Colossal enhancement in thermoelectric effect in a laterally coupled double-quantum-dot chain by the Coulomb interactions

Thermoelectric effects, including Seebeck coefficient (S), thermal conductance (κ), and figure of merit (ZT), in a laterally coupled double-quantum-dot (DQD) chain with two external nonmagnetic contacts are investigated theoretically by the nonequilibrium Green's function formalism. In this system, the DQD chain between two contacts forms a main channel for thermal electrons transporting, and each QD in the main chain couples laterally to a dangling one. The numerical calculations show that the Coulomb interactions not only lead to the splitting of the asymmetrical double-peak structure of the Seebeck coefficient, but also make the thermal spectrum show a strong violation of the Wiedemann–Franz law, leading to a colossal enhancement in ZT. These results indicate that the coupled DQD chain has potential applications in the thermoelectric devices with high thermal efficiency.     

Impurities-contributed abnormal thermoelectric effect in a parallel double quantum dot structure

We discuss the influence of local impurities on the thermoelectric effect in a parallel double quantum dot (QD) structure. It is first found that in this structure, the Fano effect contributes significantly to the enhancement of thermoelectric efficiency, especially in the case of ϕ = π. Next, impurities are introduced to couple to the QDs, respectively. We readily find that regardless of which QD is coupled to a local impurity, the thermoelectric efficiency can be enhanced by the strengthening of impurity–QD coupling. This means that the destruction of the Fano interference is not the necessary condition to suppress the thermoelectric effect. Accordingly, we hope that the numerical results can help to understand the role of impurities in adjusting the thermoelectric properties of the QD structure.     

The influence of the nanostructure geometry on the thermoelectric properties

We discuss the influence of nanostructure geometry on the thermoelectric properties in quantum ring consists of one QD in each arm, each QD connects with side QD. The calculations are based on the time-dependent Hamiltonian model, the steady state is considered to obtain an analytical expression for the transmission probability as a function of system energies. We employed the transmission probability to calculate the thermoelectric properties. We investigate thermoelectric properties through three configurations of this nanostructure. Figure of merit enhanced in configuration (II) when side QD connected to upper arm of quantum ring. The magnetic flux threads quantum ring. The effect of magnetic flux on the thermoelectric properties is examined.     

Thermoelectric transport properties through a T-shaped single quantum dot

We study the thermopower, thermal conductance, electric conductance and the thermoelectric figure of merit for a gate-defined T-shaped single quantum dot (QD). The QD is solved in the limit of strong Coulombian repulsion U→∞, inside the dot, and the quantum wire is modeled on a tight-binding linear chain. We employ the X-boson approach for the Anderson impurity model to describe the localized level within the quantum dot. Our results are in qualitative agreement with recent experimental reports and other theoretical researches for the case of a quantum dot embedded into a conduction channel, employing analogies between the two systems. The results for the thermopower sign as a function of the gate voltage (associated with the quantum dot energy) are in agreement with a recent experimental result obtained for a suspended quantum dot. The thermoelectric figure of merit times temperature results indicates that, at low temperatures and in the crossover between the intermediate valence and Kondo regimes, the system might have practical applicability in the development of thermoelectric devices.     

Thermoelectric Phenomena in a Quantum Dot Attached to Ferromagnetic Leads in Kondo Regime

We have studied the thermoelectric properties through ferromagnetic leads-QD coupled system (F-QD-F) in the Kondo regime by nonequilibrium Green's functions method. The spin-flip effect induced by ferromagnetic leads and Kondo effect influence the thermoelectric properties significantly. The peak-valley structure emerges at the low temperature due to Kondo resonance, and the peak-valley structure also relies on the polarization angle θ, the spin-dependent linewidth function Γ_γσ and the energy level of QD ε_d. Novel resonant peak also emerges in the curve of ZT_c versus polarization angle θ. The Kondo effect suppresses the figure of merit ZT_c and the spin-dependent figure of merit ZT_s. In addition, the spin-dependent figure of merit ZT_s is relate with the gap between Γ_γ↑ and Γ_γ↓.     

