442 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
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
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
Spin current and magneto-electric effect in non-collinear magnets
A new microscopic mechanism of the magneto-electric (ME) effect based on the
spin supercurrent is theoretically presented for non-collinear magnets. The
close analogy between the superconductors (charge current) and magnets (spin
current) is drawn to derive the distribution of the spin supercurrent and the
resultant electric polarization. Application to the spiral spin structure is
discussed.Comment: 5 pages, 2 figure
Spin 3/2 dimer model
We present a parent Hamiltonian for weakly dimerized valence bond solid
states for arbitrary half-integral S. While the model reduces for S=1/2 to the
Majumdar-Ghosh Hamiltonian we discuss this model and its properties for S=3/2.
Its degenerate ground state is the most popular toy model state for discussing
dimerization in spin 3/2 chains. In particular, it describes the impurity
induced dimer phase in Cr8Ni as proposed recently. We point out that the
explicit construction of the Hamiltonian and its main features apply to
arbitrary half-integral spin S.Comment: 5+ pages, 6 figures; to appear in Europhysics Letter
Universal time-evolution of a Rydberg lattice gas with perfect blockade
We investigate the dynamics of a strongly interacting spin system that is
motivated by current experimental realizations of strongly interacting Rydberg
gases in lattices. In particular we are interested in the temporal evolution of
quantities such as the density of Rydberg atoms and density-density
correlations when the system is initialized in a fully polarized state without
Rydberg excitations. We show that in the thermodynamic limit the expectation
values of these observables converge at least logarithmically to universal
functions and outline a method to obtain these functions. We prove that a
finite one-dimensional system follows this universal behavior up to a given
time. The length of this universal time period depends on the actual system
size. This shows that already the study of small systems allows to make precise
predictions about the thermodynamic limit provided that the observation time is
sufficiently short. We discuss this for various observables and for systems
with different dimensions, interaction ranges and boundary conditions.Comment: 16 pages, 3 figure
Phase separation in a lattice model of a superconductor with pair hopping
We have studied the extended Hubbard model with pair hopping in the atomic
limit for arbitrary electron density and chemical potential. The Hamiltonian
considered consists of (i) the effective on-site interaction U and (ii) the
intersite charge exchange interactions I, determining the hopping of electron
pairs between nearest-neighbour sites. The model can be treated as a simple
effective model of a superconductor with very short coherence length in which
electrons are localized and only electron pairs have possibility of
transferring. The phase diagrams and thermodynamic properties of this model
have been determined within the variational approach, which treats the on-site
interaction term exactly and the intersite interactions within the mean-field
approximation. We have also obtained rigorous results for a linear chain (d=1)
in the ground state. Moreover, at T=0 some results derived within the random
phase approximation (and the spin-wave approximation) for d=2 and d=3 lattices
and within the low density expansions for d=3 lattices are presented. Our
investigation of the general case (as a function of the electron concentration
and as a function of the chemical potential) shows that, depending on the
values of interaction parameters, the system can exhibit not only the
homogeneous phases: superconducting (SS) and nonordered (NO), but also the
phase separated states (PS: SS-NO). The system considered exhibits interesting
multicritical behaviour including tricritical points.Comment: 15 pages, 9 figures; pdf-ReVTeX, final version, corrected typos;
submitted to Journal of Physics: Condensed Matte
Anomalous Coexistence of Ferroelectric Phases ( and ) in Orthorhombic EuYMnO () Crystals
We have investigated the magnetic and dielectric properties of orthorhombic
EuYMnO () single crystals without the presence
of the 4 magnetic moments of the rare-earth ions. In , the
magnetic-structure driven ferroelectricity is observed. The ferroelectric
transition temperature is steeply reducing with increasing . In , two ferroelectric phases ( and ) are
coexistent at low temperatures. In these phases, ferroelectricity has different
origin, which is evidenced by the distinctive poling-electric-field dependence
of electric polarization. Namely, the electric polarization along the c axis
() is easily saturated by a poling electric field, therefore is
caused by the spiral antiferromagnetic order. On the other hand, the
electric polarization along the a axis () is probably attributed to the
collinear -type antiferromagnetic order, because is unsaturated even
in a poling field of V/m.Comment: 10 pages, 4figures, to be published in Journal of the Physical
Society of Japa
Entanglement between particle partitions in itinerant many-particle states
We review `particle partitioning entanglement' for itinerant many-particle
systems. This is defined as the entanglement between two subsets of particles
making up the system. We identify generic features and mechanisms of particle
entanglement that are valid over whole classes of itinerant quantum systems. We
formulate the general structure of particle entanglement in many-fermion ground
states, analogous to the `area law' for the more usually studied entanglement
between spatial regions. Basic properties of particle entanglement are first
elucidated by considering relatively simple itinerant models. We then review
particle-partitioning entanglement in quantum states with more intricate
physics, such as anyonic models and quantum Hall states.Comment: review, about 20 pages. Version 2 has minor revisions
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