Coherent Radiation by a Spherical Medium of Resonant Atoms

Radiation by the atoms of a resonant medium is a cooperative process in which the medium participates as a whole. In two previous papers we treated this problem for the case of a medium having slab geometry, which, under plane-wave excitation, supports coherent waves that propagate in one dimension. We extend the treatment here to the three-dimensional problem, focusing principally on the case of spherical geometry. By regarding the radiation field as a superposition of electric and magnetic multipole fields of different orders, we express it in terms of suitably defined scalar fields. The latter fields possess a sequence of exponentially decaying eigenmodes corresponding to each multipole order. We consider several examples of spherically symmetric initial excitations of a sphere. Small uniformly excited spheres, we find, tend to radiate superradiantly, while the radiation from a large sphere with an initially excited inner core exhibits temporal oscillations that result from the participation of a large number of coherently excited amplitudes in different modes. The frequency spectrum of the emitted radiation possesses a rich structure, including a frequency gap for large spheres and sharply defined and closely spaced peaks caused by the small frequency shifts and even smaller decay rates characteristic of the majority of eigenmodes.Physic

Axions as Quintessence in String Theory

We construct a model of quintessence in string theory based on the idea of axion monodromy as discussed by McAllister, Silverstein and Westphal arXiv:0808.0706. In the model, the quintessence field is an axion whose shift symmetry is broken by the presence of 5-branes which are placed in highly warped throats. This gives rise to a potential for the axion field which is slowly varying, even after incorporating the effects of moduli stabilization and supersymmetry breaking. We find that the resulting time dependence in the equation of state of Dark Energy is potentially detectable, depending on the initial conditions. The model has many very light extra particles which live in the highly warped throats, but these are hard to detect. A signal in the rotation of the CMB polarization can also possibly arise.Comment: 41 pages, 1 figure, v2: references adde

Comment on ``Spin Dependent Hopping and Colossal Negative Magnetoresistance in Epitaxial Nd0.52Sr0.48MnO3Nd_{0.52}Sr_{0.48}MnO_{3} Films in Fields up to 50 T''

Recently Wagner et al. [Phys. Rev. Lett. Vol. 81, P. 3980 (1998)] proposed that Mott's original model be modified to incorporate a hopping barrier which depends on the misorientation between the spins of electrons at the initial and the final states in an elementary process. They further claimed that using the model they can explain the observed scaling behavior-- negative-magnetoresistivity scaling proportional to the Brillouin function $\cal{B}$ in the ferromagnetic state and to ${\cal{B}}^2$ in the paramagnetic state. In this comment we argue that the modification needed for Mott's original model is different from that proposed by Wagner et al. and further show that our picture will successfully explain the observed scaling in the two regimes.Comment: 1 pag

Design of an area-scalable two-layer evaporator wick for high-heat-flux vapor chambers

A hybrid two-layer evaporator wick is proposed for passive, high-heat-flux dissipation over large areas using a vapor chamber heat spreader. For such applications, the evaporator wick layer must be designed to simultaneously minimize the device temperature rise and minimize the flow resistance to capillary feeding of the wick. This requires a strategy that exploits the benefits of a thin wick for reduced thermal resistance and a thick wick for liquid feeding. In the present design, a thick cap layer of wick material evenly routes liquid to a thin, low-thermal-resistance base layer through an array of vertical liquid-feeding posts. This two-layer structure decouples the functions of liquid resupply (cap layer) and capillary-fed boiling heat transfer (base layer), making the design scalable to heat input areas of ~1 cm2 for operation at 1 kW/cm2. A reduced-order model is developed to demonstrate the potential performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The two-layer wick design is found to sustain liquid feeding at higher heat fluxes, without reaching the capillary limit, compared to single-layer evaporator wick designs

Experimental Investigation of Boiling Regimes in a Capillary-Fed Two-Layer Evaporator Wick

