Thermoelectric Efficiency of a Topological Nano-Junction.

We studied the non-equilibrium current, transport coefficients and thermoelectric performance of a nano-junction, composed by a quantum dot connected to a normal superconductor and a topological superconductor leads, respectively. We considered a one-dimensional topological superconductor, which hosts two Majorana fermion states at its edges. Our results show that the electric and thermal currents across the junction are highly mediated by multiple Andreev reflections between the quantum dot and the leads, thus leading to a strong nonlinear dependence of the current on the applied bias voltage. Remarkably, we find that our system reaches a sharp maximum of its thermoelectric efficiency at a finite bias, when an external magnetic field is imposed upon the junction. We propose that this feature can be used for accurate temperature sensing at the nanoscale.

1. Introduction

Topology provides a fertile conceptual framework to many fields of modern physics, ranging from cosmology to condensed matter, but it is in the later where it has found its most powerful applications. Up to date, several topological phenomena have been discovered and studied in the context of solid state physics: The quantum spin Hall effect, topological insulators and Weyl semimetals, to name a few, and hence this constitutes a very active field of research.

An important class of topological materials which are currently gathering a lot of attention are topological superconductors (TS), mainly due to their potential application in topological quantum computers [1,2,3]. They were first proposed theoretically, in 2D [4] and 1D [5] models, where a spinless p-wave superconductor supports non-Abelian anyons or Majorana zero modes (MZM) in the vortex core in the 2D case [6], or at the edges in the one dimensional scenario. In principle, the presence of a Majorana bound state at the surface of a TS can be detected experimentally by the zero-bias anomaly [6,7,8,9,10]. In 2010 TS were experimentally fabricated [11,12] using a one dimensional semiconducting nano-wire with strong spin-orbit coupling, in proximity to an s-wave superconductor. Experimental evidence for the emergence of Majorana fermions in these systems has been reported [13]. Due to their nonlocal topological properties, Majorana fermions are protected against perturbations and different sources of local noise, thus representing a physical principle to implement fault-tolerant quantum computation.

While the electrical properties of topological materials have been intensively studied [6,7,8,9,11,12,14,15], thermoelectric performance of these topological based devices has not yet received the same level of attention from the condensed matter community, besides some interesting examples [15,16,17,18]. The heat production due to Joule heating in nano and micro-devices is one of the biggest problems in current microelectronics, and there is a growing interest on new materials capable of mitigating energy dissipation. In this context, proper thermal energy management and active cooling are nowadays highly required for the advance of new technologies [19,20,21,22]. In this work, we study a nano-device based on a topological superconductor, which can be used as a nano-thermocouple controlled by a weak magnetic field at small applied voltages. Our theoretical analysis suggests that this configuration can provide the basis for a highly sensitive temperature sensor, which can be used to detect Joule heating in micro-devices with nanometer scale resolution.

Along this article, we shall describe the transport properties and thermoelectric performance of a hybrid nano-junction formed by a quantum dot connected to an s-wave superconductor and a topological superconductor leads, respectively, as described in [14]. The Hamiltonian describing this model is presented in Section 2. The s-wave superconducting lead is submitted to a finite bias voltage, such that the BCS superconducting order parameter acquires a time-dependent Josephson phase, and therefore the junction is out of equilibrium. In Section 3 we apply the Keldysh formalism (described in Appendix A) to study the electrical current through our system. The current-voltage characteristics and the thermoelectric performance are presented in Section 4, for different externally applied magnetic fields. Finally, we conclude with the main discussion in Section 5. In Appendix B we provide the derivation of quantum-mechanical transport coefficients in the context of irreversible thermodynamics [23,24,25] and in analogy with Onsager reciprocal relations [24,26,27,28].

2. The Hybrid Nano-Junction

The Model

The system that we shall study along this work was first proposed in [14]. As sketched in Figure 1, it consists on a hybrid nano-junction composed by a semiconductor quantum dot (QD), connected to a normal s-wave superconductor (SC) and a one dimensional topological superconductor (TS), that play the role of the left (L) and right (R) leads, respectively. The system is represented, in second quantization, by the Hamiltonian

Figure 1

Sketch of the nano-junction.

