Nature464, 566-570 (25 March 2010) | doi:10.1038/nature08941; Received 15 December 2009; Accepted 18 February 2010
Anomalous structure in the single particle spectrum of the fractional quantum Hall effect
O. E. Dial1, R. C. Ashoori1, L. N. Pfeiffer2,3 & K. W. West2,3
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
Alcatel-Lucent Bell Laboratories, Murray Hill, New Jersey 07974, USA
Present address: Princeton University, Princeton, New Jersey 08544, USA.
Correspondence to: O. E. Dial1R. C. Ashoori1 Correspondence and requests for materials should be addressed to O.E.D. (Email: dial@alum.mit.edu) or R.C.A. (Email: ashoori@mit.edu).
The two-dimensional electron system is a powerful laboratory for investigating the physics of interacting particles. Application of a large magnetic field produces massively degenerate quantum levels known as Landau levels; within a Landau level the kinetic energy of the electrons is suppressed, and electron–electron interactions set the only energy scale1. Coulomb interactions break the degeneracy of the Landau levels and can cause the electrons to order into complex ground states. Here we observe, in the high energy single particle spectrum of this system, salient and unexpected structure that extends across a wide range of Landau level filling fractions. The structure appears only when the two-dimensional electron system is cooled to very low temperatures, indicating that it arises from delicate ground state correlations. We characterize this structure by its evolution with changing electron density and applied magnetic field, and present two possible models for understanding these observations. Some of the energies of the features agree qualitatively with what might be expected for composite fermions, which have proven effective for interpreting other experiments in this regime. At the same time, a simple model with electrons localized on ordered lattice sites also generates structure similar to that observed in the experiment. Neither of these models alone is sufficient to explain the observations across the entire range of densities measured. The discovery of this unexpected prominent structure in the single particle spectrum of an otherwise thoroughly studied system suggests that there exist core features of the two-dimensional electron system that have yet to be understood.
