Nanoacoustic effects in type-II superconductors and decoherence of two-state systems

In this thesis we focus on two areas of research: nanoacoustic effects in superconductors, and decoherence of two-state systems due to radiation of acoustic phonons. In the first part of this thesis we propose two new nanoacoustic effects: induction of voltage by mechanical stress, and nucleation of a superconducting vortex by an acoustic standing wave. Both of these effects take place in type-II superconductors. In the second part we study relaxation processes via acoustic phonons of a particle in a double-well potential and of a flux qubit.;Part 1. Mechanical stress causes motion of dislocations in solids. In a type-II superconductor a moving dislocation generates a pattern of current that exerts a force on the surrounding vortex lattice capable of depinning it. We show that the concentration and the speed of dislocations needed to produce depinning currents are within practical range. When external magnetic field and transport current are present, this effect generates voltage across the superconductor. In this manner, a type-II superconductor can serve as an electrical sensor of the mechanical stress. Nucleation of vortices in a superconductor below the first critical field can be assisted by transverse sound in the GHz frequency range. We work out from energy considerations that, in the presence of a sound wave, vortices enter and exit the superconductor at the frequency of the sound. The computed threshold parameters of the sound are shown to be within experimental reach.;Part 2. We propose a method of computing phonon-induced relaxation of two-state systems that is based on symmetry arguments. This allows one to express the rates in terms of independently measurable parameters. For translationally and rotationally invariant systems the conservation of linear and angular momenta must be taken into account when formulating the interaction Hamiltonian. For a particle (e.g., electron or proton) in a rigid double-well potential embedded in a solid the rate is proportional to the seventh power of temperature. For a flux qubit the two-phonon relaxation is important only if the size of the qubit is much smaller than the phonon wavelength. Due to symmetry the two-phonon rate of both systems is proportional to the square of the bias. This allows for additional control of the relaxation rate.