2,780 research outputs found
Heat-transfer tests on the Rockwell International space shuttle orbiter with and without simulated protuberances
Aerothermodynamic tests on the forward half of the Rockwell International Space Shuttle Orbiter Configuration 140C were conducted at Mach number 8. The phase-change paint and thin-skin thermocouple techniques were used to determine the aerodynamic heating rates on the Orbiter models during simulated atmospheric reentry. Smooth 0.04-scale models and models with scaled protuberances and indentations which simulated the windshields, cargo bay door hinges, vents, and thruster nozzles were tested over an angle-of-attack range from 20 to 45 deg at yaw angles from -5 to 5 deg and at Reynolds numbers, based on the total Orbiter scaled length, from 2.15 to 15.9 million. Comparisons of the model heat-transfer rates obtained with a smooth surface and with scaled protuberances are presented
Barkhausen-type noise in the resistance of antiferromagnetic Cr thin films
We present an experimental study of the changes generated on the electrical
resistance of epitaxial Cr thin films by the transformation of quantized
spin density wave domains as the temperature is changed. A characteristic
resistance noise appears only within the same temperature region where a
cooling-warming cycle in displays hysteretic behavior. We propose an
analysis based on an analogy with the Barkhausen noise seen in ferromagnets.
There fluctuations in the magnetization occur when the magnetic field
is swept. By mapping and , where
corresponds to the order parameter of the spin density wave, we
generalize the Preisach model in terms of a random distribution of {\it
resistive hysterons} to explain our results. These hysterons are related to
distributions of quantized spin density wave domains with different sizes,
local energies and number of nodes.Comment: 5 pages, 3 figures. To be published in Europhysics Letter
Strongly Inhomogeneous Phases and Non-Fermi Liquid Behavior in Randomly Depleted Kondo Lattices
We investigate the low-temperature behavior of Kondo lattices upon random
depletion of the local -moments, by using strong-coupling arguments and
solving SU() saddle-point equations on large lattices. For a large range of
intermediate doping levels, between the coherent Fermi liquid of the dense
lattice and the single-impurity Fermi liquid of the dilute limit, we find
strongly inhomogeneous states that exhibit distinct non-Fermi liquid
characteristics. In particular, the interplay of dopant disorder and strong
interactions leads to rare weakly screened moments which dominate the bulk
susceptibility. Our results are relevant to compounds like Ce_{x}La_{1-x}CoIn_5
and Ce_{x}La_{1-x}Pb_3Comment: 4 pages, 5 figure
Spin fluctuations with two-dimensional XY behavior in a frustrated S = 1/2 square-lattice ferromagnet
The spin dynamics of the layered square-lattice vanadate Pb2VO(PO4)2 is
investigated by electron spin resonance at various magnetic fields and at
temperatures above magnetic ordering. The linewidth divergence towards low
temperatures seems to agree with isotropic Heisenberg-type spin exchange
suggesting that the spin relaxation in this quasi-two dimensional compound is
governed by low-dimensional quantum fluctuations. However, a weak easy- plane
anisotropy of the g factor points to the presence of a planar XY type of
exchange. Indeed, we found that the linewidth divergence is described best by
XY-like spin fluctuations which requires a single parameter only. Therefore,
ESR-probed spin dynamics could establish Pb2VO(PO4)2 as the first frustrated
square lattice system with XY-inherent spin topological fluctuations.Comment: 5 pages, 3 figure
On the equivalence between topologically and non-topologically massive abelian gauge theories
We analyse the equivalence between topologically massive gauge theory (TMGT)
and different formulations of non-topologically massive gauge theories (NTMGTs)
in the canonical approach. The different NTMGTs studied are St\"uckelberg
formulation of (A) a first order formulation involving one and two form fields,
(B) Proca theory, and (C) massive Kalb-Ramond theory. We first quantise these
reducible gauge systems by using the phase space extension procedure and using
it, identify the phase space variables of NTMGTs which are equivalent to the
canonical variables of TMGT and show that under this the Hamiltonian also get
mapped. Interestingly it is found that the different NTMGTs are equivalent to
different formulations of TMGTs which differ only by a total divergence term.
We also provide covariant mappings between the fields in TMGT to NTMGTs at the
level of correlation function.Comment: One reference added and a typos corrected. 15 pages, To appear in
Mod. Phys. Lett.
BTZ Black Hole Entropy from Ponzano-Regge Gravity
The entropy of the BTZ black hole is computed in the Ponzano-Regge
formulation of three-dimensional lattice gravity. It is seen that the correct
semi-classical behaviour of entropy is reproduced by states that correspond to
all possible triangulations of the Euclidean black hole.Comment: 11 pages LaTeX, 3 eps figures, some minor clarifications added,
result unchange
Reductions for the Stable Set Problem
One approach to finding a maximum stable set (MSS) in a graph is to try to reduce the size of the problem by transforming the problem into an equivalent problem on a smaller graph. This paper introduces several new reductions for the MSS problem, extends several well-known reductions to the maximum weight stable set (MWSS) problem, demonstrates how reductions for the generalized stable set problem can be used in conjunction with probing to produce powerful new reductions for both the MSS and MWSS problems, and shows how hypergraphs can be used to expand the capabilities of clique projections. The effectiveness of these new reduction techniques are illustrated on the DIMACS benchmark graphs, planar graphs, and a set of challenging MSS problems arising from Steiner Triple Systems
Spin correlations and exchange in square lattice frustrated ferromagnets
The J1-J2 model on a square lattice exhibits a rich variety of different
forms of magnetic order that depend sensitively on the ratio of exchange
constants J2/J1. We use bulk magnetometry and polarized neutron scattering to
determine J1 and J2 unambiguously for two materials in a new family of vanadium
phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have
ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground
state is reduced, and the diffuse magnetic scattering is enhanced, as the
predicted bond-nematic region of the phase diagram is approached.Comment: 4 pages, 4 figure
Strong frustration due to competing ferromagnetic and antiferromagnetic interactions: magnetic properties of M(VO)2(PO4)2 (M = Ca and Sr)
We present a detailed investigation of the magnetic properties of complex
vanadium phosphates M(VO)2(PO4)2 (M = Ca, Sr) by means of magnetization,
specific heat, 31P NMR measurements, and band structure calculations.
Experimental data evidence the presence of ferro- and antiferromagnetic
interactions in M(VO)2(PO4)2 resulting in a nearly vanishing Curie-Weiss
temperature \theta_{CW} < 1 K that contrasts with the maximum of magnetic
susceptibility at 3 K. Specific heat and NMR measurements also reveal weak
exchange couplings with the thermodynamic energy scale J_c = 10-15 K.
Additionally, the reduced maximum of the magnetic specific heat indicates
strong frustration of the spin system. Band structure calculations show that
the spin systems of the M(VO)2(PO4)2 compounds are essentially
three-dimensional with the frustration caused by competing ferro- and
antiferromagnetic interactions. Both calcium and strontium compounds undergo
antiferromagnetic long-range ordering at T_N = 1.5 K and 1.9 K, respectively.
The spin model reveals an unusual example of controllable frustration in
three-dimensional magnetic systems.Comment: 14 pages, 13 figures, 3 tables; published versio
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