The specific heat of a single crystal sample of YBa2Cu307-b has been measured with magnetic fields up to 8 T applied parallel to the c axis of the crystal. The results provide strong evidence for the existence of a critical regime within which there is scaling behavior characteristic of the three-dimensional 3'-Y model with critical exponents consistent with those observed in superffuid He.

The heat capacity, magnetization, and electrical conductivity of single-crystal samples of YBa2Cu307 "have been measured and are shown to support the existence of an intermediate critical regime in the vicinity of T"governed by the XY-like critical exponent v= -.Clear evidence is found for

We report alternative types of magnetoresistance oscillations in high mobility two-dimensional electron systems subjected to large amplitude one-dimensional periodic magnetic modulations, of period 500 nm to 1 m. We observe Shubnikov-de Haas oscillations that are strongly modified in amplitude and phase, Hall resistance oscillations, and aperiodic magnetoresistance oscillations. These effects are shown to arise from the internal structure of overlapping Landau bands and are well accounted for by perturbation calculations.

We investigate a new type of magnetoresistance (MR) in which the resistivity of a near-surface two-dimensional electron gas is controlled by the magnetization of a submicron ferromagnetic grating defined on the surface of the device. We observe an increase in resistance of up to ∼1500% at a temperature of 4 K and ∼1% at 300 K. The magnitude and temperature dependence of the MR are well accounted for by a semiclassical theory. Optimization of device parameters is expected to increase considerably the magnitude of the room temperature MR.

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