Abstract
We examine the relation between the phasecoherent processes and spindependent thermoelectric effects in an AharonovBohm (AB) interferometer with a Rashba quantum dot (QD) in each of its arm by using the Green's function formalism and equation of motion (EOM) technique. Due to the interplay between quantum destructive interference and Rashba spinorbit interaction (RSOI) in each QD, an asymmetrical transmission node splits into two spindependent asymmetrical transmission nodes in the transmission spectrum and, as a consequence, results in the enhancement of the spindependent thermoelectric effects near the spindependent asymmetrical transmission nodes. We also examine the evolution of spindependent thermoelectric effects from a symmetrical parallel geometry to a configuration in series. It is found that the spindependent thermoelectric effects can be enhanced by controlling the dotelectrode coupling strength. The simple analytical expressions are also derived to support our numerical results.
PACS numbers: 73.63.Kv; 71.70.Ej; 72.20.Pa
Keywords:
Rashba spinorbit interaction; AharonovBohm interferometer; Quantum dots; Fano effectsIntroduction
With the fast development and improvement of experimental techniques [19], much important physical properties in QD molecules such as electronic structures, electronic transport, and thermoelectric effects et al have widely attracted academic attention [1029]. QDs can be realized by etching a twodimensional electron gas (2DEG) below the surface of AlGaAs/GaAs heterostructures or by an electrostatic potential. Confinement of particles in all three spatial directions results in the discrete energy levels such like an atom or a molecule. We can therefore think of QDs as artificial atoms or molecules. The small sizes of QDs make the phasecoherent of waves become more important, and quantum interference phenomena emerge when the particles moves along different transport paths. Fano resonances, known in the atomic physics, arise from quantum interference effects between resonant and nonresonant processes [30]. The main embodying of the Fano resonances is the asymmetric line profile in the transmission spectrum, which originates from the coexistence the resonant transmission peak and the resonant transmission dip. The first experiment observation of the asymmetrical Fano line shape in the QD system has been reported in a singleelectron transistor [31].
The RSOI in the QD can be introduced by an asymmetricalinterface electric field applied to the semiconductor heterostructures [32,33]. Electron spin, the intrinsic properties of electrons, become more important when electrons transport through the AB interferometer. The RSOI can couple the spin degree of freedom to its orbital motion, which provides a possible method to control the spin of transport electrons. A spin transistor by using the RSOI in a semiconductor sandwiched between two ferromagnetic electrodes has been proposed [34]. In spin Hall devices, spinup and spindown electrons flow in an opposite direction using the Rashba SOI and a longitudinal electric field such that the spin polarization becomes infinity [3537]. Some theoretical and experimental works have also shown that the spinpolarization of current based on the RSOI can reach as high as 100%[38,39] or infinite [40].
Recently, an experimental measurement of the spin Seebeck effect (the conversion of heat to spin polarization) by detecting the redistribution of spins along the length of a sample of permalloy (NiFe) induced by a temperature gradient was firstly demonstrated [41]. The new heattoelectron spin discovery can be named as "thermospintronics". More recently, the spin Seebeck effect was also observed in a ferromagnetic semiconductor GaMnAs [42]. Much academic work on spindependent thermoelectric effects in single QD attached to ferromagnetic leads with collinear magnetic moments or noncollinear magnetic moments has been reported [4346]. Up to now, we note that most of the spin Seebeck effects are obtained by using ferromagnetic materials such as ferromagnetic thin films, ferromagnetic semiconductors, or ferromagnetic electrodes et al. In our previous work, a pure spin generator consisting of a Rashba quantum dot molecule sandwiched between two nonferromagnetic electrodes via RSOI instead of ferromagnetic materials has been proposed by the coaction of the magnetic flux [24]. It should be noted that charge thermopower of QD molecular junctions in the Kondo regime and the Coulomb blockade regime have been widely investigated [2529].
