24,799 research outputs found
Phase diagrams and universality classes of random antiferromagnetic spin ladders
The random antiferromagnetic two-leg and zigzag spin-1/2 ladders are
investigated using the real space renormalization group scheme and their
complete phase diagrams are determined. We demonstrate that the first system
belongs to the same universality class of the dimerized random spin-1/2 chain.
The zigzag ladder, on the other hand, is in a random singlet phase at weak
frustration and disorder. Otherwise, we give additional evidence that it
belongs to the universality class of the random antiferromagnetic and
ferromagnetic quantum spin chains, although the universal fixed point found in
the latter system is never realized. We find, however, a new universal fixed
point at intermediate disorder.Comment: 10 pages, 10 figure
Valence-bond theory of highly disordered quantum antiferromagnets
We present a large-N variational approach to describe the magnetism of
insulating doped semiconductors based on a disorder-generalization of the
resonating-valence-bond theory for quantum antiferromagnets. This method
captures all the qualitative and even quantitative predictions of the
strong-disorder renormalization group approach over the entire experimentally
relevant temperature range. Finally, by mapping the problem on a hard-sphere
fluid, we could provide an essentially exact analytic solution without any
adjustable parameters.Comment: 5 pages, 3 eps figure
Strong-disorder renormalization-group study of the one-dimensional tight-binding model
We formulate a strong-disorder renormalization-group (SDRG) approach to study
the beta function of the tight-binding model in one dimension with both
diagonal and off-diagonal disorder for states at the band center. We show that
the SDRG method, when used to compute transport properties, yields exact
results since it is identical to the transfer matrix method. The beta function
is shown to be universal when only off-diagonal disorder is present even though
single-parameter scaling is known to be violated. A different single-parameter
scaling theory is formulated for this particular (particle-hole symmetric)
case. Upon breaking particle-hole symmetry (by adding diagonal disorder), the
beta function is shown to crossover from the universal behavior of the
particle-hole symmetric case to the conventional non-universal one in agreement
with the two-parameter scaling theory. We finally draw an analogy with the
random transverse-field Ising chain in the paramagnetic phase. The
particle-hole symmetric case corresponds to the critical point of the quantum
Ising model while the generic case corresponds to the Griffiths paramagnetic
phase.Comment: includes 12 pages, 4 figure
Unveiling shocks in planetary nebulae
The propagation of a shock wave into a medium is expected to heat the
material beyond the shock, producing noticeable effects in intensity line
ratios such as [O III]/Halpha. To investigate the occurrence of shocks in
planetary nebulae (PNe), we have used all narrowband [O III] and Halpha images
of PNe available in the HST archive to build their [O III]/Halpha ratio maps
and to search for regions where this ratio is enhanced. Regions with enhanced
[O III]/Halpha emission ratio can be ascribed to two different types of
morphological structures: bow-shock structures produced by fast collimated
outflows and thin skins enveloping expanding nebular shells. Both collimated
outflows and expanding shells are therefore confirmed to generate shocks in
PNe. We also find regions with depressed values of the [O III]/Halpha ratio
which are found mostly around density bounded PNe, where the local contribution
of [N II] emission into the F656N Halpha filter cannot be neglected.Comment: 13 pages, 9 figures, 3 tables; To appear in Astronomy & Astrophysic
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