T.S. Hahn

Substrate Temperature Dependence of Phase and Orientation of Pulsed Laser Deposited Bi-La-Ti-O Thin Films

Bi3.25La0.75Ti3O12 (BLT) thin films were grown by pulsed laser deposition at substrate temperatures ranging from 400 to 800°C. The phase, orientation, and surface morphology of the deposited films were investigated in order to understand the deposition behavior of the BLT films. As the substrate temperature was increased to 800°C, the films showed a tendency to grow with c-axis preferred orientation and a rough surface. At lower substrate temperatures, however, polycrystalline BLT films were formed. In addition, at the lowest substrate temperature of 400°C, the film consisted mostly of a nonferroelectric fluorite-like phase. This was attributed to the difficulty in forming a perfectly layered structure. The fluorite-like phase was transformed to a ferroelectric polycrystalline BLT phase through annealing in flowing oxygen at 600°C.

Determining the absolute value of penetration depth of large area films

We report a method that allows the measurement of the absolute size of the magnetic penetration depth λ(T) of large area YBa2Cu3O7−δ films using the two‐coil method where a YBa2Cu3O7−δ film is overlapped with a 1‐μm‐thick Pb film equivalent in size placed in the gap between the drive and receive coil. The drive coil has fewer counter turns on the top section than the bottom, which is closer to the film, so the film’s edge current would be suppressed and the accuracy that depends on geometry is greatly improved. The transition of the Pb film causes a steplike portion in the mutual inductance M(T) graph and the resultant step height leads to the absolute value of λ.

Voltage noise and vortex states in YBa2Cu3Ox films

Abstract The voltage-noise measurements in epitaxial YBa 2 Cu 3 O x films have been extended to the case of a high magnetic field. In a magnetic field, two noise peaks, one from the vortex motion and the other from the resistance fluctuations, were observed. The location of the vortex-motion induced peak was found to be well above the vortex-glass transition. Our interpretation is that a noise peak manifests itself in a crossover from the pinned vortex-liquid to the unpinned vortex-liquid state.

Origin of 1fnoise peaks of YBa2Cu3Ox films in a magnetic field

Abstract The temperature and magnetic-field dependence of 1 f noise from voltage fluctuations has been measured in epitaxial YBa2Cu3Ox films. In a magnetic field, two noise peaks were observed as temperature is reduced; one at higher temperature showed no magnetic-field dependent, while the peak near the zero-resistance transition temperature showed clear magnetic- field dependence. We show that the field-dependence noise is due to flux motion interacting with the pinning potential. A classical model is used to explain the flux-motion-induced noise peaks. The origin of the noise peaks is interpreted as due to the opposite temperature dependences of the flux-bundle size and the sample resistance. A rough estimate of the temperature and magnetic- field dependence of the flux-bundle size is presented.

A rapid single flux quantum 1 bit arithmetic logic unit constructed with a half-adder cell

We have designed, fabricated, and tested a rapid single flux quantum (RSFQ) 1 bit arithmetic logic unit (ALU) block. The circuit consists of three DC current driven SFQ switches and a half-adder. We successfully tested the half-adder cell at clock frequency up to 20 GHz. The switches were commutating output ports of the half-adder to produce AND, OR, XOR, or ADD functions. For a high-speed test, we attached two switches at the input ports of the half-adder to control the high-speed input data by low-frequency pattern generators. The output in this measurement was an eye-diagram. Using this set-up, the circuit was successfully tested up to 20 GHz. The chip was fabricated using a standard HYPRES 1 kA cm−2 Nb Josephson junction fabrication process.

Pinning effect on critical dynamics in YBa2Cu3O7−δ films with inherent random disorder and with columnar defects

Abstract The in-plane longitudinal resistivities of YBa 2 Cu 3 O 7− δ (YBCO) films before and after Sn ion irradiation of a matching field 1 T have been measured as a function of magnetic field H (≤6 T) and temperature T . The extracted fluctuation part of the conductivity σ xx * ( T , H ) of the unirradiated sample with inherent random disorder exhibits 3D- XY scaling behavior that reveals dynamic critical exponent z =1.86±0.1 and ν ≈0.669. The inherent random disorder increases the dynamic critical exponent z to 1.86±0.1 from the value of 1.5 that is for detwinned YBCO single crystal. After the irradiation, the fluctuation conductivity enhanced by the strong pinning of vortex shows deviation from the scaling behavior for the unirradiated one, suggesting that the critical dynamics is strongly perturbed by the strong pinning due to columnar defects.

