Time-resolved X-ray microscopy of nanoparticle aggregates under oscillatory shear

Of all current detection techniques with nanometer resolution, only X-ray microscopy allows imaging nanoparticles in suspension. Can it also be used to investigate structural dynamics? When studying response to mechanical stimuli, the challenge lies in applying them with precision comparable to spatial resolution. In the first shear experiments performed in an X-ray microscope, we accomplished this by inserting a piezo actuator driven shear cell into the focal plane of a scanning transmission X-ray microscope (STXM). Thus shear-induced reorganization of magnetite nanoparticle aggregates could be demonstrated in suspension. As X-ray microscopy proves suitable for studying structural change, new prospects open up in physics at small length scales.Comment: submitted to J. Synchrot. Radia

Collisional strong-field QED kinetic equations from first principles

Starting from nonequilibrium quantum field theory on a closed time path, we derive kinetic equations for the strong-field regime of quantum electrodynamics (QED) using a systematic expansion in the gauge coupling $e$. The strong field regime is characterized by a large photon field of order $\mathcal{O}(1/e)$, which is relevant for the description of, e.g., intense laser fields, the initial stages of off-central heavy ion collisions, and condensed matter systems with net fermion number. The strong field enters the dynamical equations via both quantum Vlasov and collision terms, which we derive to order $\mathcal{O}(e^2)$. The kinetic equations feature generalized scattering amplitudes that have their own equation of motion in terms of the fermion spectral function. The description includes single photon emission, electron-positron pair photoproduction, vacuum (Schwinger) pair production, their inverse processes, medium effects and contributions from the field, which are not restricted to the so-called locally-constant crossed field approximation. This extends known kinetic equations commonly used in strong-field QED of intense laser fields. In particular, we derive an expression for the asymptotic fermion pair number that includes leading-order collisions and remains valid for strongly inhomogeneous fields. For the purpose of analytically highlighting limiting cases, we also consider plane-wave fields for which it is shown how to recover Furry-picture scattering amplitudes by further assuming negligible occupations. Known on-shell descriptions are recovered in the case of simply peaked ultrarelativistic fermion occupations. Collisional strong-field equations are necessary to describe the dynamics to thermal equilibrium starting from strong-field initial conditions

Magnetoelectric and structural properties of Y2CoMn O6: The role of antisite defects

We have carried out an investigation on the magnetoelectric properties of the presumed multiferroic Y2CoMnO6 with different degrees of Co/Mn atomic ordering. The magnetic ground state was studied by neutron diffraction, showing a collinear ferromagnetic (FM) ordering of Co and Mn moments with a small antiferromagnetic canting. No superstructure peaks from an E-type magnetic structure were detected in our measurements. Magnetic measurements reveal FM transitions with pinned magnetic domains. The degree of Co/Mn ordering affects the Curie temperature only a little, but has strong effects on the magnetic hysteresis loops, and the FM moment signal at high field increases with increasing such order. The loops display steps at critical fields whose number and extent depends on each specimen. The most ordered sample exhibits the greatest steps ascribed to the alignment of magnetic domains separated by antiphase boundaries. All samples are insulators exhibiting low dielectric loss and dielectric constants at low temperature. On warming, they show a step increase in the real dielectric permittivity accompanied by peaks in the dielectric loss typical of thermally activated hopping processes. At room temperature, the huge values of the dielectric constant reveal the presence of Maxwell-Wagner depletion layers. Pyroelectric measurements reveal a high polarization at low temperature for these compounds that increases with increasing the Co/Mn ordering. There is no correlation between the magnetic transition and the onset of pyroelectric current. No significant changes are observed in the pyroelectric effect measured under an external magnetic field, so magnetoelectric coupling is negligible. This paper identifies the pyroelectric current as thermally stimulated depolarization current ascribed to the reorientation of defect dipoles with activation energy of about 0.05 eV. Therefore, no ferroelectric transition occurs in these compounds, discarding the existence of intrinsic magnetoelectric multiferroicity

Was the GLE on May 17, 2012 linked with the M5.1-class flare the first in the 24th solar cycle?

