8,985 research outputs found
Radical-cation salts of BEDT-TTF with lithium tris(oxalato)metallate(III)
The first radical-cation salts in the extensive family (BEDT-TTF)x[(A)M(C2O4)3]·Guest containing lithium as the counter cation have been synthesized and characterised
Remarks on Duality Transformations and Generalized Stabilizer States
We consider the transformation of Hamilton operators under various sets of
quantum operations acting simultaneously on all adjacent pairs of particles. We
find mappings between Hamilton operators analogous to duality transformations
as well as exact characterizations of ground states employing non-Hermitean
eigenvalue equations and use this to motivate a generalization of the
stabilizer formalism to non-Hermitean operators. The resulting class of states
is larger than that of standard stabilizer states and allows for example for
continuous variation of local entropies rather than the discrete values taken
on stabilizer states and the exact description of certain ground states of
Hamilton operators.Comment: Contribution to Special Issue in Journal of Modern Optics celebrating
the 60th birthday of Peter Knigh
Quantitative verification of entanglement and fidelities from incomplete measurement data
Many experiments in quantum information aim at creating multi-partite
entangled states. Quantifying the amount of entanglement that was actually
generated can, in principle, be accomplished using full-state tomography. This
method requires the determination of a parameter set that is growing
exponentially with the number of qubits and becomes infeasible even for
moderate numbers of particles. Non-trivial bounds on experimentally prepared
entanglement can however be obtained from partial information on the density
matrix. The fundamental question that needs to be addressed in this context is
then formulated as: What is the entanglement content of the least entangled
quantum state that is compatible with the available measurement data?
We formulate the problem mathematically employing methods from the theory of
semi-definite programming and then address this problem for the case, where the
goal of the experiment is the creation of graph states. The observables that we
consider are the generators of the stabilizer group, thus the number of
measurement settings grows only linearly in the number of qubits. We provide
analytical solutions as well as numerical methods that may be applied directly
to experiments, and compare the obtained bounds with results from full-state
tomography for simulated data.Comment: 7 pages, PQE 2009 special issu
Numerically stable computation of CreditRisk+
The CreditRisk+ model launched by CSFB in 1997 is widely used by practitioners in the banking sector as a simple means for the quantification of credit risk, primarily of the loan book. We present an alternative numerical recursion scheme for CreditRisk+, equivalent to an algorithm recently proposed by Giese, based on well-known expansions of the logarithm and the exponential of a power series. We show that it is advantageous to the Panjer recursion advocated in the original CreditRisk+ document, in that it is numerically stable. The crucial stability arguments are explained in detail. Furthermore, the computational complexity of the resulting algorithm is stated
The parameter space of graphene chemical vapor deposition on polycrystalline Cu
A systematic study on the parameter space of graphene CVD on polycrystalline Cu foils is
presented, aiming at a more fundamental process rationale in particular regarding the choice
of carbon precursor and mitigation of Cu sublimation. CH4 as precursor requires H2 dilution
and temperatures ≥1000°C to keep the Cu surface reduced and yield a high quality, complete
monolayer graphene coverage. The H2 atmosphere etches as-grown graphene, hence
maintaining a balanced CH4/H2 ratio is critical. Such balance is more easily achieved at low
pressure conditions, at which however Cu sublimation reaches deleterious levels. In contrast,
C6H6 as precursor requires no reactive diluent and consistently gives similar graphene quality
at 100-150°C lower temperatures. The lower process temperature and more robust processing
conditions allow the problem of Cu sublimation to be effectively addressed. Graphene
formation is not inherently self-limited to a monolayer for any of the precursors. Rather, the
higher the supplied carbon chemical potential the higher the likelihood of film inhomogeneity
and primary and secondary multilayer graphene nucleation. For the latter, domain boundaries
of the inherently polycrystalline CVD graphene offer pathways for a continued carbon supply
to the catalyst. Graphene formation is significantly affected by the Cu crystallography, i.e. the
evolution of microstructure and texture of the catalyst template form an integral part of the
CVD process.S.H. acknowledges funding from ERC grant InsituNANO (n°279342) and from EPSRC
(Grant Nr. EP/H047565/1). P.R.K. acknowledges funding from the Cambridge
Commonwealth Trust and C.D. acknowledges funding from Royal Society.This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/jp303597m
Inflation with Non-minimal Gravitational Couplings and Supergravity
We explore in the supergravity context the possibility that a Higgs scalar
may drive inflation via a non-minimal coupling to gravity characterised by a
large dimensionless coupling constant. We find that this scenario is not
compatible with the MSSM, but that adding a singlet field (NMSSM, or a variant
thereof) can very naturally give rise to slow-roll inflation. The inflaton is
necessarily contained in the doublet Higgs sector and occurs in the D-flat
direction of the two Higgs doublets.Comment: 13 pages, 1 figur
Atomic layer deposited oxide films as protective interface layers for integrated graphene transfer
The transfer of chemical vapour deposited (CVD) graphene from its parent growth catalyst has become a bottleneck for many of its emerging applications. The sacrificial polymer layers that are typically deposited onto graphene for mechanical support during transfer are challenging to fully remove and hence leave graphene and subsequent device interfaces contaminated. Here, we report on the use of atomic layer deposited (ALD) oxide films as protective interface and support layers during graphene transfer. The method avoids any direct contact of the graphene with polymers and through the use of thicker ALD layers (≥100nm), polymers can be eliminated from the transfer-process altogether. The ALD film can be kept as a functional device layer, facilitating integrated device manufacturing. We demonstrate back-gated field effect devices based on single-layer graphene transferred with a protective Al2O3 film onto SiO2 that show significantly reduced charge trap and residual carrier densities. We critically discuss the advantages and challenges of processing graphene/ALD bilayer structures.We acknowledge funding from EPSRC (Grant No. EP/K016636/1, GRAPHTED) and ERC (Grant No. 279342, InsituNANO). ACV acknowledges the Conacyt Cambridge Scholarship and Roberto Rocca Fellowship. JAA-W acknowledges the support of his Research Fellowships from the Royal Commission for the Exhibition of 1851 and Churchill College, Cambridge. RSW acknowledges a Research Fellowship from St. John's College, Cambridge and a Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union's Horizon 2020 research and innovation programme
Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space
We report on measurements of extremely high reflection rates of solar wind
particles from regolith-covered lunar surfaces. Measurements by the Sub-keV
Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1
spacecraft in orbit around the Moon show that up to 20% of the impinging solar
wind protons are reflected from the lunar surface back to space as neutral
hydrogen atoms. This finding, generally applicable to regolith-covered
atmosphereless bodies, invalidates the widely accepted assumption that regolith
almost completely absorbs the impinging solar wind.Comment: 2 figure
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