Heat Generation by Electric Current in a Quantum Dot Molecular Coupled to Ferromagnetic Leads

We study the properties of the heat flow generated by electric current in a quantum dot (QD) molecular sandwiched between two ferromagnetic leads. The heat is exchanged between the QD and the phonon reservoir coupled to it. We find that when the leads' magnetic moments are in parallel configuration, the total heat generation is independent on the leads' spin-polarization regardless of the magnitude of the intradot Coulomb interaction. This behavior is similar to that of the electronic current. In the antiparallel configuration, however, the influences of the leads' ferromagnetism on the heat generation are quite different from those on the electric current. Under the conditions of weak intradot Coulomb interaction and small bias voltage, the heat generation is monotonously suppressed by increasing leads' spin-polarization. Whereas for sufficient large intradot Coulomb interaction and bias voltage, the heat generation shows non-monotonous behavior due to the electron-phonon interaction and the spin accumulation induced on the dot. Furthermore, the magnitude of the negative differential of the heat generation previously found in a QD connected to nonmagnetic leads can be weakened by the increase of the spin-polarization of the ferromagnetic leads.     

Heat Generation by Electric Current in a Quantum Dot Molecular Coupled to Ferromagnetic Leads

We study the properties of the heat flow generated by electric current in a quantum dot(QD) molecular sandwiched between two ferromagnetic leads. The heat is exchanged between the QD and the phonon reservoir coupled to it. We find that when the leads' magnetic moments are in parallel configuration, the total heat generation is independent on the leads' spin-polarization regardless of the magnitude of the intradot Coulomb interaction. This behavior is similar to that of the electronic current. In the antiparallel configuration, however, the influences of the leads' ferromagnetism on the heat generation are quite different from those on the electric current. Under the conditions of weak intradot Coulomb interaction and small bias voltage, the heat generation is monotonously suppressed by increasing leads' spin-polarization.Whereas for sufficient large intradot Coulomb interaction and bias voltage, the heat generation shows non-monotonous behavior due to the electron-phonon interaction and the spin accumulation induced on the dot. Furthermore, the magnitude of the negative differential of the heat generation previously found in a QD connected to nonmagnetic leads can be weakened by the increase of the spin-polarization of the ferromagnetic leads.     

Photon-mediated spin-polarized current in a quantum dot under thermal bias

Spin-polarized current generated by thermal bias across a system composed of a quantum dot(QD) connected to metallic leads is studied in the presence of magnetic and photon fields. The current of a certain spin orientation vanishes when the dot level is aligned to the lead's chemical potential, resulting in a 100% spin-polarized current. The spin-resolved current also changes its sign at the two sides of the zero points. By tuning the system's parameters, spin-up and spin-down currents with equal strength may flow in opposite directions, which induces a pure spin current without the accompany of charge current. With the help of the thermal bias, both the strength and the direction of the spin-polarized current can be manipulated by tuning either the frequency or the intensity of the photon field, which is beyond the reach of the usual electric bias voltage.     

Large Negative Differential of Heat Generation in a Two-Level Quantum Dot Coupled to Ferromagnetic Leads

Heat current exchanged between a two-level quantum dot(QD) and a phonon reservoir coupled to it is studied within the nonequilibrium Green's function method. We consider that the QD is connected to the left and right ferromagnetic leads. It is found that the negative differential of the heat generation(NDHG) phenomenon,i.e.,the intensity of the heat generation decreases with increasing bias voltage,is obviously enhanced as compared to that in single-level QD system. The NDHG can emerge in the absence of the negative differential conductance of the electric current,and occurs in different bias voltage regions when the magnetic moments of the two leads are arranged in parallel or antiparallel configurations. The characteristics of the found phenomena can be understood by examining the change of the electron number on the dot.     

Large Negative Differential of Heat Generation in a Two-Level Quantum Dot Coupled to Ferromagnetic Leads

Heat current exchanged between a two-level quantum dot (QD) and a phonon reservoir coupled to it is studied within the nonequilibrium Green's function method. We consider that the QD is connected to the left and right ferromagnetic leads. It is found that the negative differential of the heat generation (NDHG) phenomenon, i.e., the intensity of the heat generation decreases with increasing bias voltage, is obviously enhanced as compared to that in single-level QD system. The NDHG can emerge in the absence of the negative differential conductance of the electric current, and occurs in different bias voltage regions when the magnetic moments of the two leads are arranged in parallel or antiparallel configurations. The characteristics of the found phenomena can be understood by examining the change of the electron number on the dot.     

