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Nakamura, O., E. Fullerton, J. Guimpel, and I. Schuller. "High Tc thin films with roughness smaller than one unit cell." Applied Physics Letters 60, no. 1 (1992): 120.
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Nakamura, O., E. E. Fullerton, J. Guimpel, and I. K. Schuller. "Disorder and superconductivity in YBa2Cu3O7−δ/GdBa2Cu3O7-δ superlattices." Physica C: Superconductivity 185-189, Part 3 (1991): 2069–2070.
Abstract: The structure of coriented YBa2Cu3O7−δ/GdBa2Cu3O7−δ superlattices is analyzed throug refinement of the X-ray diffraction spectrum. This novel technique, similar in spirit to the well known Rietveld refinement of powder X-ray diffraction spectra, allows quantitative determination of the interfacial roughness. The results show that for these superlattices the roughness consists in step disorder of one unit cell around the average thickness of the layer and interdiffusion of 15 to 25% between the Y and Gd sites within one unit cell from the interface.
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Nakamura, O., J. Guimpel, F. Sharifi, R. Dynes, and I. Schuller. "Synthesis and properties of a-axis and b-axis oriented GdBa2Cu3O7-d high Tc thin films." Applied Physics Letters 61, no. 21 (1992): 2598.
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Niebieskikwiat, D., N. Haberkorn, R. Guitarra, and J. Guimpel. "Magnetic properties of SrTiO3/Pr2/3Ca 1/3MnO3 multilayers." In Journal of Physics: Conference Series, 072069. Vol. 200., 2010.
Abstract: Nanoscale ferromagnetic (FM) clusters embedded within insulating (I) layers of the antiferromagnetic Pr2/3Ca1/3MnO3 (PCMO) are known to improve the magnetoresistance ratio of FM-I magnetoresistive multilayers. Here, we study the magnetic properties of perovskite superlattices comprised of five PCMO layers of variable thickness t separated by 5nm-thick spacers of SrTiO3 (STO). Several multilayers (1.5 nm ≤ t ≤ 8 nm) were grown on (001) STO substrates by magnetron sputtering. We show that the magnetic moment of PCMO due to the FM inclusions presents a maximum for t ∼ 3 nm, accompanied by an abrupt increase of coercivity and exchange bias field. The nonmagnetic nature of the neighboring STO layers demonstrates that the observed behavior is not related to any interlayer exchange coupling, but the geometrical matching between layer thickness and FM domain size is the key driver for the enhancement of the FM moment and anisotropy energy. These results open a new door for the optimization of perovskite based spintronic devices.
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Nieva, G., B. W. Lee, J. Guimpel, H. Iwasaki, M. B. Maple, and I. K. Schuller. "Changes in the Pr-induced TC depression of 123 compounds by chemical pressure." Physica C: Superconductivity 185-189, no. 1 (1991): 561–562.
Abstract: We present structural data refined from X-ray spectra for R1−xPrxBa2Cu3O7−δ (R=Er, Gd, Eu, Sm, Nd) systems with x=0, 0.1, and 0.4. The evolution of Tc with the ionic radius of the rare earth for different values of x can be related to the Cu2---O4 distance. The effect of chemical pressure on Tc in R1−xPrxBa2Cu3O7−δ systems is opp osite to that of hydrostatic pressure on the Y1−xPrxBa2Cu3O7−δ system.
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Nieva, G., E. Osquiguil, J. Guimpel, M. Maenhoudt, B. Wuyts, Y. Bruynseraede, M. B. Maple, and I. K. Schuller. "Photoinduced enhancement of superconductivity." Applied Physics Letters 60, no. 17 (1992): 2159–2161.
Abstract: We show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa 2Cu3Ox thin films by photoexcitation. Upon laser illumination the critical temperature increases while the resistivity of the material decreases. The relaxation back to the original state is very slow, of the order of days at room temperature. The existence of this effect opens the possibility of fabricating weak-link devices with in situ tunable superconducting characteristics.
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Nieva, G., E. Osquiguil, J. Guimpel, M. Maenhoudt, B. Wuyts, Y. Bruynseraede, M. B. Maple, and I. K. Schuller. "Photoinduced changes in the transport properties of oxygen-deficient YBa2Cu3Ox." Physical Review B 46, no. 21 (1992): 14249–14252.
Abstract: We show that illumination of oxygen-deficient metallic YBa2Cu3Ox films produces a change in the Hall coefficient, an increase in the critical temperature, and a decrease in the electrical resistivity. These changes relax to equilibrium with characteristic times of the order of days, and are due to variations in both the carrier density and mobility. The relaxation times are of the same order of magnitude as the ones measured in nonilluminated oxygen-deficient films immediately after quenching.
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Osquiguil, E., M. Maenhoudt, B. Wuyts, Y. Bruynseraede, D. Lederman, G. Nieva, J. Guimpel, and I. K. Schuller. "Photoexcitation effects in YBa2Cu3Ox." Journal of Alloys and Compounds 195, no. C (1993): 667–670.
Abstract: We show through photoexcitation experiments carried out on YBa2Cu3Ox c-axis oriented films that photoinduced phenomena are not restricted to insulating samples. Superconducting films with critical temperatures varying between 2 K and 40 K show a clear enhancement of the superconducting transition after illumination. The increase in Tc is accompanied by a corresponding decrease in the resistivity ?, and an increase in the Hall coefficient RH, indicative of an increased carrier density. Furthermore, the carrier mobility is also affected by illumination.
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Pardo, F., G. Burmeister, and J. Guimpel. "Substrate heater design for film deposition in oxidizing atmosphere." REVIEW OF SCIENTIFIC INSTRUMENTS 67, no. 6 (1996): 2370–2371.
Abstract: A simple substrate heater for oxide film deposition is presented. Its main features are (a) good thermal contact between temperature sensor, heater, and substrate and (b) compatibility with oxidizing atmosphere at high temperatures. The system shows a good response for the deposition conditions necessary in high-T-c cuprate materials. (C) 1996 American Institute of Physics.
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Pardo, F., A. P. Mackenzie, F. delaCruz, and J. Guimpel. "Effect of the reversibility region on the low-temperature vortex structure imaged by Bitter magnetic decoration." PHYSICAL REVIEW B 55, no. 21 (1997): 14610–14613.
Abstract: We have combined magnetic measurements and imaging of the vortex lattice at low fields in Tl2Ba2CuO6-delta single crystals with different oxygen content. Those with high critical temperature T-c > 80 K have a reversible region while those with low T-c < 30 K have an irreversible magnetization in the whole range of temperatures, up to T-c. The presence of the reversible region allows the formation of ordered vortex structures, while amorphous structures are obtained when no reversible region is present below T-c. The results are discussed in terms of vortex nucleation processes in the individual pinning regime.
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