In the present work, we investigate the spindependent thermoelectric effects of parallelcoupled double quantum dots embedded in an AB interferometer, in which the RSOI in each QD is considered by introducing a spindependent phase factor in the linewidth matrix elements. Due to the quantum destructive interference, an asymmetrical transmission node can be observed in the transmission spectrum in the absence of the RSOI. Using an inversion asymmetrical interface electric field, the RSOI can be introduced in the QDs. The asymmetrical transmission node splits into two spindependent asymmetrical transmission nodes in the transmission spectrum and, as a consequence, results in the enhancement of the spindependent Seebeck effects near the spindependent asymmetrical transmission nodes. We also examine the evolution of spindependent Seebeck effects from a symmetrical parallel geometry to a configuration in series. The asymmetrical couplings between QDs and nonferromagnetic electrodes induce the enhancement of spindependent Seebeck effects in the vicinity of spindependent asymmetrical transmission nodes. Although the spindependent Seebeck effects in the AB interferometer have not been realized experimentally so far, our theoretical study provides a better way to enhance spindependent Seebeck effects in the AB interferometer in the absence of the ferromagnetic materials.
Model and method
The schematic diagram for the quantum device based on parallelcoupled double quantum dots embedded in an AB interferometer in the present work is illustrated in Figure 1, and two noninteracting QDs embedded in the AB interferometer. QDs can be realized in the twodimensional electron gas of an AlGaAs/GaAs heterostructure, in which a tunable tunneling barrier between the two dots is formed by using two gate voltages. So we can set t_{c }as the coupling between the two QDs, which can be modulated by using the gate voltages [1]. The RSOI is assumed to exist inside QDs, which can produce two main effects including a spindependent extra phase factor in the tunnel matrix elements and interlevel spinflip term [47,48]. In the present paper, we only consider the first term because of only one energy level in each QD. When a temperature gradient ΔT between the two metallic electrodes is presented, a spindependent thermoelectric voltage ΔV_{↑(↓) }emerges. The proposed spindependent thermoelectric AB interferometer can be described by using the following Hamiltonian in a secondquantized form as,
Figure 1. (Color online) Schematic diagram for a thermoelectric device based on a double QD AB interferometer in the presence of magnetic flux Φ. A spindependent thermoelectric voltage ΔV_{σ }is generated when a temperature gradient ΔT is presented, where μ is the chemical potential of the metallic electrodes, and T is the temperature of the metallic electrode.
where is the creation(annihilation) operator for an electron with energy ε_{αkσ}, momentum k and spin index σ in electrode α. The electrode α can be regarded as an independent electron and thermal reservoirs, which can be described by using the FermiDirac distribution such as f_{α }= 1/{exp[(ε  μ_{α})/(k_{B}T_{α}) + 1}. Here k_{B }is the Boltzmann constant. creates (destroys) an electron with energy ε_{n }and spin index σ in the nth QD. t_{c }describes the tunnel coupling between the two QDs, which can be controlled by using the voltages applied to the gate electrodes [1]. The tunnel matrix element V_{ασn }in a symmetric gauge is assumed to be independent of momentum k, and it can be written as , , , , with the AB phase ϕ = 2πΦ/Φ_{0 }and the flux quantum Φ_{0 }= h/e. Φ can be calculated by the equation , where B is the magnetic field threading the AB interferometer and S is the corresponding area of the quantum ring consisting of the double quantum dots and metallic electrodes. The value S may be obtained in the previous wellknown experimental work [1]. So the magnitude of the magnetic field B is 16.4mT when ϕ = 2π. In the absence of the RSOI, the work will come back to the previous work [24], in which a 2πperiodic linear conductance is obtained, and it is in good agreement with the experimental work [1]. φ_{R }denotes the difference between φ_{R1 }and φ_{R2}, where φ_{Ri }is the phase factor induced by the RSOI inside the ith QD.
In the steady state, using the Green's functions and Dyson's equations, the electric current with spin index σ through the AB interferometer can be calculated by [49],
and the thermal current with spin index σ from the electrode α is calculated by [50],
where τ_{σ}(ε) is the transmission probability of electron with spin index σ, which can be given by . The spindependent linewidth matrix describes the tunnel coupling of the two QDs to the left (right) metallic electrode, which can be expressed as,
where . is the 2 × 2 matrix of the fourier transform of retarded QD Green's function, and its matrix elements in the time space can be defined as , where Θ(t) is the step function. The advanced dot Green's function can be obtained by the relation .