Vapor chambers with transformative evaporator wick designs capable of passively dissipating high heat fluxes over large areas, while maintaining low thermal resistances, can meet the thermal management needs of next-generation power semiconductor devices. Our prior work proposed a two-layer evaporator wick structure to enhance the performance of vapor chambers operating at high heat fluxes. The current study experimentally characterizes the capillary-fed boiling heat transfer behavior in such a two-layer evaporator wick, compared to a homogeneous (single-layer) wick, over a 1 cm2 evaporator area. The two-layer design comprises a thin base wick layer that is fed with liquid from a thick cap wick layer above using an array of vertical posts. The two-layer wick is fabricated using a sequence of sintering and laser-machining steps to form the base wick layer (200 mm), array of liquid-feeding posts, and cap wick layer (800 mm) using 90–106 mm copper particles. A test facility is constructed to replicate the conditions that exist at the evaporator of a vapor chamber; the novel facility design uses a physical restriction to prevent flooding of the wicks during testing. Two-layer wicks having 5 x 5 and 10 x 10 arrays of liquid feeding posts are characterized, along with a 200 mm-thick single-layer evaporator wick. The 10 x 10 array provides a \u3e400% enhancement in the dryout heat flux compared to the single-layer wick. High-speed visualizations are used to identify the characteristic regimes of boiling operations for the wicks. The single-layer wick exhibits a partial dryout mode of operation, where a dry spot formed in the center of the heated evaporator area causes an increase in the thermal resistance with heat flux. In contrast, the distributed feeding provided by the two-layer wicks mitigates the development of this partial dryout regime and maintains a constant low resistance (~0.1 K/W) during capillary-fed boiling until a complete dryout event occurs. This study demonstrates the significant enhancement in dryout heat flux offered by the liquid-feeding approach realized in the two-layer evaporator wicks characterized here

Typical Sounding Atypical Diagnostic Conundrum -- A Rare Case of Mycobacterium Avium Complex (MAC) Presenting with Multiple Cranial Nerve Involvement

Background MAC infections rarely cause symptomatic systemic infection in immunocompetent healthy individuals. This case brings to light a rare such presentation that gives us a learning point about keeping a strong pre-clinical suspicion for this condition in patients whom lung imaging reveals suspicious cavitary lesions. Starting empiric therapy while awaiting culture results may be considered after weighing risks and benefits in order to achieve the morbidity and mortality associated with the disease. Case report A 54 year-old white male presented for evaluation of progressively worsening vertigo, imbalance, vertical diplopia, facial diplegia, bilateral hyperacusis followed by hearing loss, dysphagia, and dysarthria together with unintentional 40 pound weight loss, headache, neck stiffness, and productive cough over 5 months. His neurological exam showed multiple cranial nerve abnormalities. Contrast enhanced MRI head showed mild thickening and enhancement of cranial nerves V, VII, and VIII bilaterally. Biopsy of a cavitary left upper lobe lung lesion noted on CT chest and PET scan along with specimen culture surprisingly revealed Mycobacterium avium complex (MAC). Despite starting treatment, he died 6 weeks after discharge from presumed disseminated MAC. This rare case of MAC in an immunocompetent individual presenting with only multiple cranial neuropathies has not been previously reported. Conclusion MAC related CNS infections are very unusual in immunocompetent patient. Our case highlights one such unique presentation of an immunocompetent male who presented with multiple cranial nerve palsy that was ultimately diagnosed to be secondary to MAC infection. Given its rarity, it is very likely to lead to a delay in diagnosis and this could further delay treatment and poor outcome as in our patient. More extensive reporting of this rare CNS MAC infection in immunocompetent individuals could help understand the disease presentation better and might aid in earlier diagnosis and initiation of treatment

Warm tachyonic inflation in warped background

We analyze warm tachyonic inflation, proposed in the literature, but from the viewpoint of four dimensional effective action for tachyon field on a non-BPS D3-brane. We find that consistency with observational data on density perturbation and validity of effective action requires warped compactification. The number of background branes which source the flux is found to be of the order of 10 in contrast to the order of $10^{14}$ in the standard cold inflationary scenario.Comment: 9 pages, RevTe

ОБЗОР СЕМАНТИКИ ЯЗЫКА HTML 5

HTML5 несет в себе множество изменений разного уровня и разной важности. Принципиально, ключевые изменения можно разделить на 5 блоков: семантика, мультимедиа, динамичная графика, веб-формы, JavaScript APls. В данной статье рассмотрен обзор семантики HTML5 в котором появился ряд новых семантических тегов, позволяющих более осмысленно организовывать внутреннюю структуру веб-страни