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Paper IPM / Physic / 12036  


Abstract:  
We investigate the phase diagram of an anisotropic ferrimagnetic
spin(1/2, 1) in the presence of a noncommuting (transverse) magnetic field. We
find a magnetization plateau for the isotropic case while there is no plateau for the
anisotropic ferrimagnet. The magnetization plateau can appear only when the
Hamiltonian has the U(1) symmetry in the presence of the magnetic field. The
anisotropic model is driven by the magnetic field from the N?eel phase for low fields
to the spinflop phase for intermediate fields and then to the paramagnetic phase
for high fields. We find the quantum critical points and their dependence on the
anisotropy of the aforementioned fieldinduced quantum phase transitions. The
spinflop phase corresponds to the spontaneous breaking of Z2 symmetry. We use
the numerical density matrix renormalization group and analytic spin wave theory
to find the phase diagram of the model. The energy gap, sublattice magnetization
and total magnetization parallel and perpendicular to the magnetic field are also
calculated. The elementary excitation spectra are obtained via the spin wave theory
in the three different regimes depending on the strength of the magnetic field.
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