Theoretical Study Of The Thermoelectric Properties Through Quantum-Dots System

With the miniaturization of spin devices,spin-polarized transport and thermoelectric transport of the hybrid structure show many novel physical phenomena and potential applications,such as spin separation,spin rotation,read-out technology,storage unit,and the use of waste heat,and so on.In experiment,the coherent transport of many quantum-dots system provide new field for designing and preparating new quantum device,Which has been paid close attention.Next,we constructed three kinds of quantum dot system to analyze the characteristics of the thermoelectric transport and spin-polarized transport based on nonequilibrium Green function.Firstly,We theoretically study thermoelectric transport through two quantum dot connected to ferromagnetic and superconducting electrodes.In a limited range of the gap,the thermopower increases and the thermal conductance decreases with the increasing of the gap,which leads the effective figure of merit to increase.For a given gap,the platform of thermopower spectrum in the gap vanishes with increasing of the interdot tunneling coupling and presents a nonzero value.Due to the coupling between quantum dot and electrodes,the level broadened is asymmetric about the Fermi level.So Andreev reflection also can induce the thermopower.One can obtain very large figure of merit by optimizing system parameters.For example,figure of merit can reach 30 or so when interdot tunneling coupling is near 1.Secondly,We theoretically analyze the thermoelectric properties of the single-spin state.The spectra of electrical conductance and thermal conductance exhibits the electronic Dicke-like effect in low temperature.By adjusting the level detuning,superradiant modes get wider and subradiant modes become narrower.When the level detuning is very large,the density states near subradiant modes exhibit ? like shape.Electrical conductance can achieve the two antiresonant point at the single-spin level of side quantum dot by adjusting the interdot hopping coefficient.Thermoelectric coefficients can achieve considerable values near antiresonant point because the Wiedemann–Franz law is strongly violated.With increase of the level detuning,the distance of side quantum-dot level and the subradiant state gets shorter,so Thermopower and effective figure of merit can get considerable value near the subradiant state.Effective figure of merit can greatly increase or decrease by adjusting the level detuning,the angle ? and the interdot hopping coefficient.Thirdly,We study the nonequilibrium spin-polarized transport in ferromagnetic-quantum dot–metal system.The existence of voltage bias and temperature gradient between two electrodes leads the pile-up of extra charge inside the QD,which makes the dot level renormalization.Based to electron cotunneling,the channels of spin-up and spin-down are separated due toL ?_L???_L? and z_L??z_L?.The positive and negative value of differential thermal conductance manifest physical mechanism of electron-like and hole-like transport,and that temperature strongly influence on spin-polarized transport near the Fermi level.The voltage-induce renormalization effect makes the dot level split into ?? and ??,which respectively locate above and below Fermi level.The fluxes of the thermally excited carrier?electron and hole?offset each other when ?? = ??.Regardless of temperature change,the thermal current is always zero.We can acquire the pure spin current by adjust-ing the value of voltage bias and temperature.Quantum-dot system has many advantages,such as easily preparation,easily measure date and easily control parameters,and so on.In the paper,research results of quantum-dot system provide theoretical foundation for the design of electronic devices,spin devices and thermoelectric devices,etc.