73 research outputs found
Induced paramagnetic states by localized -loops in grain boundaries
Recent experiments on high-temperature superconductors show paramagnetic
behavior localized at grain boundaries (GB). This paramagnetism can be
attributed to the presence unconventional d-wave induced -junctions. By
modeling the GB as an array of and conventional Josephson junction we
determine the conditions of the occurrence of the paramagnetic behavior.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Paramagnetic Meissner Effect in Multiply-Connected Superconductors
We have measured a paramagnetic Meissner effect in Nb-Al2O3-Nb Josephson
junction arrays using a scanning SQUID microscope. The arrays exhibit
diamagnetism for some cooling fields and paramagnetism for other cooling
fields. The measured mean magnetization is always less than 0.3 flux quantum
(in terms of flux per unit cell of the array) for the range of cooling fields
investigated. We demonstrate that a new model of magnetic screening, valid for
multiply-connected superconductors, reproduces all of the essential features of
paramagnetism that we observe and that no exotic mechanism, such as d-wave
superconductivity, is needed for paramagnetism.Comment: 4 pages, 3 figures, LaTe
Detailed investigation of the superconducting transition of niobium disks exhibiting the paramagnetic Meissner effect
The superconducting transition region in a Nb disk showing the paramagnetic
Meissner effect (PME) has been investigated in detail. From the field-cooled
magnetization behavior, two well-defined temperatures can be associated with
the appearance of the PME: T_1 (< T_c) indicates the characteristic temperature
where the paramagnetic moment first appears and a lower temperature T_p (< T_1)
defines the temperature where the positive moment no longer increases. During
the subsequent warming, the paramagnetic moment begins to decrease at T_p and
then vanishes at T_1 with the magnitude of the magnetization change between
these two temperatures being nearly the same as that during cooling. This
indicates that the nature of the PME is reversible and not associated with flux
motion. Furthermore, the appearance of this paramagnetic moment is even
observable in fields as large as 0.2 T even though the magnetization does not
remain positive to the lowest temperatures. Magnetic hysteresis loops in the
temperature range between T_1 and T_p also exhibit a distinct shape that is
different from the archetypal shape of a bulk type-II superconductor. These
behaviors are discussed in terms of the so-called 'giant vortex state'.Comment: Total 4 printed pages, 4 Figure
Vortices and Quantum tunneling in Current-Biased 0-\pi-0 Josephson Junctions of d-wave Superconductors
We study a current-biased 0-\pi-0 Josephson junction made by high-T_c
superconductors, theoretically. When a length of the \pi junction is large
enough, this junction contains a vortex-antivortex pair at both ends of the \pi
junction. Magnetic flux carried by the vortices is calculated using the
sine-Gordon equation. The result shows that the magnetic flux of the vortices
is suppressed to zero as the distance between the vortices is reduced. By
applying an external current, the orientation of the vortices is reversed, and
a voltage pulse is generated. The current needed for this transition and
generated pulse energy are calculated. Macroscopic quantum tunneling (MQT) in
this transition is also studied. The tunneling rate has been evaluated by an
effective Hamiltonian with one degree of freedom.Comment: 12 pages, LaTeX with 5 PS figures, using jpsj.st
Reentrant AC magnetic susceptibility in Josephson-junction arrays: An alternative explanation for the paramagnetic Meissner effect
The paramagnetic Meissner effect (PME) measured in high granular
superconductors has been attributed to the presence of -junctions between
the grains. Here we present measurements of complex AC magnetic susceptibility
from two-dimensional arrays of conventional (non ) Nb/Al/AlOx/Nb Josephson
junctions. We measured the susceptibility as a function of the temperature ,
the AC amplitude of the excitation field, , and the external magnetic
field, . The experiments show a strong paramagnetic contribution from
the multi-junction loops, which manifests itself as a reentrant screening at
low temperature, for values of higher than 50 mOe. A highly simplified
model, based on a single loop containing four junction, accounts for this
paramagnetic contribution and the range of parameters in which it appears. This
model offers an alternative explanation of PME which does not involve
-junctions.Comment: PDF file, 6 two-columns pages, 9 figure
Phenomenological theory of the 3 Kelvin phase in Sr2RuO4
We model the 3K-phase of Sr2RuO4 with Ru-metal inclusion as interface state
with locally enhanced transition temperatures. The resulting 3K-phase must have
a different pairing symmetry than the bulk phase of Sr2RuO4, because the
symmetry at the interface is lower than in the bulk. It is invariant under time
reversal and a second transition, in general, above the onset of bulk
superconductivity is expected where time reversal symmetry is broken. The
nucleation of the 3K-phase exhibits a ``capillary effect'' which can lead to
frustration phenomena for the superconducting states on different
Ru-inclusions. Furthermore, the phase structure of the pair wave function gives
rise to zero-energy quasiparticle states which would be visible in
quasiparticle tunneling spectra. Additional characteristic properties are
associated with the upper critical field Hc2. The 3K-phase has a weaker
anisotropy of Hc2 between the inplane and z-axis orientation than the bulk
superconducting phase. This is connected with the more isotropic nature
Ru-metal which yields a stronger orbital depairing effect for the inplane
magnetic field than in the strongly layered Sr$_2RuO4. An anomalous temperature
dependence for the z-axis critical field is found due to the coupling of the
magnetic field to the order parameter texture at the interface. Various other
experiments are discussed and new measurements are suggested.Comment: 10 pages, 5 figure
Superconducting properties of RuSr2GdCu2O8 studied by SQUID magnetometry
For polycrystalline RuSr2GdCu2O8 (Ru-1212), distinct peaks have been reported
in d.c. magnetization in the superconducting state of the sample. Sr2GdRuO6
(Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in
the same temperature regime. Based on measurements performed on both bulk and
powdered samples of Ru-1212 and Sr-2116, we exclude the possibility, that the
observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities.
