The structure of algebraic covariant derivative curvature tensors

We use the Nash embedding theorem to construct generators for the space of algebraic covariant derivative curvature tensors

Studies on electrostatic interactions of colloidal particles under two-dimensional confinement

We study the effective electrostatic interactions between a pair of charged colloidal particles without salt ions while the system is confined in two dimensions. In particular we use a simplified model to elucidate the effects of rotational fluctuations in counterion distribution. The results exhibit effective colloidal attractions under appropriate conditions. Meanwhile, long-range repulsions persist over most of our studied cases. The repulsive forces arise from the fact that in two dimensions the charged colloids cannot be perfectly screened by counterions, as the residual quadrupole moments contribute to the repulsions at longer range. And by applying multiple expansions we find that the attractive forces observed at short range are mainly contributed from electrostatic interactions among higher-order electric moments. We argue that the scenario for attractive interactions discussed in this work is applicable to systems of charged nanoparticles or colloidal solutions with macroions.Comment: 23 pages, 11 figures, 1 tabl

Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray micro-diffraction

We depict the use of x-ray diffraction as a tool to directly probe the strain status in rolled-up semiconductor tubes. By employing continuum elasticity theory and a simple model we are able to simulate quantitatively the strain relaxation in perfect crystalline III-V semiconductor bi- and multilayers as well as in rolled-up layers with dislocations. The reduction in the local elastic energy is evaluated for each case. Limitations of the technique and theoretical model are discussed in detail.Comment: 32 pages (single column), 9 figures, 39 reference

Phase-Remapping Attack in Practical Quantum Key Distribution Systems

Quantum key distribution (QKD) can be used to generate secret keys between two distant parties. Even though QKD has been proven unconditionally secure against eavesdroppers with unlimited computation power, practical implementations of QKD may contain loopholes that may lead to the generated secret keys being compromised. In this paper, we propose a phase-remapping attack targeting two practical bidirectional QKD systems (the "plug & play" system and the Sagnac system). We showed that if the users of the systems are unaware of our attack, the final key shared between them can be compromised in some situations. Specifically, we showed that, in the case of the Bennett-Brassard 1984 (BB84) protocol with ideal single-photon sources, when the quantum bit error rate (QBER) is between 14.6% and 20%, our attack renders the final key insecure, whereas the same range of QBER values has been proved secure if the two users are unaware of our attack; also, we demonstrated three situations with realistic devices where positive key rates are obtained without the consideration of Trojan horse attacks but in fact no key can be distilled. We remark that our attack is feasible with only current technology. Therefore, it is very important to be aware of our attack in order to ensure absolute security. In finding our attack, we minimize the QBER over individual measurements described by a general POVM, which has some similarity with the standard quantum state discrimination problem.Comment: 13 pages, 8 figure

Mixing by polymers: experimental test of decay regime of mixing

By using high molecular weight fluorescent passive tracers with different diffusion coefficients and by changing the fluid velocity we study dependence of a characteristic mixing length on the Peclet number, $Pe$, which controls the mixing efficiency. The mixing length is found to be related to $Pe$ by a power law, $L_{mix}\propto Pe^{0.26\pm 0.01}$, and increases faster than expected for an unbounded chaotic flow. Role of the boundaries in the mixing length abnormal growth is clarified. The experimental findings are in a good quantitative agreement with the recent theoretical predictions.Comment: 4 pages,5 figures. accepted for publication in PR

Childbearing and family planning choices of women living with HIV and AIDS: the lived experiences

Objective: The main objective of this study was to assess the reproductive choices for Women Living with HIV on ART in the urban health centres within Lusaka.Methods: Nine hundred and fifty six HIV-positive women receiving care in the zoned health centres were randomly sampled and 12 health care workers who were purposefully sampled and were available in the ART setting participated in the study. A survey questionnaire, in-depth interviews and focus group discussions were the main data collection tools used. Quantitative data was analysed using SPSS while qualitative data was analysed using qualitative content analysis rooted in grounded theory (Precaution Adoption Model (PAPM).Results: The majority of the respondents in this study n = 503 (64%) felt that it was important to be safe from becoming pregnant whereas n = 429 (46%) felt that it was not. Health workers counselled the respondents on nearly all of the available contraceptive methods, with an emphasis on the oral pill, injectable hormonal drug, and on male and female condoms – with the greatest emphasis on the male condom. The reasons that women had for selecting particular family planning methods varied temporally. Some women engaged in unprotected sex because their partner was also HIV seropositive. Even in situations when an HIV-positive woman was told of her sero status and given counselling on the risks of motherto- child-transmission (MTCT), the majority of women would still engage in unprotected sex that often led to pregnancy. Nurses preferentially discouraged the women from getting pregnant using the counselling methods described. The rationale for the counselling methods was based on age, education, medical and gynaecological complications. In the current health care setup, it was not demonstrated that reproductive health issues of people living with HIV and AIDS could be provided within the ART clinic.Conclusion: As HIV continues to spread among women of childbearing age, there is an increasing need for support programs for infected women regarding sex, safer sex, pregnancy and family planning. The healthcare challenges for this group must be addressed with a twopronged approach- women must prioritize the risk to their health with repeated exposures to HIV and the healthcare workers must empower them to make these decisions. For this to occur, it is crucial that HIV positive women have easy access to reproductive healthcare counselling in juxtaposition to the ART Clinic. The health care workers that directly counsel these women have a pivotal role in addressing these concerns. Nursing professionals who comprise the bulk of healthcare workers providing these services can play a major role in meeting these needs

Vacuum ultraviolet photoabsorption spectra of nitrile ices for their identification on Pluto

Icy bodies, such as Pluto, are known to harbor simple and complex molecules. The recent New Horizons flyby of Pluto has revealed a complex surface composed of bright and dark ice surfaces, indicating a rich chemistry based on nitrogen (N2), methane (CH4), and carbon monoxide (CO). Nitrile (CN) containing molecules such as acetonitrile (CH3CN), propionitrile (CH3CH2CN), butyronitrile (CH3CH2CH2CN), and isobutyronitrile ((CH3)2CHCN) are some of the nitrile molecules that are known to be synthesized by radiative processing of such simple ices. Through the provision of a spectral atlas for such compounds we propose that such nitriles may be identified from the ALICE payload on board New Horizons</i

Antiferromagnetic Critical Fluctuations in BaFe2_2As2_2

Magnetic correlations near the magneto-structural phase transition in the bilayer iron pnictide parent compound, BaFe$_2$As$_2$, are measured. In close proximity to the antiferromagnetic phase transition in BaFe$_2$As$_2$, a crossover to three dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about T$_N$. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe$_2$As$_2$ and their evolution near the anticipated crossover to three dimensional critical behavior and long-range order are discussed.Comment: 6 pages, 4 figures; Accepted for publication in Physical Review