Due to the presence of the normal superconductor, the Hamiltonian is conveniently represented in spin and Nambu subspaces by a matrix. The first contribution corresponds to the BCS Hamiltonian in the standard and symmetric Bogoliubov-de Gennes form where is the BCS order parameter of the superconductor. On the other hand, the QD contribution is given by where is the Zeeman-coupling to an external magnetic field h, whereas is the local energy level at the quantum dot. Finally, the tunneling Hamiltonian, which couples the QD to the normal (L) and topological superconductor (R) leads via the coupling constants , respectively, is given by

Here, we have defined and ,with , the unit and Pauli matrices acting on the spin () and Nambu () subspaces, respectively. In this spin-Nambu basis, the fermion operators and are organized in a 4-component vector , with an identical structure for the QD fermion operators and . On the other hand, the Majorana operator is associated with the spinor , where is a constant phase [14]. For simplicity we will consider , and neglect the Coulomb interaction between electrons in the QD. Even though a TS hosts two Majorana Bound states (MBS) at each one of its ends, assuming a very long, semi-infinite lead, the tunneling problem can be reduced to that of transport through the SC-QD-MBS junction only.

3. Non-Equilibrium Transport Through the Nano-Junction

Let us consider a finite bias voltage V imposed upon the s-wave superconductor lead. In consequence, the BCS order parameter acquires a time-dependence , where is a Josephson phase [7,14]. Because of this periodic time dependency, transport across the junction is highly non-linear and thus cannot be described by the standard Green-Kubo relations [23,29,30], which are strictly valid only for the linear response regime and in agreement with the fluctuation-dissipation theorem [29,30]. Therefore, transport in this quantum mechanical system must be analyzed with a formalism appropriate for non-equilibrium regimes, such as the reduced density matrix [7], equations of motion [15] or, as we chose in this work, non-equilibrium Green’s functions, also referred as the Keldysh formalism in condensed matter theory [24,25,31,32]. For readers which are not familiar with the method, we present a description of it in Appendix A.

For the nano junction described by the Hamiltonian Equation (1), the current operator is defined via continuity equation at the left SC lead by the expression where is the number operator of the conventional superconductor SC.

Starting from the full Hamiltonian Equation (1), we construct the effective action for the system on the Keldysh contour, following the argument presented in Equation (A16), by defining a source term involving the current in Equation (7)

This source couples to the effective action via the first Pauli matrix , thus breaking the symmetric time evolution on the Keldysh contour.

After integrating out the fermionic fields of the superconductor () and the quantum dot (), we obtain an effective action in terms of the Green’s function of the Majorana fermions at the TS. Therefore, the reduced generating functional, including the source, can be expressed as with the Green’s function of the TS after the integration of the SC and QD fields. When expressed in the form of a Dyson equation, we have with the TS self-energy given by and the non-interacting TS Green’s function components defined on the Keldysh space (A: Advanced, R: Retarded, K: Kinetic) as

On the other hand, the QD’s Green’s function is given by

Here, , with and the non-interacting QD Green’s function components on the Keldysh space are given by

The QD and TS kinetic Green’s functions are identically zero, because in the absence of self-energy terms for these non-interacting subsystems, they are proportional to the complex regulator and hence they vanish in the limit , in agreement with the fact that non-interacting systems are dissipationless [29,32]. Finally, the SC Green’s function in equilibrium () is with and functions obtained from the standard BCS theory by [32] whereas the kinetic component, assuming that the SC is a macroscopic system at finite temperature T, is given by Equation (17) (see Equation (A29) and Appendix A for details).

Non-Equilibrium Green’s Function in the Floquet Basis

At non-zero bias, the SC Green’s function depends on time due to the Josephson phase of the order parameter Equation (6), via the transformation with . Moreover, the time-dependence of the Josephson phase of the normal SC lead in Equation (6), determines a periodic time-dependence on the Green’s functions, and hence a Floquet representation [33] in Fourier space is the natural choice. Moreover, when Fourier transforming to the energy domain it is clear that, as a consequence of the voltage dependence of the Josephson phase, the Green’s functions acquire an energy shift , which we express as

Each , for , is a matrix, with indices . This motivates the introduction of new indices M, N to designate the matrix elements associated to the Floquet basis in each of the Green’s function components with , and D representing the Floquet mode. Therefore, the superconductor Green’s function in the Floquet basis is a block matrix expressed as where

Taking functional derivatives of the generating functional with respect to the source terms (see Appendix A), we obtain the current flowing across the nano-junction where and Tr implies taking the trace over Keldysh and Floquet subspaces, respectively. Similarly, the energy flux through the nano-junction is given by the expression

4. Results

4.1. Electrical Current

For the current J - voltage V characteristics, we considered a bias in the range , with the BCS gap of the normal superconductor. The total number n of Floquet modes to be included in the numerical evaluation of the formulas was adjusted such that the calculated electrical current converged. In our case, we found that the first Floquet modes were sufficient to achieve the convergence criteria. The contribution of the first mode is notoriously small because the Green’s function has only diagonal elements, with the off-diagonal elements being present only for the Floquet modes. The current through the nano-junction is shown in Figure 2, where the oscillating behavior as a function of bias V is a consequence of the presence of multiple Andreev reflections (see Figure 3), responsible for tunneling from the quantum dot to the non-trivial topological superconductor.

