203,138 research outputs found
How many electrons are needed to flip a local spin?
Considering the spin of a local magnetic atom as a quantum mechanical
operator, we illustrate the dynamics of a local spin interacting with a
ballistic electron represented by a wave packet. This approach improves the
semi-classical approximation and provides a complete quantum mechanical
understanding for spin transfer phenomena. Sending spin-polarized electrons
towards a local magnetic atom one after another, we estimate the minimum number
of electrons needed to flip a local spin.Comment: 3 figure
Thermal stability of metastable magnetic skyrmions: Entropic narrowing and significance of internal eigenmodes
We compute annihilation rates of metastable magnetic skyrmions using a form
of Langer's theory in the intermediate-to-high damping (IHD) regime. For a
N\'eel skyrmion, a Bloch skyrmion, and an antiskyrmion, we look at two possible
paths to annihilation: collapse and escape through a boundary. We also study
the effects of a curved vs. a flat boundary, a second skyrmion and a
non-magnetic defect. We find that the skyrmion's internal modes play a dominant
role in the thermally activated transitions compared to the spin-wave
excitations and that the relative contribution of internal modes depends on the
nature of the transition process. Our calculations for a small skyrmion
stabilized at zero-field show that collapse on a defect is the most probable
path. In the absence of a defect, the annihilation is largely dominated by
escape mechanisms, even though in this case the activation energy is higher
than that of collapse processes. Escape through a flat boundary is found more
probable than through a curved boundary. The potential source of stability of
metastable skyrmions is therefore found not to lie in high activation energies,
nor in the dynamics at the transition state, but comes from entropic narrowing
in the saddle point region which leads to lowered attempt frequencies. This
narrowing effect is found to be primarily associated with the skyrmion's
internal modes.Comment: 14 pages, 9 figure
Thermodynamics and kinetics of the undercooled liquid and the glass transition of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy
Differential scanning calorimetry (DSC) was used to determine the thermodynamic functions of the undercooled liquid and the amorphous phase with respect to the crystalline state of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5bulk metallic glass forming alloy. The specific heat capacities of this alloy in the undercooled liquid, the amorphous state and the crystal were determined. The differences in enthalpy, ∆H, entropy, ∆S, and Gibbs free energy, ∆G, between crystal and the undercooled liquid were calculated using the measured specific heat capacity data as well as the heat of fusion. The results indicate that the Gibbs free energy difference between metastable undercooled liquid and crystalline solid, ∆G, stays small compared to conventional metallic glass forming alloys even for large undercoolings. Furthermore, the Kauzmann temperature, TK, where the entropy of the undercooled liquid equals to that of the crystal, was determined to be 560 K. The Kauzmann temperature is compared with the experimentally observed rate-dependent glass transition temperature, Tg. Both onset and end temperatures of the glass transition depend linearly on the logarithm of the heating rate based on the DSC experiments. Those characteristic temperatures for the kinetically observed glass transition become equal close to the Kauzmann temperature in this alloy, which suggests an underlying thermodynamic glass transition as a lower bound for the kinetically observed freezing process
Epitaxial growth of gallium arsenide with ammonium halides as transporting agents
Epitaxial growth of gallium arsenide with ammonium halides as transporting agent
Dynamics of rapidly rotating Bose-Einstein condensates in a harmonic plus quartic trap
A two-dimensional rapidly rotating Bose-Einstein condensate in a harmonic
plus quartic trap is expected to have unusual vortex states that do not occur
in a pure harmonic trap. At a critical rotation speed , a central
hole appears in the condensate, and at some faster rotation speed ,
the system undergoes a transition to a giant vortex state with pure
irrotational flow. Using a time-dependent variational analysis, we study the
behavior of an annular condensate with a single concentric ring of vortices.
The transition to a giant vortex state is investigated by comparing the energy
of the two equilibrium states (the ring of vortices and the giant vortex) and
also by studying the dynamical stability of small excitation modes of the ring
of vortices.Comment: 12pages, 4figure
Path sampling for lifetimes of metastable magnetic skyrmions and direct comparison with Kramers' method
We perform a direct comparison between Kramers' method in many dimensions --
i.e., Langer's theory -- adapted to magnetic spin systems, and a path sampling
method in the form of forward flux sampling, as a means to compute collapse
rates of metastable magnetic skyrmions. We show that a good agreement is
obtained between the two methods. We report variations of the attempt frequency
associated with skyrmion collapse by three to four orders of magnitude when
varying the applied magnetic field by 5 of the exchange strength, which
confirms the existence of a strong entropic contribution to the lifetime of
skyrmions. This demonstrates that in complex systems, the knowledge of the rate
prefactor, in addition to the internal energy barrier, is essential in order to
properly estimate a lifetime.Comment: 5 pages, 5 figures (main text), 8 pages including supplemental
materia
Ferromagnetic bubble clusters in YCaMnO thin films
We studied the ferromagnetic topology in a YCaMnO thin
film with a combination of magnetic force microscopy and magnetization
measurements. Our results show that the spin-glass like behavior, reported
previously for this system, could be attributed to frustrated interfaces of the
ferromagnetic clusters embedded in a non-ferromagnetic matrix. We found
temperature dependent changes of the magnetic topology at low temperatures,
which suggests a non-static Mn/Mn ratio
Seismic Waveguide of Metamaterials
We have developed a new method of an earthquake-resistant design to support
conventional aseismic designs using acoustic metamaterials. We suggest a simple
and practical method to reduce the amplitude of a seismic wave exponentially.
Our device is an attenuator of a seismic wave. Constructing a cylindrical
shell-type waveguide that creates a stop-band for the seismic wave, we convert
the wave into an evanescent wave for some frequency range without touching the
building we want to protect.Comment: 4 pages, 4 figure
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Erythroderma with circulating atypical T-cells, likely Sézary syndrome
The erythrodermic patient is often challenging and requires careful evaluation. Work-up should include an extensive and careful medication history, histological and laboratory testing, and if necessary, molecular studies for the evaluation of underlying malignancy. Herein, we present an erythrodermic patient with repeated biopsies demonstrating a spongiotic process who was found to have circulating atypical T-cells concerning for an underlying erythrodermic T-cell leukemia, most closely related to Sézary syndrome
Development of oxide dispersion strengthened turbine blade alloy by mechanical alloying
There were three nickel-base alloys containing up to 18 wt. % of refractory metal examined initially for oxide dispersion strengthening. To provide greater processing freedom, however, a leaner alloy was finally selected. This base alloy, alloy D, contained 0.05C/15Cr / 2Mo/4W/2Ta/4.5Al/2.Ti/015Zr/0.01-B/Bal. Ni. Following alloy selection, the effect of extrusion, heat treatment, and oxide volume fraction and size on microstructure and properties were examined. The optimum structure was achieved in zone annealed alloy D which contained 2.5 vol. % of 35 mm Y2O3 and which was extruded 16:1 at 1038 C
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