Pressure dependence of the oxygen isotope effect in YBa2_2Cu4_4O8_8

We have carried out measurements of the pressure dependence to 1.2 GPa of the oxygen isotope effect on $T_c$ in the high-$T_c$ superconductor YBa$_2$Cu$_4$O$_8$ using a clamp cell in a SQUID magnetometer. This compound lies close to, but just above, the 1/8$^{th}$ doping point where in La$_{2-x}$Sr$_x$CuO$_4$ marked anomalies in isotope effects occur. Both isotopes show the same very large pressure dependence of $T_c$ with the result that the isotope exponent remains low ($\sim$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.

On the Dichotomy between the Nodal and Antinodal Excitations in High-temperature Superconductors

Angle-resolved photoemission data on optimally- and under-doped high temperature superconductors reveal a dichotomy between the nodal and antinodal electronic excitations. In this paper we propose an explanation of this unusual phenomenon by employing the coupling between the quasiparticle and the commensurate/incommensurate magnetic excitations.Comment: 11 pages, 9 figure

Enabling the next generation of near-net-shaping techniques for UHTCs

Hypersonics and other extreme applications rely on components that often require a complex geometry and a fully dense, fine-grained and controlled microstructure. The current state-of-the-art processing of these materials allow their full densification and fine-grained microstructures, but the shaping capability remains a challenge. Near-Net-Shaping Techniques involving the colloidal processing of UHTC powders can afford to render near-net-shaped pieces, but because of the shape, the sintering needs to be done using pressureless conditions. This work will discuss, using zirconium diboride (ZrB2) as a case study, the strategies to enhance the pressureless densification of these materials: i) particle packing in the green bodies (by the control of the interparticle forces in suspension), ii) sintering profiles and iii) incorporation of sintering aids, and how these results compare with the current state-of-the-art processing (hot-pressing). Please click Additional Files below to see the full abstract

Absence of an isotope effect in the magnetic resonance in high-TcT_c superconductors

An inelastic neutron scattering experiment has been performed in the high-temperature superconductor $\rm YBa_2Cu_3O_{6.89}$ to search for an oxygen-isotope shift of the well-known magnetic resonance mode at 41 meV. Contrary to a recent prediction (I. Eremin, {\it et al.}, Phys. Rev. B {\bf 69}, 094517 (2004)), a negligible shift (at best $\leq$ +0.2 meV) of the resonance energy is observed upon oxygen isotope substitution ($^{16}$O$\to^{18}$O). This suggests a negligible spin-phonon interaction in the high-$T_c$ cuprates at optimal doping.Comment: 3 figure

The Doping Phase Diagram of Y1-xcaxba2(Cu1-yzny)3O7-d from Transport Measurements: Tracking the Pseudogap Below Tc (y = 0)

The effects of planar hole concentration, p, on the resistivity, r(T), of sintered Y1-xCaxBa2(Cu1-yZny)3O7-d samples were investigated over a wide range of Ca, Zn, and oxygen contents. Zn was used to suppress superconductivity and this enabled us to extract the characteristic pseudogap temperature, T*(p), from r(T,p) data below Tco(p) [ = Tc (y = 0)]. We have also located the characteristic temperature, Tscf, marking the onset of significant superconducting fluctuations above Tc, from the analysis of r(T,H,p) and r(T,p) data. This enabled us to identify T*(p) near the optimum doping level where the values of T*(p) and Tscf(p) are very close and hard to distinguish. We again found that T*(p) depends only on the hole concentration p, and not on the level of disorder associated with Zn or Ca substitutions. We conclude that (i) T*(p) (and therefore, the pseudogap) persists below Tco(p) on the overdoped side and does not merge with the Tco(p) line and (ii) T*(p), and thus the pseudogap energy, extrapolates to zero at the doping p = 0.19 +/- 0.01. PACS numbers: 74.25.Dw, 74.25. 74.62.Dh, 74.72.-h Key words: Pseudoap, Superconducting fluctuationsComment: 14 pages (Text), 7 figure

Market Allocations under Ambiguity: A Survey

We review some of the (theoretical) economic implications of David Schmeidler's models of decision under uncertainty (Choquet expected utility and maxmin expected utility) in competitive market settings. We start with the portfolio inertia result of Dow and Werlang (1992), show how it does or does not generalize in an equilibrium setting. We further explore the equilibrium implications (indeterminacies, non revelation of information) of these decision models. A section is then devoted to the studies of Pareto optimal arrangements under these models. We conclude with a discussion of experimental evidence for these models that relate, in particular, to the implications for market behaviour discussed in the preceding sections