15,692 research outputs found
The quantum dynamics of atomic magnets, co-tunneling and dipolar-biased tunneling
Multi-spins tunneling cross-relaxations in an ensemble of weakly-coupled
Ho ions, mediated by weak anisotropic dipolar interactions, can be
evidenced by ac-susceptibility measurements in a high temperature regime. Based
on a four-body representation, including the rare-earth nuclear spin, two-ions
tunneling mechanisms can be attributed to both dipolar-biased tunneling and
co-tunneling processes. The co-reversal involving entangled pairs of magnetic
moments is discussed with a particular emphasis, giving new evidences to
elucidate the many-body quantum dynamics.Comment: 4 figure
First-principles Calculations of Engineered Surface Spin Structures
The engineered spin structures recently built and measured in scanning
tunneling microscope experiments are calculated using density functional
theory. By determining the precise local structure around the surface
impurities, we find the Mn atoms can form molecular structures with the binding
surface, behaving like surface molecular magnets. The spin structures are
confirmed to be antiferromagnetic, and the exchange couplings are calculated
within 8% of the experimental values simply by collinear-spin GGA+U
calculations. We can also explain why the exchange couplings significantly
change with different impurity binding sites from the determined local
structure. The bond polarity is studied by calculating the atomic charges with
and without the Mn adatoms
Role of the host cell in bacteriophage T4 development. II. Characterization of host mutants that have pleiotropic effects on T4 growth
Mutant host-defective Escherichi coli that fail to propagate bacteriophage T4 and have a pleiotropic effect on T4 development have been isolated and characterized. In phage-infected mutant cells, specific early phage proteins are absent or reduced in amount, phage DNA synthesis is depressed by about 50%, specific structural phage proteins, including some tail and collar components, are deficient or missing, and host-cell lysis is delayed and slow. Almost all phage that can overcome the host block carry mutantions that map in functionally undefined 'nonessential' regions of the T4 genome, most near gene 39. The mutant host strains are temperature sensitive for growth and show simultaneous reversion of the ts phenotype and the inability to propagate T4+. The host mutations are cotransduced with ilv (83 min) and may lie in the gene for transcription termination factor rho
Quantum interference oscillations of the superparamagnetic blocking in an Fe8 molecular nanomagnet
We show that the dynamic magnetic susceptibility and the superparamagnetic
blocking temperature of an Fe8 single molecule magnet oscillate as a function
of the magnetic field Hx applied along its hard magnetic axis. These
oscillations are associated with quantum interferences, tuned by Hx, between
different spin tunneling paths linking two excited magnetic states. The
oscillation period is determined by the quantum mixing between the ground S=10
and excited multiplets. These experiments enable us to quantify such mixing. We
find that the weight of excited multiplets in the magnetic ground state of Fe8
amounts to approximately 11.6%.Comment: Accepted in Phys. Rev. Let
Phonon-assisted tunneling in the quantum regime of Mn12-ac
Longitudinal or transverse magnetic fields applied on a crystal of Mn12-ac
allows to observe independent tunnel transitions between m=-S+p and m=S-n-p
(n=6-10, p=0-2 in longitudinal field and n=p=0 in transverse field). We observe
a smooth transition (in longitudinal) from coherent ground-state to thermally
activated tunneling. Furthermore two ground-state relaxation regimes showing a
crossover between quantum spin relaxation far from equilibrium and near
equilibrium, when the environment destroys multimolecule correlations. Finally,
we stress that the complete Hamiltonian of Mn12 should contain odd spin
operators of low order
Synchronization in disordered Josephson junction arrays: Small-world connections and the Kuramoto model
We study synchronization in disordered arrays of Josephson junctions. In the
first half of the paper, we consider the relation between the coupled
resistively- and capacitively shunted junction (RCSJ) equations for such arrays
and effective phase models of the Winfree type. We describe a multiple-time
scale analysis of the RCSJ equations for a ladder array of junctions
\textit{with non-negligible capacitance} in which we arrive at a second order
phase model that captures well the synchronization physics of the RCSJ
equations for that geometry. In the second half of the paper, motivated by
recent work on small world networks, we study the effect on synchronization of
random, long-range connections between pairs of junctions. We consider the
effects of such shortcuts on ladder arrays, finding that the shortcuts make it
easier for the array of junctions in the nonzero voltage state to synchronize.
