4,281 research outputs found
Yang-Mills Magneto-Fluid Unification
We generalize the hybrid magneto-fluid model of a charged fluid interacting
with an electromagnetic field to the dynamics of a relativistic hot fluid
interacting with a non-Abelian field. The fluid itself is endowed with a
non-Abelian charge and the consequences of this generalization are worked out.
Applications of this formalism to the Quark Gluon Plasma are suggested.Comment: 6 pages, RevTex
Spin liquid behaviour in Jeff=1/2 triangular lattice Ba3IrTi2O9
Ba3IrTi2O9 crystallizes in a hexagonal structure consisting of a layered
triangular arrangement of Ir4+ (Jeff=1/2). Magnetic susceptibility and heat
capacity data show no magnetic ordering down to 0.35K inspite of a strong
magnetic coupling as evidenced by a large Curie-Weiss temperature=-130K. The
magnetic heat capacity follows a power law at low temperature. Our measurements
suggest that Ba3IrTi2O9 is a 5d, Ir-based (Jeff=1/2), quantum spin liquid on a
2D triangular lattice.Comment: 10 pages including supplemental material, to be published in Phys.
Rev. B (Rapid Comm.
31P NMR study of Na2CuP2O7: a S=1/2 two-dimensional Heisenberg antiferromagnetic system
The magnetic properties of Na2CuP2O7 were investigated by means of 31P
nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity
measurements. We report the 31P NMR shift, the spin-lattice 1/T1, and spin-spin
1/T2 relaxation-rate data as a function of temperature T.
The temperature dependence of the NMR shift K(T) is well described by the
S=1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an
intraplanar exchange of J/k_B \simeq 18\pm2 K and a hyperfine coupling A =
(3533\pm185) Oe/mu_B. The 31P NMR spectrum was found to broaden abruptly below
T \sim 10 K signifying some kind of transition. However, no anomaly was noticed
in the bulk susceptibility data down to 1.8 K. The heat capacity appears to
have a weak maximum around 10 K. With decrease in temperatures, the
spin-lattice relaxation rate 1/T1 decreases monotonically and appears to agree
well with the high temperature series expansion expression for a S = 1/2 2D
square lattice.Comment: 12 pages, 8 figures, submitted to J. Phys.: Cond. Ma
A Genetic Algorithm Approach for Model Reference Adaptive Control of Ionic Polymer Metal Composites
Electroactive polymers undergo physical deformation to external voltage stimuli. These electrically activated polymers possess extraordinary features making them capable as lightweight sensors and actuators in manifold applications. The characteristics of applied voltage and environmental conditions, especially the moisture content surrounding the polymer, have a combined influence on the dynamical behavior of these polymers. In order to characterize these polymers under varying environmental conditions, this paper discusses the experimental procedure and modeling techniques used to derive a representative model. Ionic polymer metal composite polymers are used for this humidity relative electrodynamical study. Insight on the numerous applications of electroactive polymers as actuators and the built model enabled a controller is designed for a typical tracking problem. The control architecture includes a model reference adaptive scheme along with pole-placement control strategies to achieve the goal of tracking. A genetic algorithm approach is implemented to carryout an optimized control action. Tracking control of ionic polymer metal composites as actuator resembling that of a real-world scenario is simulated and reveals promising results
Bose-Einstein condensation of triplons in the S=1 tetramer antiferromagnet K2Ni2(MoO4)3: A compound close to quantum critical point
The structure of K2Ni2(MoO4)3 consists of S=1 tetramers formed by Ni^{2+}
ions. The magnetic susceptibility chi(T) and specific heat Cp(T) data on a
single crystal show a broad maximum due to the low-dimensionality of the system
with short-range spin correlations. A sharp peak is seen in chi(T) and Cp(T) at
about 1.13 K, well below the broad maximum. This is an indication of magnetic
long-range order i.e., the absence of spin-gap in the ground state.