The effect is related to the superconductivity of Ru-1212, but it is not an
intrinsic property of this compound. We provide evidence that the observation
of magnetization peaks in the superconducting state of Ru-1212 is due to flux
motion generated by the movement of the sample in an inhomogeneous field,
during the measurement in the SQUID magnetometer. We propose several tests,
that help to decide, whether the features observed in a SQUID magnetization
measurement of Ru-1212 represent a property of the compound or not.Comment: 22 pages, 9 figure
Species interactions and climate change: How the disruption of species co-occurrence will impact on an avian forest guild
Cardiovascular Mortality Can Be Predicted by Heart Rate Turbulence in Hemodialysis Patients
Background: Excess mortality in hemodialysis patients is mostly of cardiovascular origin. We examined the association of heart rate turbulence (HRT), a marker of baroreflex sensitivity, with cardiovascular mortality in hemodialysis patients.
Methods: A population of 290 prevalent hemodialysis patients was followed up for a median of 3 years. HRT categories 0 (both turbulence onset [TO] and slope [TS] normal), 1 (TO or TS abnormal), and 2 (both TO and TS abnormal) were obtained from 24 h Holter recordings. The primary end-point was cardiovascular mortality. Associations of HRT categories with the endpoints were analyzed by multivariable Cox regression models including HRT, age, albumin, and the improved Charlson Comorbidity Index for hemodialysis patients. Multivariable linear regression analysis identified factors associated with TO and TS.
Results: During the follow-up period, 20 patients died from cardiovascular causes. In patients with HRT categories 0, 1 and 2, cardiovascular mortality was 1, 10, and 22%, respectively. HRT category 2 showed the strongest independent association with cardiovascular mortality with a hazard ratio of 19.3 (95% confidence interval: 3.69-92.03;P < 0.001). Age, calcium phosphate product, and smoking status were associated with TO and TS. Diabetes mellitus and diastolic blood pressure were only associated with TS.
Conclusion: Independent of known risk factors, HRT assessment allows identification of hemodialysis patients with low, intermediate, and high risk of cardiovascular mortality. Future prospective studies are needed to translate risk prediction into risk reduction in hemodialysis patients
Analyses of least cost paths for determining effects of habitat types on landscape permeability: wolves in Poland
Determining ecological corridors is crucial for conservation efforts in fragmented habitats. Commonly employed least cost path (LCP) analysis relies on the underlying cost matrix. By using Ecological Niche Factor Analysis, we minimized the problems connected with subjective cost assessment or the use of presence/absence data. We used data on the wolf presence/absence in Poland to identify LCPs connecting patches of suitable wolf habitat, factors that influence patch occupancy, and compare LCPs between different genetic subpopulations. We found that a lower proportion of cities and roads surrounds the most densely populated patches. Least cost paths between areas where little dispersal takes place (i.e., leading to unpopulated patches or between different genetic subpopulations) ran through a higher proportion of roads and human settlements. They also crossed larger maximal distances over deforested areas. We propose that, apart from supplying the basis for direct conservation efforts, LCPs can be used to determine what factors might facilitate or hinder dispersal by comparing different subsets of LCPs. The methods employed can be widely applicable to gain more in-depth information on potential dispersal barriers for large carnivores
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