69,464 research outputs found
Electronic states of trans-polyacetylene, poly(p-phenylene vinylene) and sp-hybridised carbon species in amorphous hydrogenated carbon probed by resonant Raman scattering
Inclusions of sp-hybridised, trans-polyacetylene [trans-(CH)x] and
poly(p-phenylene vinylene) (PPV) chains are revealed using resonant Raman
scattering (RRS) investigation of amorphous hydrogenated carbon (a-C:H) films
in the near IR - UV range. The RRS spectra of trans-(CH)x core Ag modes and the
PPV CC-H phenylene mode are found to transform and disperse as the laser
excitation energy \hbar{\omega}L is increased from near IR through visible to
UV, whereas sp-bonded inclusions only become evident in UV. This is attributed
to \hbar{\omega}L probing of trans-(CH)x chain inhomogeneity and the
distribution of chains with varying conjugation length; for PPV to the resonant
probing of phelynene ring disorder; and for sp segments, to \hbar{\omega}L
probing of a local band gap of end-terminated polyynes. The IR spectra analysis
confirmed the presence of sp, trans-(CH)x and PPV inclusions. The obtained RRS
results for a-C:H denote differentiation between the core Ag trans-(CH)x modes
and the PPV phenylene mode. Furthermore, it was found that at various laser
excitation energies the changes in Raman spectra features for trans-(CH)x
segments included in an amorphous carbon matrix are the same as in bulk
trans-polyacetylene. The latter finding can be used to facilitate
identification of trans-(CH)x in the spectra of complex carbonaceous materials.Comment: 31 page, 9 figure
Evidence for heat losses via party wall cavities in masonry construction
This paper presents empirical evidence and analysis that supports the existence of a significant heat loss mechanism resulting from air movement through cavities in party walls in masonry construction. A range of heat loss experiments were undertaken as part of the Stamford Brook housing field trial in Altrincham in the United Kingdom. Co-heating tests showed a large discrepancy between the predicted and measured whole house heat loss coefficients. Analysis of the co-heating results, along with internal temperature data, thermal imaging and a theoretical analysis indicated that the most likely explanation for the discrepancy was bypassing of the thermal insulation via the uninsulated party wall cavities. The data show that such a bypass mechanism is potentially the largest single contributor to heat loss in terraced dwellings built to the 2006 revision of the Building Regulations. A comparable convective heat bypass associated with masonry party walls was identified in the late 1970s during the course of the Twin Rivers Project in the United States, albeit in a somewhat different construction from that used at Stamford Brook. A similar effect was also reported in the United Kingdom in the mid 1990s. However, it appears that no action was taken at that time either to confirm the results, to develop any technical solutions, or to amend standards for calculating heat losses from buildings. Current conventions for heat loss calculations in the United Kingdom do not take account of heat losses associated with party walls and it is suggested by the authors that such conventions may need to be updated to take account of the effect described in this paper. In the final part of the paper, the authors propose straightforward solutions to prevent bypassing of roof insulation via party walls by for example filling the cavity of the party wall with mineral fibre insulation, or by inserting a cavity closer across the cavity in the plane of the roof insulation.Practical application: The heat bypass mechanism described in this paper is believed by the authors to contribute to a significant proportion of heat loss from buildings in the UK constructed with clear cavities such as those found in separating walls between cavity masonry dwellings. It is proposed that relatively simple design changes could be undertaken to eliminate such heat loss pathways from new buildings. In addition, simple and cost effective measures are envisaged that could be used to minimise or eliminate the bypass from existing buildings. Such an approach could give rise to a significant reduction in carbon emissions from UK housing
Maximal abelian and Curci-Ferrari gauges in momentum subtraction at three loops
The vertex structure of QCD fixed in the maximal abelian gauge (MAG) and
Curci-Ferrari gauge is analysed at two loops at the fully symmetric point for
the 3-point functions corresponding to the three momentum subtraction (MOM)
renormalization schemes. Consequently the three loop renormalization group
functions are determined for each of these three schemes in each gauge using
properties of the renormalization group equation.Comment: 23 latex pages, 4 figures, anc directory contains txt files with
electronic version of renormalization group functions, coupling constant
