EVIDENCE OF SOFT-MODE QUANTUM PHASE-TRANSITIONS IN ELECTRON DOUBLE-LAYERS

Inelastic Light scattering by low-energy spin-excitations reveals thre e distinct configurations of spin of electron double Layers in gallium arsenide quantum wells at even-integer quantum Hall states. The trans formations among these spin states appear as quantum phase transitions driven by the interplay between Coulomb interactions and Zeeman split tings. One of the transformations correlates with the emergence of a s pin-flip intersubband excitation at vanishingly Low energy and provide s direct evidence of a Link between quantum phase transitions and soft collective excitations in a two-dimensional electron system.