On May 17, 2012 an M5.1-class flare exploded from the sun. An O-type coronal mass ejection (CME) was also associated with this flare. There was an instant increase in proton flux with peak at $\geq 100$ MeV, leading to S2 solar radiation storm level. In about 20 minutes after the X-ray emission, the solar particles reached the Earth.It was the source of the first (since December 2006) ground level enhancement (GLE) of the current solar cycle 24. The GLE was detected by neutron monitors (NM) and other ground based detectors. Here we present an observation by the Tupi muon telescopes (Niteroi, Brazil, $22^{0}.9 S$, $43^{0}.2 W$, 3 m above sea level) of the enhancement of muons at ground level associated with this M5.1-class solar flare. The Tupi telescopes registered a muon excess over background $\sim 20\%$ in the 5-min binning time profile. The Tupi signal is studied in correlation with data obtained by space-borne detectors (GOES, ACE), ground based neutron monitors (Oulu) and air shower detectors (the IceTop surface component of the IceCube neutrino observatory). We also report the observation of the muon signal possibly associated with the CME/sheath striking the Earth magnetosphere on May 20, 2012. We show that the observed temporal correlation of the muon excess observed by the Tupi muon telescopes with solar transient events suggests a real physical connection between them. Our observation indicates that combination of two factors, the low energy threshold of the Tupi muon telescopes and the location of the Tupi experiment in the South Atlantic Anomaly region, can be favorable in the study and detection of the solar transient events. Our experiment provides new data complementary to other techniques (space and ground based) in the study of solar physics.Comment: 9 pages, 10 figure

Self-organised reactivation maintains and reinforces memories despite synaptic turnover

Long-term memories are believed to be stored in the synapses of cortical neuronal networks. However, recent experiments report continuous creation and removal of cortical synapses, which raises the question how memories can survive on such a variable substrate. Here, we study the formation and retention of associative memory in a computational model based on Hebbian cell assemblies in the presence of both synaptic and structural plasticity. During rest periods, such as may occur during sleep, the assemblies reactivate spontaneously, reinforcing memories against ongoing synapse removal and replacement. Brief daily reactivations during rest-periods suffice to not only maintain the assemblies, but even strengthen them, and improve pattern completion, consistent with offline memory gains observed experimentally. While the connectivity inside memory representations is strengthened during rest phases, connections in the rest of the network decay and vanish thus reconciling apparently conflicting hypotheses of the influence of sleep on cortical connectivity

The Electronic and Magnetic Properties of Cation Ordered Sr2Mn2.23Cr0.77As2O2

ACKNOWLEDGMENT This research is supported by the EPSRC (research grant EP/L002493/1). We also acknowledge the UK Science and Technology Facilities Council (STFC) for provision of beam time at the ILL.Peer reviewedPostprin

A different look at the spin state of Co3+^{3+} ions in CoO5_{5} pyramidal coordination

Using soft-x-ray absorption spectroscopy at the Co-$L_{2,3}$ and O-$K$ edges, we demonstrate that the Co$^{3+}$ ions with the CoO$_{5}$ pyramidal coordination in the layered Sr$_2$CoO$_3$Cl compound are unambiguously in the high spin state. Our result questions the reliability of the spin state assignments made so far for the recently synthesized layered cobalt perovskites, and calls for a re-examination of the modeling for the complex and fascinating properties of these new materials.Comment: 5 pages 3 figure

Magnetoelectric and structural properties of Y2CoMnO6: The role of antisite defects

We have carried out an investigation on the magnetoelectric properties of the presumed multiferroic Y2CoMnO6 with different degrees of Co/Mn atomic ordering. The magnetic ground state was studied by neutron diffraction, showing a collinear ferromagnetic (FM) ordering of Co and Mn moments with a small antiferromagnetic canting. No superstructure peaks from an E-type magnetic structure were detected in our measurements. Magnetic measurements reveal FM transitions with pinned magnetic domains. The degree of Co/Mn ordering affects the Curie temperature only a little, but has strong effects on the magnetic hysteresis loops, and the FM moment signal at high field increases with increasing such order. The loops display steps at critical fields whose number and extent depends on each specimen. The most ordered sample exhibits the greatest steps ascribed to the alignment of magnetic domains separated by antiphase boundaries. All samples are insulators exhibiting low dielectric loss and dielectric constants at low temperature. On warming, they show a step increase in the real dielectric permittivity accompanied by peaks in the dielectric loss typical of thermally activated hopping processes. At room temperature, the huge values of the dielectric constant reveal the presence of Maxwell-Wagner depletion layers. Pyroelectric measurements reveal a high polarization at low temperature for these compounds that increases with increasing the Co/Mn ordering. There is no correlation between the magnetic transition and the onset of pyroelectric current. No significant changes are observed in the pyroelectric effect measured under an external magnetic field, so magnetoelectric coupling is negligible. This paper identifies the pyroelectric current as thermally stimulated depolarization current ascribed to the reorientation of defect dipoles with activation energy of about 0.05 eV. Therefore, no ferroelectric transition occurs in these compounds, discarding the existence of intrinsic magnetoelectric multiferroicity.For financial support we thank the Spanish Ministerio de Economía y Competitividad (MINECO) (Projects No. MAT2012-38213-C02-01 and -02 and No. MAT2015-68760-C1-1 and -2-P, cofunded by the European Regional Development Fund [ERDF] from the European Union) and Diputación General de Aragón (DGA, project E-69). J.A. Rodríguez-Velamazan acknowledges CSIC for the JAEdoc contract.Peer Reviewe