Wilson Lopera

In-plane electrodynamics of the superconductivity inBi2Sr2CaCu2O8+δ: Energy scales and spectral weight distribution

The in-plane infrared and visible (3 meV–3 eV) reflectivity of Bi2Sr2CaCu2O8+� (Bi-2212) thin films is measured between 300 K and 10 K for different doping levels with unprecedented accuracy. The optical conductivity is derived through an accurate fitting procedure. We study the transfer of spectral weight from finite energy into the superfluid as the system becomes superconducting. In the over-doped regime, the superfluid develops at the expense of states lying below 60 meV, a conventional energy of the order of a few times the superconducting gap. In the underdoped regime, spectral weight is removed from up to 2 eV, far beyond any conventional scale. The intraband spectral weight change between the normal and superconducting state, if analyzed in terms of a change of kinetic energy is � 1 meV. Compared to the condensation energy, this figure addresses the issue of a kinetic energy driven mechanism.

Plasma characterization of a high-pressure d(-sputtering system used for the in situ preparation of high-Tc superconducting thin films

Abstract A characterization of the high-pressure dc-Sputtering system, used for the “in situ” preparation of high-quality high-T c superconducting thin films, was performed by plasma electrical measurements using a single Langmuir probe. A correlation between plasma characteristics and thin film properties (transition temperature, T c , transition width, ΔT c and residual resistivity ratio RRR) is presented. The parameters of the plasma were obtained from measurements at pressures ranging from 2.0 to 4.0 mbar with unheated and heated substrates (around 750°C) during the “in situ” preparation of YBa 2 Cu 3 O 7− x thin films. We found that anode heating contributes to the formation of positive ions and reduces the formation of negative ions, which minimizes the re-sputtering effects during film growth. Optimal superconducting properties of the films were obtained at an oxygen pressure of 3.25 mbar; at this pressure the plasma has the largest density of positive ions and the lowest density of both electron and negative ions.

INFLUENCE OF EPITAXIAL PROPERTIES ON THE MUTUAL INDUCTANCE RESPONSE OF HIGH-QUALITY YBCO THIN FILMS

Abstract High-quality YBa 2 Cu 3 O 7− x films have been produced on SrTiO 3 (001) and NdGaO 3 (001) substrates. X-ray Φ scans show a good in-plane texture but do not exclude the presence of domains at 90°. Diamagnetic shielding properties obtained from mutual-inductance measurements show a double peak structure which may be ascribed to the presence of those domains. This point is further supported with an analysis of the normal-state resistivity. Films with an interdomain resistivity of 10 −5 Ω cm show a double peak structure in the mutual-inductance measurements and relatively low values of the critical current densities (1.5 × 10 5 A cm −2 at 77 K), while films with interdomain resistivities lower than 10 −6 Ω cm show a narrow single peak and higher values of the critical current (10 6 A cm −2 at 77 K).

Superconducting and structural properties of epitaxial Bi2Sr2CaCu2O8−δ/Bi2Sr2YCu2O8−δ/Bi2Sr2CaCu2O8−δ heterostructures

Abstract We have prepared Bi 2 Sr 2 CaCu 2 O 8−δ /Bi 2 Sr 2 YCu 2 O 8−δ /Bi 2 Sr 2 CaCu 2 O 8−δ trilayer structures, using a “in situ” DC-sputtering process at high oxygen pressures on (001) SrTiO 3 substrates. Bi 2 Sr 2 CaCu 2 O 8−δ films were used for superconducting electrodes and semiconductor-like Bi 2 Sr 2 YCu 2 O 8−δ films with thickness down 5 nm were used as barrier layers. Both materials have the same crystalline structures and allowed the epitaxial growth of the heterstructure as shown by X-ray, RBS measurements and cross section TEM analysis. The superconducting properties were examined by resistance, AC-susceptibility and current-voltage characteristics as function of the temperature. For junctions with areas of 120×120μm 2 defined by photolitography a RSJ behavior was found at temperatures below T c .

Enhanced electric and magnetic properties in LCMO/BTO bilayer deposited by Pulsed Laser Deposition

We have deposited bilayers of the FM La2/3Ca1/3MnO3 and FE BaTiO3 as a route to design systems with artificial magnetoelectric coupling. We have optimized the growth parameters for depositing LCMO/BTO/(001)Nb:STO by PLD at pure oxygen atmosphere and a substrate temperature of 820°C, keeping fixed the magnetic layer thickness (tLCMO~50 nm) and varying the thickness of the ferroelectric layer (tBTO=17.8, 44.4, 98.4 nm). Magnetization and transport measurements indicate possible multiferroic behaviour in the bilayer. We found an increase in the Curie and metal-insulator transition temperature in the bilayer in comparison with those for LCMO (48nm)/STO. Hysteresis loops on bilayers show ferromagnetic behaviour.

