10,789 research outputs found
Tests of a full-scale annular ram-induction combustor for a Mach 3 cruise turbojet engine
Full-scale annular ram-induction combustor tests for Mach 3 cruise turbojet engin
Full scale tests of a short length, double annular ram induction turbojet combustor for supersonic flight
Performance tests and characteristics of short length, double annular ram induction turbojet combustion chambers for supersonic fligh
Raman scattering in correlated thin films as a probe of chargeless surface states
Several powerful techniques exist to detect topologically protected surface
states of weakly-interacting electronic systems. In contrast, surface modes of
strongly interacting systems which do not carry electric charge are much harder
to detect. We propose resonant light scattering as a means of probing the
chargeless surface modes of interacting quantum spin systems, and illustrate
its efficacy by a concrete calculation for the 3D hyperhoneycomb Kitaev quantum
spin liquid phase. We show that resonant scattering is required to efficiently
couple to this model's sublattice polarized surface modes, comprised of
emergent Majorana fermions that result from spin fractionalization. We
demonstrate that the low-energy response is dominated by the surface
contribution for thin films, allowing identification and characterization of
emergent topological band structures.Comment: 7 pages, 4 figures; added supplemental materia
Theory of Raman response in three-dimensional Kitaev spin liquids: application to and LiIrO compounds
We calculate the Raman response for the Kitaev spin model on the
-, -, and - harmonic
honeycomb lattices. We identify several quantitative features in the Raman
spectrum that are characteristic of the spin liquid phase. Unlike the dynamical
structure factor, which probes both the Majorana spinons and flux excitations
that emerge from spin fractionalization, the Raman spectrum in the Kitaev
models directly probes a density of states of pairs of fractional, dispersing
Majorana spinons. As a consequence, the Raman spectrum in all these models is
gapless for sufficiently isotropic couplings, with a low-energy power law that
results from the Fermi lines (or points) of the dispersing Majorana spinons. We
show that the polarization dependence of the Raman spectrum contains crucial
information about the symmetry of the ground state. We also discuss to what
extent the features of the Raman response that we find reflect generic
properties of the spin liquid phase, and comment on their possible relevance to
, and LiIrO compounds.Comment: 19 pages, 10 figures. VERSION 2: Corrected Figure 5 and fixed
inconsistencies between A and B chain-labelings. Also- a few typos and two
new ref
Resonant Raman scattering theory for Kitaev models and their Majorana fermion boundary modes
We study the inelastic light scattering response in two- (2D) and
three-dimensional (3D) Kitaev spin-liquid models with \ms band structures in
the symmetry classes BDI and D leading to protected gapless surface modes. We
present a detailed calculation of the resonant Raman/Brillouin scattering
vertex relevant to iridate and ruthenate compounds whose low-energy physics is
believed to be proximate to these spin-liquid phases. In the symmetry class
BDI, we find that while the resonant scattering on thin films can detect the
gapless boundary modes of spin liquids, the non-resonant processes do not
couple to them. For the symmetry class D, however, we find that the coupling
between both types of light-scattering processes and the low-energy surface
states is strongly suppressed. Additionally, we describe the effect of weak
time-reversal symmetry breaking perturbations on the bulk Raman response of
these systems.Comment: 23 pages, 20 figures, 4 appendices, 2 ancillary file
Open string axions and the flavor problem
We consider extensions of the standard model inspired by intersecting D-brane
constructions, in order to address flavor mass textures. We include additional
anomalous gauge symmetries, and scalar fields to break them and to generate
Froggatt-Nielsen mass terms. Green-Schwarz axions are included to cancel mixed
anomalies rendering the models consistent. At low energies, a residual
anomalous global symmetry remains, and its associated pseudo-Goldstone mode
becomes the physical axion, which can be interpreted as an axion arising from
open string modes. General considerations show that such axions are very common
in D-brane models and can be completely incompatible with current bounds.
Astrophysical constraints are placed on the axion both by including neutrino
masses in the Froggatt-Nielsen scheme and considering QCD instanton
contributions to the axion mass. We find simple models where the axion decay
constant is in the allowed range, but only one such minimal model with this
property is free from excessive fine tunings elsewhere. We also note that
generically addressing flavor textures for the CKM matrix leads to
deconstructed extra dimensions.Comment: 30 pages, 2 figures. v2: references added. v3:typos fixe
Mapping the dynamic interactions between vortex species in highly anisotropic superconductors
Here we use highly sensitive magnetisation measurements performed using a
Hall probe sensor on single crystals of highly anisotropic high temperature
superconductors to study the dynamic interactions
between the two species of vortices that exist in such superconductors. We
observe a remarkable and clearly delineated high temperature regime that
mirrors the underlying vortex phase diagram. Our results map out the parameter
space over which these dynamic interaction processes can be used to create
vortex ratchets, pumps and other fluxonic devices.Comment: 7 pages, 3 figures, to be published in Supercond. Sci. Techno
Effective Vortex Pinning in MgB2 thin films
We discuss pinning properties of MgB2 thin films grown by pulsed-laser
deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are
identified that contribute most effectively to the pinning of vortices in
randomly oriented films. The EB process produces low defected crystallites with
small grain size providing enhanced pinning at grain boundaries without
degradation of Tc. The PLD process produces films with structural disorder on a
scale less that the coherence length that further improves pinning, but also
depresses Tc
A DC magnetic metamaterial
Electromagnetic metamaterials are a class of materials which have been
artificially structured on a subwavelength scale. They are currently the focus
of a great deal of interest because they allow access to previously
unrealisable properties like a negative refractive index. Most metamaterial
designs have so far been based on resonant elements, like split rings, and
research has concentrated on microwave frequencies and above. In this work, we
present the first experimental realisation of a non-resonant metamaterial
designed to operate at zero frequency. Our samples are based on a
recently-proposed template for an anisotropic magnetic metamaterial consisting
of an array of superconducting plates. Magnetometry experiments show a strong,
adjustable diamagnetic response when a field is applied perpendicular to the
plates. We have calculated the corresponding effective permeability, which
agrees well with theoretical predictions. Applications for this metamaterial
may include non-intrusive screening of weak DC magnetic fields.Comment: 6 pages, 3 figure
The Fluctuations of the Quark Number and of the Chiral Condensate
The distributions of the quark number and chiral condensate over the gauge
fields are computed for QCD in Euclidean space at nonzero quark chemical
potential. As both operators are non-hermitian the distributions are in the
complex plane. Moreover, because of the sign problem, the distributions are not
real and positive. The computations are carried out within leading order chiral
perturbation theory and give a direct insight into the delicate cancellations
that take place in contributions to the total baryon number and the chiral
condensate.Comment: 19 pages, 2 figure
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