19,930 research outputs found
Spin-guides and spin-splitters: Waveguide analogies in one-dimensional spin chains
Here we show a direct mapping between waveguide theory and spin chain
transport, opening an alternative approach to quantum information transport in
the solid-state. By applying temporally varying control profiles to a spin
chain, we design a virtual waveguide or 'spin-guide' to conduct individual spin
excitations along defined space-time trajectories of the chain. We explicitly
show that the concepts of confinement, adiabatic bend loss and beamsplitting
can be mapped from optical waveguide theory to spin-guides (and hence
'spin-splitters'). Importantly, the spatial scale of applied control pulses is
required to be large compared to the inter-spin spacing, and thereby allowing
the design of scalable control architectures.Comment: 5 figure
Dark Matter Scaling Relations
We establish the presence of a dark matter core radius, for the first time in
a very large number of spiral galaxies of all luminosities. Contrary to common
opinion we find that the sizes of these cores and the " DM core problem" are
bigger for more massive spirals. As a result the Burkert profile provides an
excellent mass model for dark halos around disk galaxies. Moreover, we find
that the spiral dark matter core densities and core radii
lie in the same scaling relation of dwarf galaxies with core radii upto ten times more
smaller.Comment: 4 pages, 4 figures, Accepted for Publication in Apj Let
A network-based ranking system for American college football
American college football faces a conflict created by the desire to stage
national championship games between the best teams of a season when there is no
conventional playoff system to decide which those teams are. Instead, ranking
of teams is based on their record of wins and losses during the season, but
each team plays only a small fraction of eligible opponents, making the system
underdetermined or contradictory or both. It is an interesting challenge to
create a ranking system that at once is mathematically well-founded, gives
results in general accord with received wisdom concerning the relative
strengths of the teams, and is based upon intuitive principles, allowing it to
be accepted readily by fans and experts alike. Here we introduce a
one-parameter ranking method that satisfies all of these requirements and is
based on a network representation of college football schedules.Comment: 15 pages, 3 figure
Modal Approach to Casimir Forces in Periodic Structures
We present a modal approach to calculate finite temperature Casimir
interactions between two periodically modulated surfaces. The scattering
formula is used and the reflection matrices of the patterned surfaces are
calculated decomposing the electromagnetic field into the natural modes of the
structures. The Casimir force gradient from a deeply etched silicon grating is
evaluated using the modal approach and compared to experiment for validation.
The Casimir force from a two dimensional periodic structure is computed and
deviations from the proximity force approximation examined.Comment: 13 pages, 7 figure
Cosmological Feedback from High-Redshift Dwarf Galaxies
We model how repeated supernova explosions in high-redshift dwarf starburst
galaxies drive superbubbles and winds out of the galaxies. We compute the
efficiencies of metal and mass ejection and energy transport from the galactic
potentials, including the effect of cosmological infall of external gas. The
starburst bubbles quickly blow out of small, high-redshift, galactic disks, but
must compete with the ram pressure of the infalling gas to escape into
intergalactic space. We show that the assumed efficiency of the star formation
rate dominates the bubble evolution and the metal, mass, and energy feedback
efficiencies. With star formation efficiency f*=0.01, the ram pressure of
infall can confine the bubbles around high-redshift dwarf galaxies with
circular velocities v_c>52 km/s. We can expect high metal and mass ejection
efficiencies, and moderate energy transport efficiencies in halos with
v_c~30-50 km/s and f*~0.01 as well as in halos with v_c~100 km/s and f*>>0.01.
Such haloes collapse successively from 1-2 sigma peaks in LambdaCDM Gaussian
density perturbations as time progresses. These dwarf galaxies can probably
enrich low and high-density regions of intergalactic space with metals to
10^-3-10^-2 Zsun as they collapse at z~8 and z<5 respectively. They also may be
able to provide adequate turbulent energy to prevent the collapse of other
nearby halos, as well as to significantly broaden Lyman-alpha absorption lines
to v_rms~20-40 km/s. We compute the timescales for the next starbursts if gas
freely falls back after a starburst, and find that, for star formation
efficiencies as low as f*<0.01, the next starburst should occur in less than
half the Hubble time at the collapse redshift. This suggests that episodic star
formation may be ubiquitous in dwarf galaxies.Comment: Accepted for ApJ v613, 60 pages, 15 figure
The negatively charged nitrogen-vacancy centre in diamond: the electronic solution
The negatively charged nitrogen-vacancy centre is a unique defect in diamond
that possesses properties highly suited to many applications, including quantum
information processing, quantum metrology, and biolabelling. Although the
unique properties of the centre have been extensively documented and utilised,
a detailed understanding of the physics of the centre has not yet been
achieved. Indeed there persists a number of points of contention regarding the
electronic structure of the centre, such as the ordering of the dark
intermediate singlet states. Without a sound model of the centre's electronic
structure, the understanding of the system's unique dynamical properties can
not effectively progress. In this work, the molecular model of the defect
centre is fully developed to provide a self consistent model of the complete
electronic structure of the centre. The application of the model to describe
the effects of electric, magnetic and strain interactions, as well as the
variation of the centre's fine structure with temperature, provides an
invaluable tool to those studying the centre and a means to design future
empirical and ab initio studies of this important defect.Comment: 24 pages, 6 figures, 10 table
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