In an ultraclean 2D electron system (2DES) subjected to crossed millimeterwave (30–150 GHz) and weak ($B<2\mathrm{k}\mathrm{G}$) magnetic fields, a series of apparently dissipationless states emerges as the system is detuned from cyclotron resonances. Such states are characterized by an exponentially vanishing low-temperature diagonal resistance and a classical Hall resistance. The activation energies associated with such states exceed the Landau level spacing by an order of magnitude. Our findings are likely indicative of a collective ground state previously unknown for 2DES.

• Received 6 September 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.046807