The nature of composite fermions and the role of particle hole symmetry: A microscopic account

TL;DR

This paper investigates the microscopic origins of composite fermions in the fractional quantum Hall effect, explores their Dirac nature and particle-hole symmetry, and assesses the compatibility of Chern-Simons theory with the lowest Landau level.

Contribution

It provides a microscopic derivation linking particle-hole symmetry to effective time reversal for composite fermions and introduces a normal form for wave functions of certain fractional states.

Findings

Particle-hole symmetry corresponds to time reversal for composite fermions.

The $ ext{Lambda}$ level spacing is nearly independent of level index.

Wave functions from Chern-Simons theory are accurate after lowest Landau level projection.

Abstract

Motivated by the issue of particle-hole symmetry for the composite fermion Fermi sea at the half filled Landau level, Dam T. Son has made an intriguing proposal [Phys. Rev. X {\bf 5}, 031027 (2015)] that composite fermions are Dirac particles. We ask what features of the Dirac-composite fermion theory and its various consequences may be reconciled with the well established microscopic theory of the fractional quantum Hall effect and the 1/2 state, which is based on {\em non-relativistic} composite fermions. Starting from the microscopic theory, we derive the assertion of Son that the particle-hole transformation of electrons at filling factor $\nu=1/2$ corresponds to an effective time reversal transformation (i.e. $\{\vec{k}_j\}$$\rightarrow$$\{-\vec{k}_j\}$) for composite fermions, and discuss how this connects to the absence of $2k_{\rm F}$ backscattering in the presence of a particle-hole symmetric disorder. By considering bare holes in various composite-fermion $\Lambda$ levels (analogs of electronic Landau levels) we determine the $\Lambda$ level spacing and find it to be very nearly independent of the $\Lambda$ level index, consistent with a parabolic dispersion for the underlying composite fermions. Finally, we address the compatibility of the Chern-Simons theory with the lowest Landau level constraint, and find that the wave functions of the mean-field Chern-Simons theory, as well as a class of topologically similar wave functions, are surprisingly accurate when projected into the lowest Landau level. These considerations lead us to introduce a "normal form" for the unprojected wave functions of the $n/(2pn-1)$ states that correctly capture the topological properties even without lowest Landau level projection.