1,458 research outputs found

    Power-efficiency enhanced thermally tunable Bragg grating for silica-on-silicon photonics

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    A thermally tunable Bragg grating device has been fabricated in a silica-on-silicon integrated optical chip, incorporating a suspended microbeam improving power efficiency. A waveguide and Bragg grating are defined through the middle of the microbeam via direct ultraviolet writing. A tuning range of 0.4 nm (50 GHz) is demonstrated at the telecommunication wavelength of 1550 nm. Power consumption during wavelength tuning is measured at 45 pm/mW, which is a factor of 90 better than reported values for similar bulk thermally tuned silica-on-silicon planar devices. The response time to a step change in heating is longer by a similar factor, as expected for a highly power-efficient device. The fabrication procedure involves a deep micromilling process, as well as wet etching and metal deposition. With this response, the device would be suitable for trimming applications and wherever low modulation frequencies are acceptable. A four-point-probe-based temperature measurement was also done to ascertain the temperature reached during tuning and found an average volume temperature of 48 °C, corresponding to 0.4 nm of tuning. The role of stress-induced buckling in device fabrication is included

    A Proposed Test of the Einstein Theory of Gravitation by Means of an Unshielded Orbiting Gyro Using Passive Telemetry

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    Coordinated Science Laboratory was formerly known as Control Systems LaboratoryJoint Services Electronics Program / DAAB-07-67-C-0199National Aeronautics and Space Administration / NsG-44

    An integrated optical Bragg grating refractometer for volatile organic compound detection

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    We report an integrated optical Bragg grating detector, fabricated using a direct UV-writing approach, that when coated with a thin-film of a hydrophobic siloxane co-polymer can perform as an all-optically accessed detector for hydrocarbon vapour. Upon exposure to a series of organic solvent vapours, both negative and positive Bragg wavelength shifts of differing magnitudes were measured. This was attributed to a combination of swelling and/or hydrocarbon solvent filling the free volume within the polymer film. A quantitative structural property relationship (QSPR) approach was utilised to create a multiple variable linear regression model, built from parameters that chemically described the hydrocarbons and the intermolecular interactions present between the co-polymer and hydrocarbon molecules. The resulting linear regression model indicated that the degree of swelling of the polysiloxane thin film when exposed to vapours of different hydrocarbons was due to the physico-chemical properties of the hydrocarbons and that this was the main causative factor of the measured Bragg wavelength shifts. Furthermore, this linear regression model allows for the prediction of the Bragg wavelength shift that would be measured upon exposure to vapours of another defined hydrocarbon. This detector is intrinsically safe in flammable environments. It includes on-chip thermal compensation, operates at telecoms wavelengths and has a predictable response to a variety of hydrocarbons making it ideal for detection of flammable hydrocarbon vapours in industrial and domestic processes

    A migrant study of pubertal timing and tempo in British-Bangladeshi girls at varying risk for breast cancer

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    Introduction: Earlier menarche is related to subsequent breast cancer risk, yet international differences in the age and tempo of other pubertal milestones and their relationships with body mass index (BMI) are not firmly established in populations at differing risk for breast cancer. We compared age and tempo of adrenarche, thelarche, pubarche, and menarche in a migrant study of Bangladeshi girls to the United Kingdom (UK) and assessed whether differences by migration were explained by differences in BMI. Methods: Included were groups of Bangladeshi (n =168, British-Bangladeshi (n =174) and white British (n =54) girls, aged 5 to 16 years. Interviewer-administered questionnaires obtained pubertal staging; height and weight were measured. Salivary dehydroepiandrosterone-sulfate concentrations >400 pg/ml defined adrenarche. Median ages of pubertal milestones and hazard ratios (HR) with 95% confidence intervals (CI) were estimated from Weibull survival models. Results: In all three groups, adrenarche occurred earliest, followed by thelarche, pubarche, and finally menarche. Neither median age at adrenarche (Bangladeshi = 7.2, British-Bangladeshi = 7.4, white British = 7.1; P-trend = 0.70) nor at menarche (Bangladeshi = 12.5, BritishBangladeshi = 12.1, white British = 12.6; P-trend = 0.70) differed across groups. In contrast, median age at thelarche (Bangladeshi = 10.7, British-Bangladeshi = 9.6, white British = 8.7; P-trend <0.01) occurred earlier among girls living in the UK. Compared with Bangladeshi girls, HRs (95% CI) for earlier thelarche were 1.6 (1.1 to 2.4) for British-Bangladeshi girls and 2.6 (1.5 to 4.4) for white British girls (P-trend <0.01), but were attenuated after adjustment for BMI (British-Bangladeshi = 1.1 (0.7 to 1.8), white British = 1.7(1.0 to 3.1); Ptrend =0.20). Conclusions: Thelarche occurred earlier, but puberty progressed slower with increasing exposure to the UK environment; differences were partially explained by greater BMI. The growth-environment might account for much of the ethnic differences in pubertal development observed across and within countries

