On Behind the Physics of the Thermoelectricity of Topological Insulators
Daniel Baldomir Daniel Faílde









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 On Behind the Physics of the Thermoelectricity of Topological Insulators 
    Daniel Baldomir^1∗ and Daniel Faílde^1



^1Departamento de Física Aplicada, Instituto de Investigacións Tecnolóxicas. Universidade de 

Santiago de Compostela E-15782 Campus Vida s/n, Santiago de Compostela, Spain



^∗daniel.baldomir@usc.es

    
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  Topological Insulators are the best thermoelectric materials involving a sophisticated physics beyond their solid state and electronic structure. We show that exists a topological contribution to the thermoelectric effect that arise between topological and thermal quantum field theories applied at very low energies. This formalism provides us with a quantized topological mass proportional to the temperature T, being both quantities directly related with an electric potential V and getting a Seebeck coefficient where we identify an anomalous contribution that we associate to the creation of real electron-hole Schwinger's pairs close to the topological bands. Finally, we find a general expression, considering the electronic contribution, for the dimensionless figure of merit of these topological materials, getting a value of 2.73 that is applicable to the Bi_2Te_3, for which it was reported a value of 2.4, using only the most basic topological numbers (0 or 1).







§ INTRODUCTION


Nowadays topological insulators (TI) are the best thermoelectrics (TE) at room temperature,<cit.> specially if they are combined with nanotechnological structures which are able to reduce the phononic thermal conductivity. A good example is<cit.> the bismuth telluride, Bi_2Te_3, that has a small band gap giving a good number of carriers at room temperature (300K) and reaching 2.4 for its dimensionless figure of merit ZT for p-type using alternating layers in a superlattice with Sb_2Te_3. This is the highest value of thermoelectricity ever observed <cit.> so far at room temperature. Despite the fact that the electronic structure of these materials was exhaustively studied  <cit.>  in relation with their thermoelectricity<cit.>, there is still lacking in the literature a physical model<cit.> which could take explicitly into account their common topological and physical features. Its importance might appears obvious with a counterexample, the Pb_1-xSn_xTe has a good electronic structure to be a topological insulator with thermoelectricity <cit.>, but due to have an even number of band inversions, this prevents it to have time reversal symmetry T̂ and thus to be a TI, although it is a good thermoelectric at higher temperatures. Hence, the whole topology of a TI is not fully necessary for having good thermoelectricity as we are going to see. Understanding the physics behind these phenomena is not an easy task because links scientific branches which were developed independently: particle physics, statistical mechanics, condensed matter and algebraic topology. This is a characteristic of materials which exhibit linear dispersion laws instead of quadratic ones, allowing a quantum field interpretation where the spinors play a fundamental role substituting the usual non relativistic wave function. 

We organize the paper as follows. We examine the basic concepts of topology and physics for topological insulators trying to show how they are related in a same structure. After that we show that the Riemann-Hurwitz formula plays an important role which was not considered in the literature so far. This allows us to find five topological regions which are connected by four bands  defined using the periodicity of the instanton solutions associated to the non-Abelian Berry fields introduced within the bulk of the TI. In the case that there is not time reversal symmetry T̂ we have only three topological regions, connected by only two bands. Finally we present a straightforward relationship between the temperature T and the topological index μ with the scalar electric potential V. This leads to a Seebeck coefficient for which we identify two terms, one related to the topological electron pump, and another associated to a change in the topological index that might be associated to the creation of real electron-hole pairs as we will analyze. We end with the calculation of a general expression for the edge states topological figure of merit in TI, taking into account the electronic contribution and neglecting the phononic part <cit.>.

