39,873 research outputs found
Large magnetocrystalline anisotropy in tetragonally distorted Heuslers: a systematic study
With a view to the design of hard magnets without rare earths we explore the
possibility of large magnetocrystalline anisotropy energies in Heusler
compounds that are unstable with respect to a tetragonal distortion. We
consider the Heusler compounds FeYZ with Y = (Ni, Co, Pt), and CoYZ
with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find
that for the CoNiZ, CoPtZ, and FePtZ families the cubic phase is
always, at , unstable with respect to a tetragonal distortion, while, in
contrast, for the FeNiZ and FeCoZ families this is the case for only 2
compounds -- FeCoGe and FeCoSn. For all compounds in which a tetragonal
distortion occurs we calculate the MAE finding remarkably large values for the
Pt containing Heuslers, but also large values for a number of the other
compounds (e.g. CoNiGa has an MAE of -2.11~MJ/m). The tendency to a
tetragonal distortion we find to be strongly correlated with a high density of
states at the Fermi level in the cubic phase. As a corollary to this fact we
observe that upon doping compounds for which the cubic structure is stable such
that the Fermi level enters a region of high DOS, a tetragonal distortion is
induced and a correspondingly large value of the MAE is then observed.Comment: 8 pages, 5 figure
Thermoelectric Amplification of Phonons in Graphene
Amplification of acoustic phonons due to an external temperature gredient
() in Graphene was studied theoretically. The threshold temperature
gradient at which absorption switches over to amplification
in Graphene was evaluated at various frequencies and temperatures
. For and frequency , . The calculation was done in the regime at . The
dependence of the normalized () on the frequency
and the temperature gradient are evaluated numerically and
presented graphically. The calculated for Graphene is lower
than that obtained for homogeneous semiconductors () , Superlattices , Cylindrical Quantum Wire . This makes Graphene a much better material for thermoelectric
phonon amplifier.Comment: 12 Pages, 6 figure
High-soft to low-hard state transition in black hole X-ray binaries with GRMHD simulations
To understand the decaying phase of outbursts in the black hole (BH) X-ray
binaries (BH-XRBs), we performed very long general relativistic
magneto-hydrodynamic (GRMHD) simulations of a geometrically thin accretion disk
around a Kerr BH with slowly rotating matter injected from outside. We
thoroughly studied the flow properties, dynamical behavior of the accretion
rate, magnetic flux rate, and jet properties during the temporal evolution. Due
to the interaction between the thin disk and injected matter, the accretion
flow near the BH goes through different phases. The sequence of phases is: soft
state soft-intermediate state hard-intermediate
state hard state quiescent state. For the accretion
rate (and hence the luminosity) to decrease (as observed) in our model, the
mass injection should not decay slower than angular momentum injection. We also
observed quasi-periodic oscillations (QPOs) in the accretion flow. Throughout
the evolution, we observed low-frequency QPOs (~10Hz) and high-frequency QPOs
(\sim 200Hz). Our simple unified accretion flow model for state transitions is
able to describe outbursts in BH-XRBs.Comment: 13 pages, 11 figures, Accepted for publication on Ap
Nuclear Breathing Mode in the Relativistic Mean Field Theory
The breathing-mode giant monopole resonance is studied within the framework
of the relativistic mean-field (RMF) theory. Using a broad range of parameter
sets, an analysis of constrained incompressibility and excitation energy of
isoscalar monopole states in finite nuclei is performed. It is shown that the
non-linear scalar self-interaction and the resulting surface properties
influence the breathing-mode considerably. It is observed that dynamical
surface properties respond differently in the RMF theory than in the Skyrme
approach. A comparison is made with the incompressibility derived from the
semi-infinite nuclear matter and with constrained nonrelativistic Skyrme
Hartree-Fock calculaions.Comment: Latex (12 pages) and 3 figures (available upon request) J. Phys. G
(in press
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