Jens Broe Rix

A thesis for the degree of Doctor of Philosophy defended September 2019.

The PhD School of Science, Faculty of Science, Condensed Matter Physics, Niels Bohr Institute, University of Copenhagen

Principal Supervisor:
Per Hedegård

Nonequilibrium Effects in Nanoscale Systems

Thermoelectrically driven ring currents Coupled spins out of equilibrium

The field of thermoelectricity has gained increasing interest during the last decades,partly due to the advances in nanoscience.In the first part of this thesis, different aspects of the Seebeck effect in nanoscale systems are studied.
We derive a mathematical tool to calculate local electric currents in single molecule junctions and use it to analyze thermoelectrically driven ring currents. Additionally, the expression is used to study the connection between ring currents and destructive interference in both electrically and thermoelectrically driven junctions.

The Seebeck coefficient of single molecules is rather small, but when organic molecules are put together to form organic semiconductors, the Seebeck coefficient increases significantly. We will show that the thickness dependent Seebeck coefficient of a layered organic crystal can be described by band bending at the crystal-metal interfaces.Recent research has shown that a spin-polarized current can flip a magnetic moment so that it points antiparallel to the magnetic field. In the second part of this hesis, we will show that even an ordinary charge current can flip magnetic moments, when at least two moments are present. An equation of motion is derived for two spins that couple to a current carrying metal and the explicit expressions and length
dependence of the nonequilibrium torques are presented. We show that the nonequilibrium torques can drive the spin system into several unexpected configurations such as pointing antiparallel to the magnetic field.