Twist Boundary Conditions

Based on the Bethe-Ansatz solution of the one-dimensional Heisenberg model under twist boundary conditions, we study the spectra of the persistent current carried by the low-lying excited states. It is shown that though the energy spectra of spin-singlet and spin-triplet excitations are degenerate, their persistent current spectra are quite different.     

Macroscopic Quantum Coherence of an Antiferromagnetic Spin Chain with Biaxial Anisotropy in an External Magnetic Field

We in this paper provide a sine-Gordon model of (1+1)-dimensions in which macroscopic quantum coherence of domain walls may be observed. The tunneling amplitude in field models of (1+1)-dimensions, such as sine-Gordon, is negligible small at zero temperature. In the long, but finite-length chain at finite temperature it is possible to have a finite, but still very small one. In the present model of the antiferromagnetic spin chain with biaxial anisotropy in an external magnetic field, in which the height of the sine-Gordon barrier can be tuned, the tunneling amplitude is shown to be enhanced by adjusting the external parameter.     

Geometric Phase of Anisotropic Quantum Dots in the Presence of Time-Dependent Magnetic Field

By using of the invariant theory, we have studied the geometric phase of anisotropic quantum dots in the presence of time-dependent magnetic field. The Aharonov-Anandan phase is also obtained under the cyclical evolution.     

The Transformation Properties at the Quantum Level in Field Theories for a Gauge-Invariant System with a Higher-Order Lagrangian

Based on the configuration-space generating functional of the Green functions for the gauge-invariant system in higher-order derivatives theories, the equations of the transformation properties at the quantum level have been derived. It follows that the sufficient conditions are found which implies that there exists the conservation laws and the expressions of the quantal conserved laws are also given. Applying the results to the non-Abelian Chern-Simons higher-order derivatives theories, the quantal BRST conserved charge and other conserved charges are found, the transformation properties of the conformal transformation at the quantum level is discussed, the quantal conserved angular momentum is derived, it is pointed out that fractional spin in this system may be also preserved in quantum theories. But the connection between the symmetries and conservation laws in classical theories are not always preserved in quantum theories.     

Quantum Entropy of a Nonlinear Two-level Atom with Atomic Motion

The wave function of a system governed by the time-dependent nonlinear Jaynes-Cummings (JC) model is obtained. We compute analytically the eigenvalues of the reduced field density operator by which the dynamics of the entropy of entanglement of the cavity field are analyzed. The influences of the atomic motion, the field-mode structure and the Kerr-like medium on this phenomenon are illustrated. The population dynamics of an excited atom is also discussed for the same set of parameters. The cavity field is assumed to be initially excited in either a Fock or a coherent states. The cavity excitation in a Fock state generates a class of an entanglement without death with fixed amplitude by adjusting the parameters of the atomic motion as well as the Kerr and the field-mode structure. In case of a coherent cavity, the only phenomenon to be noted is the periodical behavior of the dynamics under study when the atomic motion is considered. Although the Kerr medium affects the strength of the entanglement negatively, the entropy of entanglement loses its zeros where the Kerr is taken into consideration.     

Quantum-Confinement Effects on Binding Energies and Optical Properties of Excitons in Quantum Dots

Quantum-confinement effects on the binding energy and the linear optical susceptibility of excitons in quantum dots are studied. It is found that the binding energy and the linear optical susceptibility are sensitive to the barrier height and the dot size. For an infinite barrier, the binding energy of excitons decreases monotonically with the increasing dot radius, and the dbsorption intensity has almost the same amplitude with the increasing photon energy. For a finite barrier, the binding energy has a maximum value with the increasing dot radius, and the absorption intensity damps rapidly with the increasing photon energy. The effective mass ratio is also found to have an influence on the binding energy. The results could be confirmed by future experiments on excitons in quantum dots.     

Giant Persistent Current in a Mesoscopic Ring with Parallel-Coupled Double Quantum Dots

We theoretically study the properties of the ground state of the parallel-coupled double quantum dots embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. Our results show that in this system, the persistent current depends sensitively on both the parity of this system and the size of the ring. Two dots can be coupled coherently, which is reflected in the giant current peak in the strong coupling regime. This system might be a candidate for future device applications.     

The Properties of the Polarons’ Ground State in Coupling Spherical Quantum Dots

International Journal of Theoretical Physics - Investigations on the properties of polarons in coupling quantum dots (QDs) are useful for the designs of quantum devices and applications of QDs....     

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect o~ temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.     

Stationary Properties and Stochastic Resonance for a Saturation Laser Model with Cross-correlation Between Quantum Noise Terms

The stationary properties of a saturation laser model with cross-correlation between the real and imaginary parts of the quantum noise are investigated theoretically. Using the Novikov theorem and the Sargent technique, we obtain the analytic expressions of the stationary probability density distribution, the mean, the variance and the skewness of the saturation laser model. The cross-correlation coefficient λ and other parameters can make the stationary probability density distribution P _st (I) generate interesting two-extrema structure, one-extremum structure, or no-extremum structure. It is clearly found that a first- order-like-transition is induced by the coupling strength |λ| of the complex quantum noise terms in the saturation laser model. When the laser system is operated above the threshold, the mean 〈I〉 becomes larger and the output of the laser intensity increases; however the coupling strength |λ| attenuates the output of the laser intensity. When the laser is operated near and below the threshold, the mean 〈I〉 becomes smaller, the output of the laser intensity decreases, and |λ| still attenuates the output of the laser intensity. When a periodic signal is added to a saturation laser model with cross-correlation between quantum noise terms, the interesting stochastic resonance phenomena occur at λ=0. The noise intensity Q decreases the values of the resonance peak, however, the amplitude of the periodic signal B enhances the values of the resonance peak.     

Coherent Coupling of Double Quantum Dots Embedded in a Mesoscopic Ring

We theoretically study the properties of the ground state of a series-coupled double quantum dot embedded in a mesoscopic ring in the Kondo regime by means of the two-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that two dots can be coupled coherently,which is reflected in the appearance of parity effects and the complex current-phase relation in this system. This system might be a possible candidate for future device applications.     

Entanglement and Quantum Superposition of a Macroscopic-Macroscopic system

Two quantum Macro-states and their Macroscopic Quantum Superpositions (MQS) localized in two far apart, space-like separated sites can be non-locally correlated by any entangled couple of single-particles having interacted in the past. This novel “Macro-Macro” paradigm is investigated on the basis of a recent study on an entangled Micro-Macro system involving N≈10⁵ particles. Crucial experimental issues as the violation of Bell’s inequalities by the Macro-Macro system are considered.     

Quantum Dynamics of a Harmonic Oscillator in a Deformed Bath

The dissipative quantum dynamics of a harmonic oscillator in the presence of a deformed bath is investigated. The deformed bath is modelled by a collection of deformed quantum harmonic oscillators as a generalization of Hopfield model. The transition probabilities between energy levels of the oscillator are obtained perturbatively and discussed.