We consider the quantum system in the linear response regime such as an infinitesimal temperature gradient ΔT raised in the right metallic electrode, which will induce an infinitesimal spindependent thermoelectric voltage ΔV_{σ }since the two tunneling channels related to spin are opened. We divide the tunneling current into two parts: one is from the temperature gradient ΔT, which is calculated by ; the other is from the Seebeck effects, which can be calculated by . The spindependent Seebeck coefficient S_{σ }can be calculated by [50],
After expanding the FermiDirac distribution function to the first order in ΔT and ΔV_{σ}, we obtain the spindependent Seebeck coefficient by S_{σ }= ΔV_{σ}/ΔT as,
where . f = {1 + exp[(ε  μ)/(k_{B}T)]}^{1 }denotes the zero bias fermi distribution (μ = μ_{L }= μ_{R}) and zero temperature gradient (T = T_{L }= T_{R}). The spindependent Seebeck effects can be measured in the experiments as the following descriptions. First, the AB interferometer based on DQD molecular junction can be realized by using a twodimensional electron gas below the surface of an AlGaAs/GaAs heterostructure [1]. The RSOI in the QD can be introduced by using an asymmetricalinterface electric field. The temperature of the left electrode is kept at a constant, and that of the right electrode can be heated to a desired temperature by using an electric heater. So a temperature gradient can be generated in the DQD molecular junction. Second, the spindependent thermoelectric voltage can be measured by using the spindetection technique involving inversespinHall effect [51,52]. Accompanying the electric charge flowing, the energy of electrons can also be carried from one metallic electrode to the other metallic electrode. In the linear response regime (μ_{L }= μ_{R }= μ), we assume that an infinitesimal temperature gradient ΔT is raised in the right metallic electrode, and the heat current is divided into two parts following one from the temperature gradient and the other from the Seebeck effects . They can be obtained by the equations and . The total thermal current can be calculated by the sum of two terms as [50],
The corresponding electronic thermal conductance κ_{el }can be defined by . After expanding the FermiDirac distribution function to the first order in ΔT and ΔV_{σ }to Eq. (7), we obtain the electronic thermal conductance from the temperature gradient,
and the electronic thermal conductance from the Seebeck effects,
The differential conductance with spin index σ may be expressed as . In the linear response regime, the charge and spin figureofmerits (FOMs) can be defined as,
and
respectively, where and . In this study, the phonon thermal conductance of the junction, which is typically limited by the QDselectrode contact, has been ignored in the case of the poor link for phonon transport.
Results and discussion
In the following numerical calculations, we set Г = 1ev as the energy unit in this paper. For simplicity, the energy levels of QDs are identical (ε_{1 }= ε_{2 }= 0).
In Figure 2, we plot the spindependent transmission probability τ_{σ}, spindependent Seebeck coefficient S_{σ}, and spindependent Lorenz number as functions of the chemical potential μ under several different values of ϕ at room temperature (T = 300 K). The phase factor ϕ_{R }due to the RSOI inside the QD is fixed at , which is reasonable in semiconductor heterostructures [5154]. We first consider the case of the AB interferometer with symmetrical parallel geometry λ = 1 and a magnetic flux ϕ threading through the AB interferometer. When the interdot tunnel coupling is considered (t_{c }= Г_{0}), the transmission probability τ_{σ }has an exact expression,
Figure 2. (Color online) Spindependent transmission probability τ_{σ}(logarithmic scale), spindependent Seebeck coefficient S_{σ}, and spindependent Lorenz number L_{σ }(in units of as functions of the chemical potential μ under different values of ϕ at room temperature (T = 300 K). The black solid (red dashed) lines in (a n), (b n) and (c n) (n = 1,..., 6) represent spinup (spindown) transmission probability, spinup (spindown) Seebeck coefficient, and spinup (spindown) Lorenz number, respectively.
