A Sound (and Light) Way to Measure Confined Electrons

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Science  22 May 2009:
Vol. 324, Issue 5930, pp. 1022-1023
DOI: 10.1126/science.1173828

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The conservation of momentum and energy underlies many powerful spectroscopic techniques. Absorption spectroscopy is based on the principle that a wave incident on an object can only be absorbed if both its momentum and energy match that of an excitation mode of the object. On page 1044 of this issue, Kukushkin et al. (1) describe a variant of this technique for measuring the energy and momentum dependence of the excitations of a two-dimensional (2D) electron system. In this technique, momentum is imparted with sound and, separately, energy is imparted with light. This approach allows the spectrum of “magnetorotons”—characteristic excitations of the states associated with the fractional quantum Hall effect—to be measured directly. The observed spectral features were predicted many years ago (24) but have eluded direct measurement until now.