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Abstract

The observation of vanishing electrical resistance in condensed
matter has led to the discovery of new phenomena such as, for
example, superconductivity, where a zero-resistance state can
be detected in a metal below a transition temperature T-c (ref.
1). More recently, quantum Hall effects were discovered from
investigations of zero-resistance states at low temperatures
and high magnetic fields in two-dimensional electron systems
(2DESs)(2-4). In quantum Hall systems and superconductors,
zero-resistance states often coincide with the appearance of a
gap in the energy spectrum(1,2,4). Here we report the
observation of zero-resistance states and energy gaps in a
surprising setting(5) : ultrahigh-mobility GaAs/AlGaAs
heterostructures that contain a 2DES exhibit vanishing diagonal
resistance without Hall resistance quantization at low
temperatures and low magnetic fields when the specimen is
subjected to electromagnetic wave excitation. Zero-resistance
states occur about magnetic fields B=4/5B(f) and B=4/9B(f),
where B-f=2pifm*/e;m* is the electron mass, e is the electron
charge, and f is the electromagnetic-wave frequency. Activated
transport measurements on the resistance minima also indicate
an energy gap at the Fermi level(6). The results suggest an
unexpected radiation-induced, electronic-state-transition in
the GaAs/AlGaAs 2DES.