Transference of Transport Anisotropy to Composite Fermions

When interacting two-dimensional electrons are placed in a large perpendicular magnetic field, to minimize their energy, they capture an even number of flux quanta and create new particles called composite fermions (CFs). These complex electron-flux-bound states offer an elegant explanation for the fractional quantum Hall effect. Furthermore, thanks to the flux attachment, the effective field vanishes at a half-filled Landau level and CFs exhibit Fermi-liquid-like properties, similar to their zero-field electron counterparts. However, being solely influenced by interactions, CFs should possess no memory whatever of the electron parameters. Here we address a fundamental question: Does an anisotropy of the electron effective mass and Fermi surface (FS) survive composite fermionization? We measure the resistance of CFs in AlAs quantum wells where electrons occupy an elliptical FS with large eccentricity and anisotropic effective mass. Similar to their electron counterparts, CFs also exhibit anisotropic transport, suggesting an anisotropy of CF effective mass and FS.Comment: 5 pages, 5 figure

Coherent-potential-approximation study of excitonic absorption in orientationally disordered molecular aggregates

We study the dynamics of a single Frenkel exciton in a disordered molecular chain. The coherent-potential approximation is applied to the situation where the single-molecule excitation energies as well as the transition dipole moments, both their absolute values and orientations, are random. Such a model is believed to be relevant for the description of the linear optical properties of one-dimensional J aggregates. We calculate the exciton density of states, the linear absorption spectra, and the exciton coherence length which reveals itself in the linear optics. A detailed analysis of the low-disorder limit of the theory is presented. In particular, we derive asymptotic formulas relating the absorption linewidth and the exciton coherence length to the strength of disorder. Such expressions account simultaneously for all the above types of disorders and reduce to well-established form when no disorder in the transition dipoles is present. The theory is applied to the case of purely orientational disorder and is shown to agree well with exact numerical diagonalization