Microwave properties of high-Tc YBa2Cu3O7−δ microstrip resonators with different surface morphologies and finite thicknesses

Abstract Microwave properties of YBa 2 Cu 3 O 7−δ (YBCO) microstrip ring resonators with finite thicknesses and different surface morphologies were investigated. The unloaded quality factor of a YBCO resonator made of thick film appeared smaller than that of a different YBCO resonator made of a less thick film, due to the smaller ratio of the film thickness to the penetration depth. The measured penetration depth appeared to increase due to surface roughness, resulting in a decrease of the ratio of film thickness to the penetration depth and an increase of the effective surface resistance. A method is introduced to calculate the intrinsic surface resistance of a high- T c microstrip resonator with finite thickness. YBa 2 Cu 3 O 7−δ (YBCO) microstrip ring resonators with their thicknesses in the order of the magnetic field penetration depth were prepared from YBCO thin films grown by laser ablation on LaAlO 3 and their microwave properties were measured at temperatures above the liquid nitrogen temperature and frequencies in 8–12 GHz.

Comparison of microstructures of pulsed laser deposited YBa2Cu3O7−δ thin films using solid-state sintered and modified melt-textured grown targets

Abstract YBa 2 Cu 3 O 7−δ (YBCO) thin films were deposited by pulsed laser deposition (PLD) using differently prepared targets. The targets were fabricated using both solid-state sintering (SSS) and modified melt-textured growth (MTG) methods. The films deposited from MTG targets had better superconducting properties, such as T c , Δ T and J c , than the films deposited from SSS targets. In order to understand the enhanced superconducting properties, the film microstructures were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As the laser energy density was increased, the films showed an increase of a -axis outgrowth. However, the observed a -axis outgrowth formation indicated that the deposition using MTG targets would result in a more homogeneous and stable film growth as compared to SSS targets. Also, TEM analysis implied that the film deposited from MTG targets had an enhanced grain connectivity that would reduce weak-link effects in the film. Thus, these microstructural differences of films deposited from MTG targets were considered to result in higher J c than the films deposited from SSS targets.

Enhanced transport properties in YBa2Cu3Ox films with crossed columnar defects of various inclination angles

Abstract We have studied the effect of heavy-ion irradiation on the transport properties of YBa 2 Cu 3 O x thin films. Columnar defects parallel to the c axis (0°) and crossed columnar defects with various inclination angles ( θ = ± 10°, ± 20°, and ± 40°) were introduced into films by 1.3 GeV U-ion irradiation with dose equivalent to 1 T vortex density. Transport properties including irreversibility fields, activation energies, and critical current densities have been measured in magnetic fields applied parallel to the c axis. The best transport properties were observed in samples of θ = ± 10° in most of the H-T plane we studied, consistent with the splay model. However, the relative enhancement of the critical current density in the θ = ± 10° compared to the 0° sample was not as large as in the case of a single crystal suggesting that in films the point defects also play an important part in flux pinning.

Origin of a-axis outgrowth in pulsed-laser-deposited YBa2Cu3O7-δ thin films from modified melt-textured grown targets

YBa2Cu3O7-δ (YBCO) thin films were deposited using pulsed laser deposition (PLD) from modified melt-textured grown targets. As the laser energy density was increased, the surface of the films was covered with enhanced a-axis outgrowths. In order to determine the origin of these formations, the microstructures of films deposited at 2 and 4 J cm-2 were investigated using x-ray diffraction, transmission electron microscopy and high-resolution electron microscopy. It was found that a significant number of Y2O3 inclusions were formed during the growth of c-axis-oriented films at 4 J cm-2. These inclusions formed nucleation sites for the a-axis outgrowths. The formation of Y2O3 inclusions was attributed to the Y-rich targets. In addition, the non-equilibrium characteristics of PLD and the enhanced ablation of Y-rich phases attributed to the increase of laser energy density led to these Y2O3 inclusions. As for the a-axis outgrowth nucleation, it is considered that, due to the unstable growth conditions with a high flux density of incident vapour species and the strain induced by the surrounding c-axis films, the Y2O3 inclusions would prefer the nucleation of a-axis grains.