355 research outputs found
Controlled switching of intrinsic localized modes in a 1-D antiferromagnet
Nearly steady-state locked intrinsic localized modes (ILMs) in the quasi-1d
antiferromagnet (C2H5NH3)2CuCl4 are detected via four-wave mixing emission or
the uniform mode absorption. Exploiting the long-time stability of these locked
ILMs, repeatable nonlinear switching is observed by varying the sample
temperature, and localized modes with various amplitudes are created by
modulation of the microwave driver power. This steady-state ILM locking
technique could be used to produce energy localization in other atomic
lattices.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. v.2 :
clarifications of text and figures in response to comment
Four-terminal resistance of an interacting quantum wire with weakly invasive contacts
We analyze the behavior of the four-terminal resistance, relative to the
two-terminal resistance of an interacting quantum wire with an impurity, taking
into account the invasiveness of the voltage probes. We consider a
one-dimensional Luttinger model of spinless fermions for the wire. We treat the
coupling to the voltage probes perturbatively, within the framework of
non-equilibrium Green function techniques. Our investigation unveils the
combined effect of impurities, electron-electron interactions and invasiveness
of the probes on the possible occurrence of negative resistance.Comment: 10 pages, 7 figure
Magnetic Transformations in the Organic Conductor kappa-(BETS)2Mn[N(CN)2]3 at the Metal-Insulator Transition
A complex study of magnetic properties including dc magnetization, 1H NMR and
magnetic torque measurements has been performed for the organic conductor
kappa-(BETS)2Mn[N(CN)2]3 which undergoes a metal-insulator transition at
T_MI~25K. NMR and the magnetization data indicate a transition in the manganese
subsystem from paramagnetic to a frozen state at T_MI, which is, however, not a
simple Neel type order. Further, a magnetic field induced transition resembling
a spin flop has been detected in the torque measurements at temperatures below
T_MI. This transition is most likely related to the spins of pi-electrons
localized on the organic molecules BETS and coupled with the manganese 3d spins
via exchange interaction.Comment: 6 pages, 5 Figures, 1 Table; Submitted to Phys.Rev.B (Nov.2010
First principles study of the multiferroics BiFeO, BiFeCrO, and BiCrO: Structure, polarization, and magnetic ordering temperature
We present results of an {\it ab initio} density functional theory study of
three bismuth-based multiferroics, BiFeO, BiFeCrO, and
BiCrO. We disuss differences in the crystal and electronic structure of
the three systems, and we show that the application of the LDA+ method is
essential to obtain realistic structural parameters for BiFeCrO. We
calculate the magnetic nearest neighbor coupling constants for all three
systems and show how Anderson's theory of superexchange can be applied to
explain the signs and relative magnitudes of these coupling constants. From the
coupling constants we then obtain a mean-field approximation for the magnetic
ordering temperatures. Guided by our comparison of these three systems, we
discuss the possibilities for designing a multiferroic material with large
magnetization above room temperature.Comment: 8 Pages, 4 Figure
Thermal Casimir Force between Magnetic Materials
We investigate the Casimir pressure between two parallel plates made of
magnetic materials at nonzero temperature. It is shown that for real
magnetodielectric materials only the magnetic properties of ferromagnets can
influence the Casimir pressure. This influence is accomplished through the
contribution of the zero-frequency term of the Lifshitz formula. The
possibility of the Casimir repulsion through the vacuum gap is analyzed
depending on the model used for the description of the dielectric properties of
the metal plates.Comment: 9 pages, 3 figures. Contribution to the Proceedings of QFEXT09,
Norman, OK, September 21-25, 200
Changing times in England: the influence on geography teachersâ professional practice