Solid state physics allow us to tackle the problem of a crystal, with translation symmetry, reducing the analysis of the different physical properties to the first Brillouin zone. This is permitted thanks to Bloch theorem, i.e. for a periodic potential V(x+a)=V(x), where a is the spatial period or the lattice constant, the wave function associated to the electrons have also a periodicity ψ_k(x+a)= e^ikaψ_k(x). Being the eigenvalues ξ_k=ξ_k+T,ξ_k=ξ_-k also periodic in time. These are the necessary conditions for calculating the energy bands in a solid. It is easy to see that parity P̂, or space inversion, is straightforwardly followed, while time reversal symmetry T̂ is not so obviously fulfilled. For example, the Schrödinger equation iħ∂/∂ tψ(t) =H ψ(t) under time reversal gives iħ∂/∂ (-t)ψ(-t) =H ψ(-t), where ψ(-t) is not a solution due to the first order time derivative. This can be solved if the T̂ operator has also associated a complex conjugation K̂ operator. In fact, we must define T̂=UK̂ for spinless states where U is a unitary operator. In the case of having half integer spin particles, the unitary operator can be written in function of the σ_y Pauli matrix as U=exp(-iπ/2σ_y), which for spin 1/2, given that σ_y^2=1, allow us to write the time reversal operator as T̂=-iσ_y K̂, which acting on the multiparticle state gives T^2=1 for an even number of fermions or T^2=-1 when the number is odd. More generally written, T^2 has eigenvalue (-1)^2s for a particle of spin s, and if |n> is an energy eigenstate, then T|n> too, sharing the same energy and being orthogonal to each other. Thus, if there is an odd number of electrons there must be (at least) a twofold degeneracy; this is known as Kramers degeneracy. This is Kramers theorem, which when it is completed with the previous Bloch theorem for the bands, should provide us the first tools<cit.> to examine topologically the first Brillouin Zone (BZ) of a TI. 
  
In Fig.1 we represent schematically how the 2D BZ evolves under different translation operations transforming the square BZ into two equivalent cylinders S_1. Consequently, is also easy to see that combining the two lattice periodicities available in two dimensions, a 2D torus S^1 × S^1 can arises. That is, the existence of translation symmetry allow us to transform a topologically trivial square into a non-trivial torus with genus g≠0. Given that all crystals are able to develop these fundamental properties, non-trivial topology is not enough when we study TI and the presence of singularities which are usually counted on the band structure are needed. Obviously, without extra-information, these singularities break the translation symmetry or the periodicity of the Bloch states <cit.>. So, let's analyze a little bit the possibility of these translations symmetry breaking. Physically the infinitesimal translations are generated by the linear momentum operator L̂(dx)=1̂-i/ħp̂(dx), where [x̂,p̂]=i ħ1̂  follows the Heisenberg's principle of uncertainty. Extending this operation over the whole crystal one gets the finite translation operator L̂= lim_N →∞ [1̂-i/ħp̂ (a/N)]^N =exp (-i/ħp̂ a). Taking into account the periodicity of the crystal potential V(x)=V(x+a), which involves the periodicity of the states, we can write L_a|ψ(x)>=|ψ(x-a)>=exp(ika) |ψ(x)>   where k is the wavelength number associated the momentum p=ħ k. On the contrary, the presence of singularities make that |ψ(x-a)>=exp(ika) exp(i γ) |ψ(x)>, being γ the Berry phase, giving to electrons an anomalous phase factor during a complete cycle in the order parameter. Thus, in order to maintain the periodicity of the Bloch states, preserving the non-trivial topology that results for having a torus, singularities must take place on the edges of the crystal, where translation symmetry is no longer satisfied. Since singularities manifest in energy bands by means of Fermi points, bands are good hosts for feeding the non trivial topology of a material whose electronic structure is appropriate and which its surface offers the breaking of translation symmetry, besides other well known fields features for lower dimensions. In this way, the Berry phase joints the non-trivial topology of the crystal employing its curvature and connection on the bands<cit.> as we are going to see.




