Conductivity measurements reflect vortex solid melting in YBa 2 Cu 3 O 72d films. Field-independent glass exponents n g Ӎ 1.9 and z g Ӎ 4.0 describe the transition T g ͑H͒ for 0 , H # 26 T. At low fields, 3D XY exponents n XY Ӎ 0.63 and z XY Ӎ 1.25 are also observed, with z XY smaller than expected. These compete with glass scaling according to multicritical theory. A predicted power-law form of T g ͑H͒ is observed for 0.5T c , T g , T c . For T g , 0.5T c , 3D XY scaling fails, but a selfconsistent lowest Landau level analysis becomes possible, obtaining T c2 ͑H͒ with positive curvature.[S0031-9007(97)02912-8] PACS numbers: 74.25.Bt, 74.25.Dw, 74.40. + k, The nature of fluctuations near the superconducting to normal state transition in high-temperature superconductors (HTSCs) is still a matter of controversy. Several distinct fluctuation types and regions have been proposed, e.g., 3D XY fluctuations at low fields, lowest Landau level (LLL) fluctuations at high fields, and glasslike fluctuations (for disordered HTSCs) near the finite-field transition T g ͑H͒. However, experimental analyses based upon the different scaling theories lead to conflicting results. This situation is most evident for competing 3D XY and LLL fluctuations, both of which are supported experimentally, in the same region of the phase diagram, in spite of being incompatible [1].The 3D XY transition is driven by phase fluctuations of a complex order parameter (OP) which fall into the universality class of the l transition in 4 He. The zerofield, "intermediate" (nonelectrodynamic) phase fluctuations of the HTSCs are thought to be of this type [2]. At T T c (and H 0), these fluctuations diverge in size, driving the resistive phase transition. Recent experimental evidence supporting this picture is found in specific heat [3,4], magnetization [4,5], penetration depth [6], and current-voltage (J-E) measurements [4,7]. The finite-field transition T g ͑H͒, which is similarly driven by phase fluctuations of the OP, joins smoothly to T c ϵ T g ͑H 0͒. However, the glass and 3D XY fluctuations exhibit distinct scaling functions and exponents [2].Fluctuations of the OP amplitude occur near the upper critical (mean-field) temperature T c2 ͑H͒. These fluctuations drive the Cooper pair density to zero, but do not correspond to a true transition; superconducting order vanishes at the slightly lower temperature T g ͑H͒. In the lowfield region, the distinction between OP amplitude and phase fluctuations results in the dominance of 3D XY critical behavior near T T c . At high fields, this distinction is not present, yielding a different type of behavior, most conveniently described in terms of the Ginzburg-Landau LLL approximation, with its corresponding scaling theory [8,9]. Experimental evidence in support of this behavior is found in specific heat [10 -12], magnetization [10,12], and J-E characteristics [10,13]. A crossover is expected between the low-field (3D XY ) and high-field (LLL) behaviors, and its clarification is fundamental in the investigation of H...

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Hall effect and electrical resistivity measurements were carried out on undoped InAs thin films grown by molecular-beam epitaxy directly on ͑001͒ GaP substrates. The large lattice mismatch between these two compounds results in a high density array of misfit dislocations at the heterointerface and threading dislocations in the InAs epilayer. The threading dislocation density varies with epilayer thickness, with the largest proportion being present near the heterointerface. This leads to variation of both the carrier concentration and electron mobility with thickness. Consequently, a multilayer analysis was used to interpret the transport data. This analysis yields a temperature-independent carrier concentration, which indicates degenerate donor levels in this narrow band-gap material. Room temperature mobilities in excess of 10 000 cm 2 /V s were obtained for thick InAs layers despite dislocation densities of 10 10 cm Ϫ2 . The relative insensitivity of the mobility to temperature suggests that temperature-independent scattering dominates over ionized impurity/defect and phonon scattering.

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