5,244 research outputs found
Low-temperature critical current of Y1-xCaxBa2Cu3O7-delta thin films as a function of hole content and oxygen deficiency
The effects of hole content (p) and oxygen deficiency (delta) on the
zero-field critical current density, Jc0, were investigated for high-quality
c-axis oriented Y1-xCaxBa2Cu3O7-delta (x = 0, 0.05, 0.10, and 0.20) thin films.
Ca was used to introduce hole carriers in the CuO2 planes, independent of the
oxygen deficiency in the CuO1-delta chains. Low-temperature Jc0(16K) of these
films above the optimum doping were found to be high (> 10^7 Amp/cm^2) and were
primarily determined by the hole concentration, reaching a maximum at p ~
0.185+/-0.005, irrespective of oxygen deficiency. This implies that oxygen
disorder plays only a secondary role and the intrinsic Jc0 is mainly governed
by the carrier concentration and consequently by the superconducting
condensation energy which also peaks at p ~ 0.19 where the pseudogap
correlation vanishes
Effects of Zn on the grain boundary properties of La2-xSrxCu1-yZnyO4 superconductors
The properties of the grain boundaries (GBs) are of significant importance in
high-Tc cuprates. Most large scale applications of cuprate superconductors
involve usage of sintered compounds. The critical current density and the
ability to trap high magnetic flux inside the sample depend largely on the
quality of the GBs. Zn has the ability to pin vortices but it also degrades
superconductivity. In this study we have investigated the effect of Zn impurity
on the intergrain coupling properties in high-quality La2-xSrxCu1-yZnyO4
sintered samples with different hole concentrations, p (\equiv x), over a wide
range of Zn contents (y) using field-dependent ac susceptibility (ACS)
measurements. The ACS results enabled us to determine the superconducting
transition temperature Tc, and the temperature Tgcp, at which the randomly
oriented superconducting grains become coupled as a function of hole and
disorder contents. We have analyzed the behavior of the GBs from the systematic
evolution of the values of Tgcp(p, y), Tc(p, y), and from the contribution to
the field-dependent ACS signal coming from the intergrain shielding current. Zn
suppresses both Tc and Tgcp in a similar fashion. The hole content and the
carrier localization due to Zn substitution seem to have significant effect on
the coupling properties of the GBs. We have discussed the possible implications
of these findings in detail in this article.
PACS: 74.72.Dn; 74.62.Dh; 74.25.Sv Keywords: Zn doped La214; Critical current
density; Grain boundaryComment: To appear in Physica
New Conundrums: Public Policy and the Emerging Health Care Marketplace. 8th annual Herbert Lourie Memorial Lecture on Health Policy
There is a fundamentally new dynamic in American health care, one that has yet to be fully experienced but that threatens to leave a large portion of the American population without access to the quality health care they have received in the past. While the federal government has not completely abandoned the goal of assuring universal health care, a goal that dates back to the creation of Medicare and Medicaid in the 1960s and even earlier, the mechanisms to pursue that goal have changed. The implicit contract between government and health care providers--mostly doctors and not-for-profit hospitals--under which subsidized care was provided to those unable to pay has been broken in favor of more market-driven forces that promise a more cost-effective system, but a system that fails to protect a growing uninsured population. This new purchaser-driven system--in which costs increasingly determine the services that are provided--is likely to fall short of providing quality care to all who need it. Health care is different from other services, and unless this difference is recognized we are in danger of permanently denying quality health care to a significant minority of our population. Regulation of the emerging "free market" in health care is needed and government must assure that role.
Diversification, Convex Preferences and Non-Empty Core
We show, in the Choquet expected utility model, that preference for diversification, that is, convex preferences, is equivalent to a concave utility index and a convex capacity. We then introduce a weaker notion of diversification, namely ``sure diversification.'' We show that this implies that the core of the capacity is non-empty. The converse holds under concavity of the utility index. This property is shown to be equivalent to the notion of comonotone diversification\,; notion that we introduce in the paper. Finally, in the expected utility model, all these notions of diversification are equivalent and are represented by the concavity of the utility index.
Thermodynamic properties of Bi2Sr2CaCu2O8 calculated from the electronic dispersion
The electronic dispersion for Bi2Sr2CaCu2O(8+d) has been determined from
angle-resolved photoelectron spectroscopy (ARPES). From this dispersion we
calculate the entropy and superfluid density. Even with no adjustable
parameters we obtain an exceptional match with experimental data across the
entire phase diagram, thus indirectly confirming both the ARPES and
thermodynamic data. The van Hove singularity is crossed in the overdoped region
giving a distinctive linear-in-T temperature dependence in the superfluid
density there.Comment: 5 pages, 4 figures, submitted to Physical Review Letter
Fairness under Uncertainty
Ever since its introduction by Foley (1967) and Varian (1974), the notion of fairness has been one of the most extensively used notion to evaluate allocations on an ethical basis. Whereas thereis an extensive literature on the efficiency properties of allocations in economies with uncertainty the concept of an envy-free allocation has not been widely studied in economies with uncertainty. We introduce two very natural notions of equity in an economy under uncertainty, namely ex ante and ex post equity, show they can contradict efficiency requirements. In particular, the set of ex ante efficient and ex post envy-free allocations may be empty. We nevertheless show that, under special circumstances, one may prove the existence of allocations that are both ex ante efficient and ex post envy-free. Such is the case, in particular, in an economy with individual risk and no aggregate risk.
Pressure dependence of the oxygen isotope effect in YBaCuO
We have carried out measurements of the pressure dependence to 1.2 GPa of the
oxygen isotope effect on in the high- superconductor
YBaCuO using a clamp cell in a SQUID magnetometer. This compound
lies close to, but just above, the 1/8 doping point where in
LaSrCuO marked anomalies in isotope effects occur. Both
isotopes show the same very large pressure dependence of with the result
that the isotope exponent remains low (0.08) but increases slightly with
increasing pressure. This is discussed in terms of stripe suppression, a
competing pseudogap and the effect of superconducting fluctuations.Comment: 4 pages, 1 figures, submitted to Phys. Rev.
Beliefs and Pareto Efficient Sets: A Remark
We show that, in a two-period economy with uncertainty in the second period, if an allocation is Pareto optimal for a given set of beliefs and remains optimal when these beliefs are changed, then the set of optimal allocations of the two economies must actually coincide. We identify equivalence classes of beliefs giving rise to the same set of Pareto optimal allocations.Beliefs, Pareto Optimality
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