Figure 2

Tunneling current through the nano-junction for zero (a) and (b) magnetic field. We set and the coupling constants and with . Here represents the BCS order parameter, chosen as the natural energy scale in the model.

Figure 3

(a) A Cooper pair suffers an AR at the interface with the QD (red dashed line), and two AR’s at the TS interface and at the SC interface (dotted line) respectively, thus generating Andreev bound states; (b) The Cooper pair splits into two electrons of opposite spin, in order to occupy the dot’s level, thus producing two backscattered holes at the TS interface. The AR of an electron (hole) is equivalent to the transfer of a single Cooper pair in (out) of the superconducting condensate.

The current is notoriously sensitive to small changes in the externally applied magnetic field. Indeed, the Zeeman energy splitting of the dot energy level contributes to a local spin filtering effect for the current, as seen in the highest peak in the inset of Figure 2. For small magnetic fields, the principal peak will be present, varying smoothly its position, but for the highest Zeeman-splitted level will not be reachable for electron states at the dot, thus reducing the transport rate. As a consequence, configurations with single electronic occupation at the dot dominate transport in this high-field regime in favour of double occupation, as verified from the local dot’s occupation number where is the Fermi distribution function. In Figure 4, we represent the bias dependence of the local occupation number at the quantum dot, at different values of the external magnetic field expressed in terms of the Zeeman splitting H.

Figure 4

Occupation number for different values of the magnetic field h, with effective Zeeman coupling . The curves represent the dot occupation at (dotted line), (solid line) and (dashed line), respectively, with the BCS order parameter, chosen as the natural energy scale in the model.

4.2. Thermoelectric Performance

In order to study the thermoelectric properties of the nano-junction, we need to consider the temperature dependence of the electric current. We assume that the s-wave superconductor (L-lead) is in local thermal equilibrium at temperature , while the topological superconductor (R-lead) is at zero temperature , such that the thermal flux is generated by the temperature difference between the leads . As before, we set . Transport coefficients are usually defined within the linear response regime [23,27,29,30,34], where Onsager relations [23,26,27,30] and Green-Kubo [25,29,32] formulas can be applied. For non-equilibrium quantum systems with strong non-linearities, however, it is possible to define generalizations of those transport coefficients [24]. In particular, for electric transport in non-linear systems it is customary to define the differential conductance [24,35,36]

For thermal transport coefficients, starting from the entropy flux , it is possible to define a thermal flux , such that differential thermal conductance in the nonlinear response regime [24] becomes (see Appendix B for details)

The condition of vanishing electrical current involved in the definition of the thermal conductance, establishes an implicit (nonlinear) relation between the temperature difference and the bias voltage across the junction, i.e., . Therefore, applying the implicit function theorem for partial differentiation, we have that under the condition

Solving from the equation above, we obtain a natural generalization of the Seebeck coefficient to the nonlinear response regime [24],

The differential conductance in Equation (25) follows an operational definition that trivially matches the linear response value in the limit . For the generalized nonlinear Seebeck coefficient and thermal conductance, despite they clearly match the standard definitions in the linear response limit [24,27,28,37], a more careful analysis is required to justify them from the perspective of entropy flow and production in the context of irreversible thermodynamics [23,24,26,30], as presented in Appendix B.

The thermoelectric performance of the nano-junction is represented by the thermoelectric figure of merit, commonly expressed by the dimensionless quantity [20,21,27,37,38] where and are the electrical and thermal differential conductances defined by Equations (25) and (26), respectively, and is the Seebeck coefficient calculated from Equation (28).

