Physics – Condensed Matter – Strongly Correlated Electrons
Scientific paper
1999-08-03
Int. Jou. Mod. Phys. B Vol. 16 No.9, 1363 (2002)
Physics
Condensed Matter
Strongly Correlated Electrons
11 pages, 4 eps figures
Scientific paper
10.1142/S0217979202010099
We investigate both numerically and analytically the behaviour of a spin-1 antiferromagnetic (AFM) isotropic Heisenberg chain in an external magnetic field. Extensive DMRG studies of chains up to N=80 sites extend previous analyses and exhibit the well known phenomenon of the closure of the Haldane gap at a lower critical field H_c1. We obtain an estimate of the gap below H_c1. Above the lower critical field, when the correlation functions exhibit algebraic decay, we obtain the critical exponent as a function of the net magnetization as well as the magnetization curve up to the saturation (upper critical) field H_c2. We argue that, despite the fact that the SO(3) symmetry of the model is explicitly broken by the field, the Haldane phase of the model is still well described by an SO(3) nonlinear sigma-model. A mean-field theory is developed for the latter and its predictions are compared with those of the numerical analysis and with the existing literature.
Ercolessi Elisa
Morandi Giuseppe
Pieri Pierbiagio
Roncaglia Marco
Venuti Lorenzo Campos
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