1,391 research outputs found
Efficient Peltier refrigeration by a pair of normal metal/ insulator/superconductor junctions
We suggest and demonstrate in experiment that two normal metal /insulator/
superconductor (NIS) tunnel junctions combined in series to form a symmetric
SINIS structure can operate as an efficient Peltier refrigerator. Specifically,
it is shown that the SINIS structure with normal-state junction resistances 1.0
and 1.1 k is capable of reaching a temperature of about 100 mK starting
from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total
junction area of 0.8 m at mK.Comment: 7 pages, revtex, 3 figures by fax/conventional mail upon reques
On the Column Density of AGN Outflows: the Case of NGC 5548
We re-analyze the HST high resolution spectroscopic data of the intrinsic
absorber in NGC 5548 and find that the C IV absorption column density is at
least four times larger than previously determined. This increase arises from
accounting for the kinematical nature of the absorber and from our conclusion
that the outflow does not cover the narrow emission line region in this object.
The improved column density determination begins to bridge the gap between the
high column densities measured in the X-ray and the low ones previously
inferred from the UV lines. Combined with our findings for outflows in high
luminosity quasars these results suggest that traditional techniques for
measuring column densities: equivalent width, curve-of-growth and Gaussian
modeling, are of limited value when applied to absorption associated with AGN
outflows.Comment: Published ApJ version (566, 699), including a new figure with FUSE
data and a useful algebraic expression for the optical depth solutio
3D loop models and the CP^{n-1} sigma model
Many statistical mechanics problems can be framed in terms of random curves;
we consider a class of three-dimensional loop models that are prototypes for
such ensembles. The models show transitions between phases with infinite loops
and short-loop phases. We map them to sigma models, where is the
loop fugacity. Using Monte Carlo simulations, we find continuous transitions
for , and first order transitions for . The results are
relevant to line defects in random media, as well as to Anderson localization
and -dimensional quantum magnets.Comment: Published versio
Emergent Symmetry at the N\'eel to Valence-Bond-Solid Transition
We show numerically that the `deconfined' quantum critical point between the
N\'eel antiferromagnet and the columnar valence-bond-solid, for a square
lattice of spin-1/2s, has an emergent symmetry. This symmetry allows
the N\'eel vector and the valence-bond-solid order parameter to be rotated into
each other. It is a remarkable 2+1-dimensional analogue of the symmetry that appears in the scaling limit for the
spin-1/2 Heisenberg chain. The emergent is strong evidence that the
phase transition in the 2+1D system is truly continuous, despite the violations
of finite-size scaling observed previously in this problem. It also implies
surprising relations between correlation functions at the transition. The
symmetry enhancement is expected to apply generally to the critical
two-component Abelian Higgs model (non-compact model). The result
indicates that in three dimensions there is an -symmetric conformal
field theory which has no relevant singlet operators, so is radically different
to conventional Wilson-Fisher-type conformal field theories.Comment: 4+epsilon pages, 6 figure
Deconfined Quantum Criticality, Scaling Violations, and Classical Loop Models
Numerical studies of the N\'eel to valence-bond solid phase transition in 2D
quantum antiferromagnets give strong evidence for the remarkable scenario of
deconfined criticality, but display strong violations of finite-size scaling
that are not yet understood. We show how to realise the universal physics of
the Neel-VBS transition in a 3D classical loop model (this includes the
interference effect that suppresses N\'eel hedgehogs). We use this model to
simulate unprecedentedly large systems (of size ). Our results are
compatible with a direct continuous transition at which both order parameters
are critical, and we do not see conventional signs of first-order behaviour.
However, we find that the scaling violations are stronger than previously
realised and are incompatible with conventional finite-size scaling over the
size range studied, even if allowance is made for a weakly/marginally
irrelevant scaling variable. In particular, different determinations of the
anomalous dimensions and yield very
different results. The assumption of conventional finite-size scaling gives
estimates which drift to negative values at large , in violation of
unitarity bounds. In contrast, the behaviour of correlators on scales much
smaller than is consistent with large positive anomalous dimensions.
Barring an unexpected reversal in behaviour at still larger sizes, this implies
that the transition, if continuous, must show unconventional finite-size
scaling, e.g. from a dangerously irrelevant scaling variable. Another
possibility is an anomalously weak first-order transition. By analysing the
renormalisation group flows for the non-compact model (-component
Abelian Higgs model) between two and four dimensions, we give the simplest
scenario by which an anomalously weak first-order transition can arise without
fine-tuning of the Hamiltonian.Comment: 20 pages, 19 figure
Length Distributions in Loop Soups
Statistical lattice ensembles of loops in three or more dimensions typically
have phases in which the longest loops fill a finite fraction of the system. In
such phases it is natural to ask about the distribution of loop lengths. We
show how to calculate moments of these distributions using or
and O(n) models together with replica techniques. The
resulting joint length distribution for macroscopic loops is Poisson-Dirichlet
with a parameter fixed by the loop fugacity and by symmetries of the
ensemble. We also discuss features of the length distribution for shorter
loops, and use numerical simulations to test and illustrate our conclusions.Comment: 4.5 page
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