Physical Review E
Bulk and surface phase transitions in the three-dimensional O(4) spin model
Youjin Deng
Abstract
We investigate the $O(4)$ spin model on the simple-cubic lattice by means of the Wolff cluster algorithm. Using the toroidal boundary condition, we locate the bulk critical point at coupling ${K}_{c}=0.935\phantom{\rule{0.2em}{0ex}}856(2)$, and determine the bulk thermal magnetic renormalization exponents as ${y}_{t}=1.337\phantom{\rule{0.2em}{0ex}}5(15)$ and ${y}_{h}=2.482\phantom{\rule{0.2em}{0ex}}0(2)$, respectively. The universal ratio $Q={⟨{m}^{2}⟩}^{2}∕⟨{m}^{4}⟩$ is also determined as 0.9142(1). The precision of these estimates significantly improves over that of the existing results. Then, we simulate the critical $O(4)$ model with two open surfaces on which the coupling strength ${K}_{1}$ can be varied. At the ordinary transitions, the surface magnetic exponent is determined as ${y}_{h1}^{(\mathrm{o})}=1.020\phantom{\rule{0.2em}{0ex}}2(12)$. Further, we find a so-called special surface transition at $\ensuremath{\kappa}={K}_{1}∕K\ensuremath{-}1=1.258(20)$. At this point, the surface thermal exponent ${y}_{t1}^{(\mathrm{s})}$ is rather close to zero, and we cannot exclude that the corresponding surface transition is Kosterlitz-Thouless-like. The surface magnetic exponent is ${y}_{h1}^{(\mathrm{s})}=1.816(2)$.