693 research outputs found
Spin Dynamics near the Superconductor-to-Insulator Transition in Impurity-Doped YBa2Cu4O8
We studied low-frequency spin dynamics near the impurity-induced
superconductor-to-insulator transition for underdoped high-Tc superconductor
YBa2(Cu1-xMx)4O8 (M=Ni, Zn) using the Cu nuclear quadrupole resonance (NQR)
spin-echo technique. We observed remarkable suppression of the normal-state
pseudo spin-gap and a loss of Cu NQR spectrum intensity at low temperatures
around the critical impurity concentration.Comment: 6 pages, 4 figures. To be published in J. Phys. Soc. Jpn. Vol.70,
No.7 (2001
Functional Integration Approach to Hysteresis
A general formulation of scalar hysteresis is proposed. This formulation is
based on two steps. First, a generating function g(x) is associated with an
individual system, and a hysteresis evolution operator is defined by an
appropriate envelope construction applied to g(x), inspired by the overdamped
dynamics of systems evolving in multistable free energy landscapes. Second, the
average hysteresis response of an ensemble of such systems is expressed as a
functional integral over the space G of all admissible generating functions,
under the assumption that an appropriate measure m has been introduced in G.
The consequences of the formulation are analyzed in detail in the case where
the measure m is generated by a continuous, Markovian stochastic process. The
calculation of the hysteresis properties of the ensemble is reduced to the
solution of the level-crossing problem for the stochastic process. In
particular, it is shown that, when the process is translationally invariant
(homogeneous), the ensuing hysteresis properties can be exactly described by
the Preisach model of hysteresis, and the associated Preisach distribution is
expressed in closed analytic form in terms of the drift and diffusion
parameters of the Markovian process. Possible applications of the formulation
are suggested, concerning the interpretation of magnetic hysteresis due to
domain wall motion in quenched-in disorder, and the interpretation of critical
state models of superconducting hysteresis.Comment: 36 pages, 9 figures, to be published on Phys. Rev.
Cu Nuclear Quadrupole Resonance Study of Site-Disorder and Chemical Pressure Effects on Y(Ba1-xSrx)2Cu4O8
We report a zero-field Cu nuclear quadrupole resonance (NQR) study on the
effects of nonmagnetic Sr substitution for high-Tc superconductors,
Y(Ba1-xSrx)2Cu4O8 (Tc=82-80 K for x=0-0.4), using a spin-echo technique. The
site-disordering and chemical pressure effects associated with doping Sr were
observed in the broadened, shifted Cu NQR spectra. Nevertheless, the site
disorder did not significantly affect the homogeneity of Cu electron spin
dynamics, in contrast to the in-plane impurity. The peak shift of Cu NQR
spectrum due to Sr was different between the chain- and the plane-Cu sites,
more remarkably than those under a hydrostatic physical pressure, suggesting
anisotropic or nonuniform local structural strains. The small decrease of Tc
due to Sr can be traced back to either a cancellation effect on Tc between the
disorder and the pressure, or an anisotropic or nonuniform chemical pressure
effect on Tc.Comment: 4 pages, 5 figure
Vortex State of TlBaCuO via Tl NMR at 2 Tesla
We report a Tl NMR study of vortex state for an aligned
polycrystalline sample of an overdoped high- superconductor
TlBaCuO (85 K) with magnetic field 2 T along
the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at
=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and
vortex melting between 65 and 85 K. No evidence for local antiferromagnetic
ordering at vortex cores was found for our sample.Comment: 4 pages with 5 figure
Critical currents, flux-creep activation energy and potential barriers for the vortex motion from the flux creep experiments
We present an experimental study of thermally activated flux creep in a
superconducting ring-shaped epitaxial YBCO film as well as a new way of
analyzing the experimental data. The measurements were made in a wide range of
temperatures between 10 and 83 K. The upper temperature limit was dictated by
our experimental technique and at low temperatures we were limited by a
crossover to quantum tunneling of vortices. It is shown that the experimental
data can very well be described by assuming a simple thermally activated
hopping of vortices or vortex bundles over potential barriers, whereby the
hopping flux objects remain the same for all currents and temperatures. The new
