Orthogonality catastrophe in a composite fermion liquid

We discuss the emergence of an orthogonality catastrophe in the response of a composite fermion liquid as the filling factor \nu approaches 1/2m, where m=1,2,3.... A tunneling experiment is proposed in which dramatic changes in the I-V characteristic should be observable as \nu is varied. Explicit I-V characteristics calculated within the so-called Modified Random Phase Approximation, are provided for \nu=1/3 -> \nu=1/2.Comment: Latex two-column 6 pages including 5 figure

Optics with Quantum Hall Skyrmions

A novel type of charged excitation, known as a Skyrmion, has recently been discovered in quantum Hall systems with filling factor near \nu = 1. A Skyrmion -- which can be thought of as a topological twist in the spin density of the electron gas -- has the same charge as an electron, but a much larger spin. In this review we present a detailed theoretical investigation of the optical properties of Skyrmions. Our results provide means for the optical detection of Skyrmions using photoluminescence (PL) spectroscopy. We first consider the optical properties of Skyrmions in disordered systems. A calculation of the luminescence energy reveals a special optical signature which allows us to distinguish between Skyrmions and ordinary electrons. Two experiments to measure the optical signature are proposed. We then turn to the optical properties of Skyrmions in pure systems. We show that, just like an ordinary electron, a Skyrmion may bind with a hole to form a Skyrmionic exciton. The Skyrmionic exciton can have a lower energy than the ordinary magnetoexciton. The optical signature of Skyrmions is found to be a robust feature of the PL spectrum in both disordered and pure systems.Comment: 31 pages, LaTex, 11 eps figures. ijmpb style file included. Review article submitted to Int. J. Mod. Phys.

Kondo Insulator: p-wave Bose Condensate of Excitons

In the Anderson lattice model for a mixed-valent system, the $d-f$ hybridization can possess a $p$-wave symmetry. The strongly-correlated insulating phase in the mean-field approximation is shown to be a $p$-wave Bose condensate of excitons with a spontaneous lattice deformation. We study the equilibrium and linear response properties across the insulator-metal transition. Our theory supports the empirical correlation between the lattice deformation and the magnetic susceptibility and predicts measurable ultrasonic and high-frequency phonon behavior in mixed-valent semiconductors.Comment: 5 pages, 3 encapsulated PostScript figure

Linear and nonlinear optical characteristics of the Falicov-Kimball model

We calculate the linear and nonlinear optical properties of the Falicov-Kimball model for a mixed-valent system within the self-consistent mean-field approximation. Second-harmonic generation can only occur if the mixed-valent state has a built-in coherence between the itinerant d-electrons and the localized f-holes. By contrast, second-harmonic generation cannot occur for solutions of the model with f-site occupation as a good quantum number. As an experimental test of coherence in mixed-valent compounds we propose a measurement of the dynamic second-order susceptibility.Comment: 4 pages, 2 PostScript figures, to appear in Physical Review Letter

Skyrmionic excitons

We investigate the properties of a Skyrmionic exciton consisting of a negatively charged Skyrmion bound to a mobile valence hole. A variational wave function is constructed which has the generalized total momentum P as a good quantum number. It is shown that the Skyrmionic exciton can have a larger binding energy than an ordinary magnetoexciton and should therefore dominate the photoluminescence spectrum in high-mobility quantum wells and heterojunctions where the electron-hole separation exceeds a critical value. The dispersion relation for the Skyrmionic exciton is discussed.Comment: 9 pages, RevTex, 2 PostScript figures. Replaced with version to appear in Phys. Rev. B Rapid Communications. Short discussion of variational state adde

Comparison of unifocal, flicker, and multifocal pupil perimetry methods in healthy adults

