Optical excitations from a flux phase

Abstract

We calculate the optical conductivity o(cv) of a model "Aux phase, " i.e. , a condensed manyfermion state on a two-dimensional lattice characterized by nonvanishing orbital currents of the underlying fermions. In addition to a Drude-like term, it is found that there is a temperature-(T) independent broadband absorption corresponding to vertical interband excitations near a point Fermi surface. Scattering of the quasiparticle excitations by acoustic phonons leads to a resistivity that is proportional to T, even as T 0.A number of recent theoretical studies have suggested that a novel form of quantum dynamics can occur in the ground state of a strongly correlated fermion system on a two-dimensional (2D) lattice. The discussions of Refs. 1-7 have examined the possibility of the stabilization of a "flux phase, " namely, a condensed many-body state characterized by nonvanishing orbital currents of the underlying fermions. These phases are condensed "spin liquid" states, which nonetheless exhibit ordering of a superlattice of orbital currents around the elementary plaquettes of the underlying Bravais lattice. A number of workers have suggested that states of this form provide an appropriate description of the dynamics of a resonatingvalence-bond (RVB) state near the half-filled band, and, in a recent communication, one of us noted that a simple construction for a class of Gutzwiller projected singlet spin liquid states naturally leads to states of this form for general band filling in the 2D problem.A unique feature common to all (or nearly all) of the treatments of these orbitally ordered flux phases is the appearance of singular points in k space for the quasiparticle dispersion relations. The Fermi surface for the effective fermions of the theories of Refs. 1-7 are shrunk to a pair of Fermi points, and locally exhibit the dispersion relation of a massless Dirac cone. The resulting quasiparticle dispersion relation and spectral density exhibit the form expected for a prototypical two-dimensional semimetal, with a spectral density vanishing linearly in E near the effective Fermi energy. In the theory of Ref. 7, applied off of the half-filled band, the single-particle excitations around this cone are charged excitations and can be excited optically. In this paper, we pursue this idea and investigate the temperature and frequency dependence of the resulting optical excitation spectrum for such a system. The excitation spectrum that we obtain is unusual and quite interesting, consisting of a low-frequency intraband part, and a higher-frequency interband contribution. The first of these may be regarded as a strongly temperaturedependent free-particle absorption, roughly analogous to the Drude absorption expected for a simple metal, and results from the thermally excited free-carrier density around the effective Fermi energy. The second piece is a relatively structureless, temperature-independent broadband excitation spectrum resulting from vertical interband excitations near the point Fermi surfaces. Below, we discuss the...

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