Transition from quantum Hall to compressible states in the second Landau level:: New light on the ν=5/2 enigma

Quantum Hall states at filling fraction nu = 5/2 are examined by numerical diagonalization. Spin-polarized and spin-unpolarized states of systems with N less than or equal to 18 electrons are studied, neglecting effects of Landau level mixing. We find that the ground state is spin polarized. It is incompressible and has a large overlap with paired states Like the Pfaffian. For a given sample, the energy gap is about 11 times smaller than at nu = 1/3. Evidence is presented of phase transitions to compressible states, driven by the interaction strength at short distance. A reinterpretation of experiments is suggested. [S0031-9007(97)05250-2].