Growth kinetics effects on self-assembled InAs/InP quantum dots

A systematic manipulation of the morphology and the optical emission properties of MOVPE grown ensembles of InAs/InP quantum dots is demonstrated by changing the growth kinetics parameters. Under non-equilibrium conditions of a comparatively higher growth rate and low growth temperature, the quantum dot density, their average size and hence the peak emission wavelength can be tuned by changing efficiency of the surface diffusion (determined by the growth temperature) relative to the growth flux. We further observe that the distribution of quantum dot heights, for samples grown under varying conditions, if normalized to the mean height, can be nearly collapsed onto a single Gaussian curve.Comment: 2 figure

On the NP-Hardness of Approximating Ordering Constraint Satisfaction Problems

We show improved NP-hardness of approximating Ordering Constraint Satisfaction Problems (OCSPs). For the two most well-studied OCSPs, Maximum Acyclic Subgraph and Maximum Betweenness, we prove inapproximability of $14/15+\epsilon$ and $1/2+\epsilon$. An OCSP is said to be approximation resistant if it is hard to approximate better than taking a uniformly random ordering. We prove that the Maximum Non-Betweenness Problem is approximation resistant and that there are width-$m$ approximation-resistant OCSPs accepting only a fraction $1 / (m/2)!$ of assignments. These results provide the first examples of approximation-resistant OCSPs subject only to P $\neq$ \NP

The sign convention for quadrature Parkinson arrows in geomagnetic induction studies

Time series analysis, which is basic to modern geophysical data processing, involves a choice between working with a time dependence of e+iωt or e-iωt. In published work the choice made is sometimes not explicitly stated, leaving ambiguity in the interpretation of complex quantities with quadrature parts. Parkinson arrows are used in geomagnetic induction studies to summarize anomalous vertical magnetic fluctuations at different observing stations and to indicate regions of high electrical conductivity. Such arrows are now regularly computed as real and quadrature pairs. The general convention is often adopted of 'reversing' a calculated real arrow so that it will point toward a conductivity increase, but for quadrature arrows the practice between various published papers has generally not been so consistent. The present paper demonstrates that consistent practice for reversing or not reversing quadrature Parkinson arrows is possible when the initial convention for time dependence is taken into account. A reversal practice is determined for interpretation in terms of a simple channeling model. A related matter is the definition of phase. Phase values are also generally ambiguous unless the time dependence used (e-iωt or e+iωt) is stated

State-insensitive trapping of Rb atoms: linearly versus circularly polarized lights

We study the cancellation of differential ac Stark shifts in the 5s and 5p states of rubidium atom using the linearly and circularly polarized lights by calculating their dynamic polarizabilities. Matrix elements were calculated using a relativistic coupled-cluster method at the single, double and important valence triple excitations approximation including all possible non-linear correlation terms. Some of the important matrix elements were further optimized using the experimental results available for the lifetimes and static polarizabilities of atomic states. "Magic wavelengths" are determined from the differential Stark shifts and results for the linearly polarized light are compared with the previously available results. Possible scope of facilitating state-insensitive optical trapping schemes using the magic wavelengths for circularly polarized light are discussed. Using the optimized matrix elements, the lifetimes of the 4d and 6s states of this atom are ameliorated.Comment: 13 pages, 13 tables and 4 figure

Reentrant phase transition in charged colloidal suspensions

We report the observation of a novel phase transition in dilute aqueous suspensions of polystyrene particles as a function of ionic impurity concentration C. The suspension phase separates into dense and rare phases only for a restricted range of C which depends on particle concentration n. The dense phase has liquidlike or crystalline order depending on n and C. Free energies of the homogeneous and the phase-separated states are calculated with an effective interparticle potential. The calculated phase diagram is in qualitative agreement with the present experimental results

Smoothed Analysis of Tensor Decompositions

Low rank tensor decompositions are a powerful tool for learning generative models, and uniqueness results give them a significant advantage over matrix decomposition methods. However, tensors pose significant algorithmic challenges and tensors analogs of much of the matrix algebra toolkit are unlikely to exist because of hardness results. Efficient decomposition in the overcomplete case (where rank exceeds dimension) is particularly challenging. We introduce a smoothed analysis model for studying these questions and develop an efficient algorithm for tensor decomposition in the highly overcomplete case (rank polynomial in the dimension). In this setting, we show that our algorithm is robust to inverse polynomial error -- a crucial property for applications in learning since we are only allowed a polynomial number of samples. While algorithms are known for exact tensor decomposition in some overcomplete settings, our main contribution is in analyzing their stability in the framework of smoothed analysis. Our main technical contribution is to show that tensor products of perturbed vectors are linearly independent in a robust sense (i.e. the associated matrix has singular values that are at least an inverse polynomial). This key result paves the way for applying tensor methods to learning problems in the smoothed setting. In particular, we use it to obtain results for learning multi-view models and mixtures of axis-aligned Gaussians where there are many more "components" than dimensions. The assumption here is that the model is not adversarially chosen, formalized by a perturbation of model parameters. We believe this an appealing way to analyze realistic instances of learning problems, since this framework allows us to overcome many of the usual limitations of using tensor methods.Comment: 32 pages (including appendix

The parameterized complexity of some geometric problems in unbounded dimension

We study the parameterized complexity of the following fundamental geometric problems with respect to the dimension $d$: i) Given $n$ points in \Rd, compute their minimum enclosing cylinder. ii) Given two $n$-point sets in \Rd, decide whether they can be separated by two hyperplanes. iii) Given a system of $n$ linear inequalities with $d$ variables, find a maximum-size feasible subsystem. We show that (the decision versions of) all these problems are W[1]-hard when parameterized by the dimension $d$. %and hence not solvable in ${O}(f(d)n^c)$ time, for any computable function $f$ and constant $c$ %(unless FPT=W[1]). Our reductions also give a $n^{\Omega(d)}$-time lower bound (under the Exponential Time Hypothesis)

Use of mifepristone for termination of intrauterine fetal demise (IUFD) in previously scarred uterus in later half of pregnancy (>20 weeks)

Background: Mifepristone has the potential to be used as an agent for induction of labour by increasing the uterine contractility and increasing the sensitivity of uterus to prostaglandins. The present study is an endeavor to study the effect of mifepristone alone to induce labour in scarred uterus and its risk benefit ratio.Methods: Total 39 patients with IUFD and previous uterine surgery were included in the study after their informed consent. All women in the study were given Tablet Mifepristone 200 mg orally, thrice a day, maximum 6 doses (Max -1200 mg) over a duration of 48 hours. Patients were monitored for vitals, the uterine contractions and any bleeding per vaginum. Next dose of drug was omitted if sufficient uterine contractions or cervical dilatation ≥2.5 cm achieved. Patients were shifted to the labour room after onset of active labour. Labour was augmented with oxytocin wherever required.Results: spontaneous labour occurred in 74.3% (29/39) women while operative (cesarean/ hysterotomy) delivery occurred in 17.9% (07/39). Mean induction (first dose of mifepristone) to delivery interval was 51.5 hrs in second trimester while 59.8 hrs in third trimester women. Oxytocin augmentation was done in 8 (20.5 %) women.Conclusions: The potential advantage of mifepristone over prostaglandins and oxytocin, is mainly in situations where they are contraindicated (i.e., scarred uterus). In this study authors found that with mifepristone only regimen is quite safe and effective, inducing spontaneous labour in 74.3% (29/39) women with IUFD and in reducing the operative (cesarean/ hysterotomy) delivery (17.9%)