Cooling effect of thermal bias on a current-carrying nanodevice

We investigate the heat generation in a quantum dot (QD) coupled to two normal leads with different temperatures. It is found that heat in the QD can be conducted efficiently away through electron–phonon interaction in the QD when the QD is coupled stronger to colder lead than to the hotter one. As temperature of the colder lead is close to zero, the current through the QD peaks at the very QD level position, where the heat generation is zero, which helps to keep the stability of a working nanodevice. Then an ideal condition for nanodevice operation can be found.     

声子和温度对球型量子点中极化子性质的影响

采用求解能量本征方程、幺正变换及变分相结合的方法,研究声子和温度对球型量子点中极化子性质的影响。数值计算表明,声子效应导致极化子的基态能量低于电子能量,且极化子基态能量随电子—声子耦合强度的增大而降低。数值计算还表明,温度较低时,声子不会被激发,极化子的基态能量不随温度而变;温度较高时,声子会被激发,导致极化子能量随温度升高而增大。     

声子和温度对球型量子点中极化子性质的影响

采用求解能量本征方程、幺正变换及变分相结合的方法,研究声子和温度对球型量子点中极化子性质的影响.数值计算表明,声子效应导致极化子的基态能量低于电子能量,且极化子基态能量随电子-声子耦合强度的增大而降低.数值计算还表明,当温度较低,使得电子热运动能量小于声子能量时,声子不会被激发,极化子的基态能量不随温度的变化而变化;在温度较高,使得电子热运动能量大于声子能量时,电子和晶格热运动加剧,更多的声子被激发.极化子的基态能量随温度的升高而增大.     

Electron transport through a quantum dot in the presence of electron-photon and electron-phonon coupling

In this study, electron transport through a mesoscopic quantum dot system (QD-system) in the presence of electron-photon (el-pt) and electron-phonon (el-ph) interactions is discussed. The role of both of these interactions is to induce additional steps in the current, and sideband peaks in the differential conductance. By calculating the current and differential conductance of a QD-system in the presence of el-pt or el-ph coupling, we have shown that photon or phonon steps and sideband peaks are induced in the current and differential conductance whenever the applied voltage resonates with their frequency. Furthermore, additional side band peaks are induced in the differential conductance when el-pt and el-ph interactions are simultaneously included in the QD. These extra sideband peaks (ESBPs) are induced when the applied voltage and the photon frequency are in close proximity with the phonon frequency. To investigate the relationship that exists between the photon and phonon frequency in inducing ESBPs in the differential conduction, we have discussed zero applied voltage differential conductance. Under such conditions, ESBP is induced only when the photon frequency resonates with the phonon frequency. With increasing el-ph coupling amplitude, more ESBPs are induced in the differential conductance.     

Heat generation properties determined by chemical potentials

We investigate the current-induced heat generation in a quantum dot (QD) coupled to four spin chemical potentials, which originate from the magnetic pumping field applied on the QD. Both resonant and non-resonant electron tunneling process is analyzed. It is found that the heat generation characteristic is mainly determined by the two spin chemical potentials lying nearest to the dot level. In particular, when the difference of this two potentials is less than two phonon energy, the heat generation exhibits quantum properties, unique behavior to nanosystems and absent in macroscopic bulks.     

Rashba效应影响下量子点中弱耦合束缚极化子的性质

采用改进的线性组合算符和幺正变换的方法研究了Rashba效应影响下量子点中弱耦合束缚极化子的性质,导出了Rashba效应影响下量子点中弱耦合束缚极化子的振动频率、有效质量、基态分裂能和相互作用能。数值计算结果表明随Rashba自旋-轨道耦合常数的增加,由于声子作用产生的附加能量能对零磁场时自旋分裂能的影响占有绝对优势。库仑势对束缚极化子的基态能量的影响同时也占有绝对优势。所以,研究Rashba自旋轨道相互作时声子的影响不可忽略。     

柱型量子点中极化子的重整化质量

应用线性组合算符方法和幺正变换方法,研究在量子阱和抛物势作用下的柱型量子点中极化子的重整化质量.结果表明,量子点中极化子的重整化质量随量子点高度的增加而减小;随耦合强度的增加而增加,这是由于电子与晶格振动之间的相互作用增强所致.而基态能量与量子点的尺度、特征频率、耦合强度、磁场等均有关,当极化子运动速度不变时,基态能量随量子点柱高的增加而减小;随特征频率和磁场强度的增加而增加.     