To this day, the most popular method of choice for testing visual field defects (VFDs) is subjective standard automated perimetry. However, a need has arisen for an objective, and less time-consuming method. Pupil perimetry (PP), which uses pupil responses to onsets of bright stimuli as indications of visual sensitivity, fulfills these requirements. It is currently unclear which PP method most accurately detects VFDs. Hence, the purpose of this study is to compare three PP methods for measuring pupil responsiveness. Unifocal (UPP), flicker (FPP), and multifocal PP (MPP) were compared by monocularly testing the inner 60 degrees of vision at 44 wedge-shaped locations. The visual field (VF) sensitivity of 18 healthy adult participants (mean age and SD 23.7 ± 3.0 years) was assessed, each under three different artificially simulated scotomas for approximately 4.5 minutes each (i.e. stimulus was not or only partially present) conditions: quadrantanopia, a 20-, and 10-degree diameter scotoma. Stimuli that were fully present on the screen evoked strongest, partially present stimuli evoked weaker, and absent stimuli evoked the weakest pupil responses in all methods. However, the pupil responses in FPP showed stronger discriminative power for present versus absent trials (median d-prime = 6.26 ± 2.49, area under the curve [AUC] = 1.0 ± 0) and MPP performed better for fully present versus partially present trials (median d-prime = 1.19 ± 0.62, AUC = 0.80 ± 0.11). We conducted the first in-depth comparison of three PP methods. Gaze-contingent FPP had best discriminative power for large (absolute) scotomas, whereas MPP performed slightly better with small (relative) scotomas

Theory of Electronic Ferroelectricity

We present a theory of the linear and nonlinear optical characteristics of the insulating phase of the Falicov-Kimball model within the self-consistent mean-field approximation. The Coulomb attraction between the itinerant d-electrons and the localized f-holes gives rise to a built-in coherence between the d and f-states, which breaks the inversion symmetry of the underlying crystal, leading to: (1) electronic ferroelectricity, (2) ferroelectric resonance, and (3) a nonvanishing susceptibility for second-harmonic generation. As experimental tests of such a built-in coherence in mixed-valent compounds we propose measurements of the static dielectric constant, the microwave absorption spectrum, and the dynamic second-order susceptibility.Comment: 15 pages, 5 PostScript figures, submitted to Physical Review

Anomalous magnetic response of the spin-one-half Falicov-Kimball model

The infinite-dimensional spin one-half Falicov-Kimball model in an external magnetic field is solved exactly. We calculate the magnetic susceptibility in zero field, and the magnetization as a function of the field strength. The model shows an anomalous magnetic response from thermally excited local moments that disappear as the temperature is lowered. We describe possible real materials that may exhibit this kind of anomalous behavior.Comment: 17 pages, 6 encapsulated postscript figures (included), submitted to Phys. Rev.

The Trade-Off Between Luminance and Color Contrast Assessed With Pupil Responses

Purpose: A scene consisting of a white stimulus on a black background incorporates strong luminance contrast. When both stimulus and background receive different colors, luminance contrast decreases but color contrast increases. Here, we sought to charac-terize the pattern of stimulus salience across varying trade-offs of color and luminance contrasts by using the pupil light response. Methods: Three experiments were conducted with 17, 16, and 17 healthy adults. For all experiments, a flickering stimulus (2 Hz; alternating color to black) was presented super-imposed on a background with a complementary color to the stimulus (i.e., opponency colors in human color perception: blue and yellow for Experiment 1, red and green for Experiment 2, and equiluminant red and green for Experiment 3). Background luminance varied between 0% and 45% to trade off luminance and color contrast with the stimulus. By comparing the locus of the optimal trade-off between color and luminance across different color axes, we explored the generality of the trade-off. Results: The strongest pupil responses were found when a substantial amount of color contrast was present (at the expense of luminance contrast). Pupil response ampli-tudes increased by 15% to 30% after the addition of color contrast. An optimal pupillary responsiveness was reached at a background luminance setting of 20% to 35% color contrast across several color axes. Conclusions: These findings suggest that a substantial component of pupil light responses incorporates color processing. More sensitive pupil responses and more salient stimulus designs can be achieved by adding subtle levels of color contrast between stimulus and background. Translational Relevance: More robust pupil responses will enhance tests of the visual field with pupil perimetry

Spin degree of freedom in two dimensional exciton condensates

We present a theoretical analysis of a spin-dependent multicomponent condensate in two dimensions. The case of a condensate of resonantly photoexcited excitons having two different spin orientations is studied in detail. The energy and the chemical potentials of this system depend strongly on the spin polarization . When electrons and holes are located in two different planes, the condensate can be either totally spin polarized or spin unpolarized, a property that is measurable. The phase diagram in terms of the total density and electron-hole separation is discussed.Comment: 4 pages, 3 figures, Accepted for publication in Physical Review Letter