STM in the fractional quantum Hall effect: Spectroscopy of composite-fermion bound states

The fractional quantum Hall states are non-Fermi liquids of electrons, in that their ground states and low energy excitations are described not in terms of electrons but in terms of composite fermions which are bound states of electrons and $2p$ quantized vortices. An electron or a hole at filling factor $\nu=n/(2pn+1)$, where $p,n$ are integers, is a complex molecule of $2pn+ 1$ quasiparticles (excited composite fermions) or quasiholes (missing composite fermions) and has its own internal excitations. Recent scanning tunneling microscopy experiments have succeeded in measuring the electron spectral functions of these states, which provides valuable information on the nature of these strongly correlated molecules and thereby on the short-distance correlations in the fractional quantum Hall liquids. These experiments exhibit several sharp peaks in the tunneling spectra. Detailed calculations based on the composite-fermion theory demonstrate multiple peaks in the local density of states, and we argue that the separation between the peaks represents interaction-corrected composite-fermion cyclotron energy. We discuss what aspects of experiments are explained by our model and which ones remain to be explained.