Phonon Measurements of the Energy Gap in the Fractional Quantum Hall State
The excitation spectrum A(k) of the magnetoroton excitations of the fractional quantum Hall (FQH) state is believed to fall from a maximum A(0) at k = 0 to a minimum of value Δmin~0.5Δ(0) at k~1B -1(lB=magnetic length) and ultimately approaches A(∞)=0.67Δ(0) (Girvin et al, 1986). Experimental measurements of the excitation spectrum have previously been limited to transport and optical luminescence techniques. We have made initial phonon absorption studies of the FQHE which we believe give a direct measurement of the energy gap near to the magnetoroton minimum.
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