Figure 5 shows the thermoelectric figure of merit of the junction for different bias voltages, reaching its maximum at eV/ when (the inset). The magnetic field breaks the electronic spin symmetry, giving rise to different energy contributions for spin up or spin down fermions: electrons with a spin parallel to the field dominate in the whole transport process as compared to those with anti-parallel spin, thus leading to a spin-filtering effect in the current through the junction. This filtering effect is reflected on the sharp thermal response of the figure of merit at finite magnetic fields, as seen in Figure 5. Until now it has been very hard to achieve high in thermoelectric devices, because and S cannot be independently controlled in the linear response regime: a material with large electrical conductivity has a large thermal conductivity when the Wiedemann-Franz law applies [24,27,37]. It is possible, however, to achieve some decoupling in materials where electron-phonon scattering is weak, while phonon-phonon scattering is high [19,20,21,22]. In this system, the comparatively small values of are a consequence of the Seebeck coefficient of the device, due to the weak dependence of the electrical current on the temperature (see Equation (28)). This feature is not so surprising, considering that the model does not involve lattice phonons besides the BCS Hamiltonian for the normal s-wave superconductor, where they are implicitly involved through the BCS gap .

Figure 5

Thermoelectric figure of merit as a function of temperature at a fixed bias of 0.7 eV/, at different magnetic fields h, with the effective Zeeman coupling (a). Notice the sharp response at at an applied magnetic field (b). Here represents the BCS order parameter, chosen as the natural energy scale in the model.

We remark the strong and sharp response of the thermoelectric performance of the system as a function of temperature, when , as seen for instance in Figure 5b. We propose that this interesting feature can be applied for thermal sensing with nano-metric spatial resolution in microelectronics and future nano-electronics, in order to mitigate and prevent the deleterious effects of excessive Joule heating in device components.

5. Discussion

Along this work, we studied the non-equilibrium electric and thermal transport through a topological nano-junction, in the context of the Keldysh formalism, by exploring the non-linear response of the junction at different applied magnetic fields. Due to the diversity of quasi-particle excitations present in the system, such as Majorana fermions and Cooper pairs, the electrical current presents a high nonlinear behavior as a characteristic signature of virtual processes known as multiple Andreev reflections between the leads and the quantum dot (see Figure 3), in agreement with the literature [7,8]. The transport coefficients in the nonlinear regime are defined as generalizations of the ones in linear response, closely following the analysis in [24]. In particular, the definition of thermal transport coefficients are tied to the notion of entropy production and fluxes through the junction. We have provided a detailed discussion on the implication of those definitions in the context of classical Onsager reciprocal relations [23,26,30], particularly on the restrictions imposed by the positive-definiteness of the entropy production rate [23,24,26,27,30] (see Appendix B for details).

As seen in Figure 6, the Lorenz number for this system strongly differs from the Wiedemann-Franz law, which is only valid at low temperatures and in the linear-response regime [27,37]. On the first hand, as stated in Section 3, the junction corresponds to a system which is far from the linear response regime, due to the oscillatory effect of the bias voltage on the Josephson phase of the BCS superconducting lead. Therefore, the system is far from the domain of applicability of Wiedeman-Franz law [24,27,37]. In addition, as depicted in Figure 3, the multiple Andreev reflections tend to decouple the net particle flow from the electric charge flow. While the energy flux is always in the same direction as the particle flux, the charge flow may have the opposite direction. Consider for instance the process depicted in Figure 3b, where an electron pair flows from left to right, representing two quanta of negative charge in this direction, while a pair of positively charged holes is backscattered in the opposite direction: The net charge flow associated to this process is equivalent to four quanta of negative charge transported from left to right. However, the energy carried by electrons and holes, that determines the heat flux associated to the process, moves in counterflow for both types of particles. This decoupling between the electric and thermal flux directions is what leads to an extremely nontrivial behavior of the Lorenz number, thus exhibiting strong deviations from the Wiedemann-Franz law.

Figure 6

Lorenz number for . The minimum at coincides with the sharp response in the thermoelectric figure of merit in Figure 5.

The applied magnetic field imposes a filtering effect over the electronic spin components of the current, thus suppressing transport of the antiparallel spin component at sufficiently strong fields. The magnetic field also contributes to the thermoelectric efficiency: at higher magnetic fields, the higher the electrical-heat conversion efficiency. We reported a maximum efficiency of at a bias of 0.7 eV/, for an applied magnetic field of . We remark that the sharp thermoelectric response obtained at finite magnetic fields, could be used to construct a highly sensitive temperature sensor based on this topological nano-junction. A nanoscale thermocouple with nanometer-scale resolution may be an important contribution to mitigate and prevent the deleterious effects of excessive Joule heating in nano and micro-device components.