Ferroelectric/Ferromagnetic Bilayers Based on Oxide Materials by Pulsed-Laser Deposition

BaTiO₃ (BTO)/La_2/3Ca_1/3MnO₃ (LCMO) bilayers were deposited on (001) SrTiO₃ substrates via pulsed-laser deposition (PLD) at pure oxygen atmosphere at a substrate temperature of 820 ^°C. For LCMO layers, magnetization measurements indicate that electric and magnetic properties are affected by the ferroelectric layer, which is attributed to crystallographic strains at the interface and the corresponding relaxation with increasing BTO layer thickness.

Scaling Laws in PZT Thin Films Grown on Si(001) and Nb-Doped SrTiO3(001) Substrates

A dynamic scaling and kinetic roughening study was done on digitized atomic force microscope (AFM) images of Pb(Zr0.52,Ti0.48)O3 (PZT) thin films. The films were grown on Si(001) and Nb - SrTiO3(001) (STNO) substrates via rf-sputtering technique at high oxygen pressures at substrate temperatures of 600 oC by varying the deposition time and keeping other growth parameters fixed. By using a specific self-designed algorithm, we can extract from digitized 512-pixel resolution AFM-images, quantitative values of roughness parameters, i.e., interface width (s(l)), lateral correlation length (x||) and, roughness exponent (a). Herein, we report on the sputter-time deposition dependence of the parameters describing roughness for both kinds of substrates. We report a-values for different time depositions (between 15 and 60 minutes) close to 0.55 for Si substrates and 0.63 for Nb - SrTiO3 substrates, indicating that the surface becomes more correlated in STNO substrates. The a-values are associated to the Lai-Das-Sarma-Villain model.

Epitaxial growth and tunneling properties of YBa2Cu3O7−x/PrBa2Cu3O7−x/YBa2Cu3O7−x trilayer structures

Abstract Using a multitarget high oxygen pressure sputtering system, we have produced in situ high quality epitaxial YBa 2 Cu 3 O 7−x / PrBa 2 Cu 3 O 7−x /YBa 2 Cu 3 O 7−x trilayer structures on SrTiO 3 (001) substrates with PrBa 2 Cu 3 O 7−x barrier thicknesses ranging from 10 to 30 nm. The structural properties were determined by x-ray diffraction. Rutherford Back Scattering (RBS) and Transmission Electron Microscopy (TEM) analysis, which showed an epitaxial growth of the trilayers with sharp and clean interfaces. I–V and dI/dV vs V measurements showed a clear SIS quasiparticle tunneling behavior with well developed peaks at ± 45m V indicating the presence of energy gap structures at ±2Δ and flat background conductances at high bias. Ratios of the conductance at zero bias, G(0), to the normal conductance at 150 mV, G(150), of about 10 % are observed. These characteristics can be related to the very good interfaces and superconducting properties observed in our heterostructures.

Grain boundary junctions with Ag-doped YBa2Cu3O7−x epitaxial thin films

Abstract Using a dc sputtering method at high oxygen pressures we have deposited epitaxial Ag-doped YBa 2 Cu 3 O 7−x thin films on SrTiO 3 bicrystals with missorientation angle of 36.8°. A 15 % Ag-doped YBa 2 Cu 3 O 7−x sintered target was used to sputter the films. Critical current of 4–5 10 6 A/cm 2 at 77 K were measured in doped films. When compared with 1 × 10 6 A/cm 2 for undoped films, indicated an improvement of the superconducting properties by Ag doping. 200–300 nm thick Ag-doped films were patterned across the bicrystal line to form 5–20 μm-wide junctions. Current-voltage characteristics of the grain boundary junctions at temperatures from 10 K to T C showed a resistively shunted junction (RSJ) behavior with I C R N values of 160–170 μV at 77 K which are higher than the measured values for undoped junctions.

Superconducting, surface and interface properties of Ho(123) and Bi(2212) films on sapphire with cerium oxide buffer layers

Abstract We report on the X-ray diffraction, secondary ion mass spectrometry, and atomic force microscopy on Ho(123) and Bi(2212) films dc sputtered in pure oxygen atmosphere onto heated sapphire substrates with CeO 2 buffer layers. The films were c-axis oriented. The Ho(123) films had a T c of 88 K but had a relatively high room temperature resistivity of 400 μΩcm. The Bi(2212) films showed a broad transition and a low T c of 46 K. The data may be explained by a certain amount of Al diffusion and inhomogenous grain growth.