634 research outputs found
Topological Invariants for Polyacetylene, Kagome and Pyrochlore lattices
Adiabatic invariants by quantized Berry phases are defined for gapped
electronic systems in -dimensions (). This series includes
Polyacetylene, Kagome and Pyrochlore lattice respectively for and 3.
The invariants are quantum -multimer order parameters to characterize the
topological phase transitions by the multimerization. This fractional
quantization is protected by the global equivalence. As for the chiral
symmetric case, a topological form of the -invariant is explicitly given
as well.Comment: 4 pgages, 4 figure
Universal relationship between crystallinity and irreversibility field of MgB2
The relationship between irreversibility field, Hirr, and crystallinity of
MgB2 bulks including carbon substituted samples was studied. The Hirr was found
to increase with an increase of FWHM of MgB2 (110) peak, which corresponds to
distortion of honeycomb boron sheet, and their universal correlation was
discovered even including carbon substituted samples. Excellent Jc
characteristics under high magnetic fields were observed in samples with large
FWHM of (110) due to the enhanced intraband scattering and strengthened grain
boundary flux pinning. The relationship between crystallinity and Hirr can
explain the large variation of Hirr for MgB2 bulks, tapes, single crystals and
thin films.Comment: 3 pages, 4 figures, to be published in Appl. Phys. Lett. (in press
Strong Shift Equivalence of -correspondences
We define a notion of strong shift equivalence for -correspondences and
show that strong shift equivalent -correspondences have strongly Morita
equivalent Cuntz-Pimsner algebras. Our analysis extends the fact that strong
shift equivalent square matrices with non-negative integer entries give stably
isomorphic Cuntz-Krieger algebras.Comment: 26 pages. Final version to appear in Israel Journal of Mathematic
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
On the relation between entanglement and subsystem Hamiltonians
We show that a proportionality between the entanglement Hamiltonian and the
Hamiltonian of a subsystem exists near the limit of maximal entanglement under
certain conditions. Away from that limit, solvable models show that the
coupling range differs in both quantities and allow to investigate the effect.Comment: 7 pages, 2 figures version2: minor changes, typos correcte
Topological Classification of Gapped Spin Chains :Quantized Berry Phase as a Local Order Parameter
We characterize several phases of gapped spin systems by local order
parameters defined by quantized Berry phases. This characterization is
topologically stable against any small perturbation as long as the energy gap
remains finite. The models we pick up are dimerized Heisenberg chains
and S=2 Heisenberg chains with uniaxial single-ion-type anisotropy.
Analytically we also evaluate the topological local order parameters for the
generalized Affleck-Kennedy-Lieb-Tasaki (AKLT) model. The relation between the
present Berry phases and the fractionalization in the integer spin chains are
discussed as well.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Theory of magnetoelectric resonance in two-dimensional antiferromagnet via spin-dependent metal-ligand hybridization mechanism
We investigate magnetic excitations in an Heisenberg model
representing two-dimensional antiferromagnet . In
terahertz absorption experiment of the compound, Goldstone mode as well as
novel magnetic excitations, conventional magnetic resonance at 2 meV and both
electric- and magnetic-active excitation at 4 meV, have been observed. By
introducing a hard uniaxial anisotropy term , three modes can
be explained naturally. We also indicate that, via the spin-dependent
metal-ligand hybridization mechanism, the 4 meV excitation is an
electric-active mode through the coupling between spin and electric-dipole.
Moreover, at 4 meV excitation, an interference between magnetic and electric
responses emerges as a cross correlated effect. Such cross correlation effects
explain the non-reciprocal linear directional dichroism observed in .Comment: 5 pages, 3 figure
Extreme sensitivity of a frustrated quantum magnet: Cs_2CuCl_4
We report a thorough theoretical study of the low temperature phase diagram
of Cs_2CuCl_4, a spatially anisotropic spin S=1/2 triangular lattice
antiferromagnet, in a magnetic field. Our results, obtained in a
quasi-one-dimensional limit in which the system is regarded as a set of weakly
coupled Heisenberg chains, are in excellent agreement with experiment. The
analysis reveals some surprising physics. First, we find that, when the
magnetic field is oriented within the triangular layer, spins are actually most
strongly correlated within planes perpendicular to the triangular layers. This
is despite the fact that the inter-layer exchange coupling in Cs_2CuCl_4 is
about an order of magnitude smaller than the weakest (diagonal) exchange in the
triangular planes themselves. Second, the phase diagram in such orientations is
exquisitely sensitive to tiny interactions, heretofore neglected, of order a
few percent or less of the largest exchange couplings. These interactions,
which we describe in detail, induce entirely new phases, and a novel
commensurate-incommensurate transition, the signatures of which are identified
in NMR experiments. We discuss the differences between the behavior of
Cs_2CuCl_4 and an ideal two-dimensional triangular model, and in particular the
occurrence of magnetization plateaux in the latter. These and other related
results are presented here along with a thorough exposition of the theoretical
methods, and a discussion of broader experimental consequences to Cs_2CuCl_4
and other materials.Comment: 43 pages, 20 figures; typos correcte
Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites
Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and
Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic
model, and then analyze the model using the Monte-Carlo method. We reproduce
the diagrams, which contain two different multiferroic states, i.e., the
ab-plane spin cycloid with electric polarization P//a and the bc-plane spin
cycloid with P//c. We reveal that their competition originates from a conflict
between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction,
which is controlled by the second-neighbor spin exchanges enhanced by the
GdFeO3-type distortion. This leads to a P flop from a to c with increasing x in
agreement with the experiments.Comment: 5 pages, 5 figures. Recalculated results after correcting errors in
the assignment of DM vectors. The conclusion is not affecte
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
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