435 research outputs found
Properties of electrons near a Van Hove singularity
The Fermi surface of most hole-doped cuprates is close to a Van Hove
singularity at the M point. A two-dimensional electronic system, whose Fermi
surface is close to a Van Hove singularity shows a variety of weak coupling
instabilities. It is a convenient model to study the interplay between
antiferromagnetism and anisotropic superconductivity. The renormalization group
approach is reviewed with emphasis on the underlying physical processes.
General properties of the phase diagram and possible deformations of the Fermi
surface due to the Van Hove proximity are described.Comment: Proceedings of SNS-01 to appear in the Journal of Physics and
Chemistry of Solids, SNS-0
Fermi surface renormalization in Hubbard ladders
We derive the one-loop renormalization equations for the shift in the
Fermi-wavevectors for one-dimensional interacting models with four Fermi-points
(two left and two right movers) and two Fermi velocities v_1 and v_2. We find
the shift to be proportional to (v_1-v_2)U^2, where U is the Hubbard-U. Our
results apply to the Hubbard ladder and to the t_1-t_2 Hubbard model. The
Fermi-sea with fewer particles tends to empty. The stability of a saddle point
due to shifts of the Fermi-energy and the shift of the Fermi-wavevector at the
Mott-Hubbard transition are discussed.Comment: 5 pages, 4 Postscript figure
Isotopic liftings of Clifford algebras and applications in elementary particle mass matrices
Isotopic liftings of algebraic structures are investigated in the context of
Clifford algebras, where it is defined a new product involving an arbitrary,
but fixed, element of the Clifford algebra. This element acts as the unit with
respect to the introduced product, and is called isounit. We construct
isotopies in both associative and non-associative arbitrary algebras, and
examples of these constructions are exhibited using Clifford algebras, which
although associative, can generate the octonionic, non-associative, algebra.
The whole formalism is developed in a Clifford algebraic arena, giving also the
necessary pre-requisites to introduce isotopies of the exterior algebra. The
flavor hadronic symmetry of the six u,d,s,c,b,t quarks is shown to be exact,
when the generators of the isotopic Lie algebra su(6) are constructed, and the
unit of the isotopic Clifford algebra is shown to be a function of the six
quark masses. The limits constraining the parameters, that are entries of the
representation of the isounit in the isotopic group SU(6), are based on the
most recent limits imposed on quark masses.Comment: 19 page
Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6
We present the results of measurements of the thermal conductivity
of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and
300 K and with the heat flow directed either along or perpendicular to the
chain direction. The analysis of the anisotropy of the heat transport allowed
for the identification of a small but non-negligible magnon contribution
along the chains, superimposed on the dominant phonon contribution
. At temperatures above about 100 K the energy diffusion constant
D_E(T), calculated from the data, exhibits similar features as
the spin diffusion constant D_S(T), previously measured by NMR. In this regime,
the behaviour of both transport parameters is consistent with a diffusion
process that is caused by interactions inherent to one-dimensional S=1 spin
systems.Comment: 6 pages, 4 figure
Characterization of neutrino signals with radiopulses in dense media through the LPM effect
We discuss the possibilities of detecting radio pulses from high energy
showers in ice, such as those produced by PeV and EeV neutrino interactions. It
is shown that the rich radiation pattern structure in the 100 MHz to few GHz
allows the separation of electromagnetic showers induced by photons or
electrons above 100 PeV from those induced by hadrons. This opens up the
possibility of measuring the energy fraction transmitted to the electron in a
charged current electron neutrino interaction with adequate sampling of the
angular distribution of the signal. The radio technique has the potential to
complement conventional high energy neutrino detectors with flavor information.Comment: 5 pages, 4 ps figures. Submitted to Phys. Rev. Let
Quantum Monte Carlo simulation for the conductance of one-dimensional quantum spin systems
Recently, the stochastic series expansion (SSE) has been proposed as a
powerful MC-method, which allows simulations at low for quantum-spin
systems. We show that the SSE allows to compute the magnetic conductance for
various one-dimensional spin systems without further approximations. We
consider various modifications of the anisotropic Heisenberg chain. We recover
the Kane-Fisher scaling for one impurity in a Luttinger-liquid and study the
influence of non-interacting leads for the conductance of an interacting
system.Comment: 8 pages, 9 figure
Superconducting and pseudogap phases from scaling near a Van Hove singularity
We study the quantum corrections to the Fermi energy of a two-dimensional
electron system, showing that it is attracted towards the Van Hove singularity
for a certain range of doping levels. The scaling of the Fermi level allows to
cure the infrared singularities left in the BCS channel after renormalization
of the leading logarithm near the divergent density of states. A phase of
d-wave superconductivity arises beyond the point of optimal doping
corresponding to the peak of the superconducting instability. For lower doping
levels, the condensation of particle-hole pairs due to the nesting of the
saddle points takes over, leading to the opening of a gap for quasiparticles in
the neighborhood of the singular points.Comment: 4 pages, 6 Postscript figures, the physical discussion of the results
has been clarifie
Prevalence of iron deficiency anaemia and risk factors in 1,010 adolescent girls from rural Maharashtra, India: a cross-sectional survey
Objective: Iron deficiency anaemia (IDA) is the most common nutritional disorder observed in adolescent girls in India. Our aim was to investigate the prevalence and risk factors associated with IDA in rural Maharashtra, India to address current evidence gaps.
