We discuss the current progress on study of cooling of levitated microparticles under high vacuum conditions (6 x 10(-7) Torr). Particle levitation as well as feedback cooling are done using an electrical AC trap (quadrupole ion trap). It is shown that we can cool particle motion in all three degrees of freedom to 50-100K and increase their trapping time from a few hours without feedback to times exceeding weeks when the particles are cooled. The cooling is achieved by FM modulation of the AC trap frequency, and data on its dependence on gain settings is presented.
Thin-film YBa2Cu3O7−δ has been studied with in-plane magnetic fields B to 850 T, with good signal-to-noise to 300 T, using a GHz-transmission-line technique for electrical transport measurements in explosive-driven microsecond pulsed magnetic fields. At an ambient temperature of 1.6 K with B⊥c axis we find an onset of dissipation at 150 T, the highest field reported, with a saturation of resistivity at 240 T. Comparison with the Pauli limit Bp=155 T expected for this material suggests that the critical field is limited by spin paramagnetism.Received 20 February 1998DOI:https://doi.org/10.1103/PhysRevB.57.R14084©1998 American Physical Society