Landau-Zener transitions in noisy environment and many-body systems

Abstract

This dissertation discusses the Landau-Zener (LZ) theory and its application in noisy environments and in many-body systems. The first project considers the effect of fast quantum noise on LZ transitions. There are two important time intervals separated by the characteristic LZ time. For each interval we derive and solve the evolution equation, and match the solutions at the boundaries to get a complete solution. Outside the LZ time interval, we derive the master equation, which differs from the classical equation by a quantum commutation term. Inside the LZ time interval, the mixed longitudinal-transverse noise correlation renormalizes the LZ gap and the system evolves according to the renormalized LZ gap. In the extreme quantum regime at zero temperature our theory gives a beautiful result which coincides with that of other authors. Our initial attempts to solve two experimental puzzles - an isotope effect and the quantized hysteresis curve of a single molecular magnet - are also discussed. The second project considers an ultracold dilute Fermi gas in a magnetic field sweeping across the broad Feshbach resonance. The broad resonance condition allows us to use the single mode approximation and to neglect the energy dispersion of the fermions. We then propose the Global Spin Model Hamiltonian, whose ground state we solve exactly, which yields the static limit properties of the BEC-BCS crossover. We also study the dynamics of the Global Spin Model by converting it to a LZ problem. The resulting molecular production from the initial fermions is described by a LZ-like formula with a strongly renormalized LZ gap that is independent of the initial fermion density. We predict that molecular production during a field-sweep strongly depends on the initial value of magnetic field. We predict that in the inverse process of molecular dissociation, immediately after the sweeping stops there appear Cooper pairs with parallel electronic spins and opposite momenta.