We report a systematic investigation of dynamical fluctuation effects in the frequency-dependent microwave conductivity (45 MHz-45 GHz) of YBa 2 Cu 3 O 72d thin films for T * T c . Our measurements directly yield a dynamical critical exponent z in the range 2.3 -3.0, and the fluctuation lifetime t fl , which diverges more quickly than Gaussian theory predicts as the temperature approaches T c from above, independent of sample quality. In addition, both the temperature and the frequency dependence of the fluctuation conductivity s fl exhibit scaling behavior for temperatures 1 -2 K above T c , and can be collapsed onto the same universal curve. [S0031-9007(96)01664-X] PACS numbers: 74.25.Nf, 74.40.tk Dynamic fluctuation effects have become an intriguing subject of research in statistical and condensed matter physics, as they depend on the equations of motion of the system, and are not simply determined by the equilibrium distribution of particles at a given instant of time. Over the years, there have been both theoretical and experimental investigations of the static and dynamic fluctuations of many different types of systems. In the high T c superconductors (HTSC), measurements of fluctuation effects reveal unusual behavior, such as 3D XY critical behavior observed in the microwave penetration depth l͑T ͒ over a range of temperature 5-10 K wide below T c [1]. Heat capacity measurements are also consistent with 3D XY critical fluctuations for T T c 6 10 K [2]. In contrast to these results, temperature-dependent dc conductivity measurements have been interpreted in terms of simple 2D or 3D Gaussian fluctuation theory for T . T c [3,4]; also, low frequency penetration depth measurements have been interpreted as consistent with Gaussian fluctuations [5]. Furthermore, analysis of the nonlinear currentvoltage characteristics near T c in the HTSC yield a wide range of values for the relevant critical exponents [6,7].In this Letter we report a systematic experimental study of the microwave fluctuation conductivity s fl obtained from thin films and a single crystal of YBa 2 Cu 3 O 72d (YBCO). The study provides direct information about the fluctuation lifetime t fl , which is one of the important parameters for the identification of the universality class of the phase transition, and which describes the relevant dynamics of the fluctuations in the HTSC. We employ a swept-frequency technique (45 MHz -45 GHz) to measure both the frequency and temperature dependence of the conductivity of YBCO thin films in the vicinity of T c with nominally zero external magnetic field. The extra experimental degree of freedom afforded by access to the measured conductivity over three decades in frequency provides essential additional information about the dynamics of the system not available to temperature dependent measurements alone. Previous experiments have exploited frequency dependent techniques to verify the Gaussian dynamics of fluctuations in thin films of conventional superconductors [8,9]. More recently, frequencydependent co...

We have studied the transport properties of disordered WSi films near the metal/insulator transition (MIT) and we have also reviewed the data for several other disordered materials near their MIT. In all cases, we found the presence of enhanced superconductivity. We constructed a superconductivity "phase diagram" (i.e., T(c) versus sigma) for each system, which reveals a striking correlation: In all cases, T(c) values are significantly enhanced only for samples whose conductivities lie within a narrow range on the metallic side of, and moderately near, the MIT. We present a heuristic model to explain this phenomenon.

We have measured the transport spin polarization of epitaxial thin films of the conductive ferromagnetic oxide, SrRuO 3 , using point contact Andreev reflection spectroscopy. In spite of the fact that spin-up and spin-down electronic densities of states at the Fermi level for SrRuO 3 calculated from band structure theory are practically the same, the experimental transport spin polarization for these films was found to be about 50%. This is a direct consequence of the Fermi velocity disparity between the majority and minority bands. The experimental results are compared with our theoretical estimates of the spin polarization in the ballistic and diffusive limits.

The discovery of low-dimensional metallic systems such as high-mobility metal oxide field-effect transistors, the cuprate superconductors, and conducting oxide interfaces (e.g., LaAlO3/SrTiO3) has stimulated research into the nature of electronic transport in two-dimensional systems given that the seminal theory for transport in disordered metals predicts that the metallic state cannot exist in two dimensions (2D). In this report, we demonstrate the existence of a metal–insulator transition (MIT) in highly disordered RuO2 nanoskins with carrier concentrations that are one-to-six orders of magnitude higher and with mobilities that are one-to-six orders of magnitude lower than those reported previously for 2D oxides. The presence of an MIT and the accompanying atypical electronic characteristics place this form of the oxide in a highly diffusive, strong disorder regime and establishes the existence of a metallic state in 2D that is analogous to the three-dimensional case.

Thin films of a new, metastable phase of superconducting niobium nitride have been grown using pulsed laser deposition (PLD). This new NbN phase is stabilized by heteroepitaxial growth on (100) Mgo and is shown to be a primitive cubic (Pm3m) distortion from the typical B1, or rocksalt structure. Structural and electrical characterization reveals that this NbN phase has a highersuperconducting critical temperature and a larger lattice parameter when compared with films of B1-NbN. Growth of this new phase demonstrates that PLD can be used as a synthesis tool to deposit new, metastable materials.The next generation of microelectronic devices will be based on the discovery of new materials and the development of related processing technology. The traditional methods of synthesis and preparation, typically involving high temperatures and long reaction times, have been used to investigate a large number of combination of elements. These reactions usually occur near chemical equilibrium, producing the thermodynamically stable products. The materials of the future will require the production of metastable compounds, synthesized far from chemical equilibrium by new reaction methods. Currently, high-powered lasers are being incorporated in a new physical vapor deposition technique called pulsed laser deposition (PLD). ' PLD is being used in the development of new synthesis routes to the formation of metastable compounds so that their intrinsic properties can be accurately measured and evaluated for use in future applications. The deposition of thin films of metastable compounds, such as the infinite layer oxides Ca, "Sr"CuOz,cubic boron nitride (c-BN), carbon nitride (CN"), and amorphous Ge, C",using PLD has been reported.In this paper, we report on the synthesis of a new phase of superconducting NbN by PLD and the structural and electrical characterization of this material. Niobium nitride is a refractory material with a bulk superconducting transition temperature (T, ) of -16 K.Thin films of NbN, typically deposited by sputtering methods, ' also are being studied for applications in electronics. The low chemical reactivity, mechanical durability, high T, ( T, ))4.2 K), and ease with which Josephson junctions can be reproducibly manufactured make NbN a good candidate for use in low-temperature digital electronics. We have demonstrated that, depending on the pressure of the reactive gas atmosphere [Nz/H2 (10%)], any one of several different NbN"(0(x (1.4) phases could be grown on 600'C (100) MgO substrates by ablating niobium targets. ' '" At deposition pressures between 1 and 20 mTorr, metallic Nb2N was deposited, and at pressures greater than 100 mTorr, insulating Nb3N~was grown. Superconducting NbN films were deposited at 60 mTorr. The NbN films grew in one of two different structures. One of these structures has never been reported. In this paper, we describe this new phase of NbN and compare it to the well-known NbN phase with the B1 structure.The films were grown on heated (100) MgO by ablating Nb foil with a pulsed ...

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It is demonstrated here that granular films of Y-Ba-Cu-O may serve as optical detectors, operating at wavelengths from the visible to the far infrared, at temperatures well above that of liquid helium. Preliminary measurements using a blackbody source show that an upper bound of the minimum detectable power is 1 μW. The response time as determined by a pulsed far-infrared source is of the order 20 ns. Methods to improve the sensitivity will be discussed.

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