18,543 research outputs found
Flux-vector model of spin noise in superconducting circuits: Electron versus nuclear spins and role of phase transition
Superconducting Quantum Interference Devices (SQUIDs) and other
superconducting circuits are limited by intrinsic flux noise with spectral
density with whose origin is believed to be due to
spin impurities. Here we present a theory of flux noise that takes into account
the vectorial nature of the coupling of spins to superconducting wires. We
present explicit numerical calculations of the flux noise power (spectral
density integrated over all frequencies) for electron impurities and lattice
nuclear spins under several different assumptions. The noise power is shown to
be dominated by surface electron spins near the wire edges, with bulk lattice
nuclear spins contributing % of the noise power in aluminum and niobium
wires. We consider the role of electron spin phase transitions, showing that
the spin-spin correlation length (describing e.g. the average size of
ferromagnetic spin clusters) greatly impacts the scaling of flux noise with
wire geometry. Remarkably, flux noise power is exactly equal to zero when the
spins are polarized along the flux vector direction, forming what we call a
poloidal state. Flux noise is non-zero for other spin textures, but gets
reduced in the presence of correlated ferromagnetic fluctuations between the
top and bottom wire surfaces, where the flux vectors are antiparallel. This
demonstrates that engineering spin textures and/or inter-surface correlation
provides a method to reduce flux noise in superconducting devices.Comment: New version accepted in PRB. Contains new discussion about the
poloidal stat
The Penna model for biological ageing on a lattice: spatial consequences of child-care
We introduce a square lattice into the Penna bit-string model for biological
ageing and study the evolution of the spatial distribution of the population
considering different strategies of child-care. Two of the strategies are
related to the movements of a whole family on the lattice: in one case the
mother cannot move if she has any child younger than a given age, and in the
other case if she moves, she brings these young children with her. A stronger
condition has also been added to the second case, considering that young
children die with a higher probability if their mothers die, this probability
decreasing with age. We show that a highly non uniform occupation can be
obtained when child-care is considered, even for an uniform initial occupation
per site. We also compare the standard survival rate of the model with that
obtained when the spacial lattice is considered (without any kind of
child-care).Comment: 8 pages, 6 Postscript figure
Effect of an inhomogeneous external magnetic field on a quantum dot quantum computer
We calculate the effect of an inhomogeneous magnetic field, which is
invariably present in an experimental environment, on the exchange energy of a
double quantum dot artificial molecule, projected to be used as a 2-qubit
quantum gate in the proposed quantum dot quantum computer. We use two different
theoretical methods to calculate the Hilbert space structure in the presence of
the inhomogeneous field: the Heitler-London method which is carried out
analytically and the molecular orbital method which is done computationally.
Within these approximations we show that the exchange energy J changes slowly
when the coupled dots are subject to a magnetic field with a wide range of
inhomogeneity, suggesting swap operations can be performed in such an
environment as long as quantum error correction is applied to account for the
Zeeman term. We also point out the quantum interference nature of this slow
variation in exchange.Comment: 12 pages, 4 figures embedded in tex
Site-dependent hydrogenation on graphdiyne
Graphene is one of the most important materials in science today due to its
unique and remarkable electronic, thermal and mechanical properties. However in
its pristine state, graphene is a gapless semiconductor, what limits its use in
transistor electronics. In part due to the revolution created by graphene in
materials science, there is a renewed interest in other possible graphene-like
two-dimensional structures. Examples of these structures are graphynes and
graphdiynes, which are two-dimensional structures, composed of carbon atoms in
sp2 and sp-hybridized states. Graphdiynes (benzenoid rings connecting two
acetylenic groups) were recently synthesized and some of them are intrinsically
nonzero gap systems. These systems can be easily hydrogenated and the relative
level of hydrogenation can be used to tune the band gap values. We have
investigated, using fully reactive molecular dynamics (ReaxFF), the structural
and dynamics aspects of the hydrogenation mechanisms of graphdiyne membranes.
Our results showed that the hydrogen bindings have different atom incorporation
rates and that the hydrogenation patterns change in time in a very complex way.
The formation of correlated domains reported to hydrogenated graphene is no
longer observed in graphdiyne cases.Comment: Submitted to Carbo
Applications and Sexual Version of a Simple Model for Biological Ageing
We use a simple model for biological ageing to study the mortality of the
population, obtaining a good agreement with the Gompertz law. We also simulate
the same model on a square lattice, considering different strategies of
parental care. The results are in agreement with those obtained earlier with
the more complicated Penna model for biological ageing. Finally, we present the
sexual version of this simple model.Comment: For Int.J.Mod.Phys.C Dec. 2001; 11 pages including 6 fig
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