Abstract
We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of
the canonical half-doped manganite Pr0.5Ca0.5MnO3 in its magnetic and orbitally ordered phase.
Comparison of the data, which cover multiple Brillouin zones and the entire energy range of the
excitations, with several different models shows that only the CE-type ordered state provides an
adequate description of the magnetic ground state, provided interactions beyond nearest neighbor
are included. We are able to rule out a ground state in which there exist pairs of dimerized spins
which interact only with their nearest neighbors. The Zener polaron ground state, which comprises
strongly bound magnetic dimers, can be ruled out on the basis of gross features of the observed spin
wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be
ruled out on the basis of subtle discrepancies between the calculated and observed structure factors
at certain positions in reciprocal space. Adding further neighbor interactions results in almost no
dimerization, i.e. interpolating back to the CE model. These results are consistent with theoretical
analysis of the degenerate double exchange model for half-doping.
