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Tunneling Anisotropic Thermopower and Seebeck Effects in Magnetic Tunnel Junctions  [PDF]
Carlos López-Monís,Alex Matos-Abiague,Jaroslav Fabian
Physics , 2013, DOI: 10.1103/PhysRevB.90.174426
Abstract: The Tunneling Anisotropic Magneto-Thermopower (TAMT) and the Tunneling Anisotropic Spin-Seebeck (TASS) effects are studied for a magnetic tunnel junction (MTJ) composed of a ferromagnetic electrode, a zinc-blende semiconductor and a normal metal. We develop a theoretical model for describing the dependence of a thermally induced tunneling current across the MTJ on the in-plane orientation of the magnetization in the ferromagnetic layer. The model accounts for the specific Bychkov-Rashba and Dresselhaus spin-orbit interactions present in these systems, which are responsible for the $C_{2v}$ symmetry we find in the TAMT and the TASS.
Time resolved spin Seebeck effect experiments  [PDF]
Niklas Roschewsky,Michael Schreier,Akashdeep Kamra,Felix Schade,Kathrin Ganzhorn,Sibylle Meyer,Hans Huebl,Stephan Gepr?gs,Rudolf Gross,Sebastian T. B. Goennenwein
Physics , 2013, DOI: 10.1063/1.4879462
Abstract: In this Letter, we present the results of transient thermopower experiments, performed at room temperature on yttrium iron garnet/platinum bilayers. Upon application of a time-varying thermal gradient, we observe a characteristic low-pass frequency response of the ensuing thermopower voltage with cutoff frequencies of up to 37 MHz. We interpret our results in terms of the spin Seebeck effect, and argue that small wavevector magnons are of minor importance for the spin Seebeck effect in our thin film hybrid structures.
The Kelvin Formula for Thermopower  [PDF]
Michael R. Peterson,B. Sriram Shastry
Physics , 2010, DOI: 10.1103/PhysRevB.82.195105
Abstract: Thermoelectrics are important in physics, engineering, and material science due to their useful applications and inherent theoretical difficulty, especially in strongly correlated materials. Here we reexamine the framework for calculating the thermopower, inspired by ideas of Lord Kelvin from 1854. We find an approximate but concise expression, which we term as the Kelvin formula for the the Seebeck coefficient. According to this formula, the Seebeck coefficient is given as the particle number $N$ derivative of the entropy $\Sigma$, at constant volume $V$ and temperature $T$, $S_{\text{Kelvin}}=\frac{1}{q_e}\{\frac{\partial {\Sigma}}{\partial N} \}_{V,T}$. This formula is shown to be competitive compared to other approximations in various contexts including strongly correlated systems. We finally connect to a recent thermopower calculation for non-Abelian fractional quantum Hall states, where we point out that the Kelvin formula is exact.
Seebeck effects in two-dimensional spin transistors  [PDF]
M. I. Alomar,Lloren? Serra,David Sanchez
Physics , 2014, DOI: 10.1103/PhysRevB.91.075418
Abstract: We consider a spin-orbit-coupled two-dimensional electron system under the influence of a thermal gradient externally applied to two attached reservoirs. We discuss the generated voltage bias (charge Seebeck effect), spin bias (spin Seebeck effect) and magnetization-dependent thermopower (magneto-Seebeck effect) in the ballistic regime of transport at linear response. We find that the charge thermopower is an oscillating function of both the spin-orbit strength and the quantum well width. We also observe that it is always negative for normal leads. We carefully compare the exact results for the linear response coefficients and a Sommerfeld approximation. When the contacts are ferromagnetic, we calculate the spin-resolved Seebeck coefficient for parallel and antiparallel magnetization configuration. Remarkably, the thermopower can change its sign by tuning the Fermi energy. This effect disappears when the Rashba coupling is absent. Additionally, we determine the magneto-Seebeck ratio, which shows dramatic changes in the presence of a the Rashba potential.
Carrier transport in layered nanolaminated films  [PDF]
Aniruddha Konar,Rajan Kumar Pandey,Tamilmani Ethirajan
Physics , 2015,
Abstract: Analyzing {\it{ab-initio}} electronic and phonon band structure, temperature-dependent carrier transport in layered Ti$_{2}$AlC is investigated. It is found that cylindrical Fermi surface is the origin of the anisotropic carrier effective mass (infinite effective mass along $c$ axis ) leading to strong anisotropic (insulator along $c$ axis and metallic along the layer) carrier transport in these films. Using electronic and phonon bandstructures, we develop an analytical model of electron-phonon interaction as well as in-plane carrier conductivity originating from strong inter-valley (s$\rightarrow$d) scattering in Ti$_{2}$AlC. We invoke density functional theory to calculate the deformation potential corresponding to acoustic phonon vibration. The calculated deformation potential is in well agreement with the extracted deformation potential value from the transport data. Extracted deformation potential will be useful for prediction of transport quantities for application of these metals at elevated temperatures.
