Spin instabilities and quantum phase transitions in integral and fractional quantum Hall states -: art. no. 201301

The spin excitations of quantum Hall states at filling factors nu=2 and 4/3 are investigated numerically in the systems with comparable cyclotron ((h) over bar omega(c)) and Zeeman (E-Z) gaps. The relevant quasiparticles and their interactions are studied, including spin wave and skyrmion bound states. For nu=2, a spin instability at a finite value of epsilon=(h) over bar omega(c)-E-Z leads to an abrupt paramagnetic to ferromagnetic transition, in agreement with the mean-field approximation. However, for nu=4/3 a new quantum phase transition is found in finite-size droplets that involves a gradual change from para- to ferromagnetic occupancy.