procedure of data analysis also allows to establish the current and temperature
dependencies of the flux-creep activation energy U, as well as the temperature
dependence of the critical current Ic, from the flux-creep rates measured at
different temperatures. The variation of the activation energy with current,
U(I/Ic), is then used to reconstruct the profile of the potential barriers in
real space.Comment: 12 pages, 13 Postscript figures, Submitted to Physical Review
Flux Creep and Flux Jumping
We consider the flux jump instability of the Bean's critical state arising in
the flux creep regime in type-II superconductors. We find the flux jump field,
, that determines the superconducting state stability criterion. We
calculate the dependence of on the external magnetic field ramp rate,
. We demonstrate that under the conditions typical for most of the
magnetization experiments the slope of the current-voltage curve in the flux
creep regime determines the stability of the Bean's critical state, {\it i.e.},
the value of . We show that a flux jump can be preceded by the
magneto-thermal oscillations and find the frequency of these oscillations as a
function of .Comment: 7 pages, ReVTeX, 2 figures attached as postscript file
Magnetic anisotropy and crystalline texture in BaO(Fe2O3)(6) thin films deposited on GaN/Al2O3
BaO(Fe2O3)6 (BaM) thin films were deposited by pulsed laser deposition on GaN∕Al2O3 substrates. A pole figure obtained from the (006) reflection indicated that ∼81% of the film volume had the c axis tilted less than 5° from the film normal. A low anisotropy field was inferred from vector coil vibrating sample magnetometer (VVSM) measurements. The reduction in Hafrom literature values and a two-step switching of the easy axis magnetization is postulated to result from interdiffusion and misalignment effects. To alleviate interdiffusion and to improve the c-axis alignment, experiments were repeated with lower deposition temperatures, thinner films, and MgO buffer layers. The features of the hysteresis loop due to two-step switching and the in-plane coercivity were reduced while the anisotropy field (Ha) was larger. Films deposited with MgO buffer layers are observed to have single-step switching of the easy axis magnetization, larger anisotropy fields, and sharp ferromagnetic resonance (FMR) peaks. Films with MgO buffer layers were determined to have anisotropy fields Ha=1.57T by FMR and Ha∼1.5–1.6Tas determined from the difference in the saturation fields for the easy and hard axis loops
Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Ni-substituted sites and the effect on Cu electron spin dynamics of YBa2Cu{3-x}NixO{7-\delta}
We report Cu nuclear quadrupole resonance experiment on magnetic impurity
Ni-substituted YBaCuNiO. The distribution of
Ni-substituted sites and its effect on the Cu electron spin dynamics are
investigated. Two samples with the same Ni concentration =0.10 and nearly
the same oxygen content but different 's were prepared: One is an
as-synthesized sample (7-=6.93) in air (), and the
other is a quenched one (7-=6.92) in a reduced oxygen atmosphere
(). The plane-site Cu(2) nuclear spin-lattice
relaxation for the quenched sample was faster than that for the as-synthesized
sample, in contrast to the Cu(1) relaxation that was faster for the
as-synthesized sample. This indicates that the density of plane-site Ni(2) is
higher in the quenched sample, contrary to the chain-site Ni(1) density which
is lower in the quenched sample. From the analysis in terms of the Ni-induced
nuclear spin-lattice relaxation, we suggest that the primary origin of
suppression of is associated with nonmagnetic depairing effect of the
plane-site Ni(2).Comment: 7 pages, 5 figure
Research on the Geography of Agricultural Change: Redundant or Revitalized?
Future research directions for agricultural geography were the subject of debate in Area in the late 1980s. The subsequent application of political economy ideas undoubtedly revived interest in agricultural research. This paper argues that agricultural geography contains greater diversity than the dominant political economy discourse would suggest. It reviews ‘other’ areas of agricultural research on policy, post-productivism, people, culture and animals, presenting future suggestions for research. They should ensure that agricultural research continues revitalized rather than redundant into the next millennium
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