    Non-Fermi liquid normal state of the Heavy Fermion superconductor UBe13

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    Non-Fermi liquid (NFL) behavior in the normal state of the heavy-fermion superconductor UBe13 is studied by means of low-temperature measurements of the specific heat, C, and electrical resistivity, \rho, on a high-quality single crystal in magnetic fields up to 15.5 T. At B=0, unconventional superconductivity forms at Tc=0.9 K out of an incoherent state, characterized by a large and strongly temperature dependent \rho(T). In the magnetic field interval 4 T \leq B \leq 10 T, \rho(T) follows a T^3/2 behavior for Tc(B)\leq T \leq 1 K, while \rho is proportional to T at higher temperatures. Corresponding Non-Fermi liquid behavior is observed in C/T as well and hints at a nearby antiferromagnetic (AF) quantum critical point (QCP) covered by the superconducting state. We speculate that the suppression of short-range AF correlations observed by thermal expansion and specific heat measurements below T_L \simeq 0.7 K (B=0) yields a field-induced QCP, T_L \to 0, at B=4.5 T.Comment: Presented at the M2S-2003 conference in Rio / Brazi

    Domain wall generation by fermion self-interaction and light particles

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    A possible explanation for the appearance of light fermions and Higgs bosons on the four-dimensional domain wall is proposed. The mechanism of light particle trapping is accounted for by a strong self-interaction of five-dimensional pre-quarks. We obtain the low-energy effective action which exhibits the invariance under the so called \tau-symmetry. Then we find a set of vacuum solutions which break that symmetry and the five-dimensional translational invariance. One type of those vacuum solutions gives rise to the domain wall formation with consequent trapping of light massive fermions and Higgs-like bosons as well as massless sterile scalars, the so-called branons. The induced relations between low-energy couplings for Yukawa and scalar field interactions allow to make certain predictions for light particle masses and couplings themselves, which might provide a signature of the higher dimensional origin of particle physics at future experiments. The manifest translational symmetry breaking, eventually due to some gravitational and/or matter fields in five dimensions, is effectively realized with the help of background scalar defects. As a result the branons acquire masses, whereas the ratio of Higgs and fermion (presumably top-quark) masses can be reduced towards the values compatible with the present-day phenomenology. Since the branons do not couple to fermions and the Higgs bosons do not decay into branons, the latter ones are essentially sterile and stable, what makes them the natural candidates for the dark matter in the Universe.Comment: 34 pages, 2 figures, JHEP style,few important refs. adde

    A preferential attachment model with random initial degrees

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    In this paper, a random graph process G(t)t1{G(t)}_{t\geq 1} is studied and its degree sequence is analyzed. Let (Wt)t1(W_t)_{t\geq 1} be an i.i.d. sequence. The graph process is defined so that, at each integer time tt, a new vertex, with WtW_t edges attached to it, is added to the graph. The new edges added at time t are then preferentially connected to older vertices, i.e., conditionally on G(t1)G(t-1), the probability that a given edge is connected to vertex i is proportional to di(t1)+δd_i(t-1)+\delta, where di(t1)d_i(t-1) is the degree of vertex ii at time t1t-1, independently of the other edges. The main result is that the asymptotical degree sequence for this process is a power law with exponent τ=min{τW,τP}\tau=\min\{\tau_{W}, \tau_{P}\}, where τW\tau_{W} is the power-law exponent of the initial degrees (Wt)t1(W_t)_{t\geq 1} and τP\tau_{P} the exponent predicted by pure preferential attachment. This result extends previous work by Cooper and Frieze, which is surveyed.Comment: In the published form of the paper, the proof of Proposition 2.1 is incomplete. This version contains the complete proo

    The effect of memory on relaxation in a scalar field theory

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    We derive a kinetic equation with a non-Markovian collision term which includes a memory effect, from Kadanoff-Baym equations in ϕ4\phi^4 theory within the three-loop level for the two-particle irreducible (2PI) effective action. The memory effect is incorporated into the kinetic equation by a generalized Kadanoff-Baym ansatz.Based on the kinetic equations with and without the memory effect, we investigate an influence of this effect on decay of a single particle excitation with zero momentum in 3+1 dimensions and the spatially homogeneous case. Numerical results show that, while the time evolution of the zero mode is completely unaffected by the memory effect due to a separation of scales in the weak coupling regime, this effect leads first to faster relaxation than the case without it and then to slower relaxation as the coupling constant increases.Comment: 12 pages, 6 eps figure

    Magnetic structure of CeRhIn_5 as a function of pressure and temperature

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    We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn5_{5} at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ce and the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and NQR measurements confirm the presence of bulk superconductivity. This work places new constraints on an interpretation of the relationship between antiferromagnetism and unconventional superconductivity in CeRhIn5_{5}.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
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