In 2D arrays we find that the additional shortcut junctions are only marginally
effective at inducing synchronization of the active junctions. The differences
in the effects of shortcut junctions in 1D and 2D can be partly understood in
terms of an effective phase model.Comment: 31 pages, 21 figure
Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo_xY_{1-x}F_4
The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5%
and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While
this system is considered as the archetypal quantum spin glass, the detailed
analysis of our magnetization data indicates the absence of a phase transition,
not only in a transverse applied magnetic field, but also without field. A
zero-Kelvin phase transition is also unlikely, as the magnetization seems to
follow a non-critical exponential dependence on the temperature. Our analysis
thus unmasks the true, short-ranged nature of the magnetic properties of the
LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations
suggesting the lack of phase transition in this system.Comment: 5 pages, 4 figure
Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness
Using a large sample of spiral galaxies for which 21 cm single-dish and/or
long-slit optical spectra are available, we make a detailed comparison between
various estimates of rotational widths. Different optical width estimators are
considered and their limitations discussed, with emphasis on biases associated
with rotation curve properties (shape and extent) and disk central surface
brightness. The best match with HI rotational velocities is obtained with
Polyex widths, which are measured at the optical radius (encompassing a fixed
fraction of the total light of the galaxy) from a model fit to the rotation
curve. In contrast with Polyex widths, optical rotational velocities measured
at 2.15 disk scale lengths r_d deviate from HI widths by an amount that
correlates with the central surface brightness of the disk. This bias occurs
because the rotation curves of galaxies are in general still rising at 2.15
r_d, and the fraction of total mass contained within this radius decreases with
increasing disk surface brightness. Statistical corrections, parameterized by
the radial extent of the observed rotation curve, are provided to reduce Polyex
and HI width measurements into a homogeneous system. This yields a single
robust estimate of rotational velocity to be used for applications of disk
scaling relations.Comment: 13 pages, 8 figures. To appear in the Astronomical Journal (August
2007
Polarization probes of vorticity in heavy ion collisions
We discuss the information that can be deduced from a measurement of particle
(hyperon or vector meson) polarization in ultrarelativistic nuclear collisions.
We describe the sensitivity of polarization to initial conditions, hydrodynamic
evolution and mean free path, and find that the polarization observable is
sensitive to all details and stages of the system's evolution. We suggest that
an experimental investigation covering production plane and reaction plane
polarizations, as well as the polarization of jet-associated particles in the
plane defined by the jet and particle direction, can help in disentangling the
factors contributing to this observable. Scans of polarization in energy and
rapidity might also point to a change in the system's properties.Comment: In press, Phys.Rev.C. One new figure, text streamlined and edited,
physics conclusions and reasoning not change
Absence of Ferromagnetism in Mn-doped Tetragonal Zirconia
In a recent letter, it has been predicted within first principle studies that
Mn-doped ZrO2 compounds could be good candidate for spintronics application
because expected to exhibit ferromagnetism far beyond room temperature. Our
purpose is to address this issue experimentally for Mn-doped tetragonal
zirconia. We have prepared polycrystalline samples of Y0.15(Zr0.85-yMny)O2
(y=0, 0.05, 0.10, 0.15 & 0.20) by using standard solid state method at
equilibrium. The obtained samples were carefully characterized by using x-ray
diffraction, scanning electron microscopy, elemental color mapping, X-ray
photoemission spectroscopy and magnetization measurements. From the detailed
structural analyses, we have observed that the 5% Mn doped compound
crystallized into two symmetries (dominating tetragonal & monoclinic), whereas
higher Mn doped compounds are found to be in the tetragonal symmetry only. The
spectral splitting of the Mn 3s core-level x-ray photoelectron spectra confirms
that Mn ions are in the Mn3+ oxidation state and indicate a local magnetic
moment of about 4.5 {\mu}B/Mn. Magnetic measurements showed that compounds up
to 10% of Mn doping are paramagnetic with antiferromagnetic interactions.
However, higher Mn doped compound exhibits local ferrimagnetic ordering. Thus,
no ferromagnetism has been observed for all Mn-doped tetragonal ZrO2 samples.Comment: 20 pages, 4 figure
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