The Fractional Quantum Hall Effect at Even Denominators

Abstract

The recent discovery of the fractional quantum Hall effect (FQHE) at the even—denominator filling fraction v=5/2 represents yet another surprise in the area of two-dimensional electron physics. Previous to the work of WILLETT, et al.[1] the FQHE was strongly believed to occur solely at odd-denominator rational fractional fillings (v=p/q,q-odd) of the Landau levels. This belief was generated by both a wealth of experimental data and strong theoretical arguments[2] going as deep as the Pauli exclusion principle. How then does this new observation fit in with our overall understanding of the FQHE and 2D correlation effects in general? Here we present data[3] strongly suggesting that the underlying many-body ground state for the v=5/2 FQHE is not spin polarized, in contrast to the canonical v=1/3 and 2/3 incompressible quantum liquid states. Further, the apparent absence of even-denominator states in the lowest Landau level v<2 points to important differences between electron correlations in the first and second Landau levels. The purpose of the present paper is to review the existing experimental data[1,3] on the v=5/2 FQHE and show how this startling new result can, in principle at least, be accommodated by a generalized picture of the FQHE.