CRITICAL-BEHAVIOR OF THE 2-DIMENSIONAL ISING-MODEL IN A TRANSVERSE FIELD - A DENSITY-MATRIX RENORMALIZATION CALCULATION

We have adjusted the density-matrix renormalization method to handle t wo-dimensional systems of limited width. The key ingredient for this e xtension is the incorporation of symmetries in the method. The advanta ge of our approach is that we can force certain symmetry properties to the resulting ground-state wave function. Combining the results obtai ned for system sizes up to 30x6 and finite-size scaling, we derive the phase-transition point and the critical exponent for the gap in the I sing model in a transverse field on a two-dimensional square lattice.