2,924 research outputs found
Neutron diffraction, magnetization and ESR studies of pseudocubic Nd(0.75)Ba(0.25)MnO3 and its unusual critical behavior above Tc
Results of structural neutron diffraction study, magnetization and ESR
measure-ments are presented for insulating Nd0.75Ba0.25MnO3, Tc = 129 K. The
crystal structure is refined in the range 4.2-300 K. The compound is found to
exhibit the Jahn-Teller (JT) transition at 250 K. The field cooled (FC)
magnetization data are in a reasonable agreement with the predictions for a 3D
isotropic ferromagnet above Tc. However, these measurements reveal a difference
between the FC and zero FC data in the paramagnetic region. ESR results are
also in a correspondence with behavior of a cubic ferromagnet above T* = 143 K.
It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic
moments can give a substantial contribution in ESR linewidth masking its
critical enhan-cement. The different temperature treatments of the sample
reveal a temperature hysteresis of the ESR spectra below T* indicating an
anomalous response in the paramagnetic region. The study of phase transition in
this manganite suggests change in its character from the second to first order
at T*. The conventional free energy including the magnetization and magnetic
field is not found to describe the first order transition. This suggests that
the charge, orbital and JT phonon degrees of freedom, in addition to
magnetization, may be the critical variables, the unusual character of the
transition being determined by their coupling. Unconventional critical behavior
is attributed to orbital liquid metallic phase that coexists with the initial
orbital ordered phase below T*.Comment: 18 pages, 5 figures, submitted to Phys. Rev.
Degenerate approach to the mean field Bose- Hubbard Hamiltonian
A degenerate variant of mean field perturbation theory for the on-site
Bose-Hubbard Hamiltonian is presented. We split the perturbation into two terms
and perform exact diagonalization in the two-dimensional subspace corresponding
to the degenerate states. The final relations for the second order ground state
energy and first order wave function do not contain singularities at integer
values of the chemical potentials. The resulting equation for the phase
boundary between superfluid and Mott states coincides with the prediction based
on the conventional mean field perturbation approach.Comment: 4 pages, 1 figur
- …