Quantum lattice fluctuations in a frustrated Heisenberg spin-Peierls chain

As a simple model for spin-Peierls systems we study a frustrated Heisenberg chain coupled to optical phonons. In view of the anorganic spin-Peierls compound CuGeO3 we consider two different mechanisms of spin-phonon coupling. Combining variational concepts in the adiabatic regime and perturbation theory in the anti-adiabatic regime we derive effective spin Hamiltonians which cover the dynamical effect of phonons in an approximate way. Ground-state phase diagrams of these models are determined, and the effect of frustration is discussed. Comparing the properties of the ground state and of low-lying excitations with exact diagonalization data for the full quantum spin phonon models, good agreement is found especially in the anti-adiabatic regime.Comment: 9 pages, 7 figures included, submitted to Phys. Rev.

Psychometric Properties of Clinical Performance Ratings

A total of 7931 ratings of 482 third- and fourth-year medical students were gath ered over twelve four-week periods. Ratings were made by multiple raters, house officers, and attending faculty, on fifteen behaviorally-anchored rating scales. The data were factor analyzed separately for each of the twelve periods.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67886/2/10.1177_016327878000300106.pd