A study of two systems of correlated electrons

In this dissertation, we review two avenues of experimental study.;The first is the attempt to measure the charge of a vortex in a type-II superconductor. The vortex, long known to permit the penetration of a single quantum of magnetic flux, has also been proposed as carrying a small but nonzero electric charge, as well. A number of theoretical approaches to high- Tc are suggestive of this vortex charging, and evidence has been seen in Hall effect measurements which might also suggest that moving vortices may carrying their bound charge with them, creating an additional source of transport current.;An experimental scheme was designed and built in order to isolate the voltage due to these moving, charged vortices via phase sensitive measurements on a mechanical excitation of vortex motion.;Unfortunately, the mechanical oscillation in a magnetic field is by its nature prone to the generation unwanted noise voltages. Numerous techniques were devised to reduce or eliminate these signals, but they fail to reduce the noise to the degree which would be required to isolate the vortex charge.;The second is the measurement of the specific heat of a novel magnetic material in order to characterize an unusual magnetic transition.;The crystal RbNa3Fe 7(Po4)6 had been seen to exhibit an apparently ferromagnetic transition at approximately 15 K. Furthermore, unusual "steps" in the magnetization of the material at low field and temperature prompted a look at the heat capacity of the material in order to look for unusual features, as well as to characterize the nature of the transition.;The transition was found to be assocaited with an unusually low change in entropy, comparable to the loss of only a single degree of freedom per unit cell for a gas of free spins. This suggests that considerable spin ordering is occuring well below the critical temperature.;Furthermore, the scaling behavior of the heat capacity near Tc was investigated, suggesting that the change in the order parameter was of dimension 4, suggesting the importance of non-nearest neighbor spin interaction terms.