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B D Simons

Publications and source records attributed to B D Simons.

At least 19 recordsLinked to original sources
PubMed2005

Phase bifurcation and quantum fluctuations in Sr3Ru2O7.

The bilayer ruthenate Sr3Ru2O7 has been cited as a textbook example of itinerant metamagnetic quantum criticality. However, recent studies of the ultrapure system have revealed striking anomalies in magnetism and transport in the vicinity of the quantum critical point. Drawing on fresh experimental data, we show that the complex phase behavior reported here can be fully accommodated within the framework of a simple Landau theory. We discuss the potential physical mechanisms that underpin the phenomenology, and assess the capacity of the ruthenate system to realize quantum tricritial behavior.

Journal Article↗
PubMed2005

Pattern formation as a signature of quantum degeneracy in a cold exciton system.

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. The stability of an electron-hole interface which describes recently observed exciton rings is analyzed. Interface instability occurs below a critical temperature, with a periodic 1D pattern developing via a continuous (type II) transition, in a qualitative agreement with observations.

Journal Article↗
PubMed2005

Dynamics of the BCS-BEC crossover in a degenerate Fermi gas.

We study the short-time dynamics of a degenerate Fermi gas positioned near a Feshbach resonance following an abrupt jump in the atomic interaction resulting from a change of magnetic field. We investigate the dynamics of the condensate order parameter and pair wave function for a range of field strengths. When the jump is sufficient to span the BCS to Bose-Einstein condensation crossover, we show that the rigidity of the momentum distribution precludes any atom-molecule oscillations in the entrance channel dominated resonances observed in 40K and 6Li. Focusing on material parameters tailored to the 40K Feshbach resonance at 202.1 G, we comment on the integrity of the fast sweep projection technique as a vehicle to explore the condensed phase in the crossover region.

Journal Article↗
PubMed2004

Formation mechanism and low-temperature instability of exciton rings.

The macroscopic rings observed in the photoluminescence patterns of excitons in coupled quantum wells are explained by a mechanism of carrier imbalance, transport, and recombination. The rings originate from the spatial separation of p and n carriers, and occur at the interface of the p and n domains, where excitons are generated. We explore the states of excitons in the ring over a range of temperatures down to 380 mK and report a transition of the ring into a periodic array of aggregates, a new low-temperature ordered exciton state.

Journal Article↗
PubMed2003

Universality of parametric spectral correlations: local versus extended perturbing potentials.

We explore the influence of an arbitrary external potential perturbation V on the spectral properties of a weakly disordered conductor. In the framework of a statistical field theory of a nonlinear sigma-model type we find, depending on the range and the profile of the external perturbation, two qualitatively different universal regimes of parametric spectral statistics (i.e., cross correlations between the spectra of Hamiltonians H and H+V). We identify the translational invariance of the correlations in the space of Hamiltonians as the key indicator of universality, and find the connection between the coordinate system in this space which makes the translational invariance manifest, and the physically measurable properties of the system. In particular, in the case of localized perturbations, the latter turn out to be the eigenphases of the scattering matrix for scattering off the perturbing potential V. They also have a purely statistical interpretation in terms of the moments of the level velocity distribution. Finally, on the basis of this analysis, a set of results obtained recently by the authors using random matrix theory methods is shown to be applicable to a much wider class of disordered and chaotic structures.

Journal Article↗
PubMed2003

Parametric statistics of individual energy levels in random Hamiltonians.

We establish a general framework to explore parametric statistics of individual energy levels in disordered and chaotic quantum systems of unitary symmetry. The method is applied to the calculation of the universal intralevel parametric velocity correlation function and the distribution of level shifts under the influence of an arbitrary external perturbation.

Journal Article↗
PubMed2002

Parametric spectral correlations in disordered and chaotic structures.

We explore the influence of external perturbations on the energy levels of a Hamiltonian drawn at random from the Gaussian unitary distribution of Hermitian matrices. By deriving the joint distribution function of eigenvalues, we obtain the (n,m)-point parametric correlation function of the initial and the final density of states for perturbations of arbitrary rank and strength. A further generalization of these results allows for the incorporation of short-range spatial correlations in diffusive as well as ballistic chaotic structures.

Journal Article↗
PubMed2001

Quantized adiabatic charge transport in a carbon nanotube.

