Scatterer that leaves "footprints" but no "fingerprints"

We calculate the exact transmission coefficient of a quantum wire in the presence of a single point defect at the wire's cut-off frequencies. We show that while the conductance pattern (i.e., the scattering) is strongly affected by the presence of the defect, the pattern is totally independent of the defect's characteristics (i.e., the defect that caused the scattering cannot be identified from that pattern).Comment: 4 pages, 3 figure

Resonances in a two-dimensional electron waveguide with a single delta-function scatterer

We study the conductance properties of a straight two-dimensional electron waveguide with an s-like scatterer modeled by a single delta-function potential with a finite number of modes. Even such a simple system exhibits interesting resonance phenomena. These resonances are explained in terms of quasi-bound states both by using a direct solution of the Schroedinger equation and by studying the Green's function of the system. Using the Green's function we calculate the survival probability as well as the power absorption and show the influence of the quasi-bound states on these two quantities.Comment: 5 pages, 6 figures, to be published in Physical Review

Hall of Mirrors Scattering from an Impurity in a Quantum Wire

This paper develops a scattering theory to examine how point impurities affect transport through quantum wires. While some of our new results apply specifically to hard-walled wires, others--for example, an effective optical theorem for two-dimensional waveguides--are more general. We apply the method of images to the hard-walled guide, explicitly showing how scattering from an impurity affects the wire's conductance. We express the effective cross section of a confined scatterer entirely in terms of the empty waveguide's Green's function, suggesting a way in which to use semiclassical methods to understand transport properties of smooth wires. In addition to predicting some new phenomena, our approach provides a simple physical picture for previously observed effects such as conductance dips and confinement-induced resonances.Comment: 19 pages, 8 figures. Accepted for publication in Physical Review B. Minor additions to text, added reference

Transport through a quantum ring, a dot and a barrier embedded in a nanowire in magnetic field

We investigate the transport through a quantum ring, a dot and a barrier embedded in a nanowire in a homogeneous perpendicular magnetic field. To be able to treat scattering potentials of finite extent in magnetic field we use a mixed momentum-coordinate representation to obtain an integral equation for the multiband scattering matrix. For a large embedded quantum ring we are able to obtain Aharanov-Bohm type of oscillations with superimposed narrow resonances caused by interaction with quasi-bound states in the ring. We also employ scattering matrix approach to calculate the conductance through a semi-extended barrier or well in the wire. The numerical implementations we resort to in order to describe the cases of weak and intermediate magnetic field allow us to produce high resolution maps of the ``near field'' scattering wave functions, which are used to shed light on the underlying scattering processes.Comment: RevTeX, 13 pages with included postscript figures, high resolution version available at http://hartree.raunvis.hi.is/~vidar/Rann/VG_04.pd

Children’s friendships in middle childhood: how number of friends, reciprocity and friendship quality relate to peer and school identification, and general self-worth

Children’s friendships are important for well-being and school adjustment, but few studies have examined multiple indices of friendships together in middle childhood. The current study surveyed 7 to 11-year olds (n = 314) about their friendships, best friendships, friendship quality and indices of self-worth, identification with peers and identification with school. Peer relationships were positively related to self-worth, but not identification with peers or school. Best friendship quality moderated the relationship between number of reciprocated friendship nominations and self-worth. Children with a reciprocated best friend had higher friendship quality and peer identification than others. Where best friendship was reciprocated, the relationship with identification with peers was mediated via positive friendship quality. The results suggest that friendship reciprocity is particularly relevant for children’s self-worth and identification with peers. The findings are discussed in relation to the importance of fostering the development of reciprocated friendships

Quantum current magnification in a multi-channel mesoscopic ring

We have studied the current magnification effect in a multi-channel open mesoscopic ring. We show that the current magnification effect is robust even in the presence of several propagating modes inspite of mode mixing and cancellation effects. The magnitude of circulating currents in the multi-channel regime can be much larger than that in a single channel case. Impurities can enhance or degrade the current magnification effect depending sensitively on the system parameters. Circulating currents are mostly associated with Fano resonances in the total transport current. We further show that system-lead coupling qualitatively changes the current magnification effect.Comment: 12 pages, 11 figure

Radiographic and Clinical Factors in Pediatric Patients With Surgical Small-bowel Intussusception

Background When evaluating a pediatric patient with abdominal pain, identification of a small bowel–to–small bowel intussusception (SBI) on radiologic imaging can create a diagnostic dilemma. The clinical significance and need for surgical exploration of SBI is highly variable, as most of them are considered clinically insignificant. We hypothesize that combination of clinical and radiologic factors in an exclusively SBI population will yield factors that guide the clinician in making operative decisions. Methods A comprehensive database from a pediatric tertiary hospital was reviewed from January 1, 2011, to December 31, 2016, for any radiographic study mentioning intussusception. Results were reviewed for patients having only SBI (i.e., not ileocolic intussusception), and this comprised the study cohort. The electronic medical records for these patients were reviewed for clinical presentation variables, need for operative intervention, and identification of the intussusception during surgery. Patients with SBI due to enteral feeding tubes were excluded from the study. Results Within the study period, 139 patients were identified with an SBI on radiologic imaging. Univariate analysis yielded numerous clinical and radiologic factors highly predictive of the need for surgical intervention. However, upon multivariate analysis, only a history of prior abdominal surgery (odds ratio [OR]: 7.2; CI: 1.1-46.3), the presence of focal abdominal pain (OR: 22.1; CI: 4.2-116.3), and the intussusception length (cm; OR: 10.6; CI: 10.3-10.8) were correlated with the need for surgical intervention. Conclusions SBI is a disease process with a highly variable clinical significance. The presence of focal abdominal pain, a history of prior abdominal surgery, and the intussusception length are the greatest predictors of the need for operative intervention

Mechanically induced current and quantum evaporation from Luttinger liquids

We investigate transport through a tunnelling junction between an uncorrelated metallic lead and a Luttinger liquid when the latter is subjected to a time dependent perturbation. The tunnelling current as well as the electron energy distribution function are found to be strongly affected by the perturbation due to generation of harmonics in the density oscillations. Using a semiconducting lead instead of a metallic one results in electrons being injected into the lead even without applied voltage. Some applications to carbon nanotubes are discussed.Comment: 7 pages, 2 figures (eps files