The topological elements of the TI have been found, but now it remains to see how they work together employing their associated invariants. The Riemann-Hurwitz formula <cit.>, which generalizes the Euler topological invariant, enable us to to construct an equation relating the genus g and g' of two compact surfaces, i.e., whose boundary is zero. Actually this formula establishes the conditions for a map f: T → S being subjective and holomorphic, reducing the several topological invariants introducing the genus of T, the genus of S, N the degree of the map f and the amount of ramifications e_f(k). Riemann used the mentioned formula in the case that S  were zero, i.e., spheres. Much later the proofs were given by Zeuthen and Hurwitz. The formula is 


    2(g_T -1)=2N(g_S -1)+∑_k∈ T (e_f(k)-1)


In our case we have a 2D torus T^2 with genus g_T=1 mapped in a S^3 sphere with genus g_S=0 and the degree of the map N=2 due to the Kramers double degeneracy, see Fig. 2. Hence we obtain five ramifications branches, i.e., e_f(k)=5. This can be directly related with the second Chern number as we shall see later, playing a fundamental role in the transformation of heat in electricity. The number of ramification branches diminish to three when there are not Kramer pairs, that is, N=1 with e_f(k)=3. But it is very remarkable to observe that this formula doesn't depend of the dimensions of the involved sphere or torus, which justifies us to work with a T^2 torus instead of a T^3 or T^4 without being worry about new results.


 
In the presence of singularities in the band structure the map c of Fig. 2 can be interpreted as the d_a map within the Hamiltonian H introduced<cit.> to study TI in (4+1)D,

    H=∑_kψ_k ^+ d_a (k) Γ^a ψ_k

being d_a (k)=(m+p∑_i cos k_i,sin k_x, sin k_y, sin k_z, sin k_w) and Γ's the Clifford matrices {Γ^μ , Γ^ν}= 2 g^μν 1_5 × 5. We can calculate then, the second Chern number C_2 associated to this Hamiltonian

    C_2 = 3/8 π^2ϵ^abcde∫dk^4 d̂_a (k) ∂d̂_b (k) /∂ k_x∂d̂_c (k) /∂ k_y∂d̂_d (k) /∂ k_z∂d̂_e (k) /∂ k_w

which is no more than the winding number resulting from the map d̂_̂â(̂k̂)̂≡d_a(k)/|d(k)|  of a four dimensional torus T^4 to a sphere S^4, having the mass m associated to the spinor's Γ's five critical values  given by the condition ∑_k,μ d_a^2=0 allowing us to identify, in the same way as the Riemann-Hurwitz theorem, five different branches for the second Chern number

    C_2 ≡μ={[  0  ,  m ∉ (-4p,4p); +1 ,  m ∈ (-4p,-2p);   -3 ,  m ∈ (-2p,0);    +3 ,  m ∈ (0,2p);   -1 ,  m ∈ (2p,4p);                     ].
 
where the parameter p is taken as p=2k_BT/v^2_F  and must be equal to the background kinetic energy of the particles for keeping its physical meaning. This parameter p=2k_BT/v_F^2 is chosen as a mass parameter that is mainly associated to the quantization of the temperature<cit.>, as we are going to see very soon. It is easy to see that for non-T̂ symmetric Hamiltonians there are only three ramifications available for the Second Chern number, in the same way as it happens when we break time reversal symmetry in a Quantum Spin Hall system. As we saw, this can be interpreted, in one straighforward form, into the Riemann-Hurwitz formalism as two different maps, where we change the degree of the map N from 2 to 1. 

