2,973 research outputs found
Observation of superspin glass state in magnetically textured ferrofluid (gamma-Fe2O3)
Magnetic properties in a magnetically textured ferrofluid made out of
interacting maghemite (gamma-Fe2O3) nanoparticles suspended in glycerin have
been investigated. Despite the loss of uniform distribution of anisotropy axes,
a superspin glass state exists at low temperature in a concentrated, textured
ferrofluid as in the case of its non-textured counterpart. The onset of
superspin glass state was verified from the sample's AC susceptibility. The
influence of the anisotropy axis orientation on the aging behavior in the
glassy states is also discussed
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
Reconstruction of a latest Paleocene shallow-marine eutrophic paleoenvironment at Sidi Nasseur (Central Tunisia) based on foraminifera, ostracoda, calcareous nannofossils and stable isotopes (d13C, d18O)
In order to unravel faunal and paleoenvironmental parameters in shallow marine settings prior to the Paleocene- Eocene thermal maximum, we investigated the Sidi Nasseur section (NAS) in Central Tunisia. This section exposes Paleocene to lower Eocene shales and marls of the El Haria Formation. The uppermost Paleocene part of the Sidi Nasseur section is marked by poor to moderately rich, but fairly diversified nannofossil associations, containing the typical latest Paleocene taxa of the top of NP9a. The ostracode record displays an almost continuous record in the uppermost Paleocene part of the section. Representatives of Aegyptiana, Paracosta, Reticulina and Reymenticosta make up the major part of the ostracode fauna. The benthic foraminiferal assemblage consists of numerous small calcareous benthic foraminifera, like Anomalinoides midwayensis and Lenticulina spp. and many large Frondicularia phosphatica, Pyramidulina spp. These, together with the non-calcareous agglutinated foraminifera and the rare planktic foraminifera, indicate an inner neritic to coastal environment with eutrophic conditions, regularly interrupted by oxygen deficiency. The dominance of non-calcareous benthic foraminifera between intervals with abundant calcareous benthic foraminifera suggests post-mortem dissolution. The foraminiferal d13C record (based upon Pyramidulina latejugata) of the latest Paleocene in the Sidi Nasseur area is very similar to these from coeval sediments at Gebel Duwi and Gebel Aweina in Egypt. Oxygen isotopic ratios indicate a marine setting with a water composition affected by evaporation. During the latest Paleocene, the highly productive shallow water environment evolved to shallower water depths with higher salinity and increasing dominance of A. midwayensis
Universal dephasing in a chiral 1D interacting fermion system
We consider dephasing by interactions in a one-dimensional chiral fermion
system (e.g. a Quantum Hall edge state). For finite-range interactions, we
calculate the spatial decay of the Green's function at fixed energy, which sets
the contrast in a Mach-Zehnder interferometer. Using a physically transparent
semiclassical ansatz, we find a power-law decay of the coherence at high
energies and zero temperature (T=0), with a universal asymptotic exponent of 1,
independent of the interaction strength. We obtain the dephasing rate at T>0
and the fluctuation spectrum acting on an electron.Comment: 5 pages, 3 figures; minor changes, version as published
Effective Nucleon-Nucleon Interaction and Fermi Liquid Theory
We present two novel relations between the quasiparticle interaction in
nuclear matter and the unique low momentum nucleon-nucleon interaction in
vacuum. These relations provide two independent constraints on the Fermi liquid
parameters of nuclear matter. Moreover, the new constraints define two
combinations of Fermi liquid parameters, which are invariant under the
renormalization group flow in the particle-hole channels. Using empirical
values for the spin-independent Fermi liquid parameters, we are able to compute
the major spin-dependent ones by imposing the new constraints as well as the
Pauli principle sum rules.Comment: 4 pages, 5 figures, in Proc. 11th International Conference on Recent
Progress in Many-Body Theories, Manchester, UK, July 9-13, 200
Reconstruction of a latest Paleocene shallow-marine eutrophic paleoenvironment at Sidi Nasseur (Central Tunisia) based on foraminifera, ostracoda, calcareous nannofossils and stable isotopes (delta<sup>13</sup>C delta<sup>18</sup>O)
In order to unravel faunal and paleoenvironmental parameters in shallow marine settings prior to the Paleocene-Eocene thermal maximum, we investigated the Sidi Nasseur section (NAS) in Central Tunisia. This section exposes Paleocene to lower Eocene shales and marls of the El Haria Formation. The uppermost Paleocene part of the Sidi Nasseur section is marked by poor to moderately rich, but fairly diversified nannofossil associations, containing the typical latest Paleocene taxa of the top of NP9a. The ostracode record displays an almost continuous record in the uppermost Paleocene part of the section. Representatives of Aegyptiana, Paracosta, Reticulina and Reymenticosta make up the major part of the ostracode fauna. The benthic foraminiferal assemblage consists of numerous small calcareous benthic foraminifera, like Anomalinoides