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Low-dimensional thermoelectricity is based on the redistribution-accumulation of the electron density of states by reducing the dimension of thermoelectric structures. Superlattices are the archetype of these structures due to the formation of energy minibands and minigaps. Here, we study for the first time the thermoelectric response of gated bilayer graphene superlattices (GBGSLs). The study is based on the four-band effective Dirac Hamiltonian, the hybrid matrix method and the Landauer-Büttiker formalism. We analyze the Seebeck coefficient, the power factor, figure of merit, output power and efficiency for different temperatures and different superlattice structural parameters. We pay special attention to the impact of not only minibands and minigaps on the thermoelectric properties, but also to intrinsic resonances in bilayer graphene structures such as Breit-Wigner, Fano and hybrid resonances. In particular, we analyze the interplay between minibands and Fano resonances as a possible mechanism to improve the thermoelectric response of GBGSLs. We also compute the density of states to know if the redistribution-accumulation of electron states is implicated in the thermoelectric response of GBGSLs.

When external electric ( E x ) and magnetic ( H y ) fields are applied to a semiconductor nonequilibrium charge carriers appear that are redistributed perpendicularly to the fields. We show that under weak recombination the nonequilibrium electron together with the hole concentration and nonequilibrium electron temperature are related by a linear function of the coordinate z. The nonequilibrium electron temperature and electron and the hole concentration have a term proportional to H y E x and another term proportional to E x 2 . The nonequilibrium temperature shows a dependence of the z-coordinate and includes the effects of the electron-phonon interaction as well as the absorption of heat over the surfaces of the sample. Additionally, the nonequilibrium temperature and charge carriers modified the density of current from the Ohm law. The redistribution of charge carriers originate two new contributions to the magnetoresistance, one of the contributions is linked to the diffusion of charge carriers along the z-axis, the other one is related to the thickness 2b of the sample and the heat absorption over the surfaces. Another effect that modified the density of current is associated with the absorption of heat over the surfaces of the sample, this contribution is proportional to H y E x and can change sign if the electric or magnetic field is inverted. In the case of a null magnetic field, the nonlinear temperature and electron (and hole) concentration are originated by the external electric field and the relaxation mechanism over the surfaces of the sample. The current-voltage characteristic deviates from the linear response due to the nonequilibrium charge carriers.

We study the tunneling magnetoresistance and the spin-valley transport in silicene-based magnetic superlattices. The superlattice profile is obtained by the periodic modulation of the exchange field induced by ferromagnetic electrodes. The silicene band gap in the superlattice structure is also modulated through an external perpendicular electric field. The concept of parallel and antiparallel magnetization configurations of single magnetic junctions is extended to the periodic case by switching the magnetization orientation of the adjacent magnetic barriers in parallel and antiparallel fashion. The transfer matrix method and the Landauer-B\"uttiker formalism are used to obtain the transmission and transport properties, respectively. We find an oscillating conductance once the periodic modulation is incorporated. By tuning the external perpendicular electric field a conductance gap is obtained for the antiparallel configuration, which results in an enhancement of the tunneling magnetoresistance with respect to single magnetic junctions. In the case of the spin-valley polarization it is not possible to obtain two well-defined polarization states by simply switching the magnetization orientation, as in single magnetic junctions, due to the equivalence of the spin-valley conductance components. However, by inducing structural asymmetry in the width of barriers-wells, two well-defined polarization states can be reached. Moreover, an additional enhancement of the tunneling magnetoresistance is induced by the structural asymmetry. Our findings indicate that magnetic periodic modulation can be an option to improve the tunneling magnetoresistance and the spin-valley polarization of silicene-based structures.

Low-dimensional thermoelectricity is a key concept in modern thermoelectricity. This concept refers to the possibility to improve thermoelectric performance through redistribution of the density of states by reducing the dimensionality of thermoelectric devices. Among the most successful low-dimensional structures we can find superlattices of quantum wells, wires and dots. In this work, we show that this concept can be extended to cutting-edge materials like graphene. In specific, we carry out a systematic assessment of the thermoelectric properties of quantum well gated graphene superlattices. In particular, we find giant values for the Seebeck coefficient and the power factor by redistributing the density of states through the modulation of the fundamental parameters of the graphene superlattice. Even more important, these giant values can be further improved by choosing appropriately the angle of incidence of Dirac electrons, the number of superlattice periods, the width of the superlattice unit cell as well as the height of the barriers. We also find that the power factor presents a series of giant peaks, clustered in twin fashion, associated to the oscillating nature of the conductance. Finally, we consider that low-dimensional thermoelectricity in graphene and related 2D materials is promising and constitutes a possible route to push forward this exciting field.

