27 research outputs found
Nucleation of Stable Superconductivity in YBCO-Films
By means of the linear dynamic conductivity, inductively measured on
epitaxial films between 30mHz and 30 MHz, the transition line to
generic superconductivity is studied in fields between B=0 and 19T. It follows
closely the melting line described recently in terms of a blowout of
thermal vortex loops in clean materials. The critical exponents of the
correlation length and time near , however, seem to be dominated by
some intrinsic disorder. Columnar defects produced by heavy-ion irradiation up
to field-equivalent-doses of lead to a disappointing reduction
of while for the generic line of the pristine film
is recovered. These novel results are also discussed in terms of a loop-driven
destruction of generic superconductivity.Comment: 11 pages including 7 EPS figures, accepted for publication in the
Proceedings of the Spring Meeting of the German Physical Society, Muenster
1999,Festkoerperprobleme/Advances in Solid State Physics 199
Vortex Flow and Transverse Flux Screening at the Bose Glass Transition
We investigate the vortex phase diagram in untwinned YBaCuO single crystals
with columnar defects. These randomly distributed defects, produced by heavy
ion irradiation, are expected to induce a ``Bose Glass'' phase of localized
vortices characterized by a vanishing resistance and a Meissner effect for
magnetic fields transverse to the defect axis. We directly observe the
transverse Meissner effect using an array of Hall probe magnetometers. As
predicted, the Meissner state breaks down at temperatures Ts that decrease
linearly with increasing transverse magnetic field. However, Ts falls well
below the conventional melting temperature Tm determined by a vanishing
resistivity, suggesting an intermediate regime where flux lines are effectively
localized even when rotated off the columnar defects.Comment: 15 pages, 5 figure
Universality of Frequency and Field Scaling of the Conductivity Measured by Ac-Susceptibility of a Ybco-Film
Utilizing a novel and exact inversion scheme, we determine the complex linear
conductivity from the linear magnetic ac-susceptibility
which has been measured from 3\,mHz to 50\,MHz in fields between 0.4\,T and
4\,T applied parallel to the c-axis of a 250\,nm thin disk. The frequency
derivative of the phase and the dynamical scaling of
above and below provide clear evidence for a
continuous phase transition at to a generic superconducting state. Based
on the vortex-glass scaling model, the resulting critical exponents and
are close to those frequently obtained on films by other means and
associated with an 'isotropic' vortex glass. The field effect on
can be related to the increase of the glass coherence length,
.Comment: 8 pages (5 figures upon request), revtex 3.0, APK.94.01.0
Vortex dynamics and states of artificially layered superconducting films with correlated defects
Linear resistances and -characteristics have been measured over a wide
range in the parameter space of the mixed phase of multilayered a-TaGe/Ge
films. Three films with varying interlayer coupling and correlated defects
oriented at an angle from the film normal were investigated.
Experimental data were analyzed within vortex glass models and a second order
phase transition from a resistive vortex liquid to a pinned glass phase.
Various vortex phases including changes from three to two dimensional behavior
depending on anisotropy have been identified. Careful analysis of
-characteristics in the glass phases revealed a distinctive and
-dependence of the glass exponent . The vortex dynamics in the
Bose-glass phase does not follow the predicted behavior for excitations of
vortex kinks or loops.Comment: 16 pages, 10 figures, 3 table
Is there a vortex-glass transition in high-temperature superconductors?
We show that DC voltage versus current measurements of a YBCO micro-bridge in
a magnetic field can be collapsed onto scaling functions proposed by Fisher,
Fisher, and Huse, as is widely reported in the literature. We find, however,
that good data collapse is achieved for a wide range of critical exponents and
temperatures. These results strongly suggest that agreement with scaling alone
does not prove the existence of a phase transition. We propose a criterion to
determine if the data collapse is valid, and thus if a phase transition occurs.
To our knowledge, none of the data reported in the literature meet our
criterion.Comment: 4 pages, 4 figure
Contemporary midwifery practice: Art, science or both?
Current midwifery practice is regulated by the Nursing and Midwifery Council (NMC), whose primary role is to safeguard the public through setting standards for education and practice and regulating fitness to practise, conduct and performance through rules and codes (NMC, 2012; 2015a). Practice is informed by evidence-based guidelines developed and implemented by the National Institute for Health and Care Excellence based on hierarchies of evidence, with meta-analyses and systematic reviews being identified as the ‘gold standard’. This positivist epistemological approach as developed by Auguste Comte (1798–1857), with scientific evidence at the top of a knowledge hierarchy, fails to acknowledge the ‘art of midwifery’, where a constructivist paradigm of experiential, intuitive and tacit knowledge is used by reflective practitioners to provide high-quality care. As midwifery pre-registration education is now degree-level, is the essence of midwifery practice being ‘with woman’ providing holistic care under threat, as the drive for a systematic and analytical approach to decision-making gathers momentum
Genetic Control of Organ Shape and Tissue Polarity
A combination of experimental analysis and mathematical modelling shows how the genetic control of tissue polarity plays a fundamental role in the development and evolution of form