Physically the non-Abelian Berry phase takes into account the allowed bulk degenerate states which are directly related with the change of temperature, as we shall see soon. First, we take the definition of the non-Abelian Berry connection a^αβ_j(k)=i<α,k|∂/∂_k_j|β,k> and the associated field (or curvature) f^αβ_ij=∂_i a^αβ_j - ∂_j a^αβ_i + i[a_i,a_j]^αβ and second Chern number C_2=1/32π^2∫ d^4k ϵ^ijmn tr(f_ij f_mn), where the indixes of the Levi-Civita tensor stand by i,j,m,n=1,2,3,4 and α refers to the occupied bands. This can be written within a pure gauge Yang-Mills formalism, where its solutions transform in general by a_μ=T^i a^i_μ and f_αβ=T^i f_αβ^i being the T^i's the generators of the inner symmetry group, which in our case will be SU(2) <cit.>. For a U ∈ SU(2) being position dependent, we have the gauge transformations for the potentials a_α→ a_α^U=U^-1a_α U + U^-1∂_α U and the fields f_αβ→ f_αβ^U=U^-1 f_αβ U. It is immediate to observe that the non-Abelian fields are not gauge invariant, at difference of what happens with the Abelian ones, which tell us that the curvature depends of the group representation but not the topological numbers. On the other hand the Lorentz invariant associated to them, ϵ^ijmn f_ijf_mn, classically doesn't appear in the action because it is a surface term negligible under variations, but quantum mechanically it must be introduced due to have the action on an exponent and being this term which accounts basically for the topology of the motion equations where they are defined. Thus, given that tr(ϵ^ijmn f_ijf_mn)=4ϵ^ijmn∂_i[tr(a_j∂_m a_n-2/3a_j a_m a_n)] this allows to define the Chern-Simons term W(a)=-μ/8π^2∫ d^3x ϵ^ijm tr(a_i∂_j a_m-2/3a_i a_j a_m) where μ is a topological mass, which is not gauge invariant under gauge transformations 


    W(a^U) → W(a)-μ/8π^2∫ d^3x[ ϵ^ijm∂_i tr(∂_j U U^-1 a_m)+μ/24 π^2∫ d^3x ϵ^ijmtr(U^-1∂_i U U^-1∂_j U U^-1∂_m U)]


The first term of the integral is a total derivative which can be made zero in a manifold without boundary, while the second, is an integral written in short as w(U), which provide us topological information of the manifold as a winding number. This integral is actually an integer number coming from the homotopy group π_3(S^3) for the SU(2) group and we can take the  Abelian Chern-Simons gauge symmetry associate the background electrodynamics, i.e., the U(1) as a subgroup of transformations close to the surface. In this case, we can rewrite the Chern-Simons transformation for both fields. Hence the transformation of the Chern-Simons can be rewritten as 

    W(a) → W(a)+ μ/12π 24π^2 w(U)=W(U)+2πμ w(U)


But as the path integral e^iW/ħ must be invariant, hence it means that μ is an integer. These are going to be the dynamic fields meanwhile the electromagnetic gauge potential A_α will appear as a background gauge field. In fact the electric field is due to the electric polarization P associated to the cooperative Berry phase within the energy bands of the material, whose topological origin is fundamental.

Let us go now to the thermodynamic part of the topological thermoelectricity, applying the instanton solution associated to the second Chern number developed above. Within this mathematical contest it is possible to make a direct relationship between thermodynamics and quantum formalism. Feynman path integral give us an expression for the amplitude of probability to evolve a particle from x_i(0) to x_f(t) a time later by <x_f|e^-iHt|x_i>=∫^x_f_x_iDx  e^-iS, where S is the classical action, which could be obtained by S=∫^t_0 dt'[p^2/2m-V(x)]. On the other hand, in Statistical Mechanics the partition function is defined in quite a similar form by Z(β)=tr e^-β H where β≡1/k_BT. There is a way to change from one to the other using the Wick trick, i.e., doing the time a pure complex variable and transforming the Minkowskian space-time in a Euclidean one. Thus, we have -iS=∫^-iτ_0dτ'[p^2/2m-V(x)]=S_E where S_E is the new Euclidean action. This fixes the concept of temperature relating it directly with the time by τ=ħβ making tr e^-β H=∫dx_i <x_i|e^-β H|x_i>=∫dx_i∫^x(β)=x_i_x(0)=x_iDx e^-S_E, where we have assumed a cyclic motion x(0)=x(β) i.e. the particle must come back where it started after Euclidean time τ=ħβ,<cit.>. This is exactly how the Berry's phase works and justify partially its introduction as a gauge field.

