676 research outputs found
Chiral QCD, General QCD Parameterization and Constituent Quark Models
Several recent papers -using effective QCD chiral Lagrangians- reproduced
results obtained with the general QCD parameterization (GP). These include the
baryon 8+10 mass formula, the octet magnetic moments and the coincidental
nature of the "perfect" -3/2 ratio between the magnetic moments of p and n.
Although we anticipated that the GP covers the case of chiral treatments, the
above results explicitly exemplify this fact. Also we show by the GP that -in
any model or theory (chiral or non chiral) reproducing the results of exact
QCD- the Franklin (Coleman Glashow) sum rule for the octet magnetic moments
must be violated.Comment: 10 pages, Latex; abridged version (same results), removed some
reference
A ballistic pn junction in suspended graphene with split bottom gates
We have developed a process to fabricate suspended graphene devices with
local bottom gates, and tested it by realizing electrostatically controlled pn
junctions on a suspended graphene mono-layer nearly 2 micrometers long.
Measurements as a function of gate voltage, magnetic field, bias, and
temperature exhibit characteristic Fabry-Perot oscillations in the cavities
formed by the pn junction and each of the contacts, with transport occurring in
the ballistic regime. Our results demonstrate the possibility to achieve a high
degree of control on the local electronic properties of ultra-clean suspended
graphene layers, a key aspect for the realization of new graphene
nanostructures.Comment: 10 pages, 4 figure
Nanometer-spaced platinum electrodes with calibrated separation
We have fabricated pairs of platinum electrodes with separation between 20
and 3.5 nm. Our technique combines electron beam lithography and chemical
electrodeposition. We show that the measurement of the conductance between the
two electrodes through the electrolyte provides an accurate and reproducible
way to control their separation. We have tested the robustness of the
electrodes by applying large voltages across them and by using them to measure
the transport properties of Au nano-clusters. Our results show that the
technique reliably produces metallic electrodes with a separation that bridges
the minimum scale accessible by electron beam lithography with the atomic
scale.Comment: 4 pages, 4 figure
Mapping of mutants resistant to p-fluorophenylalanine in diploid Aspergillus nidulans, lethal in haploids
In a previous paper (Babudri and Morpurgo 1990 Curr. Genet. 17:519-522) we described a new class of para-fluorophenylalanine (FPA) resistant mutants in Aspergillus nidulans. These mutants were obtained by plating UV irradiated diploid conidia on minimal medium (MM) supplemented with FPA (0.188 mg/ml)
Analysis of dynamical corrections to baryon magnetic moments
We present and analyze QCD corrections to the baryon magnetic moments in
terms of the one-, two-, and three-body operators which appear in the effective
field theory developed in our recent papers. The main corrections are extended
Thomas-type corrections associated with the confining interactions in the
baryon. We investigate the contributions of low-lying angular excitations to
the moments quantitatively and show that they are completely negligible. When
the QCD corrections are combined with the non-quark model contributions of the
meson loops, we obtain a model which describes the moments within a mean
deviation of 0.04 . The nontrivial interplay of the two types of
corrections to the quark-model moments is analyzed in detail, and explains why
the quark model is so successful. In the course of these calculations, we
parametrize the general spin structure of the baryon wave functions
in a form which clearly displays the symmetry properties and the internal
angular momentum content of the wave functions, and allows us to use spin-trace
methods to calculate the many spin matrix elements which appear in the
expressions for the moments. This representation may be useful elsewhere.Comment: 32 pages, 3 figures, submitted to Phys. Rev.
Controlling the topological sector of magnetic solitons in exfoliated CrNbS crystals
We investigate manifestations of topological order in monoaxial helimagnet
CrNbS by performing transport measurements on ultra-thin crystals.
Upon sweeping the magnetic field perpendicularly to the helical axis, crystals
thicker than one helix pitch (48 nm) but much thinner than the magnetic domain
size (1 m) are found to exhibit sharp and hysteretic resistance
jumps. We show that these phenomena originate from transitions between
topological sectors with different number of magnetic solitons. This is
confirmed by measurements on crystals thinner than 48 nm --in which the
topological sector cannot change-- that do not exhibit any jump or hysteresis.
Our results show the ability to deterministically control the topological
sector of finite-size CrNbS and to detect inter-sector transitions
by transport measurements.Comment: 7 pages, 8 figure
Aharonov-Bohm effect and broken valley-degeneracy in graphene rings
We analyze theoretically the electronic properties of Aharonov-Bohm rings
made of graphene. We show that the combined effect of the ring confinement and
applied magnetic flux offers a controllable way to lift the orbital degeneracy
originating from the two valleys, even in the absence of intervalley
scattering. The phenomenon has observable consequences on the persistent
current circulating around the closed graphene ring, as well as on the ring
conductance. We explicitly confirm this prediction analytically for a circular
ring with a smooth boundary modelled by a space-dependent mass term in the
Dirac equation. This model describes rings with zero or weak intervalley
scattering so that the valley isospin is a good quantum number. The tunable
breaking of the valley degeneracy by the flux allows for the controlled
manipulation of valley isospins. We compare our analytical model to another
type of ring with strong intervalley scattering. For the latter case, we study
a ring of hexagonal form with lattice-terminated zigzag edges numerically. We
find for the hexagonal ring that the orbital degeneracy can still be controlled
via the flux, similar to the ring with the mass confinement.Comment: 7 pages, 7 figures, replaced with considerably extended new versio
Measurement of chromatic effects in LEP
The chromaticity produced by the insertions and the lattice of LEP is corrected in the FODO lattice of the arcs which has horizontal and vertical phase advance of 90o and 60o for the LEP optics presently used. This is done with two horizontal and three vertical sextupole families. To check this correction scheme the non-linear chromaticities have been measured by observing the deviation. The differences between the results and the expected effects of the sextupoles are interpreted in terms of octupole and decapole components in the dipole magnets. To check the distribution around the ring of the chromaticities and their corrections the betatron phase advances have been measured as a function of momentum deviation. This was done by observing an excited betatron oscillation for 1024 turns in all beam position monitors for different RF-frequencies. The results clearly show the negative chromaticity produced in the straight sections and its correction in the arcs. The parasitic sextupole components are found to be spread uniformly in the arcs within the measurement errors. Under this condition one finds that they have a negligible effects on the dynamic aperture
Nonadiabatic noncyclic geometric phase and ensemble average spectrum of conductance in disordered mesoscopic rings with spin-orbit coupling
We generalize Yang's theory from the U(1) gauge field to the non-Abelian
gauge field. Based on this generalization and taking
into account the geometric Pancharatnam phase as well as an effective
Aharonov-Bohm (AB) phase in nonadiabatic noncyclic transport, we calculate the
ensemble average Fourier spectrum of the conductance in disordered mesoscopic
rings connected to two leads. Our theory can explain the experimental results
reported by Morpurgo {\sl et al.} [Phys. Rev. Lett. {\bf 80}, 1050 (1998)]
satisfactorily. In particular, we clarify that the experimentally observed
splitting, as well as some structure on the sides of the main peak in the
ensemble average Fourier spectrum, stem from the nonadiabatic noncyclic
Pancharatnam phase and the effective AB phase, both being dependent on
spin-orbit coupling.Comment: 5 pages, 1 figure. A slightly revised version, and re-submitted to
PRL on Mar. 14, 200
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