The coupling of a semimetallic carbon nanotube to a surface acoustic wave (SAW) is proposed as a vehicle to realize quantized adiabatic charge transport. We demonstrate that electron backscattering from a periodic SAW potential can be used to induce a miniband spectrum at energies near the Fermi level. Within the framework of Luttinger liquid theory, electron interaction is shown to enhance minigaps and thereby improve current quantization.

Journal Article↗
PubMed2000

Posterior fixation suture augmentation of full-tendon vertical rectus muscle transposition for abducens palsy.

OBJECTIVE: To evaluate the effect of augmenting full-tendon vertical rectus transpositions with posterior fixation sutures in patients with complete or near-complete lateral rectus palsy. METHODS: Transposition of the vertical recti to the lateral rectus muscle was performed in seven patients with unilateral lateral rectus palsy (the mean angle of preoperative horizontal deviation in primary gaze was 36.7 prism diopters (delta); range, 25-62delta of esotropia). A posterior fixation suture of 5.0 Mersilene (Ethicon, Somerville, NJ) was placed in sclera (14-16 mm posterior to the limbus) adjacent to the lateral rectus and incorporated 1/3 belly width of each transposed vertical rectus muscle. RESULTS: The mean angle of postoperative horizontal deviation in primary gaze was 7.1delta (range, 0-20delta). The mean change in primary-position horizontal deviation postoperatively was 41.2delta (range, 37-72delta). Four patients were able to fuse without prism in primary gaze; three patients were orthophoric and one patient had a consecutive intermittent exotropia. The remaining three patients required prism correction to neutralize the postoperative gaze deviation. All patients had improvement in abduction. Mild limitation of adduction was noted in three patients (range, -0.5 to -2.0). CONCLUSIONS: Augmenting full vertical rectus muscle transpositions with posterior fixation sutures improves the abducting effect of surgery without significant limitation of adduction.

Abducens Nerve Diseases↗
PubMed1999

Pediatric photoscreening for strabismus and refractive errors in a high-risk population.

OBJECTIVE: To determine the accuracy of the MTI Photoscreener in detecting strabismus and refractive errors in children. PARTICIPANTS: One hundred children underwent MTI photoscreening followed by complete ophthalmologic examination. Six observers graded the photographs for strabismus, according to the location of the corneal light reflexes, and for refractive error, according to the size and location of the light crescent. RESULTS: The sensitivity of the MTI Photoscreener in detecting any amblyogenic factor was 80% to 91%, with a specificity of 20% to 67%. The sensitivity and specificity for particular amblyogenic factors varied widely among observers. The ranges were as follows: strabismus, sensitivity = 23% to 50%, specificity = 76% to 96%; myopia, sensitivity = 89%, specificity = 48% to 76%; hyperopia, sensitivity = 20% to 80%, specificity = 88% to 96%; and astigmatism, sensitivity = 46% to 77%, specificity = 79% to 89%. CONCLUSIONS: These results suggest caution in relying on photoscreening to detect strabismus and refractive errors in children.

Child↗
PubMed1999

Calculation of relaxation rates from microscopic equations of motion.

For classical systems with anharmonic forces, Newton's equations for particle trajectories are nonlinear, while Liouville's equation for the evolution of functions of position and momentum is linear and is solved by constructing a basis of functions in which the Liouvillian is a tridiagonal matrix, which is then diagonalized. For systems that are chaotic in the sense that neighboring trajectories diverge exponentially, the initial conditions determine the solution to Liouville's equation for short times; but for long times, the solutions decay exponentially at rates independent of the initial conditions. These are the relaxation rates of irreversible processes, and they arise in these calculations as the imaginary parts of the frequencies where there are singularities in the analytic continuations of solutions to Liouville's equation. These rates are calculated for two examples: the inverted oscillator, which can be solved both analytically and numerically, and a charged particle in a periodic magnetic field, which can only be solved numerically. In these systems, dissipation arises from traveling-wave solutions to Liouville's equation that couple low and high wave-number modes allowing energy to flow from disturbances that are coherent over large scales to disturbances on ever smaller scales finally becoming incoherent over microscopic scales. These results suggest that dissipation in large scale motion of the system is a consequence of chaos in the small scale motion.

Journal Article↗
PubMed1999

Surgical management of ocular motor cranial nerve palsies.

