53,308 research outputs found
Universal nonequilibrium signatures of Majorana zero modes in quench dynamics
The quantum evolution after a metallic lead is suddenly connected to an
electron system contains information about the excitation spectrum of the
combined system. We exploit this type of "quantum quench" to probe the presence
of Majorana fermions at the ends of a topological superconducting wire. We
obtain an algebraically decaying overlap (Loschmidt echo) for large times after the quench, with
a universal critical exponent =1/4 that is found to be remarkably
robust against details of the setup, such as interactions in the normal lead,
the existence of additional lead channels or the presence of bound levels
between the lead and the superconductor. As in recent quantum dot experiments,
this exponent could be measured by optical absorption, offering a new signature
of Majorana zero modes that is distinct from interferometry and tunneling
spectroscopy.Comment: 9 pages + appendices, 4 figures. v3: published versio
Completion of evaluation of manufacturing processes for B/Al composites containing 0.2mm diameter boron fibers
Four fabricators produced a total of 54 B/1100 Al, B/6061 Al, and B/2024 Al panels for evaluation. The 8 ply unidirectional, 45 to 50 volume percent, panels were made using 0.20 mm diameter boron fibers which were obtained from a single supplier. Hot press consolidation was carried out in vacuum except for one set of dry woven tape panels which were hot pressed in air. A single testing contractor conducted nondestructive inspection, metallography, fractography and mechanical property tests. The mechanical property tests included 21 and 260 C tensile tests and 21 C shear tests. Panel quality, as measured by nondestructive evaluation, was generally good as were the 21 C tensile properties. The panels hot pressed in air delaminated in the shear tests. Shear strength values were lower in these panels. But tensile strengths were not affected by the delaminations because of the relation between the tensile loading direction and the delaminations. Composite tensile strength was found to be proportional to the volume percent boron and the aluminum matrix rather than to the tape used or fabrication technique. Suitability of these composites for 260 C service was confirmed by tensile tests
Entanglement entropy of random quantum critical points in one dimension
For quantum critical spin chains without disorder, it is known that the
entanglement of a segment of N>>1 spins with the remainder is logarithmic in N
with a prefactor fixed by the central charge of the associated conformal field
theory. We show that for a class of strongly random quantum spin chains, the
same logarithmic scaling holds for mean entanglement at criticality and defines
a critical entropy equivalent to central charge in the pure case. This
effective central charge is obtained for Heisenberg, XX, and quantum Ising
chains using an analytic real-space renormalization group approach believed to
be asymptotically exact. For these random chains, the effective universal
central charge is characteristic of a universality class and is consistent with
a c-theorem.Comment: 4 pages, 3 figure
Vibration limiting of rotors by feedback control
Experimental findings of a three mass rotor with four channels of feedback control are reported. The channels are independently controllable with force being proportional to the velocity and/or instantaneous displacement from equilibrium of the shaft at the noncontacting probe locations (arranged in the vertical and horizontal attitudes near the support bearings). The findings suggest that automatic feedback control of rotors is feasible for limiting certain vibration levels. Control of one end of a rotor does afford some predictable vibration limiting of the rotor at the other end
Chern-Simons Number Diffusion and Hard Thermal Loops on the Lattice
We develop a discrete lattice implementation of the hard thermal loop
effective action by the method of added auxiliary fields. We use the resulting
model to measure the sphaleron rate (topological susceptibility) of Yang-Mills
theory at weak coupling. Our results give parametric behavior in accord with
the arguments of Arnold, Son, and Yaffe, and are in quantitative agreement with
the results of Moore, Hu, and Muller.Comment: 43 pages, 6 figure
Unified model for vortex-string network evolution
We describe and numerically test the velocity-dependent one-scale (VOS)
string evolution model, a simple analytic approach describing a string network
with the averaged correlation length and velocity. We show that it accurately
reproduces the large-scale behaviour (in particular the scaling laws) of
numerical simulations of both Goto-Nambu and field theory string networks. We
explicitly demonstrate the relation between the high-energy physics approach
and the damped and non-relativistic limits which are relevant for condensed
matter physics. We also reproduce experimental results in this context and show
that the vortex-string density is significantly reduced by loop production, an
effect not included in the usual `coarse-grained' approach.Comment: 5 pages; v2: cosmetic changes, version to appear in PR
The Escape of Ionizing Photons from the Galaxy
The Magellanic Stream and several high velocity clouds have now been detected
in optical line emission. The observed emission measures and kinematics are
most plausibly explained by photoionization due to hot, young stars in the
Galactic disk. The highly favorable orientation of the Stream allows an
unambiguous determination of the fraction of ionizing photons, F_esc, which
escape the disk. We have modelled the production and transport of ionizing
photons through an opaque interstellar medium. Normalization to the Stream
detections requires F_esc = 6%, in reasonable agreement with the flux required
to ionize the Reynolds layer. Neither shock heating nor emission within a hot
Galactic corona can be important in producing the observed H-alpha emission. If
such a large escape fraction is typical of L_* galaxies, star-forming systems
dominate the extragalactic ionizing background. Within the context of this
model, both the three-dimensional orientation of the Stream and the distances
to high-velocity clouds can be determined by sensitive H-alpha observations.Comment: 4 pages; LaTeX2e, emulateapj.sty, apjfonts.sty; 4 encapsulated PS
figures. For correct labels, may need to print Fig. 3 separately due to psfig
limitation. Astrophysical Journal (Letters), accepte
Classical Sphaleron Rate on Fine Lattices
We measure the sphaleron rate for hot, classical Yang-Mills theory on the
lattice, in order to study its dependence on lattice spacing. By using a
topological definition of Chern-Simons number and going to extremely fine
lattices (up to beta=32, or lattice spacing a = 1 / (8 g^2 T)) we demonstrate
nontrivial scaling. The topological susceptibility, converted to physical
units, falls with lattice spacing on fine lattices in a way which is consistent
with linear dependence on (the Arnold-Son-Yaffe scaling relation) and
strongly disfavors a nonzero continuum limit. We also explain some unusual
behavior of the rate in small volumes, reported by Ambjorn and Krasnitz.Comment: 14 pages, includes 5 figure
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