Quantum Transport Through Molecular Magnets

Abstract

In this thesis we theoretically study time-dependent electronic and spin transport through a molecular orbital connected to two Fermi leads, and coupled to a molecular magnet via exchange interaction. The molecular spin is considered as a classical variable and is assumed to precess around an external magnetic field with Larmor frequency. We derive expressions for charge and spin currents using the Keldysh nonequilibrium Green's functions formalism. The coupling between the electronic spins and the magnetization dynamics of the molecule leads to inelastic tunneling processes, which contribute to the spin currents. The inelastic spin currents exert a spin-transfer torque on the molecular spin, which is compensated by external means. This back-action includes a contribution tothe Gilbert damping and a change of the precession frequency. The Gilbert damping coefficient can be controlled by the bias and gate voltages, or via the external magnetic field, and has a nonmonotonic dependence on the broadening of the molecular level.Next, we study the ac-charge and -spin transport through the molecular orbital, where we assume that the source and drain contacts have time-dependent electrochemical potentials. By means of the Keldysh nonequilibrium Green's functions method we calculate the spin and charge currents in linear order with respect to the time-dependent potentials. Oscillating electrochemical potentials allow to detect the Larmor frequency by a measurement of the conductance if the ac-frequency matches the Larmor frequency. In the low ac-frequency regime the junction behaves as an equivalent classical circuit, which can be tuned from capacitive-like to inductive-like response. Furthermore, we show that the setup can be used to generate dc-spin currents, which are controlled by the molecular magnetization direction and the relative phases between the Larmor precession and the ac-voltage.Finally, we study the nonequilibrium noise of charge and spin transport through the junction, in the presence of dc-bias voltage. Using the Keldysh Green's functions method we obtain the noise components of charge and spin currents and spin-transfer torque. Then we analyze the shot noise of charge current and observe dip-like features due to inelastic tunneling processes involving the change of energy by one Larmor frequency. These processes are driven by the molecular spin precession and lead to a quantum interference effect between correlated currents, with electron waves passing through the levels connected with inelastic processes. The spin-torque noise components are driven by both the dc-bias voltage and the molecular spin precession. The torque noise components correlating spin-transfer torques in the same spatial direction in the precession plane are contributed by elastic tunneling processes, and by inelastic processes, where current-carrying spin particles change their energy by one or two Larmor frequencies. In the end, we show that the correlations of the perpendicular components of the spin-transfer torque in the precession plane are related to the Gilbert damping coefficient at zero temperature.