Results 1 - 10 of 10141
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[en] Certain phase transitions between topological quantum field theories (TQFTs) are driven by the condensation of bosonic anyons. However, as bosons in a TQFT are themselves nontrivial collective excitations, there can be topological obstructions that prevent them from condensing. Here we formulate such an obstruction in the form of a no-go theorem. We use it to show that no condensation is possible in SO(3)_k TQFTs with odd k. We further show that a 'layered' theory obtained by tensoring SO(3)_k TQFT with itself any integer number of times does not admit condensation transitions either. Furthermore, this includes (as the case k = 3) the noncondensability of any number of layers of the Fibonacci TQFT.
[en] Here we present a theory of the nonlinear growth of zonal flows in magnetized plasma turbulence, by the mechanism of secondary instability. The theory is derived for general magnetic geometry, and is thus applicable to both tokamaks and stellarators. The predicted growth rate is shown to compare favorably with nonlinear gyrokinetic simulations, with the error scaling as expected with the small parameter of the theory.
[en] We consider active particles swimming in a convergent fluid flow in a trapezoid nozzle with no-slip walls. We use mathematical modeling to analyze trajectories of these particles inside the nozzle. By extensive Monte Carlo simulations, we show that trajectories are strongly affected by the background fluid flow and geometry of the nozzle leading to wall accumulation and upstream motion (rheotaxis). In particular, we describe the non-trivial focusing of active rods depending on physical and geometrical parameters. It is also established that the convergent component of the background flow leads to stability of both downstream and upstream swimming at the centerline. The stability of downstream swimming enhances focusing, and the stability of upstream swimming enables rheotaxis in the bulk.
[en] In this paper, we describe an inverse free electron laser (IFEL) interaction driven by a near single-cycle THz pulse that is group velocity-matched to an electron bunch inside a waveguide, allowing for a sustained interaction in a magnetic undulator. We discuss the application of this guided-THz IFEL technique for compression of a relativistic electron bunch and synchronization with the external laser pulse used to generate the THz pulse via optical rectification, as well as a laser-driven THz streaking diagnostic with the potential for femtosecond scale temporal resolution. Initial measurements of the THz waveform via an electro-optic sampling based technique confirm the predicted reduction of the group velocity, using a curved parallel plate waveguide, as a function of the varying aperture size of the guide. We also present the design of a proof-of-principle experiment based on the bunch parameters available at the UCLA PEGASUS laboratory. With a 10 MV m−1 THz peak field, our simulation model predicts compression of a 6 MeV 100 fs electron beam by nearly an order of magnitude and a significant reduction of its initial timing jitter.
[en] The Superheated Instrument for Massive Particle searches (SIMPLE 2000) will consist of an array of 8-16 large active mass (approx. 15 g) superheated droplet detectors (SDDs) to be installed in the new underground laboratory of Rustrel-Pays d'Apt. Several factors make the use of SDDs an attractive approach for the detection of weakly interacting massive particles (WIMPs), namely their intrinsic insensitivity to minimally ionizing particles, high fluorine content, low cost and operation at near ambient pressure and temperature. We comment here on the fabrication, calibration and already-competitive first limits from prototype SDDs for SIMPLE, as well as on the expected immediate increase in sensitivity of the programme, which aims at an exposure of > 25 kg day during 2000. The ability of modest-mass fluorine-rich detectors to investigate regions of neutralino parameter space beyond the reach of the most ambitious cryogenic projects is pointed out. (author)
[en] Though long-range magnetic order cannot occur at temperatures T > 0 in a perfect two-dimensional (2D) Heisenberg magnet, real quasi-2D materials will invariably possess nonzero inter-plane coupling Jperpendicular driving the system to order at elevated temperatures. This process can be studied using quantum Monte Carlo calculations. However, it is difficult to test the results of these calculations experimentally since for highly anisotropic materials in which the in-plane coupling is comparable with attainable magnetic fields Jperpendicular is necessarily very small and inaccessible directly. In addition, because of the large anisotropy, the Neel temperatures are low and difficult to determine from thermodynamic measurements. Here, we present an elegant method of assessing the calculations via two independent experimental probes: pulsed-field magnetization in fields of up to 85 T, and muon-spin rotation.
[en] We consider domain walls obtained by embedding the (1+1)-dimensional φ4-kink in higher dimensions. We show that a suitably adapted dimensional regularization method avoids the intricacies found in other regularization schemes in both supersymmetric and non-supersymmetric theories. This method allows us to calculate the one-loop quantum mass of kinks and surface tensions of kink domain walls in a very simple manner, yielding a compact d-dimensional formula which reproduces many of the previous results in the literature. Among the new results is the non-vanishing one-loop correction to the surface tension of a (2+1)-dimensional N=1 supersymmetric kink domain wall with chiral domain-wall fermions. (author)
[en] The pivotal role played by radial electric fields in the development of turbulence associated with anomalous transport is examined by means of global gyrokinetic simulations. It is shown that the stabilizing effect of ExB flows on ion temperature gradient (ITG) modes is quadratic in the shearing rate amplitude. For a given shearing rate it leads to an increase in the critical gradient. The electric fields (zonal flows) self-generated by ITG modes interact in a nonlinear way and it is shown that a saturated level of both the zonal flow and ITG turbulence is reached in the absence of any collisional mechanism being included in the model. The quality of the global nonlinear simulations is verified by the energy conservation which is allowed by the inclusion of nonlinear parallel dynamics. This demonstrates the absence of spurious damping of numerical origin and thus confirms the nonlinear character of zonal flow saturation mechanism. (author)