Abstract
Nanolaminated materials exhibit characteristic magnetic, mechanical, and thermoelectric properties, with large contemporary scientific and technological interest. Here we report on the anisotropic Seebeck coefficient in nanolaminated TiSiC single-crystal thin films and trace the origin to anisotropies in element-specific electronic states. In bulk polycrystalline form, TiSiC has a virtually zero Seebeck coefficient over a wide temperature range. In contrast, we find that the in-plane (basal ) Seebeck coefficient of TiSiC, measured on single-crystal films, has a substantial and positive value of 4–6 V/K. Employing a combination of polarized angle-dependent x-ray spectroscopy and density functional theory we directly show electronic structure anisotropy in inherently nanolaminated TiSiC single-crystal thin films as a model system. The density of Ti and C states at the Fermi level in the basal plane is about 40 higher than along the axis. The Seebeck coefficient is related to electron and hole-like bands close to the Fermi level, but in contrast to ground state density functional theory modeling, the electronic structure is also influenced by phonons that need to be taken into account. Positive contribution to the Seebeck coefficient of the element-specific electronic occupations in the basal plane is compensated by 73 enhanced Si electronic states across the laminate plane that give rise to a negative Seebeck coefficient in that direction. Strong phonon vibration modes with three to four times higher frequency along the axis than along the basal plane also influence the electronic population and the measured spectra by the asymmetric average displacements of the Si atoms. These results constitute experimental evidence explaining why the average Seebeck coefficient of TiSiC in polycrystals is negligible over a wide temperature range. This allows the origin of anisotropy in physical properties of nanolaminated materials to be traced to anisotropies in element-specific electronic states.
- Received 29 May 2011
DOI:https://doi.org/10.1103/PhysRevB.85.195134
©2012 American Physical Society