Helical nuclear spin order in two-subband quantum wires

quantum wires, the hyperfine coupling between conduction electrons and nuclear spins can lead to a (partial) ordering of both of them at low temperatures. By an interaction-enhanced mechanism, the nuclear spin order, caused by Ruderman-Kittel-Kasuya-Yosida exchange, acts back onto the electrons and gaps out part of their spectrum. In wires with two subbands characterized by distinct Fermi momenta k(F1) and k(F2), the nuclear spins form a superposition of two helices with pitches pi/k(F1) and pi/k(F2), thus exhibiting a beating pattern. This order results in a reduction of the electronic conductance in two steps upon lowering the temperature.