Onset temperature for the Kosterlitz-Thouless transition in the vt=1 bilayer quantum Hall state

We investigate the onset temperature T* of the bilayer quantum Hall state at the total Landau level filling factor v(t) = 1 together with the activation energy gap Delta. The onset temperature is the temperature at which the quantum Hall state starts to form. We make two remarkable experimental observations: (1) As the total electron density decreases, T* increases while Delta decreases on account of the deterioration of electron mobility. (2) As the in-plane magnetic field increases, T* exhibits a rise at the point where Delta becomes a minimum. The minimum of Delta occurs at the transition point from the commensurate phase to the recently discovered soliton lattice phase. We discuss the relation between T* and the Kosterlitz-Thouless transition on the basis of the XY symmetry possessed by the v(t) = 1 bilayer system and discuss the experimentally observed behavior of T*.