On the other hand, Berry's phase is directly related with the electric polarization P, i.e., Δ P=e/2π∫_0^τ dt ∫_-π/a ^π/a dk ∑_n ∈ occu f_n (k) being f_n (k) = i [∂/∂ k <ψ_nk (t)|∂/∂ t|ψ_nk (t)> -∂/∂ t <ψ_nk (t)|∂/∂ k|ψ_nk (t)>] and under a gauge transformation of the electromagnetic potentials A_μ→ A'_μ= A_μ + ∂_μΛ(x,t) the electron wave function transforms ψ→ψ=ψ e^ieΛ/ħ. The above exponential function needs to be single valued while the Λ(x,t) doesn't. Thus we can write it as Λ(x,t)=2πμħτ/eħβ=μ2πτ/eβ where μ is a winding number quantizing the temperature. This allows to find the electric potential directly related with the temperature by 

    V → V'=V+μ2π/ek_BT

which means that we have transformed the electric potential into another, plus a μ2π/eβ thermal term. This turns out to be a fundamental result: the thermal energy appears quantized by the winding number being added to one electric potential under a gauge transformation. Notice that a Chern-Simons term was necessary since it is not gauge invariant and appears as surface actions. This allows to have, in non-Abelian Chern-Simons, the coupling constant g, directly associated to the temperature, related to the Chern-Simons action transforming S_CS→ S'_CS=S_CS+2πħ^2μ/g^2 where μ is the topological mass.

Getting back to the topological electric potential equation (7), we can easy calculate the Seebeck coefficient S=μ2π/ek_B+2π/e∂μ/∂ T k_B T choosing V'=0. We can identify two contributions, the first, that comes purely from the temperature gradient in a topological branch, and the second, which takes into account the contribution due to a change in the topological index μ, i.e, a jump between ramifications. As we will see, the first term is equivalent to that we find when we compute the surface Seebeck coefficient while the second, doesn't appear since there is only one ramification available on the surface. Once we have seen that the instanton solutions allow us to relate the electric potential V with the temperature T and hence the electric field E with ∇ T assuming no spatial homogeneity due to have topological bands, we can give a microscopic picture of how the thermal energy is invested in electricity. The main idea is that there is a creation (Fig. 3) of electron-hole Schwinger's pairs <cit.>, provided that the electric field is big enough. Calculating the critical electric field E_c, following Heisenberg-Euler <cit.>, we notice that the limit for obtaining a great number of real electron-hole pairs is 


    E_c= m^ 2v_F^3/eħ≃ 0.152 V/nm

where we have considered that the energy gap is 0.21 eV and the Fermi velocity v_F≃ 6 × 10^5 m/s. This provides a critical electric field of almost ten orders of magnitude lower than the critical electric field in QED and with one equivalent temperature of ζ=1.74 × 10^-6 K nm^-1, being these values accesible in these topological materials at so small scales as at hundredths of volt at distances of angstroms. In this way, we can rewrite the second term of the Seebeck coefficient considering that the dependence of μ with the temperature, as we show in equation (4), is represented by a Heaviside function, leading to the following expresion for the Seebeck


