Progress report on solar age calibration

We report on an ongoing investigation into a seismic calibration of solar models designed for estimating the main-sequence age and a measure of the chemical abundances of the Sun. Only modes of low degree are employed, so that with appropriate modification the procedure could be applied to other stars. We have found that, as has been anticipated, a separation of the contributions to the seismic frequencies arising from the relatively smooth, glitch-free, background structure of the star and from glitches produced by helium ionization and the abrupt gradient change at the base of the convection zone renders the procedure more robust than earlier calibrations that fitted only raw frequencies to glitch-free asymptotics. As in the past, we use asymptotic analysis to design seismic signatures that are, to the best of our ability, contaminated as little as possible by those uncertain properties of the star that are not directly associated with age and chemical composition. The calibration itself, however, employs only numerically computed eigenfrequencies. It is based on a linear perturbation from a reference model. Two reference models have been used, one somewhat younger, the other somewhat older than the Sun. The two calibrations, which use BiSON data, are more-or-less consistent, and yield a main-sequence age $t_\odot=4.68\pm0.02$Gy, coupled with a formal initial heavy-element abundance $Z=0.0169\pm0.0005$. The error analysis has not yet been completed, so the estimated precision must be taken with a pinch of salt.Comment: 8 pages, 3 figures, in L. Deng, K.L. Chan, C. Chiosi, eds, The Art of Modelling Stars in the 21st Century, Proc. IAU Symp. No. 252, invited contributed pape

On the seismic age and heavy-element abundance of the Sun

We estimate the main-sequence age and heavy-element abundance of the Sun by means of an asteroseismic calibration of theoretical solar models using only low-degree acoustic modes from the BiSON. The method can therefore be applied also to other solar-type stars, such as those observed by the NASA satellite Kepler and the planned ground-based Danish-led SONG network. The age, 4.60+/-0.04 Gy, obtained with this new seismic method, is similar to, although somewhat greater than, today's commonly adopted values, and the surface heavy-element abundance by mass, Zs=0.0142+/-0.0005, lies between the values quoted recently by Asplund et al. (2009) and by Caffau et al. (2009). We stress that our best-fitting model is not a seismic model, but a theoretically evolved model of the Sun constructed with `standard' physics and calibrated against helioseismic data.Comment: 16 pages, 11 figures, 5 tables, accepted for publication in MNRA

Antiferromagnetic Alignment and Relaxation Rate of Gd Spins in the High Temperature Superconductor GdBa_2Cu_3O_(7-delta)

The complex surface impedance of a number of GdBa$_2$Cu$_3$O$_{7-\delta}$ single crystals has been measured at 10, 15 and 21 GHz using a cavity perturbation technique. At low temperatures a marked increase in the effective penetration depth and surface resistance is observed associated with the paramagnetic and antiferromagnetic alignment of the Gd spins. The effective penetration depth has a sharp change in slope at the N\'eel temperature, $T_N$, and the surface resistance peaks at a frequency dependent temperature below 3K. The observed temperature and frequency dependence can be described by a model which assumes a negligibly small interaction between the Gd spins and the electrons in the superconducting state, with a frequency dependent magnetic susceptibility and a Gd spin relaxation time $\tau_s$ being a strong function of temperature. Above $T_N$, $\tau_s$ has a component varying as $1 / (T - T_N)$, while below $T_N$ it increases $\sim T^{-5}$.Comment: 4 Pages, 4 Figures. Submitted to Phys. Rev.

Quasiparticle tunnelling and field-dependent critical current in 2212-BSCCO

Intrinsic c-axis tunnelling in the superconducting state has been measured in zero and finite fields in small mesa structures fabricated on the surface of 2212-BSCCO single crystals. The temperature dependence of the zero-field critical current and quasi-particle conductance is related to microscopic d-wave models in the presence of impurity scattering. The strong field dependence of the c-axis critical current provides information on the correlation of flux pancakes across adjacent superconducting bi-layers. An instability in the IV characteristics is observed below 20K, which accounts for the apparent drop in critical current at low temperatures previously reported

Interlayer tunnelling in Bi2Sr2CaCu2O8+d single crystals

We present measurements of the intrinsic quasi-particle conductivity along the c-axis of 2212-BSCCO single-crystal mesa structures in the superconducting and normal states. Direct measurement of the mesa temperature enables corrections to be made for self-heating and permits the acquisition of reliable I-V characteristics over a wide range of temperatures and voltages. Unlike a conventional superconductor, there is no evidence for any change in the quasiparticle conductivity at Tc, consistent with precursor pairing of electrons in the normal state. At low temperatures the initial low-voltage linear conductivity exhibits a T2 dependence, approaching a limiting value at zero temperature

Intrinsic c-axis transport in 2212-BSCCO

We describe two experimental approaches to circumvent the problem of self-heating in IV measurements on small mesa samples of 2212-BSCCO. Simultaneous dc and temperature measurements have been performed, allowing corrections for heating to be made. Short pulse measurements have also been made, where the IV characteristics and the mesa temperature can be measured on a $\mu$s time-scale enabling intrinsic IV characteristics to be derived, even in the presence of appreciable self-heating. Self-heating leads to an appreciable depression of the apparent energy gap and also accounts, in major part, for the s-shaped characteristics often reported at high currents. By correcting for the temperature rise, we derive the intrinsic temperature dependence of the tunnelling characteristics for crystals with a range of doping. Results are compared with recent theoretical models for c-axis transport in d-wave superconductors