Transient behaviour of quantum-dot saturable absorber mirrors at varying excitation fluence

We present results from studying the carrier dynamics in self-assembled InAs/GaAs quantum-dot saturable absorbers intended for mode-locking of solid-state lasers. Four samples are examined, featuring controlled variations in the resonance condition of the electric field inside the absorber, the number of quantum-dot (QD) layers and the thickness of the GaAs barriers between these QD layers. Pump-probe experiments are conducted at a wide range of excitation fluences and reveal a fast relaxation component of the initial bleaching at low excitation fluences, while a slowly relaxing induced transparency becomes dominant at higher excitation fluences. Time-resolved photoluminescence measurements reveal a large and slowly relaxing induced transparency due to a capture of excess carriers from the barrier bands into the QDs and a slow radiative recombination there. The resonance condition as well as the thickness of the barriers between the QD layers can be used to control the relaxation behaviour. The fastest response is obtained in a structure with an increased number of QD layers at each individual anti-node of the electric field, which is attributed to the appearance of efficient non-radiative recombination channels and capture centres. These centres are probably related to dislocations and other defects appearing in thick QD stacks.     

耗散环境下二比特体系的量子关联性质研究

利用Quantum Discord(QD)判据,研究了与热库相互作用的二比特体系的量子关联性质.讨论了在不同初态下体系量子关联随时间的变化,以及热库的平均光子数m,n和原子的自发辐射率γ对体系量子关联性质的影响.结果表明,在不同的初态下,体系可以得到不同性质的量子关联;而且当原子自发辐射率γ取固定值时,QD的衰减会随m,n取值的减小而减慢,热库平均光子数都为零的情况下能够得到最大范围的量子关联;此外,当热库平均光子数m,n取确定值时,随着γ取值的减小,QD的衰减也会随之减慢,此时同样会得到较大范围的量子关联.说明较小的热库平均光子数以及原子自发辐射率γ能够减弱QD的衰减,从而获得生命力较强的体系量子关联.     

Effects of hydrostatic pressure on the donor impurity in a cylindrical quantum dot with Morse confining potential

The effects of hydrostatic pressure and size quantization on the binding energies of a hydrogen-like donor impurity in cylindrical GaAs quantum dot (QD) with Morse confining potential are studied using the variational method and effective-mass approximation. In the cylindrical QD, the effect of hydrostatic pressure on the binding energy of electron has been investigated and it has been found that the application of the hydrostatic pressure leads to the blue shift. The dependence of the absorption edge on geometrical parameters of cylindrical QD is obtained. Selection rules are revealed for transitions between levels with different quantum numbers. It is shown that for the radial quantum number, transitions are allowed between the levels with the same quantum numbers, and any transitions between different levels are allowed for the principal quantum number.     

Microwave-mediated heat transport in a quantum dot attached to leads

The thermoelectric effect in a quantum dot (QD) attached to two leads in the presence of microwave fields is studied by using the Keldysh nonequilibrium Green function technique. When the microwave is applied only on the QD and in the linear response regime, the main peaks in the thermoelectric figure of merit and the thermopower are found to decrease, with the emergence of a set of photon-induced peaks. Under this condition the microwave field cannot generate heat current or electrical bias voltage. Surprisingly, when the microwave field is applied only to one (bright) lead and not to the other (dark) lead or the QD, heat flows mostly from the dark to the bright lead, almost irrespective of the direction of the thermal gradient. We attribute this effect to microwave-induced opening of additional transport channels below the Fermi energy. The microwave field can change both the magnitude and the sign of the electrical bias voltage induced by the temperature gradient.     

Time dilation of a bound half-fluxon pair in a long Josephson junction with a ferromagnetic insulator

The fluxon dynamics in a long Josephson junction with a ferromagnetic insulating layer is investigated. It is found that the Josephson phase obeys a double sine-Gordon equation involving a bound pi fluxon solution, and the internal oscillations of the bound pair acting as a clock exhibit Lorentz reductions in their frequencies regarded as a relativistic effect in the time domain, i.e., time dilation. This is the complement to the Lorentz contraction of fluxons with no clock. A possible observation scheme is also discussed.     

Revival of metastable behavior in phase separated La0.5Ca0.5MnO3+δ

The La_xCa_1−xMnO_3+δ compositions close to charge ordering (x∼0.5) show a gradual relaxation from a metallic/ferromagnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.     

