a system with a fully ordered ferromagnetic ground state;
a system at aT _{c}=0 critical point.
For both situations it is found that the exact results are considerably more complex than has been anticipated on the basis of approximate approaches which are considered to be appropriate and reliable for such situations. A still higher degree of complexity is expected for the dynamics of quantum spin systems which are nonintegrable. The paper concludes with some observations concerning nonintegrability effects and quantum chaos in spin systems.
]]>This is a study of magnetic-field effects on various zero-temperature properties of the one-dimensional model with exchange alternation or anisotropy. Alternation and anisotropy in the interaction cause different types of spin ordering in the ground state, which in turn are responsible for characteristic effects in various physical properties. The presence of an external magnetic field perpendicular to the XY-plane has the tendency to counteract these orderings. The latter are suppressed when the field reaches a critical strength.
Various ground state properties of the two models are calculated, with emphasis on dynamic correlation functions. In some respects the alternating and the anisotropic models exhibit surprisingly similar properties, particularly in zero magnetic field. Important differences arise in other respects due to the different symmetry properties of the two models.
Our calculations are exact and most results are presented as closed-form expressions.
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