4,818 research outputs found
Flow enhanced pairing and other novel effects in Fermi gases in synthetic gauge fields
Recent experiments on fermions in synthetic gauge fields result in systems
with a spin-orbit coupling along one spatial axis, a detuning field, and a
Zeeman field. We show theoretically that the presence of all three results in
interesting and unusual phenomena in such systems in the presence of a contact
singlet attraction between the fermions (described by a scattering length). For
two particles, bound states appear over certain range of the centre of mass
momenta when a critical positive scattering length is attained, with the
deepest bound state appearing at a nonzero centre of mass momentum. For the
centre of mass momenta without a bound state, the gauge field induces a
resonance like feature in the scattering continuum resulting in a large
scattering phase shift. For many particles, we demonstrate that the system, in
a parameter range, shows flow enhanced pairing, i.e., a more robust superfluid
at finite centre of mass momentum. Yet another regime of parameters offers the
opportunity to study strongly interacting normal states of spin-orbit coupled
fermionic systems utilizing the resonance like feature induced by the synthetic
gauge field.Comment: 5 pages, 5 figure
Size-dependent Rigidities of Nanosized Torsional Elements
A theory for the prediction of the size dependence of torsional rigidities of
nanosized structural elements is developed. It is shown that, to a very good
approximation, the torsional rigidity (D) of a nanosized bar differs from the
prediction of standard continuum mechanics as
where A is a non-dimensional constant, a is the size scale of the cross-section
of the bar and is a material length equal to the ratio of the surface
elastic constant to the bulk elastic constant. The theory developed is compared
with direct atomistic calculations (``numerical experiment'') of the torsional
rigidity bars made of several FCC metals modeled using the embedded atom
method. Very good agreement is obtained between theory and simulation. The
framework presented here can aid the development of design methodologies for
nanoscale structural elements without the need for full scale atomistic
simulations.Comment: 18 Pages, 5 Figures, Submitted to Int. J. Sol. Struc
Continuum Theory of Edge States of Topological Insulators: Variational Principle and Boundary Conditions
We develop a continuum theory to model low energy excitations of a generic
four-band time reversal invariant electronic system with boundaries. We propose
a variational energy functional for the wavefunctions which allows us derive
natural boundary conditions valid for such systems. Our formulation is
particularly suited to develop a continuum theory of the protected edge/surface
excitations of topological insulators both in two and three dimensions. By a
detailed comparison of our analytical formulation with tight binding
calculations of ribbons of topological insulators modeled by the
Bernevig-Hughes-Zhang (BHZ) hamiltonian, we show that the continuum theory with
the natural boundary condition provides an appropriate description of the low
energy physics. As a spin-off, we find that in a certain parameter regime, the
gap that arises in topological insulator ribbons of finite width due to the
hybridization of edges states from opposite edges, depends non-monotonically on
the ribbon width and can nearly vanish at certain "magic widths".Comment: 8 pages, 5 figure
Strange Half Metals and Mott Insulators in SYK Models
We study a dual flavor fermion model where each of the flavors form a
Sachdev-Ye-Kitaev (SYK) system with arbitrary and possibly distinct -body
interactions. The crucial new element is an arbitrary all-to-all -body
interaction between the two flavors. At high temperatures the model shows a
strange metal phase where both flavors are gapless, similar to the usual single
flavor SYK model. Upon reducing temperature, the coupled system undergoes phase
transitions to previously unseen phases - first, a strange half metal (SHM)
phase where one flavor remains a strange metal while the other is gapped, and,
second, a Mott insulating phase where both flavors are gapped. At a fixed low
temperature we obtain transitions between these phases by tuning the relative
fraction of sites for each flavor. We discuss the physics of these phases and
the nature of transitions between them. This work provides an example of an
instability of the strange metal with potential to provide new routes to study
strongly correlated systems through the rich physics contained in SYK like
models.Comment: 7 pages, 3 figure
A Tight Lower Bound on the Sub-Packetization Level of Optimal-Access MSR and MDS Codes
The first focus of the present paper, is on lower bounds on the
sub-packetization level of an MSR code that is capable of carrying out
repair in help-by-transfer fashion (also called optimal-access property). We
prove here a lower bound on which is shown to be tight for the case
by comparing with recent code constructions in the literature.
We also extend our results to an MDS code over the vector alphabet.
Our objective even here, is on lower bounds on the sub-packetization level
of an MDS code that can carry out repair of any node in a subset of
nodes, where each node is repaired (linear repair) by
help-by-transfer with minimum repair bandwidth. We prove a lower bound on
for the case of . This bound holds for any and
is shown to be tight, again by comparing with recent code constructions in the
literature. Also provided, are bounds for the case .
We study the form of a vector MDS code having the property that we can repair
failed nodes belonging to a fixed set of nodes with minimum repair
bandwidth and in optimal-access fashion, and which achieve our lower bound on
sub-packetization level . It turns out interestingly, that such a code
must necessarily have a coupled-layer structure, similar to that of the Ye-Barg
code.Comment: Revised for ISIT 2018 submissio
What Do CFTs Tell Us About Anti-de Sitter Spacetimes?
The AdS/CFT conjecture relates quantum gravity on Anti-de Sitter (AdS) space
to a conformal field theory (CFT) defined on the spacetime boundary. We
interpret the CFT in terms of natural analogues of the bulk S-matrix. Our first
approach finds the bulk S-matrix as a limit of scattering from an AdS bubble
immersed in a space admitting asymptotic states. Next, we show how the
periodicity of geodesics obstructs a standard LSZ prescription for scattering
within global AdS. To avoid this subtlety we partition global AdS into patches
within which CFT correlators reconstruct transition amplitudes of AdS states.
Finally, we use the AdS/CFT duality to propose a large N collective field
theory that describes local, perturbative supergravity. Failure of locality in
quantum gravity should be related to the difference between the collective 1/N
expansion and genuine finite N dynamics.Comment: 33 pages, 7 figures, uses harvmac, reference adde
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