Spin Hall effect generated by a temperature gradient and heat current in a two-dimensional electron gas

We predict an intrinsic thermo-spin Hall effect, namely, that a transverse spin current is generated by the temperature gradient and the heat current in a disorder-free two-dimensional electron gas (2DEG) with finite spin–orbit coupling. There exist two classes of contributions to the thermal spin Hall effect, corresponding to a 2DEG contacting two reservoirs at different temperatures and to a 2DEG separated from the reservoirs by insulating spacers, respectively. It is shown that the thermal spin Hall current can be generated not only by the temperature gradient directly but also by the thermoelectric effect.     

Electronic structures of alloy quantum dots with nonuniform composition

In this study, the significant effect of the nonuniform composition in alloy quantum dots (QDs) on electronic structure is analyzed in depth. The equilibrium composition profiles in experimentally observed dome and barn shaped GeSi/Si QDs are determined by combining the finite element method and the method of moving asymptotes. Due to the composition variation, the total band edge of heavy hole is dominated by the band offset and spin-orbit coupling rather than the strain effect. The numerical results reveal that the wave function of heavy hole trends to be localized in the Ge-rich region at the top of the large QD. Moreover, the size effect gradually compensates the composition effect as the size of QD decreases.     

Large spin figure of merit in a double quantum dot coupled to noncollinear ferromagnetic electrodes

The spin thermoelectric effects are studied in a Rashba double quantum dot (QD) attached to ferromagnetic leads with noncollinear magnetic moments. The spin conductance G(s), spin thermopower S(s), electron thermal conductance κ(el) and spin thermoelectric figure of merit Z(s)T are calculated by using Green's function method. We find that the magnitude of the spin figure of merit can be remarkably enhanced by the coexistence of the Rashba spin-orbit interaction in the QDs and the leads' spin polarization, and can reach even as high as 3 by optimizing the parameters of the structure. The angle between the leads' magnetic moments can act as a powerful means to manipulate the properties of the spin figure of merit.     

Universal functions for estimating total vibration-rotation band absorptance—I. Linear molecules and spherical tops

We estimate total band absorptances and their derivatives for nonoverlapping lines of vibration-rotation bands for linear molecules and spherical tops. We use universal functions obtained by replacing the sums of line contributions by integrals over the rotational quantum numbers. An optical path is introduced for the total band. Only general information is utilized on vibrational transitions and line shapes. Power and asymptotic series have been obtained for Doppler and Lorentz line shapes. For a linear molecule and the Lorentz shape, approximate formulae have been derived for the universal functions.     

绕圆柱体自由表面磁流体流动和传热的研究

本文对在不同雷诺数下,绕圆柱体的磁流体自由表面流动及传热进行了模拟,分析了磁场对绕流圆柱尾迹和涡分离的影响,获得了两种雷诺数下的电磁力密度、流场和温度场分布。结果表明,磁场不仅影响了流动的形态,而且对湍流有抑制作用,降低了自由表面的更新机制,从而减少了传热能力;在相同的Hartmann数下,相比低雷诺数下的流动换热情况,高雷诺数下的湍流不能被完全抑制,自由表面与尾迹的相互作用也较强,因而自由表面换热也较强。     

A method to obtain ground state electroluminescence from 1.3μm emitting InAs/GaAs quantum dots grown by molecular beam epitaxy

1.3μm emitting InAs/GaAs quantum dots(QDs) have been grown by molecular beam epitaxy and QD light emitting diodes(LEDs) have been fabricated.In the electroluminescence spectra of QD LEDs,two clear peaks corresponding to the ground state emission and the excited state emission are observed.It was found that the ground state emission could be achieved by increasing the number of QDs contained in the active region because of the state filling effect.This work demonstrates a way to control and tune the emitting wavelength of QD LEDs and lasers.     

Effect of interband scattering on the thermoelectric properties of semiconductors and semimetals

This paper reports on the study of the effect of interband scattering on the thermoelectric figure-of-merit of semiconductors and semimetals. It has been shown that the inclusion of interband scattering in the case of two types of carriers of the same sign leads to a decrease in the thermoelectric figure-of-merit. By contrast, if the material contains carriers of two types of opposite signs and the conduction and valence bands overlap, as is the case with semimetals, the interband scattering favorably affects on the magnitude of the thermoelectric figure-of-merit.