2,009 research outputs found
Long-range order in the A-like phase of superfluid 3He in aerogel
A mutual action of the random anisotropy brought in the superfluid 3He by
aerogel and of the global anisotropy caused by its deformation is considered.
Strong global anisotropy tends to suppress fluctuations of orientation of the
order parameter and stabilizes ABM order parameter. In a limit of vanishing
anisotropy these fluctuations are getting critical. It is argued that still in
a region of small fluctuations the average order parameter can acquire "robust"
component. This component maintains a long-range order even in a limit of
vanishing global anisotropy.Comment: A contribution to QFS 2007 in Kazan, revised for publication in the
Proceeding
Robust superfluid phases of 3He in aerogel
Within a phenomenological approach possible forms of the order parameter of
the superfluid phases of 3He in a vicinity of the transition temperature are
discussed. Effect of aerogel is described by a random tensor field interacting
with the orbital part of the order parameter. With respect to their interaction
with the random tensor field a group of "robust" order parameters which can
maintain long-range order in a presence of the random field is specified.
Robust order parameters, corresponding to Equal Spin Pairing (ESP) states are
found and proposed as candidates for the observed A-like superfluid phase of
liquid 3He in aerogel.Comment: 5 pages, prepared for QFS 200
Space-time in light of Karolyhazy uncertainty relation
General relativity and quantum mechanics provide a natural explanation for
the existence of dark energy with its observed value and predict its dynamics.
Dark energy proves to be necessary for the existence of space-time itself and
determines the rate of its stability.Comment: 5 pages, Two misprints are correcte
Polynomial kernelization for removing induced claws and diamonds
A graph is called (claw,diamond)-free if it contains neither a claw (a
) nor a diamond (a with an edge removed) as an induced subgraph.
Equivalently, (claw,diamond)-free graphs can be characterized as line graphs of
triangle-free graphs, or as linear dominoes, i.e., graphs in which every vertex
is in at most two maximal cliques and every edge is in exactly one maximal
clique.
In this paper we consider the parameterized complexity of the
(claw,diamond)-free Edge Deletion problem, where given a graph and a
parameter , the question is whether one can remove at most edges from
to obtain a (claw,diamond)-free graph. Our main result is that this problem
admits a polynomial kernel. We complement this finding by proving that, even on
instances with maximum degree , the problem is NP-complete and cannot be
solved in time unless the Exponential Time
Hypothesis fai
Cluster algebras in algebraic Lie theory
We survey some recent constructions of cluster algebra structures on
coordinate rings of unipotent subgroups and unipotent cells of Kac-Moody
groups. We also review a quantized version of these results.Comment: Invited survey; to appear in Transformation Group
Network and Seiberg Duality
We define and study a new class of 4d N=1 superconformal quiver gauge
theories associated with a planar bipartite network. While UV description is
not unique due to Seiberg duality, we can classify the IR fixed points of the
theory by a permutation, or equivalently a cell of the totally non-negative
Grassmannian. The story is similar to a bipartite network on the torus
classified by a Newton polygon. We then generalize the network to a general
bordered Riemann surface and define IR SCFT from the geometric data of a
Riemann surface. We also comment on IR R-charges and superconformal indices of
our theories.Comment: 28 pages, 28 figures; v2: minor correction
Minimizing the Cost of Team Exploration
A group of mobile agents is given a task to explore an edge-weighted graph
, i.e., every vertex of has to be visited by at least one agent. There
is no centralized unit to coordinate their actions, but they can freely
communicate with each other. The goal is to construct a deterministic strategy
which allows agents to complete their task optimally. In this paper we are
interested in a cost-optimal strategy, where the cost is understood as the
total distance traversed by agents coupled with the cost of invoking them. Two
graph classes are analyzed, rings and trees, in the off-line and on-line
setting, i.e., when a structure of a graph is known and not known to agents in
advance. We present algorithms that compute the optimal solutions for a given
ring and tree of order , in time units. For rings in the on-line
setting, we give the -competitive algorithm and prove the lower bound of
for the competitive ratio for any on-line strategy. For every strategy
for trees in the on-line setting, we prove the competitive ratio to be no less
than , which can be achieved by the algorithm.Comment: 25 pages, 4 figures, 5 pseudo-code
Superconductivity in a Mesoscopic Double Square Loop: Effect of Imperfections
We have generalized the network approach to include the effects of
short-range imperfections in order to analyze recent experiments on mesoscopic
superconducting double loops. The presence of weakly scattering imperfections
causes gaps in the phase boundary or for certain intervals of
, which depend on the magnetic flux penetrating each loop. This is
accompanied by a critical temperature , showing a smooth transition
between symmetric and antisymmetric states. When the scattering strength of
imperfections increases beyond a certain limit, gaps in the phase boundary
or appear for values of magnetic flux lying in intervals
around half-integer . The critical temperature corresponding to
these values of magnetic flux is determined mainly by imperfections in the
central branch. The calculated phase boundary is in good agreement with
experiment.Comment: 9 pages, 6 figure
Unit Interval Editing is Fixed-Parameter Tractable
Given a graph~ and integers , , and~, the unit interval
editing problem asks whether can be transformed into a unit interval graph
by at most vertex deletions, edge deletions, and edge
additions. We give an algorithm solving this problem in time , where , and denote respectively
the numbers of vertices and edges of . Therefore, it is fixed-parameter
tractable parameterized by the total number of allowed operations.
Our algorithm implies the fixed-parameter tractability of the unit interval
edge deletion problem, for which we also present a more efficient algorithm
running in time . Another result is an -time algorithm for the unit interval vertex deletion problem,
significantly improving the algorithm of van 't Hof and Villanger, which runs
in time .Comment: An extended abstract of this paper has appeared in the proceedings of
ICALP 2015. Update: The proof of Lemma 4.2 has been completely rewritten; an
appendix is provided for a brief overview of related graph classe
Wigner crystallization in a polarizable medium
We present a variational study of the 2D and 3D Wigner crystal phase of large
polarons. The method generalizes that introduced by S. Fratini,P.\
Qu{\'{e}}merais [Mod. Phys. Lett. B {\bf 12} 1003 (1998)]. We take into account
the Wigner crystal normal modes rather than a single mean frequency in the
minimization procedure of the variational free energy. We calculate the
renormalized modes of the crystal as well as the charge polarization
correlation function and polaron radius. The solid phase boundaries are
determined via a Lindemann criterion, suitably generalized to take into account
the classical-to-quantum cross-over.
In the weak electron-phonon coupling limit, the Wigner crystal parameters are
renormalized by the electron-phonon interaction leading to a stabilization of
the solid phase for low polarizability of the medium. Conversely, at
intermediate and strong coupling, the behavior of the system depends strongly
on the polarizability of the medium.
For weakly polarizable media, a density crossover occurs inside the solid
phase when the renormalized plasma frequency approaches the phonon frequency.
At low density, we have a renormalized polaron Wigner crystal, while at higher
densities the electron-phonon interaction is weakened irrespective of the {\it
bare} electron-phonon coupling.
For strongly polarizable media, the system behaves as a Lorentz lattice of
dipoles. The abrupt softening of the internal polaronic frequency predicted by
Fratini and Quemerais is observed near the actual melting point only at very
strong coupling, leading to a possible liquid polaronic phase for a wider range
of parameters.Comment: 24 pages, 13 figures v1.
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