981 research outputs found
Dynamical magneto-electric coupling in helical magnets
Collective mode dynamics of the helical magnets coupled to electric
polarization via spin-orbit interaction is studied theoretically. The soft
modes associated with the ferroelectricity are not the transverse optical
phonons, as expected from the Lyddane-Sachs-Teller relation, but are the spin
waves hybridized with the electric polarization. This leads to the Drude-like
dielectric function in the limit of zero magnetic
anisotropy. There are two more low-lying modes; phason of the spiral and
rotation of helical plane along the polarization axis. The roles of these soft
modes in the neutron scattering and antiferromagnetic resonance are revealed,
and a novel experiment to detect the dynamical magneto-electric coupling is
proposed.Comment: 5 pages, 1 figur
Stable Bosonic Topological Edge Modes in the Presence of Many-Body Interactions
Many magnetic materials are predicted to exhibit bosonic topological edge
modes in their excitation spectra, because of the nontrivial topology of their
magnon, triplon or other quasi-particle band structures. However, there is a
discrepancy between theory prediction and experimental observation, which
suggests some underlying mechanism that intrinsically suppresses the expected
experimental signatures, like the thermal Hall current. Many-body interactions
that are not accounted for in the non-interacting quasi-particle picture are
most often identified as the reason for the absence of the topological edge
modes. Here we report stable bosonic edge modes at the boundaries of a ladder
quantum paramagnet with gapped triplon excitations in the presence of the full
many-body interaction. For the first time, we use tensor network methods to
resolve topological edge modes in the time-dependent spin-spin correlations and
the dynamical structure factor, which is directly accessible experimentally. We
further show that these edge modes have anomalously long time coherence,
discuss the topological phase diagram of the model, demonstrate the
fractionalization of its low-lying excitations, and propose potential material
candidates
Characterizing and Improving Generalized Belief Propagation Algorithms on the 2D Edwards-Anderson Model
We study the performance of different message passing algorithms in the two
dimensional Edwards Anderson model. We show that the standard Belief
Propagation (BP) algorithm converges only at high temperature to a paramagnetic
solution. Then, we test a Generalized Belief Propagation (GBP) algorithm,
derived from a Cluster Variational Method (CVM) at the plaquette level. We
compare its performance with BP and with other algorithms derived under the
same approximation: Double Loop (DL) and a two-ways message passing algorithm
(HAK). The plaquette-CVM approximation improves BP in at least three ways: the
quality of the paramagnetic solution at high temperatures, a better estimate
(lower) for the critical temperature, and the fact that the GBP message passing
algorithm converges also to non paramagnetic solutions. The lack of convergence
of the standard GBP message passing algorithm at low temperatures seems to be
related to the implementation details and not to the appearance of long range
order. In fact, we prove that a gauge invariance of the constrained CVM free
energy can be exploited to derive a new message passing algorithm which
converges at even lower temperatures. In all its region of convergence this new
algorithm is faster than HAK and DL by some orders of magnitude.Comment: 19 pages, 13 figure
Entanglement Entropy in the Calogero-Sutherland Model
We investigate the entanglement entropy between two subsets of particles in
the ground state of the Calogero-Sutherland model. By using the duality
relations of the Jack symmetric polynomials, we obtain exact expressions for
both the reduced density matrix and the entanglement entropy in the limit of an
infinite number of particles traced out. From these results, we obtain an upper
bound value of the entanglement entropy. This upper bound has a clear
interpretation in terms of fractional exclusion statistics.Comment: 14 pages, 3figures, references adde
Geometry versus Entanglement in Resonating Valence Bond Liquids
We investigate the behavior of bipartite as well as genuine multipartite
entanglement of a resonating valence bond state on a ladder. We show that the
system possesses significant amounts of bipartite entanglement in the steps of
the ladder while no substantial bipartite entanglement is present in the rails.
Genuine multipartite entanglement present in the system is negligible. The
results are in stark contrast with the entanglement properties of the same
state on isotropic lattices in two and higher dimensions, indicating that the
geometry of the lattice can have important implications on the quality of
quantum information and other tasks that can be performed by using multiparty
states on that lattice.Comment: 6 pages, 8 figures, RevTeX
Voltage dependence of Landau-Lifshitz-Gilbert damping of a spin in a current driven tunnel junction
We present a theory of Landau-Lifshitz-Gilbert damping for a
localized spin in the junction coupled to the conduction electrons
in both leads under an applied volatege . We find the voltage dependence of
the damping term reflecting the energy dependence of the density of states. We
find the effect is linear in the voltage and cotrolled by particle-hole
asymmetry of the leads.Comment: 6 pages, 3 figure
Relations between some invariants of algebraic varieties in positive characteristic
We discuss relations between certain invariants of varieties in positive
characteristic, like the a-number and the height of the Artin-Mazur formal
group. We calculate the a-number for Fermat surfacesComment: 13 page
Electric-dipole active two-magnon excitation in {\textit{ab}} spiral spin phase of a ferroelectric magnet GdTbMnO
A broad continuum-like spin excitation (1--10 meV) with a peak structure
around 2.4 meV has been observed in the ferroelectric spiral spin phase of
GdTbMnO by using terahertz (THz) time-domain spectroscopy.
Based on a complete set of light-polarization measurements, we identify the
spin excitation active for the light vector only along the a-axis, which
grows in intensity with lowering temperature even from above the magnetic
ordering temperature but disappears upon the transition to the -type
antiferromagnetic phase. Such an electric-dipole active spin excitation as
observed at THz frequencies can be ascribed to the two-magnon excitation in
terms of the unique polarization selection rule in a variety of the
magnetically ordered phases.Comment: 11 pages including 3 figure
Cartan subalgebras and the UCT problem, II
We show that outer approximately represenbtable actions of a finite cyclic
group on UCT Kirchberg algebras satisfy a certain quasi-freeness type property
if the corresponding crossed products satisfy the UCT and absorb a suitable UHF
algebra tensorially. More concretely, we prove that for such an action there
exists an inverse semigroup of homogeneous partial isometries that generates
the ambient C*-algebra and whose idempotent semilattice generates a Cartan
subalgebra. We prove a similar result for actions of finite cyclic groups with
the Rokhlin property on UCT Kirchberg algebras absorbing a suitable UHF
algebra. These results rely on a new construction of Cartan subalgebras in
certain inductive limits of Cartan pairs. We also provide a characterisation of
the UCT problem in terms of finite order automorphisms, Cartan subalgebras and
inverse semigroups of partial isometries of the Cuntz algebra .
This generalizes earlier work of the authors.Comment: minor revisions; final version, accepted for publication in Math.
Ann.; 26 page
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