Yitong Zhang

Effect of electric field relaxation on the V-I curve

The effects of sweeping rate of applied current (dI/dt) on the characteristic (V - I) and electric field relaxation (V - t) curves have been investigated numerically as well as experimentally. The calculation is based on the nonlinear electric field diffusion equation, while the experiment is conducted by electric transport measurements of V - I and V - t curves on three Ag-Bi-2223 tape samples. It is found that the V - I curves shift toward smaller current with increasing dI/dt and V decays with time apparently. We show that a certain dI/dt causes a highly spatially varied electric field which is the natural result of the fact that flux lines can only diffuse over barriers with certain velocity. We also show that the electric field diffusion causes the two phenomena in the bulk sample: (1) the V - I curve is affected by the sweeping rate of the applied current; (2) the resistance relaxes with time.

Effect of flux creep on Ic measurement of electric transport

The influence of flux creep on Ic measurement was studied by numerically solving the nonlinear flux creep equation at different dI/dt, Vc and n*, where dI/dt is the sweeping rate of the applied current (I), Ic, Vc and n* are the critical current, the criterion and the material parameter, respectively. It is shown that the V-I curve consists of two parts converging at Ip, Vp and (dI/dt)p at which the current fully penetrates the sample. In the segment where I Ip, the curve is a power law, VIn*, reflecting the material equation. It is suggested that the appropriate region in the V-I curve to determine n* is where I>Ip. Based on the V-I curve, it is concluded that if dI/dt>(dI/dt)p, Ic decreases with increasing dI/dt. On the other hand, if dI/dt<(dI/dt)p, Ic is independent of dI/dt and is therefore suitable. The three critical parameters ((dI/dt)p, Vp and Ip) are dependent on each other. The parabolic V-I relation can be observed in the giant flux creep state (small n*), whereas in the critical state (large n*), V in the parabolic V-I relation is too small to be detected by a standard voltmeter. This also indicates that the pulsed method may underestimate Ic in the case of high temperatures or in strong applied fields, but it will not affect Ic in the case of low temperatures, weak applied fields and strong pinning.

Dynamic response of Ag–Bi2223 tapes in applied fields by electric transport measurement

Abstract Electric transport measurement of Ag-sheathed Bi 2−x Pb x Sr 2 Ca 2 Cu 3 O y tapes in magnetic fields H was conducted by the four-probe method to study the effect of H and sweeping rate of applied current d I/ d t on critical current. It is found that H and d I/ d t apparently affect the critical current and the V–I characteristic curve. To understand the experimental result, numerical simulation was carried out based on the collective creep model of the vortex glass. The numerical result is in agreement qualitatively with the experiment. It is shown that the influence of the applied field, temperature and vortex pinning on the normalized critical current can be reduced to a single critical current parameter n .

Study of the relationship between the V -I curve and the flux dynamics in superconductors

The equivalence of the experimental V-I characteristic (V∝Im) and the material E-j curve (E∝jn) was studied. We show numerically that only when the current I is larger than Ihom, the fully penetrating current, can the V-I curve be equivalent to the E-j one and thus be used to determine the material parameter n, whereas if I<Ihom, j is inhomogeneous and the V-I curve is not appropriate for use in probing the properties of the sample. The inhomogeneity of j can be checked by simply measuring the voltage relaxation curve at a given I. Furthermore, it is shown that Ihom varies with dI/dt and n. The dependence of Ihom on dI/dt indicates that current cannot go directly into a homogeneous region in practical transport measurements. Moreover, we argue that all of the V-I curves with different values of the flux-creep barrier exponent µ merge at large current. Since current is probably inhomogeneous in the small-current region, the V-I curve might not be appropriate for use in studying the properties of the sample in this region. Therefore, the V-I curve may not be appropriate for determining µ in the U-j relation at all sensitively.

Magnetic properties of high-Tc superconductive materials in low fields

Abstract Magnetic hysteresis loops in low fields at 77 K were measured by vibrating sample magnetometer for sintered and melt-textured high- T c superconductors. The characteristics of loops for different samples were compared and analyzed, and the difference between transport and magnetic critical current density of these granular materials is discussed. For studying the vortex pinning mechanism of grain interfaces, critical state models were used to derive critical current density j cm ( b ) by fitting model-calculated magnetic hysteresis to experimental results. The field dependence of intergranular currents determined by intergranular weak links was found to correspond very well to the exponential model.

