665 research outputs found
Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles
Measurements of the magnetic irreversibility line and time-logarithmic decay
of the magnetization are described for three samples composed of
regular amorphous, acicular amorphous and crystalline nanoparticles. The
relaxation rate is the largest and the irreversibility temperature is the
lowest for the regular amorphous nanoparticles. The crystalline material
exhibits the lowest relaxation rate and the largest irreversibility
temperature. We develop a phenomenological model to explain the details of the
experimental results. The main new aspect of the model is the dependence of the
barrier for magnetic relaxation on the instantaneous magnetization and
therefore on time. The time dependent barrier yields a natural explanation to
the time-logarithmic decay of the magnetization. Interactions between particles
as well as shape and crystalline magnetic anisotropies define a new energy
scale that controls the magnetic irreversibility. Introducing this energy scale
yields a self-consistent explanation of the experimental data.Comment: RevTex, 16 JPEG figures, to appear in PR
Orbital upper critical field and its anisotropy of clean one- and two-band superconductors
The Helfand-Werthamer (HW) scheme\cite{HW} of evaluating the orbital upper
critical field is generalized to anisotropic superconductors in general, and to
two-band clean materials, in particular. Our formal procedure differs from
those in the literature; it reproduces not only the isotropic HW limit, but
also the results of calculations for the two-band superconducting
MgB\cite{MMK,DS} along with the existing data on and its
anisotropy ( are the principal
directions of a uniaxial crystal). Using rotational ellipsoids as model Fermi
surfaces we apply the formalism developed to study for a few
different anisotropies of the Fermi surface and of the order parameters. We
find that even for a single band d-wave order parameter decreases
on warming, however, relatively weakly. For order parameters of the form ,\cite{Xu} according to our simulations
may either increase or decrease on warming even for a single band
depending on the sign of . Hence, the common belief that the multi-band
Fermi surface is responsible for the temperature variation of is
proven incorrect
Interband coupling and transport interband scattering in superconductors
A two-band model with repulsive interband coupling and interband {\it
transport} (potential) scattering is considered to elucidate their effects on
material properties. In agreement with previous work, we find that the bands
order parameters differ and the large is at the band with a
smaller normal density of states (DOS), . However, the bands
energy gaps, as determined by the energy dependence of the DOS, are equal due
to scattering. For each temperature, the gaps turn zero at a certain critical
interband scattering rate, i.e. for strong enough scattering the model material
becomes gappless. In the gapless state, the DOS at the band 2 is close to the
normal state value, whereas at the band 1 it has a V-shape with non-zero
minimum. When the normal bands DOS' are mismatched, , the
critical temperature is suppressed even in the absence of interband
scattering, has a dome-like shape. With increasing interband
scattering, the London penetration depth at low temperatures evolves from being
exponentially flat to the power-law and even to near linear behavior in the
gapless state, the latter being easily misinterpreted as caused by order
parameter nodes
Effective Demagnetization Factors of Diamagnetic Samples of Various Shapes
Effective demagnetizing factors that connect the sample magnetic moment with
the applied magnetic field are calculated numerically for perfectly diamagnetic
samples of various non-ellipsoidal shapes. The procedure is based on
calculating total magnetic moment by integrating the magnetic induction
obtained from a full three dimensional solution of the Maxwell equations using
adaptive mesh. The results are relevant for superconductors (and conductors in
AC fields) when the London penetration depth (or the skin depth) is much
smaller than the sample size. Simple but reasonably accurate approximate
formulas are given for practical shapes including rectangular cuboids, finite
cylinders in axial and transverse field as well as infinite rectangular and
elliptical cross-section strips.Comment: updated version, corrected typos et
Orbital upper critical field of type-II superconductors with pair breaking
The orbital upper critical field is evaluated for isotropic
materials with arbitrary transport and pair-breaking scattering rates. It is
shown that unlike transport scattering which enhances , the pair
breaking suppresses the upper critical field and reduces the dimensionless
ratio from the Helfand-Werthamer
value of to 0.5 for a strong pair-breaking. is evaluated
for arbitrary transport and pair-breaking scattering.
A phenomenological model for the pair-breaking suppression by magnetic fields
is introduced. It shows qualitative features such as a positive curvature of
and the low temperature upturn usually associated with multi-band
superconductivity
Effect of equatorial line nodes on upper critical field and London penetration depth
The upper critical field and its anisotropy are calculated for order
parameters with line nodes at equators, , of the Fermi surface of
uniaxial superconductors. It is shown that characteristic features found in
Fe-based materials -- a nearly linear in a broad domain, a low
and increasing on warming anisotropy -- can
be caused by competing effects of the equatorial nodes and of the Fermi surface
anisotropy. For certain material parameters, may change sign on
warming in agreement with recorded behavior of FeTeS system. It is also shown
that the anisotropy of the penetration depth decreases on warming to reach at in
agreement with data available. For some materials may
change on warming from at low s to at
high s
Anisotropic criteria for the type of superconductivity
The classical criterion for classification of superconductors as type-I or
type-II based on the isotropic Ginzburg-Landau theory is generalized to
arbitrary temperatures for materials with anisotropic Fermi surfaces and order
parameters. We argue that the relevant quantity for this classification is the
ratio of the upper and thermodynamic critical fields, , rather than
the traditional ratio of the penetration depth and the coherence length,
. Even in the isotropic case, coincides with
only at the critical temperature and they differ as
decreases, the long known fact. Anisotropies of Fermi surfaces and order
parameters may amplify this difference and render false the criterion based on
the value of
Collective flux creep: beyond the logarithmic solution
Numerical studies of the flux creep in superconductors show that the
distribution of the magnetic field at any stage of the creep process can be
well described by the condition of spatial constancy of the activation energy
independently on the particular dependence of on the field B and
current . This results from a self-organization of the creep process in the
undercritical state related to a strong non-linearity of the flux
motion. Using the spatial constancy of , one can find the field profiles
, formulate a semi-analytical approach to the creep problem and
generalize the logarithmic solution for flux creep, obtained for , to
the case of essential dependence of on . This approach is useful for the
analysis of dynamic formation of an anomalous magnetization curve
(''fishtail''). We analyze the quality of the logarithmic and generalized
logarithmic approximations and show that the latter predicts a maximum in the
creep rate at short times, which has been observed experimentally. The vortex
annihilation lines (or the sample edge for the case of remanent state
relaxation), where B=0, cause instabilities (flux-flow regions) and modify or
even destroy the self-organization of flux creep in the whole sample.Comment: 12 pages, 9 PS figure
Response to Comment by A. Bussmann-Holder (arXiv:0909.3603)
Response to Comment by A. Bussmann-Holder (arXiv:0909.3603
London Penetration Depth and Pair Breaking
The London penetration depth is evaluated for isotropic materials for any
transport and pair-breaking Born scattering rates. Besides known results, a
number of new features are found. The slope of the
normalized superfluid density at the
transition has a minimum near the value of the pair-breaking
parameter separating gapped and gapless states. The low- exponentially flat
part of for the s-wave materials is suppressed by increasing pair
breaking. For strong suppression by magnetic impurities the "Homes
scaling" with being the normal
conductivity gives way to . For the
d-wave order parameter, the transport and spin-flip Born scattering rates enter
the theory only as a sum, in particular, they affect the depression in
the same manner. We confirm that the linear low temperature behavior of
in a broad range of the combined scattering parameter turns to the
behavior only when the critical temperature is suppressed at least by a factor
of 3 relative to the clean limit . Moreover, in this range, is only weakly dependent on the scattering parameter, i.e. it is
nearly universal
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