A gallium arsenide bulk crystal highly enriched with gallium isotope 71 Ga to 99.40 at% has been grown using the seedless horizontal Bridgman method. The crystal has a total impurity concentration of about 2 × 10 17 atoms cm −3 and a high electrical resistivity ρ(300 K) ∼ 10 3 -10 8 cm. Thermal conductivity measurements have been made from 90 K to 300 K of the enriched 71 GaAs sample and three different nat GaAs samples with a natural isotope composition of gallium. The 71 GaAs has a higher value of thermal conductivity due to reduced isotope scattering of phonons: the observed isotope effect amounts to 5% at room temperature, increasing to 13% with a temperature decrease down to 100 K.

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We report the results of a 63 Cu and 17 O NMR study of the nuclear quadrupole interaction tensor, 17,63 Q,␣ , in the hole doped spin ladder system Sr 14−x Ca x Cu 24 O 41 ͑x = 0 and 12͒ performed under ambient and high pressures. NMR data show that the hole density in the Cu 2 O 3 ladder layer grows with temperature, Ca content, and an applied pressure. We have derived the hole occupation of Cu 3d and O 2p orbitals at the different ion sites in the Cu 2 O 3 ladders as a function of temperature, Ca substitution, and pressure. We also suggest that the most important role of high pressure for the stabilization of a superconducting ground state in Ca-rich two-leg ladders is an increase of the hole concentration in the conducting Cu 2 O 3 planes while Ca substitution increases the coupling between ladders in the planes. We have obtained an estimate of 0.10 hole per Cu1 in Ca12 under 32 kbar at low temperature when this compound undergoes a superconducting transition at 5 K. Such a value fits fairly well with the doping phase diagram of cuprate superconductors.

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