midwayensis and Lenticulina spp. and many large Frondicularia phosphatica, Pyramidulina spp. These, together with the non-calcareous agglutinated foraminifera and the rare planktic foraminifera, indicate an inner neritic to coastal environment with eutrophic conditions, regularly interrupted by oxygen deficiency. The dominance of non-calcareous benthic foraminifera between intervals with abundant calcareous benthic foraminifera suggests post-mortem dissolution. The foraminiferal d13C record (based upon Pyramidulina latejugata) of the latest Paleocene in the Sidi Nasseur area is very similar to these from coeval sediments at Gebel Duwi and Gebel Aweina in Egypt. Oxygen isotopic ratios indicate a marine setting with a water composition affected by evaporation. During the latest Paleocene, the highly productive shallow water environment evolved to shallower water depths with higher salinity and increasing dominance of A. midwayensis
Variational Cluster Perturbation Theory for Bose-Hubbard models
We discuss the application of the variational cluster perturbation theory
(VCPT) to the Mott-insulator--to--superfluid transition in the Bose-Hubbard
model. We show how the VCPT can be formulated in such a way that it gives a
translation invariant excitation spectrum -- free of spurious gaps -- despite
the fact that if formally breaks translation invariance. The phase diagram and
the single-particle Green function in the insulating phase are obtained for
one-dimensional systems. When the chemical potential of the cluster is taken as
a variational parameter, the VCPT reproduces the dimension dependence of the
phase diagram even for one-site clusters. We find a good quantitative agreement
with the results of the density-matrix renormalization group when the number of
sites in the cluster becomes of order 10. The extension of the method to the
superfluid phase is discussed.Comment: v1) 10 pages, 6 figures. v2) Final version as publishe
Classical Setting and Effective Dynamics for Spinfoam Cosmology
We explore how to extract effective dynamics from loop quantum gravity and
spinfoams truncated to a finite fixed graph, with the hope of modeling
symmetry-reduced gravitational systems. We particularize our study to the
2-vertex graph with N links. We describe the canonical data using the recent
formulation of the phase space in terms of spinors, and implement a
symmetry-reduction to the homogeneous and isotropic sector. From the canonical
point of view, we construct a consistent Hamiltonian for the model and discuss
its relation with Friedmann-Robertson-Walker cosmologies. Then, we analyze the
dynamics from the spinfoam approach. We compute exactly the transition
amplitude between initial and final coherent spin networks states with support
on the 2-vertex graph, for the choice of the simplest two-complex (with a
single space-time vertex). The transition amplitude verifies an exact
differential equation that agrees with the Hamiltonian constructed previously.
Thus, in our simple setting we clarify the link between the canonical and the
covariant formalisms.Comment: 38 pages, v2: Link with discretized loop quantum gravity made
explicit and emphasize
Decoupling with unitary approximate two-designs
Consider a bipartite system, of which one subsystem, A, undergoes a physical
evolution separated from the other subsystem, R. One may ask under which
conditions this evolution destroys all initial correlations between the
subsystems A and R, i.e. decouples the subsystems. A quantitative answer to
this question is provided by decoupling theorems, which have been developed
recently in the area of quantum information theory. This paper builds on
preceding work, which shows that decoupling is achieved if the evolution on A
consists of a typical unitary, chosen with respect to the Haar measure,
followed by a process that adds sufficient decoherence. Here, we prove a
generalized decoupling theorem for the case where the unitary is chosen from an
approximate two-design. A main implication of this result is that decoupling is
physical, in the sense that it occurs already for short sequences of random
two-body interactions, which can be modeled as efficient circuits. Our
decoupling result is independent of the dimension of the R system, which shows
that approximate 2-designs are appropriate for decoupling even if the dimension
of this system is large.Comment: Published versio
Growth of a dynamical correlation length in an aging superspin glass
We report on zero field cooled magnetization relaxation experiments on a
concen- trated frozen ferrofluid exhibiting a low temperature superspin glass
transition. With a method initially developed for spin glasses, we investigate
the field dependence of the relaxations that take place after different aging
times. We extract the typical number of correlated spins involved in the aging
dynamics. This brings important insights into the dynamical correlation length
and its time growth. Our results, consistent with expressions obtained for spin
glasses, extend the generality of these behaviours to the class of superspin
glasses. Since the typical flipping time is much larger for superspins than for
atomic spins, our experiments probe a time regime much closer to that of
numerical simulations
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