Anisotropic Magneto-Thermopower: the Contribution of Interband Relaxation  [PDF]
J. -E. Wegrowe,Q. Anh Nguyen,M. Al-Barki,J. -F. Dayen,T. L. Wade,H. -J. Drouhin
Physics , 2005, DOI: 10.1103/PhysRevB.73.134422
Abstract: Spin injection in metallic normal/ferromagnetic junctions is investigated taking into account the anisotropic magnetoresistance (AMR) occurring in the ferromagnetic layer. It is shown, on the basis of a generalized two channel model, that there is an interface resistance contribution due to anisotropic scattering, beyond spin accumulation and giant magnetoresistance (GMR). The corresponding expression of the thermopower is derived and compared with the expression for the thermopower produced by the GMR. First measurements of anisotropic magnetothermopower are presented in electrodeposited Ni nanowires contacted with Ni, Au and Cu. The results of this study show that while the giant magnetoresistance and corresponding thermopower demonstrates the role of spin-flip scattering, the observed anisotropic magnetothermopower indicates interband s-d relaxation mechanisms.
Time-Dependent Thermopower Effect in an Interacting Quantum Dot  [PDF]
M. Bagheri Tagani,H. Rahimpour Soleimani
Physics , 2012, DOI: 10.1007/s10765-012-1181-5
Abstract: The time-dependent thermopower is analyzed through an interacting quantum dot coupled to a time-dependent gate voltage and under the influence of an external magnetic field using the Keldysh nonequilibrium Green's function formalism. Formal expressions of the electrical and thermal conductances, thermopower, and thermoelectrical figure of merit are obtained. The influence of the magnetic field on the displacement current and the heat current is studied. Results show that although applying time-dependent gate voltage results in the enhancement of the Seebeck coefficient, the electron-electron interaction gives rise to a significant reduction in the thermopower. The reason for why applying time dependent gate voltage results in the enhancement of the thermopower is also analyzed.
Nonlinear Seebeck Effect in a Model Granular Superconductor  [PDF]
Sergei Sergeenkov
Physics , 1998, DOI: 10.1134/1.567701
Abstract: The change of the Josephson supercurrent density of a weakly-connected granular superconductor in response to externally applied arbitrary thermal gradient dT/dx (nonlinear Seebeck effect) is considered within a model of 3D Josephson junction arrays. For dT/dx>(dT/dx)_c, where (dT/dx)_c is estimated to be of the order of 10^4 K/m for YBCO ceramics with an average grain's size of 10 microns, the weak-links-dominated thermopower S (Seebeck coefficient) is predicted to become strongly dT/dx-dependent.
Thermopower for a molecule with vibrational degrees of freedom  [PDF]
Sergei Kruchinin,Thomas Pruschke
Physics , 2011,
Abstract: We propose a simple model to study resonant tunneling through an organic molecule between to conducting leads, taking into account the vibrational modes of the molecule. We solve the model approximately analytically in the weak coupling limit and give explicit expressions for the thermopower and Seebeck coefficient. The behavior of these two quantities is studied as function of model parameters and temperature. For a certain regime of parameters a rather peculiar variation of the thermopower and Seebeck coefficient is observed. Although the model is very simple, we expect it to give some nontrivial insight into thermal transport properties through nan-devices. Furthermore, because we can provide an analytical solution, it may eventually serve as benchmark for more advanced analytical or computational methods.
Spin thermopower in the overscreened Kondo model  [PDF]
Rok Zitko,Jernej Mravlje,Anton Ramsak,Tomaz Rejec
Physics , 2013, DOI: 10.1088/1367-2630/15/10/105023
Abstract: We study the spin thermopower in the two-channel spin-1/2 Kondo model which exhibits the phenomenon of impurity spin overscreening and non-Fermi-liquid properties. While magnetic field lower than the Kondo temperature does not strongly polarize the impurity spin, we show that it nevertheless strongly affects the low-energy part of the spectral function. In turn, this leads to characteristic saturation of the spin Seebeck coefficient at the value of 0.388kB/|e| at T ~ T*, where T* ~ B^2/TK is the scale of the crossover between the intermediate-temperature non-Fermi-liquid regime and the low-temperature Fermi-liquid regime. We show that measuring the spin thermopower at low magnetic fields would provide a sensitive test for distinguishing regular Fermi liquid, singular Fermi liguid, and non-Fermi liquid behaviour in nanodevices.
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