where . After a simple derivation, the transmission probability τ_{σ }has an approximate expression as,
where and . The parameter, q_{± σ }= ± t_{c}/Г_{∓σ}, describes the degree of electron phase coherence between two different paths. For example, one is the path through the bonding molecular state, and the other is the path through the antibonding molecular state. Г_{± σ }is the expanding function due to the coupling between the bonding (antibonding) molecular state and metallic electrodes, which is given by . When the spindependent electron phase is considered, the transmission spectrum is composed of four resonant peaks, and their asymmetrical degrees can thus be marked by the parameter q_{± σ}. In the absence of the interdot tunnel coupling (t_{c }= 0), a symmetrical transmission node (q_{± σ }= 0) arising from the quantum destructive interference is obtained. In the presence of the interdot tunnel coupling (t_{c }= Г_{0}) and absence of the magnetic flux (ϕ = 0), the relation between the spinup and spindown phase factors owns ϕ_{↑ }= ϕ_{↓}. The transmission probability τ_{σ}, Seebeck coefficient S_{σ }and Lorenz number L_{σ }become spinindependent as shown in Figure 2), 1), and 1), respectively. In this case, the transmission probability τ_{σ }as a function of the chemical potential displays a near symmetrical BreitWigner peak centered at the bonding molecular state and an asymmetrical Fano line shape centered at the antibonding molecular state. The degree of the asymmetry of the FanoLike peak can be attributed to the electron phase coherence. In the table 1, we calculate the approximate values of q_{± σ }of four resonate peaks for different AB phase ϕ with ϕ_{R }= 0.5π. For ϕ = 0, we find q_{+↑ }= q_{+↓}≃ 6.8 (near symmetrical BreitWigner peak at energy t_{c}) and q_{↑ }= q_{↓}≃ 1.2 (FanoLike peak at energy t_{c}). According to Eq. (12), an asymmetrical transmission node centered at energy t_{c}/cos(ϕ_{R}/2) can be found as shown in Figure 2 (a1). So we find that Seebeck coefficient S_{↑ }= S_{↓ }is enhanced strongly in the vicinity of the asymmetrical transmission node, and the corresponding value of Lorenz number L_{↑ }= L_{↓ }in units of L_{WF }at the asymmetrical transmission node approaches to a temperatureindependent value of 4.2 [55]. Once the AB phase ϕ is presented, the asymmetrical transmission node splits into two spindependent asymmetrical transmission nodes at energies t_{c}/cos(ϕ_{σ}/2). S_{↑ }and L_{↑ }are enhanced strongly in the vicinity of energy t_{c}/cos(ϕ_{↑}/2), and S_{↓ }and L_{↓ }are enhanced strongly in the vicinity of energy t_{c}/cos(ϕ_{↓}/2). Some interesting features in table 1 and Figure 2 should be noted as the following expressions. First, q_{± σ }has a negative value when the spindependent molecular states are located at the high energy region, while q_{± σ }has a positive value when they are located at the low energy region. We also find that the region of the enhanced thermoelectric effects appears at the molecular states with the lower value of q_{± σ}. For example, when ϕ = 0.25π and ϕ_{R }= 0.5π, S_{↑ }is enhanced strongly in the vicinity of the molecular states with q_{↑ }= 1.0, and S_{↓ }can be enhanced strongly in the vicinity of the molecular states with q_{↓ }= 1.4. Second, S_{σ }always has a larger positive value when q_{± σ }< 0, and S_{σ }has a smaller negative value when q_{± σ }> 0. The last feature is that one spin component of Seebeck effects can be tuned while the other spin component is retained. The behind reason is that the behavior of the spindependent transmission as a function of the chemical potential is dominated by the level expanding functions Г_{± σ}, which gives rise to a similar behavior of the Seebeck effects as a function of the chemical potential.
Table 1. Approximate values of q_{± σ }for various different values of ϕ
In Figure 3, we calculate , Z_{C}T and Z_{S}T as functions of the chemical potential for the different values of ϕ. The results show that and τ_{σ }has a similar behavior due to in the lower temperature region. has a negative value for the whole energy region due to , and it should be noted that has an obvious negative value in the vicinity of transmission peak with q_{± σ} ≃ 1.0 as shown in Figure 3 (a2), (a3), (a4), (a5), and (a6). Z_{C}T and Z_{S}T are enhanced strongly in the vicinity of transmission peaks with q_{± σ} ≃ 1.0 and q_{± σ} ≃ 1.4. The magnitude of Z_{S}T can approach to that of Z_{C}T in the vicinity of transmission peaks with q_{± σ} ≃ 1.4. The results indicate that a near pure spin thermoelectric generator can be obtained by tuning the AB phase ϕ with a fixed value of ϕ_{R}.
Figure 3. (Color online) Spindependent electronic thermal conductance and , charge FOM Z_{C}T and spin FOM Z_{S}T as function of the chemical potential μ under several different values of ϕ at room temperature (T = 300 K). Thick black solid (red dashed) lines in [an(n = 1,..., 6)] denotes spinup electronic thermal conductance . Thin black solid (red dashed) lines in [an(n = 1,..., 6)] denotes spindown electronic thermal conductance . The black solid lines in [bn(n = 1,..., 6)] represent the charge FOM Z_{C}T, and the red dashed lines in [bn(n = 1,..., 6)] represent the spin FOM.