Fano and hybrid resonances of bilayer graphene could be attractive for thermoelectric devices. The special profile presented by such resonances could significantly enhance the Seebeck coefficient and the power factor. In this work, we study the thermoelectric properties of bilayer graphene single and double barrier structures. The charge carriers are described as massive chiral particles through an effective Dirac-like Hamiltonian. The Hybrid matrix method, the Landauer-B\"{u}ttiker formalism and the Cutler-Mott formula are implemented to obtain the transmission, transport and thermoelectric properties, respectively. The Seebeck coefficient and the power factor are analyzed for gapless and gapped single and double barriers. We find that in the energy range where Fano resonances occur, the Seebeck coefficient attains values of tens of mV/K and the power factor reaches values of the order nW/K. Hybrid resonances also sustain high values for the thermoelectric properties, however not as high as Fano resonances. We also find that despite the Fano and hybrid profiles are manifested in the conductance of gapped barrier structures, the Seebeck coefficient and the power factor are systematically reduced as the bandgap gets larger. So, our findings indicate that bilayer graphene barrier structures can be used to improve the response of thermoelectric devices., Comment: 17 pages, 12 figures, J.A.B.-T. acknowledges to PRODEP-SEP-Mexico for the Posdoctoral Research Fellowship 511-6/2019.-10959

It is known that when strong electric field is applied to a semiconductor sample, the current voltage characteristic deviates from the linear response. In this letter, we propose a new point of view of nonlinearity in semiconductors which is associated with the electron temperature dependence on the recombination rate. The heating of the charge carriers breaks the balance between generation and recombination, giving rise to nonequilibrium charge carriers concentration and nonlinearity.

This paper theoretically investigates the impact of aperiodic sequences in the ballistic transport and thermoelectric effect in silicene gated superlattices. In our analysis, we have implemented the well-known Fibonacci, Thue–Morse, and triadic Cantor type sequences. The transfer matrix technique and the Landauer–Butikker formalism are used to calculate the transmission probability and the conductance, respectively. The Cutler–Mott formula is employed to estimate the Seebeck coefficient, and the thermoelectric power factor is then obtained. We found that the transmission minibands of aperiodic superlattices exhibit a much more fragmented structure in comparison to that reported in the periodic case. Consequently, the conductance curve presents a more pronounced oscillating shape, which improves the thermoelectric properties. In particular, the Seebeck coefficient has reached values up to 78.2 mV/K for Fibonacci, 233.0 mV/K for Thue–Morse, and 436.3 mV/K for Cantor. In addition, the power factor has been substantially increased, reaching peaks of approximately 8.2, 50.2, and 2.1 nW/K 2 for the mentioned sequences, respectively. The best results were obtained for s p i n d o w n ( s p i n u p) charge carriers in the K ( K ′) valley. Besides, an additional improvement is obtained by considering superior generations of the aperiodic sequences. Finally, our findings are supported through the redistribution of the density of the states, which is induced by the aperiodicity of the nanostructure as well as by the low-dimensionality of the thermoelectric device.

We investigate numerically the thermoelectric properties of aperiodic graphene superlattices generated by applying an external electric field following the Fibonacci and Thue–Morse sequences. We find that aperiodicity reduces and fragments the transmission bands natural in periodic superlattices as well as redistributes the density of states of the system. We also find an overall reduction of the conductance in aperiodic graphene superlattices with respect to periodic ones. Furthermore, as the generation of the aperiodic structure increases, the conductance decreases and a series of peaks arise on it. This behavior is more pronounced in Thue–Morse superlattices than in Fibonacci ones. In the case of the thermoelectric properties, we obtain that Fibonacci graphene superlattices present similar values for the Seebeck coefficient and the power factor as in periodic superlattices, while Thue–Morse graphene superlattices show an enhancement of the thermoelectric properties, in particular the power factor is two times larger than the corresponding one to periodic and Fibonacci graphene superlattices. So, according to our findings, aperiodicity can be used as a tuning parameter to improve the thermoelectric properties of graphene superlattices.

In linear approximations using small electric Ex and magnetic Hy fields, nonequilibrium carriers appear in bounded semiconductors orthogonal to the electric field. An expression for the longitudinal magnetoresistance in a finite bipolar semiconductor is obtained by taking into account the redistribution of the nonequilibrium carriers along the z axis (thickness 2b), the generation and recombination processes assisted by traps (Shockley-Read model), and the different surface recombination velocities. For the first time, it is shown that the redistribution of nonequilibrium carriers generates a change in the conductivity from the classical value. In this case, two new terms appear, which depend on the thickness b. The first contribution to the magnetoresistance exhibits a linear dependence on the magnetic field, where the conductivity of the semiconductor increases or decreases depending on the magnetic field orientation. The second contribution shows a quadratic dependence on the magnetic field. The change...

In this work GaAs/Mn multilayers were deposited on GaAs (001) substrates by R.F magnetron sputtering technique, varying the deposition time (tg). Scanning electron and atomic force Microscopy studies were realized on the surface of the samples in order to determine the morphology and average roughness. X-ray diffraction spectra show that our samples tend to do amorphous. Raman spectroscopy at room temperature was employed to analyze the structural properties of the samples. We found that for a GaAs film taken as reference, the Raman spectra is dominated by the transverse (TO) and longitudinal (LO) modes located at 266 cm-1 and 291 cm-1, respectively. However, for the GaAs/Mn multilayers the TO and LO modes decrease dramatically, and the Mn Raman modes in the range of 100 cm-1 and 250 cm-1 are evidenced. Additional new peaks located around 650 and 690 cm -1 are only observed for the samples with high Mn content. By using the mass reduced model we estimate that the Mn related peaks are located at 650.2 cm-1 and 695.2 cm-1, in good agreement with the experimental data, these peaks are correlated with excitations due to (Mn)mAsn localized structures.