Flux reversal in a two-state symmetric optical thermal ratchet

A Brownian particle's random motions can be rectified by a periodic potential energy landscape that alternates between two states, even if both states are spatially symmetric. If the two states differ only by a discrete translation, the direction of the ratchet-driven current can be reversed by changing their relative durations. We experimentally demonstrate flux reversal in a symmetric two-state ratchet by tracking the motions of colloidal spheres moving through large arrays of discrete potential energy wells created with dynamic holographic optical tweezers. The model's simplicity and high degree of symmetry suggest possible applications in molecular-scale motors.Comment: 4 pages, 5 figures, accepted for publication in Physical Review E, Rapid Communication

A Higher-Accuracy van der Waals Density Functional

We propose a second version of the van der Waals density functional (vdW-DF2) of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate semilocal exchange functional and the use of a large-N asymptote gradient correction in determining the vdW kernel. The predicted binding energy, equilibrium separation, and potential-energy curve shape are close to those of accurate quantum chemical calculations on 22 duplexes. We anticipate the enabling of chemically accurate calculations in sparse materials of importance for condensed-matter, surface, chemical, and biological physics.Comment: 14 pages, 10 figure

A smart environment for biometric capture

The development of large scale biometric systems require experiments to be performed on large amounts of data. Existing capture systems are designed for fixed experiments and are not easily scalable. In this scenario even the addition of extra data is difficult. We developed a prototype biometric tunnel for the capture of non-contact biometrics. It is self contained and autonomous. Such a configuration is ideal for building access or deployment in secure environments. The tunnel captures cropped images of the subject's face and performs a 3D reconstruction of the person's motion which is used to extract gait information. Interaction between the various parts of the system is performed via the use of an agent framework. The design of this system is a trade-off between parallel and serial processing due to various hardware bottlenecks. When tested on a small population the extracted features have been shown to be potent for recognition. We currently achieve a moderate throughput of approximate 15 subjects an hour and hope to improve this in the future as the prototype becomes more complete

Reply to "Comment on `First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism'"

The recent preprint by Mazin et al. [cond-mat/0212417] contains many inappropriate evaluations and/or criticisms on our published work [Phys. Rev. B 66, 020513 (2002) and Nature 418, 758 (2002)]. The preprint [cond-mat/0212417v1] was submitted to Physical Review B as a comment on one of our papers [Phys. Rev. B 66, 020513 (2002)]. In the reviewing process, Mazin et al. have withdrawn many of the statements contained in cond-mat/0212417v1, however two claims remain in their revised manuscript [cond-mat/0212417v3]: (1) the calculated variations of the superconducting energy gap within the sigma- or the pi-bands are not observable in real samples due to scatterings, and (2) the Coulomb repulsion mu(k,k') is negligibly small between sigma- and pi-states and thus should be approximated by a diagonal 2 x 2 matrix in the sigma and pi channels. Here, we point out that the former does not affect the validity of our theoretical work which is for the clean limit, and that the latter is not correct

Vortex pinning by meandering line defects in planar superconductors

To better understand vortex pinning in thin superconducting slabs, we study the interaction of a single fluctuating vortex filament with a curved line defect in (1+1) dimensions. This problem is also relevant to the interaction of scratches with wandering step edges in vicinal surfaces. The equilibrium probability density for a fluctuating line attracted to a particular fixed defect trajectory is derived analytically by mapping the problem to a straight line defect in the presence of a space and time-varying external tilt field. The consequences of both rapid and slow changes in the frozen defect trajectory, as well as finite size effects are discussed. A sudden change in the defect direction leads to a delocalization transition, accompanied by a divergence in the trapping length, near a critical angle.Comment: 9 pages, 9 figure

Ballistic miniband conduction in a graphene superlattice

Rational design of artificial lattices yields effects unavailable in simple solids, and vertical superlattices of multilayer semiconductors are already used in optical sensors and emitters. Manufacturing lateral superlattices remains a much bigger challenge, with new opportunities offered by the use of moire patterns in van der Waals heterostructures of graphene and hexagonal crystals such as boron nitride (h-BN). Experiments to date have elucidated the novel electronic structure of highly aligned graphene/h-BN heterostructures, where miniband edges and saddle points in the electronic dispersion can be reached by electrostatic gating. Here we investigate the dynamics of electrons in moire minibands by transverse electron focusing, a measurement of ballistic transport between adjacent local contacts in a magnetic field. At low temperatures, we observe caustics of skipping orbits extending over hundreds of superlattice periods, reversals of the cyclotron revolution for successive minibands, and breakdown of cyclotron motion near van Hove singularities. At high temperatures, we study the suppression of electron focusing by inelastic scattering

Physical Activity and Weight Gain Prevention in Older Men

Background: Physical activity and adiposity are important predictors of mortality, even in older individuals. However, it is unclear how much physical activity is needed to prevent weight gain in older persons. Purpose To examine the associations of different amounts of physical activity with weight gain prevention in older men. Methods: 5,973 healthy men (mean age, 65.0 y) from the Harvard Alumni Health Study were followed from 1988 to 1998. At baseline (1988), in 1993, and 1998, men reported their recreational physical activity and body weight. Physical activity was categorized as: <7.5 MET-hr/week (7.5 MET-hr/week corresponds to the minimum required by the 2008 US federal guidelines), 7.5 to <21 MET-hr/week (21 MET-hr/week corresponds to the 2002 Institute of Medicine [IOM] guideline), and ≥21 MET-hr/week. Meaningful weight gain was defined as an increase of ≥3% of body weight. Results: Overall, weight tended to be stable over any 5-year period; mean change, −0.08 (SD=4.44) kg. However, ~21% of men experienced meaningful weight gain over any 5-year period. In multivariate analyses, compared to men expending ≥21 MET-hr/week, those expending 7.5 to <21 MET-hr/week had an odds ratio (OR) of 1.35 (95% confidence interval: 1.03, 1.77) for meaningful weight gain, and men expending <7.5 MET-hr/week, an OR of 1.16 (1.01, 1.33) (p, trend = 0.09). Conclusions: Among older men, those with lesser levels of physical activity were more likely to gain weight than men satisfying the 2002 IOM guidelines of ≥21 MET-hr/week (~60 minutes per day of moderate-intensity physical activity)

Small T1−1T_1^{-1} coherence peak near TcT_c in unconventional superconductors

It is usually believed that a coherence peak just below T$_{c}$ in the nuclear spin lattice relaxation rate T$_{1}^{-1}$ in superconducting materials is a signature of conventional s-wave pairing. In this paper we demonstrate that any unconventional superconductor obeying BCS pure-case weak-coupling theory should show a small T$_{1}^{-1}$ coherence peak near T$_{c}$, generally with a height between 3 and 15 percent greater than the normal state T$_{1}^{-1}$ at T$_{c}$. It is largely due to impurity effects that this peak has not commonly been observed.Comment: 4 pages, 5 figure