This article reviews the various surgical treatment approaches currently used in the management of ocular motor cranial nerve palsies. Regardless of the approach, the final goal is improved alignment, especially in the primary and reading positions.

Eye Movements↗
PubMed1998

Outcome of surgical management of superior oblique palsy: a study of 123 cases.

PURPOSE: To determine the outcome of the surgical management of superior oblique palsy at our institution. SUBJECTS AND METHODS: Retrospective review of 123 patients who underwent surgical correction of superior oblique paresis at Bascom Palmer Eye Institute from 1976 to 1996. Subject-Patients: 67% were male and 33% female. The mean age at surgery was 30.5 years (range, 2-78 years). Etiologies of the pareses were trauma (34%), congenital (33%), and acquired/non-traumatic (33%). The mean angle of preoperative vertical deviation in primary gaze was 14.0 delta (range, 0-45 delta). SURGERY: 109/123 (89%) patients underwent single muscle surgery. Of these 109, 57 had single oblique muscle surgery: a superior oblique tuck in 34/57 (60%); an inferior oblique weakening procedure in 22/57 (38%); and a Harada-Ito procedure in 1/57 (2%). The other 14 patients (11%) had bilateral surgery. RESULTS: The final postoperative vertical deviation in primary gaze was < or =3 PD in 60% of patients and < or =7 PD in 80%. The mean change in primary position vertical deviation postoperatively was 10.4 PD for distance and 13.0 PD for near. An "excellent" outcome (final vertical deviation &le3 PD in primary and reading gazes) was achieved most frequently in those patients with congenital pareses and isolated oblique muscle surgery. COMPLICATIONS: Clinically significant Brown's Syndrome occurred in 43/72 (60%) of those cases who had undergone a superior oblique tuck. The incidence of Brown's Syndrome was unrelated to tuck size. Reoperation was three times more likely to be necessary in traumatic cases than in congenital cases (35.0% vs 11.9%, p=0.02). CONCLUSIONS: Based on these results we recommend oblique muscle surgery as the initial procedure to correct superior oblique palsy when appropriate.

Adolescent↗
PubMed1997

A pars plana approach for cataract surgery in posterior lenticonus.

PURPOSE: To report the management and outcome of cataract surgery and intraocular lens placement in a child with unilateral posterior lenticonus. METHODS: Case report. A 7-year-old boy with a best-corrected visual acuity of RE, 20/200, posterior lenticonus, and cataract underwent a pars plana lensectomy, vitrectomy, posterior chamber intraocular lens insertion into the ciliary sulcus, and central anterior capsulotomy. RESULT: At 2 years of follow-up, best-corrected visual acuity was RE, 20/40. CONCLUSION: This technique allowed complete removal of the opaque posterior lenticonus plaque while preserving the peripheral anterior capsule for sulcus fixation of the posterior chamber intraocular lens.

Cataract Extraction↗
PubMed1997

Branch retinal vein occlusion. Axial length and other risk factors.

OBJECTIVE: To determine the association between axial length, as a measurement of hyperopia, and branch retinal vein occlusion and to determine the clinical characteristics and other risk factors of patients with branch retinal vein occlusion. METHODS: A case-control study was performed using 36 patients with branch retinal vein occlusion and 36 age- and sex-matched control patients selected from a list of subjects who had undergone cataract extraction. RESULTS: There was essentially no difference in axial length between patients the disorder and control patients (23.55 mm versus 23.62 mm; P = 0.79). Although the intraocular pressure (IOP) among control eyes was somewhat higher than that in branch retinal vein occlusion eyes, the difference was not statistically significant (P = 0.32). Systemic hypertension was more common in patients with branch retinal vein occlusion (53%) than in control patients (42%) but the difference was not statistically significant (P = 0.35). Chronic open-angle glaucoma was present in 14% of patients with branch retinal vein occlusion and 22% of control patients (P = 0.37), but this difference was not statistically significant. Diabetes mellitus was two times more common in controls (28%) than in patients with branch retinal vein occlusion (14%). This difference, however, also did not reach statistical significance (P = 0.13). CONCLUSIONS: Hyperopia as measured by axial length is not a risk factor for branch retinal vein occlusion. This study provides evidence that hypertension is a risk factor for branch retinal vein occlusion and that chronic open-angle glaucoma and diabetes mellitus are not risk factors for branch retinal vein occlusion.

Adult↗