Study Design: Cross sectional survey
Methods: The study recruited 13 to 17 year old adolescent girls living in 34 villages of Osmanabad district. Data were collected on individual health, dietary, sociodemographic factors, and anthropometric measurements were taken. Haemoglobin (Hb) levels were measured using Sahli’s hemometer. Logistic and linear regressions were used to identify risk factors associated with IDA and Hb level respectively.
Results: Among 1,010 adolescent girls (response rate 97.5%), the mean Hb was 10.1 g/dl (standard deviation=1.3), and 87% had anaemia (Hb<12 g/dl). The prevalence of mild (11.0-11.9 g/dl), moderate (8.0-10.9 g/dl) and severe (Hb≤ 7.9 g/dl) anaemia was 17%, 65% and 5% respectively. Anaemia likelihood increased significantly with age (odds ratio (OR): 1.41 per year, 95% confidence interval (CI): 1.17 to 1.70). Factors associated with decreased anaemia risk were mid upper arm circumference (MUAC) ≥22 cm (OR: 0.51, 95% CI: 0.31 to 0.82), ≥3 days/week consumption of fruit (OR: 0.35, 95% CI: 0.23 to 0.54) or rice (OR: 0.39, 95% CI: 0.17 to 0.91), and incomplete schooling (OR: 0.47, 95% CI: 0.24 to 0.91). In the final model lower age, MUAC and fruit consumption were significantly associated with Hb level.
Conclusion: Anaemia prevalence was extremely high among adolescent girls in rural areas of Maharashtra. Whilst we identified risk factors that could be used for targeting interventions, there is urgent need of comprehensive preventative interventions for the whole adolescent girl population
Thermal Conductivity of Spin-1/2 Chains
We study the low-temperature transport properties of clean one-dimensional
spin-1/2 chains coupled to phonons. Due to the presence of approximate
conservation laws, the heat current decays very slowly giving rise to an
exponentially large heat conductivity, . As a result of an
interplay of Umklapp scattering and spinon-phonon coupling, the characteristic
energy scale turns out to be of order , where is
the Debye energy, rather than the magnetic exchange interaction -- in
agreement with recent measurements in SrCuO compounds. A large magnetic field
strongly affects the heat transport by two distinct mechanisms. First, it
induces a LINEAR spinon--phonon coupling, which alters the nature of the fixed point: the elementary excitations of the system are COMPOSITE
SPINON-PHONON objects. Second, the change of the magnetization and the
corresponding change of the wave vector of the spinons strongly affects the way
in which various Umklapp processes can relax the heat current, leading to a
characteristic fractal--like spiky behavior of when plotted as a
function of magnetization at fixed T.Comment: 16 pages, RevTex4, 2 figures included; revised refs. and some useful
comments on experimental relevance. On July 12 2005, added an appendix
correcting an error in the form of the phonon propagator. The main result is
unchange
Microscopic description of d-wave superconductivity by Van Hove nesting in the Hubbard model
We devise a computational approach to the Hubbard model that captures the
strong coupling dynamics arising when the Fermi level is at a Van Hove
singularity in the density of states. We rely on an approximate degeneracy
among the many-body states accounting for the main instabilities of the system
(antiferromagnetism, d-wave superconductivity). The Fermi line turns out to be
deformed in a manner consistent with the pinning of the Fermi level to the Van
Hove singularity. For a doping rate , the ground state is
characterized by d-wave symmetry, quasiparticles gapped only at the
saddle-points of the band, and a large peak at zero momentum in the d-wave
pairing correlations.Comment: 4 pages, 2 Postscript figure
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