We investigate a quantum version of the spherical model which is obtained from the classical Berlin-Kac spherical model by a simple canonical quantization scheme. We find a complete solution of the model for short-range as well as for long-range interactions. At finite temperatures the critical behavior is the same as in the classical spherical model whereas at zero temperature we find a quantum phase transition characterized by new critical exponents. Based on a functional-integral representation of the partition function the free energy of the model is shown to be equivalent to that of the nonlinear σ model in the limit of infinite order-parameter dimensionality.
T. Vojta, "Quantum version of a spherical model: Crossover from quantum to classical critical behavior," Physical Review B - Condensed Matter and Materials Physics, vol. 53, no. 2, pp. 710-714, American Physical Society (APS), Jan 1996.
The definitive version is available at http://dx.doi.org/10.1103/PhysRevB.53.710
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