65,024 research outputs found
RKKY interaction on the surface of three-dimensional Dirac semimetals
We study the RKKY interaction between two magnetic impurities located on the
surface of a three-dimensional Dirac semimetal with two Dirac nodes in the band
structure. By taking into account both bulk and surface contributions to the
exchange interaction between the localized spins, we demonstrate that the
surface contribution in general dominates the bulk one at distances larger than
the inverse node separation due to a weaker power-law decay. We find a strong
anisotropy of the surface term with respect to the spins being aligned along
the node separation axis or perpendicular to it. In the many impurity dilute
regime, this implies formation of quasi-one-dimensional magnetic stripes
orthogonal to the node axis. We also discuss the effects of a surface
spin-mixing term coupling electrons from spin-degenerate Fermi arcs.Comment: 7,5 pages, 3 figures (+4 pages of Appendixes
Generating Macroscopic Superpositions with Interacting Bose-Einstein Condensates: Multi-Mode Speed-Ups and Speed Limits
We theoretically investigate the effect of multi-mode dynamics on the
creation of macroscopic superposition states (spin-cat states) in Bose-Einstein
condensates via one-axis twisting. A two-component Bose-Einstein condensate
naturally realises an effective one-axis twisting interaction, under which an
initially separable state will evolve toward a spin-cat state. However, the
large evolution times necessary to realise these states is beyond the scope of
current experiments. This evolution time is proportional to the degree of
asymmetry in the relative scattering lengths of the system, which results in
the following trade-off; faster evolution times are associated with an increase
in multi-mode dynamics, and we find that generally multi-mode dynamics reduce
the degree of entanglement present in the final state. However, we find that
highly entangled cat-like states are still possible in the presence of
significant multi-mode dynamics, and that these dynamics impose a speed-limit
on the evolution such states
Simulating star formation in molecular cloud cores I. The influence of low levels of turbulence on fragmentation and multiplicity
We present the results of an ensemble of simulations of the collapse and
fragmentation of dense star-forming cores. We show that even with very low
levels of turbulence the outcome is usually a binary, or higher-order multiple,
system. We take as the initial conditions for these simulations a typical
low-mass core, based on the average properties of a large sample of observed
cores. All the simulated cores start with a mass of , a
flattened central density profile, a ratio of thermal to gravitational energy
and a ratio of turbulent to gravitational energy
. Even this low level of turbulence is sufficient to
produce multiple star formation in 80% of the cores; the mean number of stars
and brown dwarfs formed from a single core is 4.55, and the maximum is 10. At
the outset, the cores have no large-scale rotation. The only difference between
each individual simulation is the detailed structure of the turbulent velocity
field. The multiple systems formed in the simulations have properties
consistent with observed multiple systems. Dynamical evolution tends
preferentially to eject lower mass stars and brown dwarves whilst hardening the
remaining binaries so that the median semi-major axis of binaries formed is
au. Ejected objects are usually single low-mass stars and brown
dwarfs, yielding a strong correlation between mass and multiplicity. Our
simulations suggest a natural mechanism for forming binary stars that does not
require large-scale rotation, capture, or large amounts of turbulence.Comment: 20 pages, 12 figures submitted to A&
Critical Behavior of Coupled q-state Potts Models under Weak Disorder
We investigate the effect of weak disorder on different coupled -state
Potts models with using two loops renormalisation group. This study
presents new examples of first order transitions driven by randomness. We found
that weak disorder makes the models decouple. Therefore, it appears that no
relations emerge, at a perturbation level, between the disordered -state Potts model and the two disordered , -state Potts models
(), despite their central charges are similar according to recent
numerical investigations. Nevertheless, when two -state Potts models are
considered (), the system remains always driven in a strong coupling
regime, violating apparently the Imry-Wortis argument.Comment: 7 pages + 1 PS figure (Latex
Discs in misaligned binary systems
We perform SPH simulations to study precession and changes in alignment
between the circumprimary disc and the binary orbit in misaligned binary
systems. We find that the precession process can be described by the rigid-disc
approximation, where the disc is considered as a rigid body interacting with
the binary companion only gravitationally. Precession also causes change in
alignment between the rotational axis of the disc and the spin axis of the
primary star. This type of alignment is of great important for explaining the
origin of spin-orbit misaligned planetary systems. However, we find that the
rigid-disc approximation fails to describe changes in alignment between the
disc and the binary orbit. This is because the alignment process is a
consequence of interactions that involve the fluidity of the disc, such as the
tidal interaction and the encounter interaction. Furthermore, simulation
results show that there are not only alignment processes, which bring the
components towards alignment, but also anti-alignment processes, which tend to
misalign the components. The alignment process dominates in systems with
misalignment angle near 90 degrees, while the anti-alignment process dominates
in systems with the misalignment angle near 0 or 180 degrees. This means that
highly misaligned systems will become more aligned but slightly misaligned
systems will become more misaligned.Comment: 15 pages, 16 figures, 1 table, accepted for publication in MNRA
An effective Hamiltonian for phase fluctuations on a lattice: an extended XY model
We derive an effective Hamiltonian for phase fluctuations in an s-wave
superconductor starting from the attractive Hubbard model on a square lattice.
In contrast to the common assumption, we find that the effective Hamiltonian is
not the usual XY model but is of an extended XY type. This extended feature is
robust and leads to essential corrections in understanding phase fluctuations
on a lattice. The effective coupling in the Hamiltonian varies significantly
with temperature.Comment: 2 figure
Solvable Critical Dense Polymers on the Cylinder
A lattice model of critical dense polymers is solved exactly on a cylinder
with finite circumference. The model is the first member LM(1,2) of the
Yang-Baxter integrable series of logarithmic minimal models. The cylinder
topology allows for non-contractible loops with fugacity alpha that wind around
the cylinder or for an arbitrary number ell of defects that propagate along the
full length of the cylinder. Using an enlarged periodic Temperley-Lieb algebra,
we set up commuting transfer matrices acting on states whose links are
considered distinct with respect to connectivity around the front or back of
the cylinder. These transfer matrices satisfy a functional equation in the form
of an inversion identity. For even N, this involves a non-diagonalizable braid
operator J and an involution R=-(J^3-12J)/16=(-1)^{F} with eigenvalues
R=(-1)^{ell/2}. The number of defects ell separates the theory into sectors.
For the case of loop fugacity alpha=2, the inversion identity is solved exactly
for the eigenvalues in finite geometry. The eigenvalues are classified by the
physical combinatorics of the patterns of zeros in the complex
spectral-parameter plane yielding selection rules. The finite-size corrections
are obtained from Euler-Maclaurin formulas. In the scaling limit, we obtain the
conformal partition functions and confirm the central charge c=-2 and conformal
weights Delta_t=(t^2-1)/8. Here t=ell/2 and t=2r-s in the ell even sectors with
Kac labels r=1,2,3,...; s=1,2 while t is half-integer in the ell odd sectors.
Strikingly, the ell/2 odd sectors exhibit a W-extended symmetry but the ell/2
even sectors do not. Moreover, the naive trace summing over all ell even
sectors does not yield a modular invariant.Comment: 44 pages, v3: minor correction
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