1,020 research outputs found
Two Component Heat Diffusion Observed in CMR Manganites
We investigate the low-temperature electron, lattice, and spin dynamics of
LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance
spectroscopy. Probing the high-spin d-d transition as a function of time delay
and probe energy, we compare the responses of the Mott insulator and the
double-exchange metal to the photoexcitation. Attempts have previously been
made to describe the sub-picosecond dynamics of CMR manganites in terms of a
phenomenological three temperature model describing the energy transfer between
the electron, lattice and spin subsystems followed by a comparatively slow
exponential decay back to the ground state. However, conflicting results have
been reported. Here we first show clear evidence of an additional component in
the long term relaxation due to film-to-substrate heat diffusion and then
develop a modified three temperature model that gives a consistent account for
this feature. We confirm our interpretation by using it to deduce the bandgap
in LMO. In addition we also model the non-thermal sub-picosecond dynamics,
giving a full account of all observed transient features both in the insulating
LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434
v2: Abstract correcte
Strain-induced insulator state in La_0.7Sr_0.3CoO_3
We report on the observation of a strain-induced insulator state in
ferromagnetic La_0.7Sr_0.3CoO_3 films. Tensile strain above 1% is found to
enhance the resistivity by several orders of magnitude. Reversible strain of
0.15% applied using a piezoelectric substrate triggers huge resistance
modulations, including a change by a factor of 10 in the paramagnetic regime at
300 K. However, below the ferromagnetic ordering temperature, the magnetization
data indicate weak dependence on strain for the spin state of the Co ions. We
interpret the changes observed in the transport properties in terms of a
strain-induced splitting of the Co e_g levels and reduced double exchange,
combined with a percolation-type conduction in an electronic cluster state
Rheology of human blood plasma: Viscoelastic versus Newtonian behavior
We investigate the rheological characteristics of human blood plasma in shear
and elongational flows. While we can confirm a Newtonian behavior in shear flow
within experimental resolution, we find a viscoelastic behavior of blood plasma
in the pure extensional flow of a capillary break-up rheometer. The influence
of the viscoelasticity of blood plasma on capillary blood flow is tested in a
microfluidic device with a contraction-expansion geometry. Differential
pressure measurements revealed that the plasma has a pronounced flow resistance
compared to that of pure water. Supplementary measurements indicate that the
viscoelasticity of the plasma might even lead to viscoelastic instabilities
under certain conditions. Our findings show that the viscoelastic properties of
plasma should not be ignored in future studies on blood flow.Comment: 4 figures, 1 supplementary material Highlighted in
http://physics.aps.org/articles/v6/1
Reversible strain effect on the magnetization of LaCoO3 films
The magnetization of ferromagnetic LaCoO3 films grown epitaxially on
piezoelectric substrates has been found to systematically decrease with the
reduction of tensile strain. The magnetization change induced by the reversible
strain variation reveals an increase of the Co magnetic moment with tensile
strain. The biaxial strain dependence of the Curie temperature is estimated to
be below 4K/% in the as-grown tensile strain state of our films. This is in
agreement with results from statically strained films on various substrates
Experimental and numerical investigation of the shear behaviour of infiltrated woven fabrics
Wet compression moulding (WCM) as a promising alternative to resin transfer moulding (RTM) provides high-volume production potential for continuously fibre reinforced composite components. Lower cycle times are possible due to the parallelisation of the process steps draping, infiltration and curing during moulding. Although experimental and theoretical investigations indicate a strong mutual dependency arising from this parallelisation, no material characterisation set-ups for textiles infiltrated with low viscous fluids are yet available, which limits a physical-based process understanding and prevents the development of proper simulation tools. Therefore, a modified bias-extension test set-up is presented, which enables infiltrated shear characterisation of engineering textiles. Experimental studies on an infiltrated woven fabric reveal both, rate- and viscosity-dependent shear behaviour. The process relevance is evaluated on part level within a numerical study by means of FE-forming simulation. Results reveal a significant impact on the global and local shear angle distribution, especially during forming
Capabilities of macroscopic forming simulation for large-scale forming processes of dry and impregnated textiles
Forming of continuously fibre-reinforced polymers (CoFRP) has a significant impact on the structural performance of composite components, underlining the importance of forming simulation for CoFRP product development processes. For an integrated development of industrial composite components, efficient forming simulation methods are in high demand. Application-oriented method development is particularly crucial for industrial needs, where large and complex multi-layer components are manufactured, commercial FE software is used, and yet high prediction accuracy is required. To meet industrial demands, this contribution gives an insight in macroscopic forming simulation approaches that utilize the FE software Abaqus in combination with user-defined material models and finite elements. Three CoFRP forming technologies are considered, which are in industrial focus due to their suitability for mass production: textile forming of dry unidirectional non-crimp fabrics (UD-NCF), thermoforming of pre-impregnated UD tapes and wet compression moulding (WCM). In addition to the highly anisotropic, large-strain material behaviour that composite forming processes have in common, the three process technologies face various process-specific modelling challenges. UD-NCFs require material models that capture the deformation behaviour and the slippage of the stitching. Thermoforming of UD tapes is highly rate- and temperature-dependent, calling for rheological membrane and bending modelling. Moreover, a thermomechanical approach including crystallisation kinetics enables the prediction of potential phase-transition during forming and resulting defects in the semi-crystalline thermoplastic matrix. For simultaneous forming and infiltration in wet compression moulding, a finite Darcy-Progression-Element is superimposed with the membrane and shell elements for forming simulation, capturing infiltration-dependent material properties. The three outlined technologies illustrate the complexity and importance of further simulation method development to support future process development
Influence of strain on magnetization and magnetoelectric effect in La0.7A0.3MnO3 / PMN-PT(001) (A = Sr; Ca)
We investigate the influence of a well-defined reversible biaxial strain
<=0.12 % on the magnetization (M) of epitaxial ferromagnetic manganite films. M
has been recorded depending on temperature, strain and magnetic field in 20 -
50 nm thick films. This is accomplished by reversibly compressing the isotropic
in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT
(001) substrates by application of an electric field E <= 12 kV cm-1. The
magnitude of the total variable in-plane strain has been derived.
Strain-induced shifts of the ferromagnetic Curie temperature (Tc) of up to 19 K
were found in La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 films and are
quantitatively analysed for LSMO within a cubic model. The observed large
magnetoelectric coupling coefficient alpha=mu0 dM/dE <= 6 10-8 s m-1 at ambient
temperature results from the strain-induced M change in the
magnetic-film-ferroelectric-substrate system. It corresponds to an enhancement
of mu0 DeltaM <= 19 mT upon biaxial compression of 0.1 %. The extraordinary
large alpha originates from the combination of three crucial properties: (i)
the strong strain dependence of M in the ferromagnetic manganites, (ii) large
piezo-strain of the PMN-PT substrates and (iii) effective elastic coupling at
the film-substrate interface.Comment: 15 pages, 6 figures, 1 tabl
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