80 research outputs found
Perturbation theories for the S=1/2 spin ladder with four-spin ring exchange
The isotropic S=1/2 antiferromagnetic spin ladder with additional four-spin
ring exchange is studied perturbatively in the strong coupling regime with the
help of cluster expansion technique, and by means of bosonization in the weak
coupling limit. It is found that a sufficiently large strength of ring exchange
leads to a second-order phase transition, and the shape of the boundary in the
vicinity of the known exact transition point is obtained. The critical exponent
for the gap is found to be , in agreement both with exact results
available for the dimer line and with the bosonization analysis. The phase
emerging for high values of the ring exchange is argued to be gapped and
spontaneously dimerized. The results for the transition line from strong
coupling and from weak coupling match with each other naturally.Comment: 8 pages, 4 figures, some minor changes in text and reference
Spin-wave spectrum in La2CuO4 -- double occupancy and competing interaction effects
The recently observed spin-wave energy dispersion along the AF zone boundary
in La2CuO4 is discussed in terms of double occupancy and competing interaction
effects in the Hubbard model on a square lattice.Comment: 4 pages, 2 figure
Lepton Flavor Violating Processes and Muon g-2 in Minimal Supersymmetric SO(10) Model
In the recently proposed minimal supersymmetric SO(10) model, the neutrino
Dirac Yukawa coupling matrix, together with all the other fermion mass
matrices, is completely determined once free parameters in the model are
appropriately fixed so as to accommodate the recent neutrino oscillation data.
Using this unambiguous neutrino Dirac Yukawa couplings, we calculate the lepton
flavor violating (LFV) processes and the muon g-2 assuming the minimal
supergravity scenario. The resultant rates of the LFV processes are found to be
large enough to well exceed the proposed future experimental bound, while the
magnitude of the muon g-2 can be within the recent result by Brookhaven E821
experiment. Furthermore, we find that there exists a parameter region which can
simultaneously realize the neutralino cold dark matter abundance consistent
with the recent WMAP data.Comment: 18 pages, 10 figures. The version to be published in Phys. Rev.
Anomalous heavy-fermion and ordered states in the filled skutterudite PrFe4P12
Specific heat and magnetization measurements have been performed on
high-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to
study the high-field heavy-fermion state (HFS) and low-field ordered state
(ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields
applied along the direction, the Kondo temperature of ~ 9 K and the
existence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear
Schottky contribution, which works as a highly-sensitive on-site probe for the
Pr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03
\mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in
the ODS is nonmagnetic and most probably of quadrupolar origin, combined with
other experimental facts. Significantly suppressed heavy-fermion behavior in
the ODS suggests a possibility that the quadrupolar degrees of freedom is
essential for the heavy quasiparticle band formation in the HFS. Possible
crystalline-electric-field level schemes estimated from the anisotropy in the
magnetization are consistent with this conjecture.Comment: 7 pages and 7 figures. Accepted for publication in Phys. Rev.
Structural effect on the static spin and charge correlations in LaBaSrCuO
We report the results of elastic neutron scattering measurements performed on
1/8-hole doped LaBaSrCuO single crystals with
{\it x}=0.05, 0.06, 0.075 and 0.085. In the low-temperature less-orthorhombic
(LTLO, {\it Pccn} symmetry) phase, the charge-density-wave (CDW) and
spin-density-wave (SDW) wavevectors were found to tilt in a low-symmetric
direction with one-dimensional anisotropy in the CuO plane, while they
were aligned along the high-symmetry axis in the low-temperature tetragonal
(LTT, {\it P}4/{\it ncm} symmetry) phase. The coincident direction of two
wavevectors suggests a close relation between CDW and SDW orders. The SDW
wavevector systematically deviates from the Cu-O bond direction in the LTLO
phase upon Sr substitution and the tilt angle in the LTLO phase is smaller than
that in the low-temperature orthorhombic phase (LTO, {\it B}{\it mab} symmetry)
with comparable in-plane orthorhombic distortion. These results demonstrate a
correlation between the corrugated pattern of CuO plane and the
deviations.Comment: 6 pages, 7figure
Spin Excitations in La2CuO4: Consistent Description by Inclusion of Ring-Exchange
We consider the square lattice Heisenberg antiferromagnet with plaquette ring
exchange and a finite interlayer coupling leading to a consistent description
of the spin-wave excitation spectrum in La2CuO4. The values of the in-plane
exchange parameters, including ring-exchange J_{\Box}, are obtained
consistently by an accurate fit to the experimentally observed in-plane
spin-wave dispersion, while the out-of-plane exchange interaction is found from
the temperature dependence of the sublattice magnetization at low temperatures.
