  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 Ti3SiC2 single-crystal thin films and trace the origin to
anisotropies in element-specific electronic states. In bulk polycrystalline
form, Ti3SiC2 has a virtually zero Seebeck coefficient over a wide temperature
range. In contrast, we find that the in-plane (basal ab) Seebeck coefficient of
Ti3SiC2, measured on single-crystal films has a substantial and positive value
of 4-6 muV/K. Employing a combination of polarized angle-dependent x-ray
spectroscopy and density functional theory we directly show electronic
structure anisotropy in inherently nanolaminated Ti3SiC2 single-crystal thin
films as a model system. The density of Ti 3d and C 2p states at the Fermi
level in the basal ab-plane is about 40 % higher than along the c-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 3d 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 c-axis than along the basal
ab-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 Ti3SiC2
in polycrystals is negligible over a wide temperature range.
