Takanobu Kisu

Effect of thermal noise on the characteristics of a high Tc superconducting quantum interference device

Effects of thermal noise on the characteristics of the dc superconducting quantum interference device (SQUID) have been studied. Numerical simulation on the SQUID characteristics operating at T=77 K has been performed by taking into account the thermal noise. It is shown that the voltage versus flux relation of the dc SQUID is degraded considerably with the thermal noise. The degradation becomes significant when the inductance of the SQUID increases. Due to this degradation, there exists significant limitation for the range of the inductance available at T=77 K, unlike the case at T=4.2 K. The maximum inductance should be around 200 pH in order to avoid significant degradation of the transfer function. This limited value of the inductance must be taken into account when we realize the SQUID coupled to an input coil. The analytical expression for the degradation of the transfer function due to the thermal noise is also obtained. The theoretical result explains experimental results reported recently.

Estimate of Attainable Critical Current Density in Superconducting YBa2Cu3O7-δ

Attainable critical current density in oxide superconductor YBa2Cu3O7-δ is estimated by an ordinary analyzing method in flux pinning phenomena with superconducting parameters obtained from a magnetization measurement. The pinning mechanisms considered here are an anisotropy in the upper critical field and an electron scattering effect for grain boundaries and twinning planes. While the elementary pinning force of these planar defects is small due to a small thermodynamic critical field at 77 K, a critical current density comparable to that of Nb-Ti at 4.2 K is expected to be attained by increasing the pin concentration.

Modeling of Kinetic-Inductance Coplanar Striplin with NbN Thin Films

In order to determine transmission line parameters of superconducting coplanar striplines with thickness smaller than the magnetic penetration depth of the striplines, in which the influences of enlarged kinetic inductance and strongly nonuniform current distribution make it useless to apply the conventional design formula developed for normal conductors, we have conducted theoretical and experimental studies on the geometrical effects of the kinetic inductance and the microwave loss of the thin coplanar striplines. The analytical expression for the kinetic inductance of the coplanar stripline is given using the conformal mapping technique, and it is shown with the London equation and the two fluid-model that the microwave loss is directly proportional to the kinetic inductance. The comparison has been made between theory and experiment concerning the kinetic inductance and the microwave quality factor of NbN coplanar striplines with various thicknesses and widths. It is shown that the observed characteristics are well modeled by the analytical expression.

Effect of flux creep on current-voltage characteristics of superconducting Y-Ba-Cu-O thin films

Temperature dependence of current-voltage characteristics of Y-Ba-Cu-O thin films is studied experimentally. The observed results are analyzed with the flux creep model, and good agreement is obtained between the theory and the experiment. Using the flux creep model, the pinning potential, U, is obtained as U(T=0)\gtrsim77 meV for thin films with jc(T=0)\gtrsim2×107 A/cm2. The temperature dependence of U is also obtained, and the result is discussed based on the pinning theory.

Magnetic penetration depth and critical current in YBaCuO thin films

The ab-plane magnetic penetration depth lambda in c-axis-oriented YBaCuO thin films was estimated from inductance measurements. The temperature dependence of lambda was shown to be consistent with the Bardeen-Cooper-Schrieffer (BCS) theory under the local limit condition. It was also shown that the experimentally obtained relationship among lambda (T=0), the resistivity p, and the critical temperature T/sub c/ was consistent with the theoretical one obtained from the local limit BCS theory. From that relationship, the value of lambda (T=0) could be deduced from p and T/sub c/. Moreover, the critical current was studied on the basis of the bias current dependence of lambda .<<ETX>>

EFFECT OF CUMULATIVE ABLATION ON THE EJECTION OF PARTICULATES AND MOLECULAR SPECIES FROM YBA2CU3O7-X TARGETS

Effects of cumulative ablation on the ejection of particulates and molecular species in pulsed‐laser deposition are studied by Mie scattering and laser‐induced fluorescence spectroscopy, respectively. When a fresh target is ablated, a large amount of particulates are ejected during several initial shots and rapidly decreased within the first ten shots of ablation. This is due to the ejection of powder residues which are struck on the target surface during the polishing process. After this period, ejection of particulates increased gradually and almost saturated after 200 shots. The saturation characteristic is empirically formulated as a function of the number of cumulative ablations. On the other hand, ejection of molecular species rapidly decreases during the initial 500 ablations and afterwards decreases more slowly with further ablation. The effects of cumulative ablation on the particle ejection are discussed in conjunction with the structural modification of the ablated surface observed by the scann...

Spatial Distribution of Trapped Magnetic Flux in the High Tc Superconductor Y1Ba2Cu3O7-δ

Spatial distribution of the trapped magnetic flux density in Y1Ba2Cu3O7-δ is measured at T=4.2 K and 77 K with the GaAs Hall device as a magnetic sensor. Experimental results show that the magnetic flux density inside the sample is almost constant spatially, and suggest that superconducting grains are coupled with each other very weakly in the present sample.

Modulation of kinetic inductance of high Tc superconducting thin films with bias current

Kinetic inductance of high Tc superconducting thin films is studied using numerical simulation of the Ginzburg-Landau (GL) theory. It is shown that a current distribution in the film changes nonlinearly with a bias current supplied to the film, which results from the nonlinear properties of the GL equation. Using the current distribution, the kinetic inductance of the film is obtained as a function of the bias current. It is shown that the kinetic inductance is constant for low bias currents, and increases nonlinearly with the bias current when the bias current becomes large. The effect of the film geometry on the modulation of the kinetic inductance is quantitatively studied. Analytical solutions for the current distrubiton and the kinetic inductance are also obtained, which agree well with the simulation results. The modulation of the kinetic inductance with the bias current predicted from the theory is partly confirmed from experiments using YBaCuO films.

Relation between Pinning Potential and Critial Current Density of YBaCuO Superconducting Thin Films

The relation between the pinning potential and the critical current density of YBaCuO thin films in weak magnetic fields is studied experimentally. The pinning potential U is obtained by comparing the current-voltage characteristics of the film with the flux-creep theory. The values U=25–120 meV are obtained for films with the critical current density jc(T=0)=5.3×105–1.8×107 A/cm2. It is shown that the pinning potential increases with the increase of the critical current density. The obtained results can be qualitatively explained with the pinning model of isolated vortices.

Critical Current Distribution and Flux Creep Properties in Y1Ba2Cu3O7-δ Superconducting Thin Films

Current-voltage (I-V) characteristics of Y1Ba2Cu3O7-δ thin films were studied in a wide range of 10−4 to 102 V/m. Isolation of flux creep and flux flow was discussed with frequency dependence of I-V curve. By use of series domain network model, critical current distribution was estimated from non-linear I-V curves. Furthermore, an expression of flux creep considering the inhomogeneous critical current was derived. Comparing the expression and measurements, we determined pinning potential and the boundary between flux creep and flux flow. It was shown the obtained expression agreed well with the measurement in more than six decades.