The fitted exchange interactions J=151.9 meV and J_{\Box}=0.24 J give values
for the spin stiffness and the Neel temperature in excellent agreement with the
experimental data.Comment: 4 pages, 1 figure, RevTe
A biomechanical analysis of prognathous and orthognathous insect head capsules: evidence for a manyâtoâone mapping of form to function
Insect head shapes are remarkably variable, but the influences of these changes on biomechanical performance are unclear. Among âbasalâ winged insects, such as dragonflies, mayflies, earwigs and stoneflies, some of the most prominent anatomical changes are the general mouthpart orientation, eye size and the connection of the endoskeleton to the head. Here, we assess these variations as well as differing ridge and sclerite configurations using modern engineering methods including multibody dynamics modelling and finite element analysis in order to quantify and compare the influence of anatomical changes on strain in particular head regions and the whole head. We show that a range of peculiar structures such as the genal/subgenal, epistomal and circumocular areas are consistently highly loaded in all species, despite drastically differing morphologies in species with forwardâprojecting (prognathous) and downwardâprojecting (orthognathous) mouthparts. Sensitivity analyses show that the presence of eyes has a negligible influence on head capsule strain if a circumocular ridge is present. In contrast, the connection of the dorsal endoskeletal arms to the head capsule especially affects overall head loading in species with downwardâprojecting mouthparts. Analysis of the relative strains between species for each head region reveals that concerted changes in head substructures such as the subgenal area, the endoskeleton and the epistomal area lead to a consistent relative loading for the whole head capsule and vulnerable structures such as the eyes. It appears that bitingâchewing loads are managed by a system of strengthening ridges on the head capsule irrespective of the general mouthpart and head orientation. Concerted changes in ridge and endoskeleton configuration might allow for more radical anatomical changes such as the general mouthpart orientation, which could be an explanation for the variability of this trait among insects. In an evolutionary context, manyâtoâone mapping of strain patterns onto a relatively similar overall head loading indeed could have fostered the dynamic diversification processes seen in insects
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
Lepton Flavour Violating Leptonic/Semileptonic Decays of Charged Leptons in the Minimal Supersymmetric Standard Model
We consider the leptonic and semileptonic (SL) lepton flavour violating (LFV)
decays of the charged leptons in the minimal supersymmetric standard model
(MSSM). The formalism for evaluation of branching fractions for the SL LFV
charged-lepton decays with one or two pseudoscalar mesons, or one vector meson
in the final state, is given. Previous amplitudes for the SL LFV charged-lepton
decays in MSSM are improved, for instance the -penguin amplitude is
corrected to assure the gauge invariance. The decays are studied not only in
the model-independent formulation of the theory in the frame of MSSM, but also
within the frame of the minimal supersymmetric SO(10) model within which the
parameters of the MSSM are determined. The latter model gives predictions for
the neutrino-Dirac Yukawa coupling matrix, once free parameters in the model
are appropriately fixed to accommodate the recent neutrino oscillation data.
Using this unambiguous neutrino-Dirac Yukawa couplings, we calculate the LFV
leptonic and SL decay processes assuming the minimal supergravity scenario. A
very detailed numerical analysis is done to constrain the MSSM parameters.
Numerical results for SL LFV processes are given, for instance for tau -> e
(mu) pi0, tau -> e (mu) eta, tau -> e (mu) eta', tau -> e (mu) rho0, tau -> e
(mu) phi, tau -> e (mu) omega, etc.Comment: 36 pages, 3 tables, 5 .eps figure
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