Outcome analysis of benign vocal cord tumors treated by laryngeal endoscopy under low temperature-controlled radiofrequency

OBJECTIVE This study aimed to evaluate the outcome of benign vocal cord tumors treated using a laryngeal endoscopy under low temperature-controlled radiofrequency and to elucidate the application of a dynamic laryngoendoscopy in the operation. MATERIALS AND METHODS 85 patients with benign vocal cord tumors were treated by laryngeal endoscopy under low temperature-controlled radiofrequency from September 2011 to October 2013. A XION electronic dynamic laryngoendoscopy (Germany) was used to observe curative effects 3 months after operation. Wave images were recorded with larynx-wave recording software to analyze tumor characteristics. RESULTS Among the 85 patients, 81 showed smooth surface of operation wounds without any residue. The mucosal wave was also basically normal. Sound was generally recovered after 1-3 months. Three cases presented improved pronunciation function after the operation, whereas 1 patient with residual tumor at the front of vocal chords underwent another operation after 6 months. CONCLUSION Low temperature-controlled radiofrequency exhibited many advantages, including minimal trauma, minimal bleeding, high safety, and few complications. Moreover, treatment of benign vocal cord tumors with a laryngeal endoscopy presented satisfactory outcomes. Therefore, this technology has broad application prospects.

Voltage Relaxation and Its Influence on Critical Current Measurements

Based on the collective creep model, we numerically studied evolution of electric field and current density in superconductors and its influence on transport measurements of critical current. It is shown that many experimental facts, such as the dependence of V-I curves on sweeping rate of applied current and voltage relaxation are the results of this evolution. The simulation results are confirmed by electric transport measurements on Ag-sheathed Bi2−xPbxSr2Ca2Cu3Oy tapes. Discussions on influences of the voltage relaxation on electric transport measurements including superconducting critical current are made.

Magnetoelectrically coupled polariton excitation in a plasmonic crystal composed of nanorod dimers

In this work, the long wavelength optical properties of a plasmonic crystal, composed of gold nanorod dimers arranged parallel, have been studied. Due to the strong coupling between incident light and the oscillation of free electrons inside nanorod dimers, the magnetically induced and/or magnetoelectrically coupled plasmonic polaritons can be excited. A theoretical demonstration has been proposed and coupled equations that show similar profiles to the Huang-Kun equations for ionic crystals have been deduced, indicating the constitutive abnormalities and polaritonic bandgap effect. The analogy between the magnetoelectrically coupled metamaterials and ionic crystals may shed light on physical explanations, as well as constitutive parameter retrieval, for the magnetoelectric metamaterials.

Study of history effect of vortex matter by AC susceptibility

We have observed numerically the history effect, based on bulk flux pinning framework, in the susceptibility χ(T) (acs), experimentally measured in different samples. The dependencies of history effect in χ(T) on frequency, ac and dc fields, and bulk pinning strength are calculated. Our calculation matches well the behavior of χ in 2H–NbSe2, showing the bulk model works well. It is also shown that bulk pinning and surface pinning will result in a history effect of very different characteristics. In addition, the numerical result shows that a peak in critical current density, which is usually accompanied by the vortex melting transition, may not be observed as a dip in acs in some cases, which is a dynamic response of a sample to applied as field and is dependent on ac field frequency and amplitude.

Distribution of barriers of vortex matter at different temperatures in single crystal Bi2212

Abstract Complex ac susceptibility, systematically measured in different temperatures, dc fields, ac field amplitudes and frequencies, is analyzed to extract activation barrier U and time scale t 0 of vortex diffusion for single crystal Bi2212. The phase transition line between the vortex solid and the liquid states is determined according to the jump of t 0 as well as the change of U . The analysis, based on the phenomenological equation with an asymmetrical distribution of thermal activated barriers, shows for the first time that U in plastic deformation region is of a statistical distribution dependent on temperature, whereas there is no such a distribution for the vortex liquid state.