Bolina O., Parreira J. R.
The XY Model in a Transverse Magnetic Field
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ABSTRACT.  We show that the ground state of the XY model (ferromagnetic or 
antiferromagnetic) in a transverse magnetic field $h$, for any spin value 
in any dimension, is the state with all the spins aligned parallel to the 
field when $h$ is bigger in magnitude than some critical value $h_c$. We 
also compute the spatial and time decay correlation functions for the 
spin-1/2 linear chain and study the behavior of these quantities as a 
function of the external magnetic field