    S= 2π/eμ k_B+2π/eδ(p̅) k_B T

being p̅ the different values where μ changes, that is, 0, ± m/2, ± m/4 and where the local increase Fig. 4, represented by the second term, can be interpreted as the contribution originated in the creation of real electron-hole pairs. This equation 9 of the Seebeck coefficient has two terms, the first one quantize S in integers due to μ Chern-Simons topological mass, whereas the second corresponds to the variation of this mass respect the temperature T. The topological mass is quantized in steps of Heaviside respect to the temperature and therefore its derivative is equivalent to delta functions carrying the singularities. The relevant point is that the topological bands allow to find a new term for the Seebeck coefficient which can increase it depending of the value of temperature. But what is more important, these strings of singularities separate some regions from other thermally allowing a gradient of temperature (∇ T) which produce an electric field -E (∇ V) in formula 7. This field E⃗_2 enables to create Schwinger's pairs that have different velocities depending of the level of the occupation states where they are situated respect to the Fermi level. Thus, at difference of what happens with metals, both effects of temperature on the electrons and holes can be quite different without destroying each other and the Seebeck coefficient would be also higher than a semimetal. It is fundamental to observe that without the topology ramifications of the topological insulators we would have an homogeneous crystal without thermoelectricity.





Finally, let's calculate the topological contribution to the surface figure of merit in TI, limiting our calculations to (2+1) dimensions, where topology is determined through the first Chern number n̅ and the winding number n. We start considering that, in a given direction x or y, exists a temperature gradient in our TI in such a way that eigenstates evolve adiabatically. We can define then the Seebeck coefficient through the entropy  as S=k_B/e(ln|ψ_b/ψ_t| + inθ) where ψ_b  and ψ_t are the final an initial states, θ is the angle between the states and n is the winding number which can only take odd integers values, otherwise the topological Seebeck coefficient would give zero. In fact, we are representing the entropy of a Riemann surface. Notice that although the von Neumann entropy would give zero considering pure states, this is not true considering that these states are in entanglement due to their global topological properties. Actually there are many local Wannier bands, i.e., Ψ(k,t)=1/√(N)∑_iψ_i(k_i,t), providing separate electrons which move coherently producing what is called Thouless charge pump<cit.> (see Fig. 5) and relates Berry phase with the electric polarization P <cit.>,<cit.>. In fact, due to the previous definitions it is easy to see that ln|ψ_b/ψ_t|=0 and we can simplify the expression of the Seebeck coefficient for the TI as


    S=k_B/e inθ=nk_B/e∫_C i<ψ(r)|∇|ψ(r)> d(r)=π/e n n̅ k_B
 
being θ the angle directly associated to the Berry phase on the closed curve C. This leads to an expression very similar to that obtained in (4+1)D ,first term of equation (9), where the product μ w(U) has been transformed into nn̅, where π is the Berry phase of a non trivial material, n counts the number of times we complete a cycle in a system (number of singularities) and n̅ is the first Chern number which takes into account the whole topology on the Brillouin zone for our TI. For an adiabatical evolution of the eigenstates, we can go further in the physical interpretation of the Seebeck coefficient in TIs calculating its variation during one cycle (n=1) in the vector parameter

    Δ S=k_B/2π e∫_0 ^τ dt ∫_-π/a ^π/a dk ∑_n ∈ occu∂ a_n (k)/∂ t
 
where a is the lattice constant considered in the Brillouin zone, τ is the period of time that takes a complete cycle in the parameter space, and a_n (k) the Berry's connection in the k-space whose temporal derivative is taken on all the occuped states in the TI. Notice that τ will depend on the temperature gradient in the same way as it does on an external electric field which magnitude tell us  how fast the eigenstates evolution occur. Applying Stokes we can obtain Δ S as a function of the Berry's curvature f_n(k) given an associated electric polarization Δ P

    Δ S= k_B/ e^2Δ P
 
  where Δ P is <cit.> 
	
    Δ P= e/ 2 π∫_0 ^τ dt ∫_-π/a ^π/a dk ∑_n∈ occu f_n (k)

and the curvature  f_n (k) = i [(∂/∂ k <ψ_nk (t) )|∂/∂ t | ψ_nk (t)> - ((∂/∂ t <ψ_nk (t) )|∂/∂ k | ψ_nk (t)>]. Finally, the change in the Seebeck coefficient turns out to be a very simple quantity

    Δ S= k_B/e^2n̅ a e=  n̅ a k_B/e
 
being used the simplest expression of the polarization Δ P=n̅ ae.
 