School geography in England has been characterised as a pendulum swinging between policies that emphasise curriculum and pedagogy alternately. In this paper, I illustrate the influence of these shifts on geography teacher's professional practice, by drawing on three âmomentsâ from my experience as a student, teacher and teacher educator. Barnett's description of teacher professionalism as a continuous project of âbeingâ illuminates how geography teachers can adapt to competing influences. It reflects teacher professionalism as an unfinished project, which is responsive, but not beholden, to shifting trends, and is informed by how teachers frame and enact policies. I argue that recognising these contextual factors is key to supporting geography teachers in âbeingâ geography education professionals. As education becomes increasingly competitive on a global scale, individual governments are looking internationally for âsolutionsâ to improve educational rankings. In this climate, the future of geography education will rest on how teachers react locally to international trends. Geography teacher educators can support this process by continuing to inform the field through meaningful geography education research, in particular in making the contextual factors of their research explicit. This can be supported through continued successful international collaboration in geography education research
Characteristic crossing point ( K) in specific-heat curves of samples RuSrGdCeCuO taken for different values of magnetic field
Magnetic properties of polycrystalline samples of
RuSr(GdCe)CuO, as-prepared (by
solid-state reaction) and annealed (12 hours at 845C) in pure oxygen
at different pressure (30, 62 and 78 atm) are presented. Specific heat and
magnetization were investigated in the temperature range 1.8--300 K with a
magnetic field up to 8 T. Specific heat, , shows a jump at the
superconducting transition (with onset at K). Below 20 K, a
Schottky-type anomaly becomes apparent in . This low-temperature anomaly
can be attributed to splitting of the ground term of paramagnetic
Gd ions by internal and external magnetic fields. It is found that
curves taken for different values of magnetic field have the same
crossing point (at K) for all samples studied. At the
same time, curves taken for different temperatures have a crossing point
at a characteristic field T. These effects can be
considered as manifestation of the crossing-point phenomenon which is supposed
to be inherent for strongly correlated electron systems.Comment: 10 pages, 7 figures, submitted to J. Phys.: Condens. Matte
Sign reversal in the exchange bias and the collapse of hysteresis width across the magnetic compensation temperature in a single crystal of Nd0.75Ho0.25Al2
In the admixed Nd0.75Ho0.25Al2 system, magnetic moments of Nd and Ho
occupying the same crystallographic site are antiferromagnetically coupled and
the chosen stoichiometry displays a magnetic compensation behavior (Tcomp of
about 24 K) in all orientations. In the vicinity of Tcomp, the conduction
electron polarization (CEP) assumes the role of a soft ferromagnet exchange
coupled to a pseudo-antiferromagnet comprising Nd/Ho moments, resulting in an
asymmetry in the hysteretic (M-H) loop, i.e., the notion of an exchange bias
field (Hexch). Across Tcomp, the CEP contribution reverses sign, and in
consonance, the asymmetry in the M-H loop also undergoes a phase reversal.
Interestingly, the width of the M-H loop shows a divergence, followed by a
collapse on approaching Tcomp from either end. The observed behavior confirms a
long standing prediction based on a phenomenological model for ferrimagnetic
systems. The field induced changes across Tcomp leave an imprint of a
quasi-phase transition in the heat capacity data. Magneto-resistance (\Delta R
/ R vs T) has an oscillatory response, in which the changes across Tc and Tcomp
can be recognized.Comment: 14 text pages + 5 figure
Nonlinear spin relaxation in strongly nonequilibrium magnets
A general theory is developed for describing the nonlinear relaxation of spin
systems from a strongly nonequilibrium initial state, when, in addition, the
sample is coupled to a resonator. Such processes are characterized by nonlinear
stochastic differential equations. This makes these strongly nonequilibrium
processes principally different from the spin relaxation close to an
equilibrium state, which is represented by linear differential equations. The
consideration is based on a realistic microscopic Hamiltonian including the
Zeeman terms, dipole interactions, exchange interactions, and a single-site
anisotropy. The influence of cross correlations between several spin species is
investigated. The critically important function of coupling between the spin
system and a resonant electric circuit is emphasized. The role of all main
relaxation rates is analyzed. The phenomenon of self-organization of transition
coherence in spin motion, from the quantum chaotic stage of incoherent
fluctuations, is thoroughly described. Local spin fluctuations are found to be
the triggering cause for starting the spin relaxation from an incoherent
nonequilibrium state. The basic regimes of collective coherent spin relaxation
are studied.Comment: Latex file, 31 page
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