5,676 research outputs found
Quantum phase transitions in disordered dimerized quantum spin models and the Harris criterion
We use quantum Monte Carlo simulations to study effects of disorder on the
quantum phase transition occurring versus the ratio g=J/J' in square-lattice
dimerized S=1/2 Heisenberg antiferromagnets with intra- and inter-dimer
couplings J and J'. The dimers are either randomly distributed (as in the
classical dimer model), or come in parallel pairs with horizontal or vertical
orientation. In both cases the transition violates the Harris criterion,
according to which the correlation-length exponent should satisfy nu >= 1. We
do not detect any deviations from the three-dimensional O(3) universality class
obtaining in the absence of disorder (where nu = 0.71). We discuss special
circumstances which allow nu<1 for the type of disorder considered here.Comment: 4+ pages, 3 figure
Thermodynamically stable noncomposite vortices in mesoscopic two-gap superconductors
In mesoscopic two-gap superconductors with sizes of the order of the
coherence length noncomposite vortices are found to be thermodynamically stable
in a large domain of the phase diagram. In these phases the vortex
cores of one condensate are spatially separated from the other condensate ones,
and their respective distributions can adopt distinct symmetries. The
appearance of these vortex phases is caused by a non-negligible effect of the
boundary of the sample on the superconducting order parameter and represents
therefore a genuine mesoscopic effect. For low values of interband Josephson
coupling vortex patterns with can arise in addition to the
phases with , where and are total vorticities in the two
condensates. The calculations show that noncomposite vortices could be observed
in thin mesoscopic samples of MgB.Comment: 5 pages, 3 figures, to be published in Europhysics Letter
Scanning SQUID microscopy of vortex clusters in multiband superconductors
In type-1.5 superconductors, vortices emerge in clusters, which grow in size
with increasing magnetic field. These novel vortex clusters and their field
dependence are directly visualized by scanning SQUID microscopy at very low
vortex densities in MgB2 single crystals. Our observations are elucidated by
simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime.Comment: 4 pages, 5 figures, to be published in Physical Review
Crack propagation in concrete at very early ages
Surface defects and cracks in early-age concrete slabs have been observed to propagate under adverse conditions, impairing the performance and service life of these structures. However, the underlying mechanism of this form of crack propagation has remained largely unexplained, with very limited literature available. In this paper, simple yet sufficiently rigorous theoretical analyses of crack propagation in early-age concrete slabs, based on combined geotechnical engineering and fracture mechanics models, are presented. The results obtained clearly show how surface cracks can become unstable and propagate further, and either become stable again or develop through the full depth of the slab. They also convincingly demonstrate the roles of surface cracks and defects, pore moisture suctions and exposure conditions in this process. Importantly, the critical role of good construction practices in minimising this form of cracking is highlighted. These include proper compaction and effective curing as well as timely and adequate saw-cutting
Plastic cracking of concrete: The roles of osmotic and matric suctions
Plastic cracking of concrete is primarily attributable to desiccation by evaporation from unprotected surfaces. This causes high matric suctions to develop in the pore water in the voids adjacent to these surfaces. Dissolved salts in the pore water generate osmotic suctions. However, the effects of these suctions on the strength of plastic concrete are imperfectly understood. In this paper, equations describing total (matric plus osmotic) and osmotic suctions and the shear strength of desiccated particulate materials are discussed briefly. The development of suctions in desiccating fly ash and their effect on its shear strength are illustrated by experimental data. These show that matric suctions do but osmotic suctions do not affect the shear strength of fly ash and hence of comparable materials, including plastic concrete
Spin wave theory study of neutron intensity, magnetic field, and anisotropy of Type IIA FCC antiferromagnet
We study the spin dynamics in a 3D quantum antiferromagnet on a face-centered
cubic (FCC) lattice. The effects of magnetic field, single-ion anisotropy, and
biquadratic interactions are investigated using linear spin wave theory with
spins in a canted basis about the Type IIA FCC antiferromagnetic ground state
structure which is known to be stable. We calculate the expected finite
frequency neutron scattering intensity and give qualitative criteria for
typical FCC materials MnO and CoO. The magnetization reduction due to quantum
zero point fluctuations is also analyzed.Comment: 8 pages, 9 figures; Section IVB heading corrected, Figs. 3 - 5
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