3,379 research outputs found
On the RKKY range function of a one dimensional non interacting electron gas
We show that the pitfalls encountered in earlier calculations of the RKKY
range function for a non interacting one dimensional electron gas at zero
temperature can be unraveled and successfully dealt with through a proper
handling of the impurity potential.Comment: to appear in Phys. Re
Recent Results from the SIMPLE Dark Matter Search
SIMPLE is an experimental search for evidence of spin-dependent dark matter,
based on superheated droplet detectors using CClF. We report
preliminary results of a 0.6 kgdy exposure of five one liter devices, each
containing 10 g active mass, in the 1500 mwe LSBB (Rustrel, France). In
combination with improvements in detector sensitivity, the results exclude a
WIMP--proton interaction above 5 pb at M = 50 GeV/c.Comment: 6 pages, 2 figures,contribution to IDM2004, Sept. 6-10, 2004,
Edinburgh, U
The midwife factor in obstetric procedures and neonatal outcome
Aims. In the face of major tendency towards midwiferyled-care it was our purpose to investigate the extent of the influence of the midwife on the rates of obstetric procedures and perinatal outcome. Methods. 5384 consecutive deliveries at the Department of Obstetrics and Gynecology. University of Graz, were enrolled in the study. The following data were collected: mode of delivery, pH of umbilical artery, Apgar score. Firstly, data were investigated for interindividual differences and, secondly, for relationship with age of the midwife as a measure of experience. Results. Interindividual differences were significant for episiotomy rates (minimum: 31.6%; maximum 76.9%; p<0.001), forceps rates (minimum: 1.7%; maximum 11.1%;p=0.002) and pH of umbilical arteries (minimum: 7.21:maxinium: 7.28 p=0.001) but not for cesarean section rates and Apgar scores. Linear regression analysis was significant between age of midwives and pH of umbilical arteries (p<0.001: r=0.055) and for one-minute Apgar score (p=0.009; r=0.050) but not for episiotomy rates, cesarean section rates, forceps rates and five-minutes Apgar score. Conclusions. There are large interindividual differences in obstetric intervention rates which cannot be explained by the midwives' age. Provision of health care should be primarily determined by need and not by the personal characteristics of the health care provider, thus interindividual differences should be reduced and more often taken into account when analyzing any kind of data
Spin polarization and g-factor enhancement in graphene nanoribbons in magnetic field
We provide a systematic quantitative description of spin polarization in
armchair and zigzag graphene nanoribbons in a perpendicular magnetic field. We
first address spinless electrons within the Hartree approximation studying the
evolution of the magnetoband structure and formation of the compressible
strips. We discuss the potential profile and the density distribution near the
edges and the difference and similarities between armchair and zigzag edges.
Accounting for the Zeeman interaction and describing the spin effects via the
Hubbard term we study the spin-resolved subband structure and relate the spin
polarization of the system at hand to the formation of the compressible strips
for the case of spinless electrons. At high magnetic field the calculated
effective g-factor varies around a value of ~2.25 for armchair nanoribbons
and ~3 for zigzag nanoribbons. An important finding is that in zigzag
nanoribbons the zero-energy mode remains pinned to the Fermi-energy and becomes
fully spin-polarized for all magnetic fields, which, in turn, leads to a strong
spin polarization of the electron density near the zigzag edge.Comment: 9 pages, 4 figure
Universal finite size corrections and the central charge in non solvable Ising models
We investigate a non solvable two-dimensional ferromagnetic Ising model with
nearest neighbor plus weak finite range interactions of strength \lambda. We
rigorously establish one of the predictions of Conformal Field Theory (CFT),
namely the fact that at the critical temperature the finite size corrections to
the free energy are universal, in the sense that they are exactly independent
of the interaction. The corresponding central charge, defined in terms of the
coefficient of the first subleading term to the free energy, as proposed by
Affleck and Blote-Cardy-Nightingale, is constant and equal to 1/2 for all
0<\lambda<\lambda_0 and \lambda_0 a small but finite convergence radius. This
is one of the very few cases where the predictions of CFT can be rigorously
verified starting from a microscopic non solvable statistical model. The proof
uses a combination of rigorous renormalization group methods with a novel
partition function inequality, valid for ferromagnetic interactions.Comment: 43 pages, 1 figur
Equilibrium spin currents: Non-Abelian gauge invariance and color diamagnetism in condensed matter
The spin-orbit (SO) interaction in condensed matter can be described in terms
of a non-Abelian potential known in high-energy physics as a color field. I
show that a magnetic component of this color field inevitably generates
diamagnetic color currents which are just the equilibrium spin currents
discussed in a condensed matter context. These dissipationless spin currents
thus represent a universal property of systems with SO interaction. In
semiconductors with linear SO coupling the spin currents are related to the
effective non-Abelian field via Yang-Mills magnetostatics equation.Comment: RevTeX, 4 page
Universal conductivity and dimensional crossover in multi-layer graphene
We show, by exact Renormalization Group methods, that in multi-layer graphene
the dimensional crossover energy scale is decreased by the intra-layer
interaction, and that for temperatures and frequencies greater than such scale
the conductivity is close to the one of a stack of independent layers up to
small corrections
Universality of conductivity in interacting graphene
The Hubbard model on the honeycomb lattice describes charge carriers in
graphene with short range interactions. While the interaction modifies several
physical quantities, like the value of the Fermi velocity or the wave function
renormalization, the a.c. conductivity has a universal value independent of the
microscopic details of the model: there are no interaction corrections,
provided that the interaction is weak enough and that the system is at half
filling. We give a rigorous proof of this fact, based on exact Ward Identities
and on constructive Renormalization Group methods
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