A detail study of the spindependent thermoelectric effects is presented in Figure 4 when the configuration of the AB interferometer evolves from a symmetrical parallel geometry to a series. The AB phase ϕ and ϕ_{R }are chosen an identical value ϕ = ϕ_{R }= π. The spindependent transmission probability τ_{σ }has the following expression as,
Figure 4. (Color online) Spindependent transmission probability τ_{σ}(logarithmic scale) and spindependent Seebeck coefficient S_{σ }as functions of the chemical potential μ in the presence of different values of λ at room temperature (T = 300 K). ϕ_{R }and ϕ have same values as ϕ_{R }= ϕ = π. The black solid lines represents the spinup component, and the red dashed lines represents the spindown component.
where . When λ = 1, we have a simple expression for τ_{σ }as,
where + for spin up and  for spin down. Eq. (15) shows the symmetrical spindependent BreitWigner peaks centered at ± t_{c }as shown in Figure 4). The corresponding q_{↑ }and q_{+↓ }become infinity (see table 2). When λ = 0, the two QDs in a serial configuration are sandwiched between two metallic electrodes, in the case, the linear transmission probability become spinindependent due to the absence of the AB phase. The transmission probability can be calculated by the following expression,
Table 2. Approximate values of q_{± σ }for various different values of λ
We note that the transmission probability vanishes when t_{c }= 0, which means the full reflection for electrons happening in this AB interferometer. When 0 < λ < 1, the spindependent transmission probability τ_{σ }is composed of near BreitWigner peak and Fano line shapes as shown in Figure 4 and 3. The spindependent transmission probability can be approximated by,
where and with . From Eq. (14), we can see clearly that there are two asymmetrical transmission nodes centered at,
where + means spin up case and  represents spindown case. As a result, we find that the spindependent Seebeck effect is enhanced strongly in the vicinity the spindependent transmission nodes. The electronic thermal conductance , Z_{C}T and Z_{S}T as functions of the chemical potential under different values of λ are displayed in Figure 5. has a similar behavior with the transmission probability as the chemical potential changes. has an obvious negative values in the vicinity of the spindependent transmission node. Similarly, Z_{C}T and Z_{S}T are enhanced strongly in the vicinities of the transmission nodes. As λ increases from 0 to 1, we find the maximum values of Z_{C}T and Z_{S}T become larger. The corresponding q_{+↓ }and q_{↑} decrease, while q_{+↑ }and q_{↓} increase as λ increases (see table 2).
Figure 5. (Color online) Spindependent electronic thermal conductance and , charge and spin figure of merit Z_{C}T and Z_{S}T as function of the chemical potential μ under several different values of λ at room temperature (T = 300 K). Thick black solid (red dashed) lines in [an(n = 1,..., 4)] denotes spinup electronic thermal conductance . Thin black solid (red dashed) lines in [an(n = 1,..., 4)] denotes spindown electronic thermal conductance . The black solid lines in [bn(n = 1,..., 4)] represent the charge FOM Z_{C}T, and the red dashed lines in [bn(n = 1,..., 4)] represent the spin FOM.
Summary
We investigate the spindependent thermoelectric effects of parallelcoupled DQDs embedded in an AB interferometer in which the RSOI is considered by introducing a spindependent phase factor in the linewidth matrix elements. Due to the interplay between the quantum destructive interference and RSOI in the QDs, an asymmetrical transmission node can be observed in the transmission spectrum in the absence of the RSOI. Using an inversion asymmetrical interface electric field, we can induce the RSOI in the QDs. We find that the asymmetrical transmission node splits into two spindependent asymmetrical transmission nodes in the transmission spectrum, which induces that the spindependent Seebeck effects are enhanced strongly at different energy regimes. We also examine the evolution of spindependent Seebeck effects from a symmetrical parallel geometry to a configuration in series. The asymmetrical couplings between the QDs and metallic electrodes induce the enhancement of spindependent Seebeck effects in the vicinity of the corresponding spindependent asymmetric transmission node in the transmission spectrum.
Abbreviations
2DEG: twodimensional electron gas; AB: AharonovBohm; FOMs: figureofmerits; QD: quantum dot; RSOI: Rashba spinorbit interaction.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
All authors read and approved the final manuscript.
Acknowledgements
The authors thank the support of the National Natural Science Foundation of China (NSFC) under Grants No. 61106126, and the Science Foundation of the Education Committee of Jiangsu Province under Grant No. 09KJB140001. The authors also thank the supports of the Foundations of Changshu Institute of Technology.
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