1,060 research outputs found
On the Intrinsic Simplicity of Spectral Variability of GRBs
In this paper we present a Multi-Scale Correlation Analysis (MSCA) of the light curves of gamma-ray bursts recorded in different energy ranges. This analysis allows us to identify time intervals where emission variability can be reduced to a single physical parameter and can therefore be robustly attributed to a single physical emitter. The properties of these intervals can then be investigated separately, and the spectral properties of individual emitters can be analysed. The signatures of hidden dynamical relations between individual emitters are also discussed
Low-frequency Elastic and Thermomechanical Analysis of Ni-Mn-In(Co) Single Crystals☆
Martensitic transformation (MT) in Ni 45.0 Mn 36.7 In 13.3 Co 5.0 single crystals (SC) has been characterized by DSC and X-ray diffraction. Their elastic and thermomechanical properties have been investigated by a low-frequency dynamic-mechanical analysis in a tensile mode and by static mechanical compression made at different temperatures. The Young's modulus of the order of 10 GPa was measured in tensile tests along crystallographic axis of austenite showing soft behavior in a broad temperature range whereby revealing a lattice instability similar to the classical Ni-Mn-Ga alloys. The compression tests along , and directions have shown that despite a high brittleness the samples exhibit large martensitic plasticity, rubber-like behavior and superelasticity
Strongly nonlinear dynamics of electrolytes in large ac voltages
We study the response of a model micro-electrochemical cell to a large ac
voltage of frequency comparable to the inverse cell relaxation time. To bring
out the basic physics, we consider the simplest possible model of a symmetric
binary electrolyte confined between parallel-plate blocking electrodes,
ignoring any transverse instability or fluid flow. We analyze the resulting
one-dimensional problem by matched asymptotic expansions in the limit of thin
double layers and extend previous work into the strongly nonlinear regime,
which is characterized by two novel features - significant salt depletion in
the electrolyte near the electrodes and, at very large voltage, the breakdown
of the quasi-equilibrium structure of the double layers. The former leads to
the prediction of "ac capacitive desalination", since there is a time-averaged
transfer of salt from the bulk to the double layers, via oscillating diffusion
layers. The latter is associated with transient diffusion limitation, which
drives the formation and collapse of space-charge layers, even in the absence
of any net Faradaic current through the cell. We also predict that steric
effects of finite ion sizes (going beyond dilute solution theory) act to
suppress the strongly nonlinear regime in the limit of concentrated
electrolytes, ionic liquids and molten salts. Beyond the model problem, our
reduced equations for thin double layers, based on uniformly valid matched
asymptotic expansions, provide a useful mathematical framework to describe
additional nonlinear responses to large ac voltages, such as Faradaic
reactions, electro-osmotic instabilities, and induced-charge electrokinetic
phenomena.Comment: 30 pages, 17 eps-figures, RevTe
Influence of intermartensitic transitions on transport properties of Ni2.16Mn0.84Ga alloy
Magnetic, transport, and x-ray diffraction measurements of ferromagnetic
shape memory alloy NiMnGa revealed that this alloy undergoes
an intermartensitic transition upon cooling, whereas no such a transition is
observed upon subsequent heating. The difference in the modulation of the
martensite forming upon cooling from the high-temperature austenitic state
[5-layered (5M) martensite], and the martensite forming upon the
intermartensitic transition [7-layered (7M) martensite] strongly affects the
magnetic and transport properties of the alloy and results in a large thermal
hysteresis of the resistivity and magnetization . The
intermartensitic transition has an especially marked influence on the transport
properties, as is evident from a large difference in the resistivity of the 5M
and 7M martensite, , which is larger than the jump of resistivity at
the martensitic transition from the cubic austenitic phase to the monoclinic 5M
martensitic phase. We assume that this significant difference in between
the martensitic phases is accounted for by nesting features of the Fermi
surface. It is also suggested that the nesting hypothesis can explain the
uncommon behavior of the resistivity at the martensitic transition, observed in
stoichiometric and near-stoichiometric Ni-Mn-Ga alloys.Comment: 7 pages, 6 figures, REVTEX
Magnetic field dependence of galfenol elastic properties
Elastic shear moduli measurements on Fe100−xGax (x = 12–33) single crystals (via resonant ultrasound spectroscopy) with and without a magnetic field and within 4–300 K are reported. The pronounced softening of the tetragonal shear modulus c′ is concluded to be, based on magnetoelastic coupling, the cause of the second peak in the tetragonal magnetostriction constant λ100 near x = 28. Exceedingly high ΔE effects ( ∼ 25%), combined with the extreme softness in c′ (c′\u3c10 GPa), suggest structural changes take place, yet, gradual in nature, as the moduli show a smooth dependence on Ga concentration, temperature, and magnetic field. Shear anisotropy (c44/c′) as high as 14.7 was observed for Fe71.2Ga28.8
Magnetoelastic nature of ferromagnetic shape memory effect
Abstract. The giant magnetically-induced deformation of ferromagnetic shape memory alloys results from the magnetic field-induced rearrangement of twinned martensite under the magnetic field. This deformation is conventionally referred to as the magnetic-field-induced-strain (MFIS). The MFIS is comparable in value with the spontaneous deformation of crystal lattice during the martensitic transformation of an alloy. Although the first observations of MFIS were reported more than 30 years ago, it has got a world-wide interest 20 years later after the creation of the Ni-Mn-Ga alloy system with its practically important room-temperature martensitic structure and experimental evidence of the large magnetostriction. The underlying physics as well as necessary and sufficient conditions for the observation of MFIS are the main focus of this chapter. A magnetostrictive mechanism of the unusual magnetic and magnetomechanical effects observed in Ni-Mn-Ga alloys is substantiated and a framework of consistent theory of these effects is outlined starting from the fundamental conception of magnetoelasticity and the commonly known principles of ferromagnetism and linear elasticity theories. A reasonable agreement between the theoretical deductions and available experimental data is demonstrated and, in this way, a key role of magnetoelastic coupling in the magnetomechanical behavior of Ni-Mn-Ga alloys is proved. A correspondence of magnetostrictive mechanism to the crystallographic features of MFIS and the basic relationships of the thermodynamics of solids are discussed
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