mappings, conversion functions and amplitudes in analytic form for each gaug
Bilinear quark operator renormalization at generalized symmetric point
We compute Green's functions with a bilinear quark operator inserted at
non-zero momentum for a generalized momentum configuration to two loops. These
are required to assist lattice gauge theory measurements of the same quantity
in matching to the high energy behaviour. The flavour non-singlet operators
considered are the scalar, vector and tensor currents as well as the second
moment of the twist-2 Wilson operator used in deep inelastic scattering for the
measurement of nucleon structure functions.Comment: 19 latex pages, 4 figures, anc directory contains electronic version
of amplitude
Proposal of an experimentally accessible measure of many-fermion entanglement
We propose a measure of interaction-induced ground state entanglement in
many-fermion systems that is experimentally accessible. It is formulated in
terms of cross-correlations of currents through resonant fermion levels weakly
coupled to the probed system. The proposed entanglement measure vanishes in the
absence of many-body interactions and it is related to measures of occupation
number entanglement. We evaluate it for two examples of interacting electronic
nanostructures.Comment: 5 pages, 3 figures, published versio
Spin dynamics in a superconductor / ferromagnet proximity system
The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is
measured. By varying the temperature and thickness of the Nb the role of the
superconductivity on the whole ferromagnetic layer in these heterostructures is
explored. The change in the spin transport properties below the superconducting
transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a
sharpening in the resonance of the ferromagnet, or a decrease in the effective
Gilbert damping co-efficient. This dynamic proximity effect is in contrast to
low frequency studies in these systems, where the effect of the superconductor
is confined to a small region in the ferromagnet. We interpret this in terms of
the spin pumping model.Comment: 4 pages, 4 figures, to be submitted for publicatio
Sudden Critical Current Drops Induced in S/F Structures
In the search for new physical properties of S/F structures, we have found
that the superconductor critical current can be controlled by the domain state
of the neighboring ferromagnet. The superconductor is a thin wire of thickness
d_{s} ~ 2 xi_{S}. Nb/Co and Nb/Py (Permalloy Ni_{80}Fe_{20}) bilayer structures
were grown with a significant magnetic anisotropy. Critical current
measurements of Nb/Co structures with ferromagnet thickness d_{F} > 30nm show
sudden drops in two very defined steps when the measurements are made along the
hard axes direction (i.e. current track parallel to hard anisotropy axes
direction). These drops disappear when they are made along the easy axis
direction or when the ferromagnet thickness is below 30nm. The drops are
accompanied by vortex flux flow. In addition magnetorestistance measurements
close to Tc show a sharp increase near saturation fields of the ferromagnet.
Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet
thickness d_{F} ~ 50nm along the easy anisotropy axes. These results are
explained as being due to spontaneous vortex formation and flow induced by
Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain
walls produce a 2D vortex-antivortex lattice structure.Comment: 6 pages, 6 figure
Maximal violation of Bell inequality for any given two-qubit pure state
In the case of bipartite two qubits systems, we derive the analytical
expression of bound of Bell operator for any given pure state. Our result not
only manifest some properties of Bell inequality, for example which may be
violated by any pure entangled state and only be maximally violated for a
maximally entangled state, but also give the explicit values of maximal
violation for any pure state. Finally we point out that for two qubits systems
there is no mixed state which can produce maximal violation of Bell inequality.Comment: 3 pages, 1 figure
Unified criteria for multipartite quantum nonlocality
Wiseman and co-workers (Phys. Rev. Lett. 98, 140402, 2007) proposed a
distinction between the nonlocality classes of Bell's nonlocality, steering and
entanglement based on whether or not an overseer trusts each party in a
bipartite scenario where they are asked to demonstrate entanglement. Here we
extend that concept to the multipartite case and derive inequalities that
progressively test for those classes of nonlocality, with different thresholds
for each level. This framework includes the three classes of nonlocality above
in special cases and introduces a family of others.Comment: V2: corrected image display; V3: substantial changes including new
proofs, arguments, and result
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