Rashba效应影响下量子点中弱耦合束缚极化子的性质

采用改进的线性组合算符和幺正变换的方法研究了Rashba效应影响下量子点中弱耦合束缚极化子的性质,导出了Rashba效应影响下量子点中弱耦合束缚极化子的振动频率、有效质量、基态分裂能和相互作用能。数值计算结果表明随Rashba自旋-轨道耦合常数的增加,由于声子作用产生的附加能量能对零磁场时自旋分裂能的影响占有绝对优势。库仑势对束缚极化子的基态能量的影响同时也占有绝对优势。所以,研究Rashba自旋轨道相互作时声子的影响不可忽略。     

Plasmonic protection of the hot‐electron energy

It is shown, that hot electrons generated in a semiconductor can transfer their excess free energy into an embedded/adjacent plasmonic metallic structure (reservoir), before it is lost irreversibly to phonons in the semiconductor. Since the plasmon–phonon (and plasmon–photon) scattering in the metallic structure could be much slower than the electron–phonon scattering in the semiconductor, free energy of the hot electrons can be this way effectively protected from phonon emission for a significant amount of time. While the cubic point‐dipole crystal is proposed and studied here specifically as the plasmonic reservoir, other plasmonic structures including planar can be employed. It is also shown how the plasmon‐protected energy can by recycled in a novel, 3rd generation solar cell, be employing a planar plasmonic structure that is simultaneously also an electron collector of the cell. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phonon-Assisted Resonant Tunnelling through a Triple-Quantum-Dot： a Phonon-Signal Detector

We study the effect of electron-phonon interaction on current and zero-frequency shot noise in resonant tunnelling through a series triple-quantum-dot coupling to a local phonon mode by means of a nonperturbative mapping technique along with the Green function formulation. By fixing the energy difference between the first two quantum dots to be equal to phonon frequency and sweeping the level of the third quantum dot, we find a largely enhanced current spectrum due to phonon effect, and in particular we predict current peaks corresponding to phonon-absorption and phonon-emission assisted resonant tunnelling processes, which show that this system can be acted as a sensitive phonon-signal detector or as a cascade phonon generator.     

Heating and thermoelectric transport in a molecular junction

The energy dissipation and heat flows associated with the particle current in a system with a molecular junction are considered. In this connection, we determine the effective temperature of the molecular oscillator that is compatible with the existence of a steady state. The calculations based on the Kadanoff-Baym nonequilibrium Green function formalism are carried out supposing a strong coupling of the dot electrons with the molecular vibrations. Accordingly, the representation given by the Lang-Firsov polaron transformation is used and the dependence of results on the electron–phonon interaction strength is investigated.     

低维半导体异质结中的量子相干红外发射机理理论研究

给出了一种在非粒子反转条件下量子阱和量子点激光器的红外发射机理.此种红外发射是基于在同一作用区产生并作为红外场相干源的两种带间跃迁激光场的共振非线性混合.这种频率下转换机理并不依赖于在半导体激活媒质中的长时相干假定条件,在室温和泵注入电流条件下仍然有效.频率下转换的固有效率可以达到相当于每个可见光子产生一个红外光子的量子极限值.根据红外发射的可参变特性,这种非粒子反转的方法尤其适用于长波红外工作范围.     

量子点中束缚极化子性质的温度依赖性

采用线性组合算符和幺正变换方法研究抛物量子点中弱耦合束缚极化子性质的温度依赖性,导出了弱耦合束缚极化子的振动频率、基态能量和声子平均数随温度的变化关系。取ZnS晶体为例进行数值计算,结果表明:量子点中弱耦合束缚极化子的振动频率、基态能量和声子平均数随温度的升高而增大。     

Spin-flip mesoscopic transport through a quantum dot coupled to carbon nanotube terminals

We investigate mesoscopic transport through a system that consists of a central quantum dot (QD) and two single-wall carbon nanotube (SWCN) leads in the presence of a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunnelling current is sensitively related to the spin-flip effect. We present the calculations of charge and spin current components to show the intimate relations to the SWCN leads. Zeeman effect is important when the applied magnetic field is strong enough. The current characteristics are quite different when the source-drain bias is zero (eV=0) and nonzero (eV≠0). The asymmetric peak and valley of spin current versus gate voltage exhibit Fano resonance. Multi-resonant peaks of spin current versus photon energy ħω reflect the structure of CN quantum wires, as well as the resonant photon absorption and emission effect. The matching-mismatching of channels in the CN leads and QD results in novel spin current structure by tuning the frequency.     

Electron tunneling through a coupled system of electron and phonon

We study the electron tunneling through a single level quantum dot in the presence of electron–phonon interaction. By using the Green’s function and canonical transformation methods, we calculated exactly the current. It is found that the current vs dot level exhibits satellite peaks even without occurring of phonon-assisted tunneling processes, and properties of the current are affected heavily by the strength of electron–phonon interaction and phonon temperature.