This title appears in the Scientific Report :
2016
Thermoelectric transport through quantum dots
Thermoelectric transport through quantum dots
In this thesis the thermoelectric properties (electrical conductance, Seebeck coefficient and thermal conductance)of quantum dots described by the Anderson impurity model have been investigated by using the numerical renormalization group (NRG) method.In order to make accurate calculations for therm...
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Personal Name(s): | Merker, Lukas (Corresponding author) |
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Contributing Institute: |
Theoretische Nanoelektronik; IAS-3 |
Imprint: |
2016
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Physical Description: |
188 |
Dissertation Note: |
RWTH Aachen, Diss., 2016 |
Document Type: |
Dissertation / PhD Thesis |
Research Program: |
Controlling Spin-Based Phenomena |
Subject (ZB): | |
Publikationsportal JuSER |
In this thesis the thermoelectric properties (electrical conductance, Seebeck coefficient and thermal conductance)of quantum dots described by the Anderson impurity model have been investigated by using the numerical renormalization group (NRG) method.In order to make accurate calculations for thermoelectric properties of quantum impurity systems, a number of recent developments and refinements of the NRG have been implemented. These include the z-averaging and Campo discretization scheme, which enable the evaluation of physical quantities on an arbitrary temperature gridand at large discretization parameter Λ and the full density matrix (FDM) approach, which allows a more accurate calculation of spectral functions and transport coefficients. The implementation of the z-averaging and Campo discretization scheme has been tested within a new method for specific heats of quantum impurities. The accuracy of this new method was established by comparison with the numerical solution of the Bethe-ansatz equations for the Anderson model. The FDM approach was implemented and tested within a new approach to the calculation of impurity contributions to the uniform susceptibilities. Within this method a non-negligible contribution from the "environmental" degrees of freedom needs to be takeninto account to recover the correct susceptibility, as shown by comparison with the Bethe-ansatz approach. An accurate method to calculate the conductance of a quantum dot is implemented, enabling the extraction of the Fermi liquid scaling coefficients cT and cB to high accuracy, being able to verify the results of the renormalized super perturbation theory approach (within its regime of validity). The method was generalized to higher order moments of the local level spectral function. This, as well as reduction of the SU(2) code to the U(1) symmetry, enabled the investigation of the effect of a magnetic fieldon the thermoelectric properties of quantum dots. Furthermore the model could be used to qualitatively describe and predict the behavior of the thermopower of CeCu6−xAux in a magnetic field. Motivated by the large thermopower realized in a negative-U Anderson model, a generalized Anderson impurity model with screening to the leads was introduced and investigated with the NRG. A two-channel NRG code needed to be developed, since the decoupling of the odd parity channel is no longer valid in the presence of a screening term. Sufficiently large screening terms can make the Coulomb interaction negative (at particle-hole symmetry), thereby enhancing the thermopower by the negative-U effect. Experimentally, screening interactions are always expected to be present and may be particularly important in molecular quantum dots,where the leads are metallic. We showed the effect of the conductance electron screening term at T=0 on the conductance and on the local occupation numberas well as the charge and spin susceptibility. These results verify functional renormalization group results for these quantities within their range of validity, i.e. for small Coulomb interactions and screening interactions, but go beyond these since they allow investigating quantitatively also in the non perturbative limit. It is shown that for strong screening interaction (relative to the local Coulomb repulsion) the physics is consistent (at particle-hole symmetry) with a charge Kondo effect. |