Reports: DNI1048802-DNI10: Correlated Electron Materials for Thermoelectric Applications: A Computational DFT+DMFT Approach

Kristjan Haule, PhD , Rutgers, the State University of New Jersey

Funding from ACS Petroleum research grant enabled us to theoretically study several technologically important materials which are at the forefront of condensed matter research. With the help of ACS grant, we published numerous publications on this subject, including a Nature Physics paper, a Nature Material paper, a Physical Review Letter, four Physical Review B papers, and a chapter in a book. We were primarily interested in thermoelectricity, magnetism and superconductivity of complex correlated materials. At the same time, we are developing our theoretical computational tools into a robust electronic structure code, which can predict various spectroscopic properties of correlated materials from abinitio perspective, withouth any input from the experiment. Our workhorse is the combination of Density Functional Theory and the Dynamical Mean Field Theory (DFT+DMFT), which was developed in our group over last decade. We are currently improving the demanding quantum Monte Carlo algorithms, to increase speed of simulations, and we are also developing modules to predict various spectroscopic and transport quantities, such as one particle photoemission, two particle quantities such as optical conductivity, thermoelectric power, neutron form factor, neutron structure factor, etc.

One of the most important milestones in our research this year includes the description of the magnetic and paramagnetic state of FeAs superconductors, the new high temperature superconductors recently discowered in Japan. The paper has just been published in the Nature Materials journal (Kinetic frustration and the nature of the magnetic and paramagnetic states in iron pnictides and iron chalcogenides, Z. P. Yin, K. Haule, G. Kotliar, published online, Nature Materials (2011), doi:10.1038/nmat3120).  Detailed description of the nature of the magnetic excitations in a member of this family, BaFe2As2, was just accepted for publication in Nature Physics journal (Nature of magnetic excitations in superconducting BaFe{1.9}Ni{0.1}As2 Mengshu Liu, Leland W. Harriger, Huiqian Luo, Meng Wang,R. A. Ewings, T. Guidi, Hyowon Park, Kristjan Haule, Gabriel Kotliar, S. M. Hayden, and Pengcheng Dai) . In particular, we predicted the anysotropy of the electronic state, which was verified in detail by neutron experiment.

We worked on thermoelectricity of correlated semiconductors, in particular FeSb2 and FeSi2, the two promising correlated materials whit extraordinary large Seebeck coefficient. We showed that there is an upper bound for the value of the Seeback coefficient, which is simply given by S<Delta/T, where Delta is the semiconducting gap, and T is temperature (Thermopower of correlated semiconductors : application to FeAs2 and FeSb2, Jan M. Tomczak, K. Haule, T. Miyake, A. Georges, G. Kotliar, Phys. Rev. B 82, 085104 (2010)). This suprisingly simple relation turns out to hold even in strongly correlated semiconductors, when the thermoelectricity is of electronic origin. This does not include phonon-drag (a parasitic effect) which can substantically increases seebeck coefficient, but does not improve performance of the thermoelectric material. We showed very recently that our DFT+DMFT theoretical method can explain the optical conductivity and the thermoelectricity of FeSi in detail, including the spectral weight transfer with temperature, and the sign change of the thermoelectric coefficient with increasing temperature. This work was recently submitted to Nature Physics jorunal (Signatures of electronic correlations in FeSi Jan M. Tomczak , K. Haule, and G. Kotliar).

Converging on Alaska
Dr. Ridgway
Polyene Synthesis
Dr. O'Neil
Dr. Bali
Faults and Fluid Flow
Dr. Huntington