Interestingly, the application of a small magnetic field (H>0.1 T) induces
magnetic behavior akin to Bose-Einstein condensation (BEC) of triplon
excitations observed in some spin-gap materials. Our results demonstrate that
the temperature-field (T-H) phase boundary follows a power-law
(T-T_{N})propotional to H^(1/alpha) with the exponent 1/alpha close to 2/3, as
predicted for BEC scenario. The observation of BEC of triplon excitations in
small H infers that K2Ni2(MoO4)3 is located in the proximity of a quantum
critical point, which separates the magnetically ordered and spin-gap regions
of the phase diagram.Comment: 5 pages, 5 figures, Accepted in Phys. Rev. B Rapid Communication
Spin-gap behaviour in the 2-leg spin-ladder BiCu2PO6
We present magnetic suscceptibility and heat capacity data on a new S=1/2
two-leg spin ladder compound BiCu2PO6. From our susceptibility analysis, we
find that the leg coupling J1/k_B is ~ 80 K and the ratio of the rung to leg
coupling J2/J1 ~ 0.9. We present the magnetic contribution to the heat capacity
of a two-leg ladder. The spin-gap Delta/k_B =3 4 K obtained from the heat
capacity agrees very well with that obtained from the magnetic susceptibility.
Significant inter-ladder coupling is suggested from the susceptibility
analysis. The hopping integrals determined using Nth order muffin tin orbital
(NMTO) based downfolding method lead to ratios of various exchange couplings in
agreement with our experimental data. Based on our band structure analysis, we
find the inter-ladder coupling in the bc-plane J2 to be about 0.75J1 placing
the compound presumably close to the quantum critical limit.Comment: 8 pages, 5 figure
What measurable zero point fluctuations can(not) tell us about dark energy
We show that laboratory experiments cannot measure the absolute value of dark
energy. All known experiments rely on electromagnetic interactions. They are
thus insensitive to particles and fields that interact only weakly with
ordinary matter. In addition, Josephson junction experiments only measure
differences in vacuum energy similar to Casimir force measurements. Gravity,
however, couples to the absolute value. Finally we note that Casimir force
measurements have tested zero point fluctuations up to energies of ~10 eV, well
above the dark energy scale of ~0.01 eV. Hence, the proposed cut-off in the
fluctuation spectrum is ruled out experimentally.Comment: 4 page
Two rare cases of management of proximal phalanx spina ventosa
Tubercular dactylitis or spina ventosa is the tubercular infection of phalanges, metacarpals and metatarsals termed after cystic swelling of short tubular bones by tubercular infection. Here we presented two rare cases of spina ventosa in a 10 years old female and a 16 years old male patients. In both cases there was swelling of digits with stiffness. The radiographs were suggestive of chronic osteolytic infection with cortical destruction and diffuse sclerosis. We managed them with adequate surgical debridement, splintage and anti-tubercular therapy with good functional recovery. Spina ventosa or tubercular dactylitis is a rare condition and is often missed on diagnosis due to vague non-specific presentation, unavailability of rapid diagnostic tests and can be confused with other mimicking diseases. Any swelling over small tubular bones of hand or feet should raise the suspicion of spina ventosa, especially in endemic countries for tuberculosis. In neglected chronic infections, surgical debridement, curettage and splintage followed by anti-tubercular therapy based on histo-pathological reports is the treatment of choice with high protein diet and vitamin D supplementation ensures good outcome.
Comment on "Localized behavior near the Zn impurity in YBa2Cu4O8 as measured by nuclear quadrupole resonance"
Williams and Kramer [Phys. Rev. B {\bf 64}, 104506 (2001)] have recently
argued against the existence of staggered magnetic moments residing on several
lattice sites around Zn impurities in YBCO superconductors. This claim, which
is in line with an earlier publication by Williams, Tallon and Dupree [Phys.
Rev. B {\bf 61}, 4319 (2000)], is however in contradiction with a large body of
experimental data from different NMR groups. On the contrary, the authors argue
in favor of a very localized spin and charge density on Cu sites first
neighbors to Zn. We show that the conclusions of Williams and Kramer arise from
erroneous interpretations of NMR and NQR data.Comment: 4 page
NLO-QCD Corrections to Dilepton Production in the Randall-Sundrum Model
The dilepton production process at hadron colliders in the Randall-Sundrum
(RS) model is studied at next-to-leading order in QCD. The NLO-QCD corrections
have been computed for the virtual graviton exchange process in the RS model,
in addition to the usual gamma, Z-mediated processes of standard Drell-Yan.
K-factors for the cross-sections at the LHC and Tevatron for differential in
the invariant mass, Q, and the rapidity, Y, of the lepton pair are presented.
We find the K-factors are large over substantial regions of the phase space.Comment: 24 pages, 12 figure
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