16,715 research outputs found
A Real Space Description of Magnetic Field Induced Melting in the Charge Ordered Manganites: I. The Clean Limit
We study the melting of charge order in the half doped manganites using a
model that incorporates double exchange, antiferromagnetic superexchange, and
Jahn-Teller coupling between electrons and phonons. We primarily use a real
space Monte Carlo technique to study the phase diagram in terms of applied
field and temperature , exploring the melting of charge order with
increasing and its recovery on decreasing . We observe hysteresis in
this response, and discover that the `field melted' high conductance state can
be spatially inhomogeneous even without extrinsic disorder. The hysteretic
response plays out in the background of field driven equilibrium phase
separation. Our results, exploring , , and the electronic parameter
space, are backed up by analysis of simpler limiting cases and a Landau
framework for the field response. This paper focuses on our results in the
`clean' systems, a companion paper studies the effect of cation disorder on the
melting phenomena.Comment: 16 pages, pdflatex, 11 png fig
On the design of optimal compliant walls for turbulence control
This paper employs the theoretical framework developed by Luhar et al. (J.
Fluid Mech., 768, 415-441) to consider the design of compliant walls for
turbulent skin friction reduction. Specifically, the effects of simple
spring-damper walls are contrasted with the effects of more complex walls
incorporating tension, stiffness and anisotropy. In addition, varying mass
ratios are tested to provide insight into differences between aerodynamic and
hydrodynamic applications. Despite the differing physical responses, all the
walls tested exhibit some important common features. First, the effect of the
walls (positive or negative) is greatest at conditions close to resonance, with
sharp transitions in performance across the resonant frequency or phase speed.
Second, compliant walls are predicted to have a more pronounced effect on
slower-moving structures because such structures generally have larger
wall-pressure signatures. Third, two-dimensional (spanwise constant) structures
are particularly susceptible to further amplification. These features are
consistent with many previous experiments and simulations, suggesting that
mitigating the rise of such two-dimensional structures is essential to
designing performance-improving walls. For instance, it is shown that further
amplification of such large-scale two-dimensional structures explains why the
optimal anisotropic walls identified by Fukagata et al. via DNS (J. Turb., 9,
1-17) only led to drag reduction in very small domains. The above observations
are used to develop design and methodology guidelines for future research on
compliant walls
A transient PEMFC model with CO poisoning and mitigation by O2 bleeding and Ru-containing catalyst
In this paper we present a transient, fully two-phase, non-isothermal model of carbon monoxide poisoning and oxygen bleeding in the membraneelectrode assembly of a polymer electrolyte fuel cell. The model includes a detailed description of mass, heat and charge transport, chemisorption,electrochemical oxidation and heterogeneous catalysis (when oxygen is introduced). Example simulation results demonstrate the ability of themodel to qualitatively capture the fundamental features of the poisoning process and the extent of poisoning with respect to channel temperatureand concentration. Further examples show how the multi-step kinetics can interact with other physical phenomena such as liquid-water flooding,particularly in the anode. Carbon monoxide pulsing is simulated to demonstrate that the complicated reaction kinetics of oxygen bleeding canbe captured and even predicted. It is shown that variations in the channel temperature have a convoluted effect on bleeding, and that trends inperformance on relatively short time scales can be the precise opposite of the trends observed at steady state. We incorporate a bi-functionalmechanism for carbon monoxide oxidation on platinum–ruthenium catalysts, demonstrating the marked reduction in the extent of poisoning, theeffect of variations in the platinum–ruthenium ratio and the influence of temperature. Finally, we discuss the implications of the results, extensionsto the model and possible avenues for experimental work
CeRuSn: heavy fermions emerging from a Kondo-insulating state
The combination of low-temperature specific-heat and
nuclear-magnetic-resonance (NMR) measurements reveals important information of
the ground-state properties of CeRuSn, which has been proposed as a
rare example of a tetragonal Kondo-insulator (KI). The NMR
spin-latticerelaxation rate deviates from the Korringa law below 100 K
signaling the onset of an energy gap K. This gap is
stable against magnetic fields up to 10 T. Below 10 K, however, unusual
low-energy excitations of in-gap states are observed, which depend strongly on
the field H. The specific heat C detects these excitations in the form of an
enhanced Sommerfeld coefficient : In zero field,
increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at
J/molK. This maximum is shifted to higher temperatures with
increasing field suggesting a residual density of states at the Fermi level
developing a spin gap . A simple model, based on two narrow
quasiparticle bands located at the Fermi level - which cross the Fermi level in
zero field at 0.022 states/meV f.u. - can account qualitatively as well as
quantitatively for the measured observables. In particular, it is demonstrated
that fitting our data of both specific heat and NMR to the model, incorporating
a Ce magnetic moment of , leads to
the prediction of the field dependence of the gap. Our measurements rule out
the presence of a quantum critical point as the origin for the enhanced
in CeRuSn and suggest that this arises rather from correlated,
residual in-gap states at the Fermi level. This work provides a fundamental
route for future investigations into the phenomenon of narrow-gap formation in
the strongly correlated class of systemComment: 11 pages, 13 figure
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