Thermal transport

In this project, we are using computational physics to explore possibilities for low-temperature thermo-electric coolers. We wish to investigate devices constructed out of heavy Fermion materials with one sign of the thermopower and either Mott or Kondo insulators with the other sign of the thermopower. Since these systems often have large peaks in the thermopower at low temperature (the Kondo scale), they are good candidates for low-temperature operation. Experimental systems are being devised and grown by Frank Steglich's laboratory at the Max Planck Institute for the Physics and Chemistry of Solids. We intend to ultimately investigate nanostructures of these materials, but we have also investigated some interesting bulk properties. Namely, when we find an exponentially suppressed density of states, which creates a pseudogap feature in the insulator, then the thermoelectric figure of merit ZT can be tuned to be larger than one at low temperature. This effect disappears when there is a true gap, with vanishing density of states. In a nanostructure, the density of states never vanishes in the barrier, if the insulator is attached to metallic leads due to a normal-state proximity effects, which has the metallic density of states leak into the insulator. But it is not yet clear whether this situation can give rise to a large ZT at low temperature. In one final note, we need to say that our calculations are for the electronic part of the device only. Often, phonon thermal conductivity can dramatically lower ZT, especially at low temperature, since it rises linearly with T and the electronic contribution may rise exponentially and T, so that the phonons dominate at low temperature and sharply reduce ZT.

Annotated list of publications

Bulk thermal transport

J. K. Freericks and V. Zlatic', Nonlinear Peltier effect and the nonequilibrium Jonson-Mahan theorem, Cond. Matter Phys. 9, 603--617 (2006).
A. V. Joura, D. O. Demchenko, and J. K. Freericks, Thermal transport in the Falicov-Kimball model on a Bethe lattice, Phys. Rev. B 69, 165105--1-5 (2004).
J. K. Freericks, D. Demchenko, A. Joura, and V. Zlatic', Optimizing thermal transport in the Falicov-Kimball model: binary-alloy picture, Phys. Rev. B 68, 195120--1-12 (2003).
Here we examine what happens to thermal transport properties in the Falicov-Kimball model. We examine thermal transport in a particle-hole asymmetric Mott transition on a Beth and hypercubic lattice with dynamical mean field theory. We find regions where the thermoelectric figure of merit is larger than one both at low temperature and high temperature. But it is possible the low temperature peaks will disappear when phonon thermal conductivity is included. We also find quite different behavior on the hypercubic lattice versus the Bethe lattice which is due to anomalous features associated with the pseudogap nature of the insulating phase on the hypercubic lattice. It may be possible to exploit this anomaly in nanostructures, since their behavior appears to be closer to the hypercubic lattice than the Bethe lattice. In addition, we also examine how te peltier effect is modified when one moves into the nonlinear transport regime.

Real materials

V. Zlatic', R. Monnier, J. K. Freericks, and K. Becker, Relationship between the thermopower and entropy of strongly correlated electron systems, submitted to Phys. Rev. B.
V. Zlatic', R. Monnier, and J. K. Freericks, Thermoelectricity of EuCu₂(Ge_1-xSi_x)₂ intermetallics, (Proceedings of the Strongly Correlated Electron Systems 2005 conference, Vienna, Austria), Physica B 378--380, 661--662 (2006).
H. Wilhem, V. Zlatic', and D. Jaccard, Thermoelectric power of heavy Fermion compounds, (Proceedings of the Strongly Correlated Electron Systems 2005 conference, Vienna, Austria), Physica B 378--380, 644-647 (2006).
V. Zlatic' and R. Monnier, Theory of the thermoelectricity of intermetallic compounds with Ce or Yb ions, Physical Review B 71, 165109--1-12 (2005).
Here an Anderson impurity model, with an NCA approach and crystal field splitting is employed to examine the bulk thermal transport properties of strongly correlated materials. The focus is on both heavy Fermion compounds and intermediate valence systems. Theory presented here unifies some experimental data that find a strong correlation between the linear coefficient of the specific heat and the linear behavior of the thermopower at low temperature.

Thermal transport in nanostructures

Last modified September 12, 2004.

Jim Freericks, Professor of Physics