754 research outputs found
Magnetic strong coupling in a spin-photon system and transition to classical regime
We study the energy level structure of the Tavis-Cumming model applied to an
ensemble of independent magnetic spins coupled to a variable number of
photons. Rabi splittings are calculated and their distribution is analyzed as a
functin of photon number and spin system size . A sharp
transition in the distribution of the Rabi frequency is found at . The width of the Rabi frequency spectrum diverges as
at this point. For increased number of photons , the Rabi
frequencies converge to a value proportional to . This
behavior is interpreted as analogous to the classical spin resonance mechanism
where the photon is treated as a classical field and one resonance peak is
expected. We also present experimental data demonstrating cooperative, magnetic
strong coupling between a spin system and photons, measured at room
temperature. This points towards quantum computing implementation with magnetic
spins, using cavity quantum-electrodynamics techniques.Comment: Received 8 April 2010; revised manuscript received 17 June 2010;
published 14 July 201
Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies
On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the
ability to precisely position nanosized samples in antinodes of the
electro-magnetic field for maximal magnetic interaction. A method is developed
to entrap micro-crystals containing spins in a well defined location on a
substrate's surface. Traditional cavity ESR measurements are then performed on
a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr spins were
entrapped as well and measured while approaching the lower limit of the ESR
sensitivity. This method suggests the feasibility of on-chip ESR measurements
at dilution refrigerator temperatures by enabling the positioning of samples
atop an on-chip superconducting cavity.Comment: to appear in Journal of Applied Physic
Tunable multi-photon Rabi oscillations in an electronic spin system
We report on multi-photon Rabi oscillations and controlled tuning of a
multi-level system at room temperature (S=5/2 for Mn2+:MgO) in and out of a
quasi-harmonic level configuration. The anisotropy is much smaller than the
Zeeman splittings, such as the six level scheme shows only a small deviation
from an equidistant diagram. This allows us to tune the spin dynamics by either
compensating the cubic anisotropy with a precise static field orientation, or
by microwave field intensity. Using the rotating frame approximation, the
experiments are very well explained by both an analytical model and a
generalized numerical model. The calculated multi-photon Rabi frequencies are
in excellent agreement with the experimental data
Multi-photon Rabi oscillations in high spin paramagnetic impurity
We report on multiple photon monochromatic quantum oscillations (Rabi
oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of
Mn (S=5/2) impurities in MgO. We find that when the microwave magnetic
field is similar or large than the anisotropy splitting, the Rabi oscillations
have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model.
We show that these new frequencies come from multiple photon coherent
manipulation of the multi-level spin impurity. We develop a model based on the
crystal field theory and the rotating frame approximation, describing the
observed phenomenon with a very good agreement.Comment: International Conference: Resonance in Condensed Matter Altshuler 10
Making the Case for Global Engineering: Building Foreign Language Collaborations for Designing, Implementing, and Assessing Programs
Engineering programs must prepare students for a global engineering profession. In global markets, processes as well as products can be outsourced. Highly technical engineering work may be completed by large and diverse collaborations. Engineering students need to have foundational work in languages, cultural differences, and strategies for working with diverse colleagues. Historically only about 3% to 4% of engineering students pursue study abroad opportunities. Clearly, new and innovative programs must be devised to build global competency in undergraduate engineers. In working toward that end, the authors suggest that interdisciplinary collaborations between departments of foreign language and schools of engineering can be highly productive. To illustrate the benefits of such collaborations and to share the results of recent program assessments, this case study presents a conceptual model useful in program design and describes the evolution of a particularly intensive and effective program in global competency for undergraduate engineers
Activated polyhydroxyalkanoate meshes prevent bacterial adhesion and biofilm development in regenerative medicine applications
Regenerative medicine has become an extremely valuable tool offering an alternative to conventional therapies for the repair and regeneration of tissues. The re-establishment of tissue and organ functions can be carried out by tissue engineering strategies or by using medical devices such as implants. However, with any material being implanted inside the human body, one of the conundrums that remains is the ease with which these materials can get contaminated by bacteria. Bacterial adhesion leads to the formation of mature, alive and complex three-dimensional biofilm structures, further infection of surrounding tissues and consequent development of complicated chronic infections. Hence, novel tissue engineering strategies delivering biofilm-targeted therapies, while at the same time allowing tissue formation are highly relevant. In this study our aim was to develop surface modified polyhydroxyalkanoate-based fiber meshes with enhanced bacterial anti-adhesive and juvenile biofilm disrupting properties for tissue regeneration purposes. Using reactive and amphiphilic star-shaped macromolecules as an additive to a polyhydroxyalkanoate spinning solution, a synthetic antimicrobial peptide, Amhelin, with strong bactericidal and anti-biofilm properties, and Dispersin B, an enzyme promoting the disruption of exopolysaccharides found in the biofilm matrix, were covalently conjugated to the fibers by addition to the solution before the spinning process. Staphylococcus epidermidis is one of the most problematic pathogens responsible for tissue-related infections. The initial antibacterial screening showed that Amhelin proved to be strongly bactericidal at 12 μg/ml and caused >50% reductions of biofilm formation at 6 μg/ml, while Dispersin B was found to disperse >70% of pre-formed biofilms at 3 μg/ml. Regarding the cytotoxicity of the agents toward L929 murine fibroblasts, a CC50 of 140 and 115 μg/ml was measured for Amhelin and Dispersin B, respectively. Optimization of the electrospinning process resulted in aligned fibers. Surface activated fibers with Amhelin and Dispersin B resulted in 83% reduction of adhered bacteria on the surface of the fibers. Additionally, the materials developed were found to be cytocompatible toward L929 murine fibroblasts. The strategy reported in this preliminary study suggests an alternative approach to prevent bacterial adhesion and, in turn biofilm formation, in materials used in regenerative medicine applications such as tissue engineering
Generalised joint regression for count data: a penalty extension for competitive settings
We propose a versatile joint regression framework for count responses. The method is implemented in the R add-on package GJRM and allows for modelling linear and non-linear dependence through the use of several copulae. Moreover, the parameters of the marginal distributions of the count responses and of the copula can be specified as flexible functions of covariates. Motivated by competitive settings, we also discuss an extension which forces the regression coefficients of the marginal (linear) predictors to be equal via a suitable penalisation. Model fitting is based on a trust region algorithm which estimates simultaneously all the parameters of the joint models. We investigate the proposal’s empirical performance in two simulation studies, the first one designed for arbitrary count data, the other one reflecting competitive settings. Finally, the method is applied to football data, showing its benefits compared to the standard approach with regard to predictive performance
Multiphoton coherent manipulation in large-spin qubits
Large spin Mn2+ ions (S=5/2) diluted in a non-magnetic MgO matrix of high
crystalline symmetry are used to realize a six level system that can be
operated by means of multi-photon coherent Rabi oscillations. This spin system
has a very small anisotropy which can be tuned in-situ to reversibly transform
the system between harmonic and non-harmonic level configurations. Decoherence
effects are strongly suppressed as a result of the quasi-isotropic electron
interaction with the crystal field and with the 55Mn nuclear spins. These
results suggest new ways of manipulating, reading and resetting spin quantum
states which can be applied to encode a qubit across several quantum levels.Comment: Published versio
Early Initiation of Antiretroviral Therapy Following In Utero HIV Infection Is Associated With Low Viral Reservoirs but Other Factors Determine Viral Rebound
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