For the electronic thermal conductivity we consider the 2D density of states in the semimetal region as

    D(ξ)=2 1/(2π)^2∫δ (ξ-ħ v_F k) k dk = 1/πħ^ 2 v_F^2ξ

which allows us to obtain the electronic thermal conductivity κ_e as

    κ_e = 1/2∂/∂ T∫ dξ D(ξ) ξ f(ξ) v_F l = 3 ζ (3)/h k_B^2 T

Being f(ξ) de Fermi-Dirac distribution function and where we have supposed a temperature dependent mean free path l. On the other hand, given that we have a ballistic regime for the electronic transport, its conductivity σ_e appears given by the simple expression 

    σ_e = n̅e^2/h




Therefore, in spite of using so different expressions than the ones of the metals, where a cuadratic dispersion equation is employed instead of the linear one of the semimetal, we obtain a good Wiedemann-Franz law yielded by 

    κ_e/σ_e = 3 ζ(3)/n̅(k_B/e)^2 T=LT

where the Lorenz L number is one constant, as it ought to be, but divided by a Chern number n̅ which tell us that this expression is only valid within the context of the non-trivial topological materials that we have considered

    L = 3 ζ(3)/n̅(k_B/e)^2

being ζ (3) the Riemann zeta function of dimension three (Apery's constant).
  
Finally we can calculate the figure of merit Z for these topological insulators 


    Z=σ_e S^2/κ_e =S^2/LT

where we are not considering the phononic part of the thermal conductivity <cit.>. In this way, the  dimensionless figure of merit turns out to be a simple expression

    ZT=n^2 n̅^3π^2/3ζ(3)

This is the extra pure topological figure of merit for the edge states, which is zero in the case of trivial topological materials. Although these conditions are quite ideal and transport constraints can diminish its efficiency under real physical features of each material, this result opens a great hope because it tell us how to improve highly the thermoelectricity associated to the topological materials. In the case of Bi_2Te_3 <cit.>, for the quantum numbers equal to one we obtain a value close to the one of its present maximum, i.e. ZT=2.737.

In summary, we have shown the relationship between topological insulators, as the family Bi_2Te_3 or topological related materials without time inversion protection as the Pb_1-xSn_xTe, and their associated  thermoelectricity. We have also seen that the second Chern number obtained for the non-Abelian SU(2) field leads to a thermal topological mass on a Chern-Simons action. This is equivalent to have a quantized  temperature working in a kind of topological bands that we define as the ramification branches using the Riemann-Hurwitz formula on an Euclidean spacetime where instanton solutions substitute Bloch oscillanting states. Physically what we have is a pumped charge between bands connected by the non-Abelian Berry phase within the insulator bulk at low temperature with an electromagnetic background field on the surface. Therefore, close to the surface we have only an Abelian U(1) Chern-Simons term providing us with one transformation between electric and thermal energy because we have only one kind of states. Moreover we show that the Schwinger's electron-hole pairs, close to the topological bands, produce an increase of the Seebeck coefficient contributing to the transformation of thermal into electric energy  which is one of the key points of the model that we present in this paper. Finally we calculate a general expression to the dimensionless figure of merit in terms of the Chern number and winding number, getting a value that coincides quite well with the one experimentally measure for the Bi_2Te_3, doing zero its phononic thermal contribution. It is open for future a new class of topological materials using topological indices higher than one which can cross what is considered nowadays the efficient critical value of four for the ZT figure of merit changing the physical conditions suggesting in the presented model.




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§ ACKNOWLEDGMENTS

 Thanks to the CESGA, to AEMAT ED431E 2018/08 and the MAT2016-80762-R project for financial support.

























