Universal amplitude ratios from numerical studies of the three-dimensional O(2) model

We investigate the three-dimensional O(2) model near the critical point by Monte Carlo simulations and calculate the major universal amplitude ratios of the model. The ratio U0=A+/A- is determined directly from the specific heat data at zero magnetic field. The data do not, however, allow us to extract an accurate estimate for α. Instead, we establish a strong correlation of U0 with the value of α used in the fit. This numerical α-dependence is given by A+/A+=1-4.20(5)α+O(α2). For the special α-values used in other calculations, we find full agreement with the corresponding ratio values, e.g. that of the shuttle experiment with liquid helium. On the critical isochore, we obtain the ratio ξ+/ξ-T=0.293(9), and on the critical line the ratio ξcT/ξcL=1.957(10) for the amplitudes of the transverse and longitudinal correlation lengths. These two ratios are independent of the α - or ν-values used here. (author)