Beth Parks

Linewidth of single-photon transitions inMn12-acetate

We use time-domain terahertz spectroscopy to measure the position and linewidth of single photon transitions in Mn

Inhomogeneous broadening of single photon transitions in molecular magnets

_{12}

Effect of mechanical stress on the line width of single photon absorptions in Mn12-acetate

-acetate. This linewidth is compared to the linewidth measured in tunneling experiments. We conclude that local magnetic fields (due to dipole or hyperfine interactions) cannot be responsible for the observed linewidth, and suggest that the linewidth is due to variations in the anisotropy constants for different clusters. We also calculate a lower limit on the dipole field distribution that would be expected due to random orientations of clusters and find that collective effects must narrow this distribution in tunneling measurements.

Coherent terahertz spectroscopy of the vortex-state of cuprate superconductors

Single photon transitions in molecular magnets provide valuable probes of the widths of energy levels. We use time-domain terahertz spectroscopy to measure the width of the transition from the ground state to the first excited state in Mn12 acetate in zero applied field. The width of this transition, approximately 5.5 GHz full width at half maximum, is too large to be caused by local magnetic fields. Experiments by Mukhin et al. rule out homogeneous broadening as the source of the linewidth [Phys. Rev. B 63, 214411 (2001)]. The linewidth can be explained if the anisotropy constant in the spin Hamiltonian is not uniform due to crystal defects such as dislocations. Since similar linewidths are observed in several other materials, we conclude that whatever disorder causes this broadening is not limited to Mn12 acetate.

Re-examining the vortex state of cuprate superconductors with gap anisotropY

Abstract Quantum mechanical tunneling of the magnetic moment has been observed in several single-molecule magnets, including Mn 12 -acetate, but the tunneling mechanism is not entirely understood. It has been proposed that tunneling occurs due to defects in the crystal structure. These defects can be studied through their effect on the inhomogeneous line width of single photon (intrawell) transitions. However, most previous studies of the line width have been performed on crystals that have been pressed into pellets, which may introduce additional defects into the crystal structure. We report on measurements of the line width of loose crystals using the method of time-domain terahertz spectroscopy. The line width is not significantly reduced from the measurements on pressed pellets.

Phase-sensitive measurements of vortex dynamics in the terahertz domain

We review the progress towards an understanding of the electrodynamics in the vortex-state which arises in response to a magnetic field. B. Experimentally, the goal is to characterize the conductivity of cuprate superconductors in the presence of an applied magnetic field over a broad frequency range from microwave to infrared wavelengths. The theoretical challenge is to understand these measurements with microscopic, as well as phenomenological, models. In describing the experiments to date we will focus on a relatively new technique, coherent terahertz spectroscopy, which is uniquely capable of measuring the complete complex conductivity tensor.

Observation of the quasiparticle Hall effect in superconducting YBa2Cu3O7- delta.

We consider the effect of gap nodes on the electrodynamic response of superconductors in the vortex state. We show that for a gap with line nodes it is not possible to ignore the effect of field-induced pairbreaking on the electrodynamic response. We describe measurements of both the diagonal and the Hall components of the resistivity which support this conclusion.