72 research outputs found
Fluctuation Study of the Specific Heat of MgB2
The specific heat of polycrystalline MgB has been measured with
high resolution ac calorimetry from 5 to 45 K at constant magnetic fields. The
excess specific heat above T is discussed in terms of Gaussian
fluctuations and suggests that MgB is a bulk superconductor with
Ginzburg-Landau coherence length \AA . The transition-width
broadening in field is treated in terms of lowest-Landau-level (LLL)
fluctuations. That analysis requires that \AA . The underestimate
of the coherence length in field, along with deviations from 3D LLL
predictions, suggest that there is an influence from the anisotropy of B
between the c-axis and the a-b plane.Comment: Phys. Rev. B 66, 134515 (2002
First-Principles Dynamical Coherent-Potential Approximation Approach to the Ferromagnetism of Fe, Co, and Ni
Magnetic properties of Fe, Co, and Ni at finite temperatures have been
investigated on the basis of the first-principles dynamical CPA (Coherent
Potential Approximation) combined with the LDA (Local Density Approximation) +
Hamiltonian in the Tight-Binding Linear Muffintin Orbital (TB-LMTO)
representation. The Hamiltonian includes the transverse spin fluctuation terms.
Numerical calculations have been performed within the harmonic approximation
with 4th-order dynamical corrections. Calculated single-particle densities of
states in the ferromagnetic state indicate that the dynamical effects reduce
the exchange splitting, suppress the band width of the quasi-particle state,
and causes incoherent excitations corresponding the 6 eV satellites. Results of
the magnetization vs temperature curves, paramagnetic spin susceptibilities,
and the amplitudes of local moments are presented. Calculated Curie
temperatures () are reported to be 1930K for Fe, 2550K for Co, and
620K for Ni; for Fe and Co are overestimated by a factor of 1.8,
while in Ni agrees with the experimental result. Effective Bohr
magneton numbers calculated from the inverse susceptibilities are 3.0 (Fe), 3.0 (Co), and 1.6 (Ni), being in
agreement with the experimental ones. Overestimate of in Fe and Co
is attributed to the neglects of the higher-order dynamical effects as well as
the magnetic short range order.Comment: 10 pages, 13 figure
Nonreciprocal amplitude-frequency resonant response of metasandwiches “ferrite plate-grating of resonant elements”
New microwave nonreciprocal properties are investigated in “ferrite plate
– grating of resonant elements” metasandwiches arranged along the axis of
a rectangular waveguide in a transverse constant magnetic field. Giant
nonreciprocity in the transmission is observed at the ferromagnetic
resonance frequencies at certain values of the magnetic field under
conditions of a mutual influence between the ferromagnetic and the grating
resonances. In addition, nonreciprocal splitting of the resonance in grating
elements is observed under small magnetic field, which is much less than the
field necessary to the ferromagnetic resonance excitation.
The nonreciprocal transmission does not take place in the case of free
ferrite in the absence of a grating. Sign reversal of the nonreciprocity is
observed, when ferrite transfers to the opposite side of a grating as well
as under certain values of the constant magnetic field, when the sign
reversal of difference between frequencies of the ferromagnetic resonance
and the grating resonance takes place. Nonreciprocal effects are explained
by the interaction between precessing spins in ferrite and a magnetic field
of the surface wave, formed by a grating, and by coupling between the
resonances of grating elements. It has been shown theoretically that
microwaves in waveguide with bianisotropic layer, simulating a grating of
resonant elements, are elliptically or circularly polarized with frequency
and spatially – dependent rotating sense of the microwave magnetic field.
The nonreciprocal effects have been observed for different grating elements:
for both electric dipoles and chiral elements
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