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Register a new userFirst critical field measurements of superconducting films by third harmonic analysis
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The temperature behaviour of the first critical field ($B_{C1}$) of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the sample, a non zero third harmonic voltage $V_{3}$ is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of $B_{C1}$ using this technique are detailed. As validation test, different type II superconductors (Nb, NbN, MgB$_{2}$ and Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ under the form of thin films) have been measured. The comparison between experimental results, data presented in literature and theoretical predictions is presented and discussed.
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We propose an inductive method to measure critical current density $J_c$ in bulk superconductors. In this method, an ac magnetic field is generated by a drive current $I_0$ flowing in a small coil mounted just above the flat surface of superconductors, and the third-harmonic voltage $V_3$ induced in the coil is detected. We present theoretical calculation based on the critical state model for the ac response of bulk superconductors, and we show that the third-harmonic voltage detected in the inductive measurements is expressed as $V_3= G_3omega I_0^2/J_c$, where $omega/2pi$ is the frequency of the drive current, and $G_3$ is a factor determined by the configuration of the coil. We measured the $I_0$-$V_3$ curves of a melt-textured $rm YBa_2Cu_3O_{7-delta}$ bulk sample, and evaluated the $J_c$ by using the theoretical results.
Critical current density (Jc) is one of the major limiting factors for high field applications of iron-based superconductors. Here, we report that Mn-ions are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe films. Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm^2 under 33 T, and the vortex pinning force density monotonically increases up to 106 GN/m^3, which is the highest record so far among all iron-based superconductors. Our results demonstrate that Mn incorporation is an effective method to optimize the performance of (Li,Fe)OHFeSe films, offering a promising candidate for high-field applications.
We develop a lower critical field (Hc1) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. Parameter Hc1 is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor-Insulator-Superconductor multilayer structure (S-I-S structure). As a validation test, we measure the temperature dependence of Hc1 of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4-5 K without any problem of heat generation of the coil. This field value is higher than those reported in previous works and makes it possible to more accurately estimate Hc1 at lower temperatures.
A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can pin the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.
The key ingredient of high critical currents in a type-II superconductor is defect sites that pin vortices. Contrary to earlier understanding on nano-patterned artificial pinning, here we show unequivocally the advantages of a random pinscape over an ordered array in a wide magnetic field range. We reveal that the better performance of a random pinscape is due to the variation of its local-density-of-pinning-sites (LDOPS), which mitigates the motion of vortices. This is confirmed by achieving even higher enhancement of the critical current through a conformally mapped random pinscape, where the distribution of the LDOPS is further enlarged. The demonstrated key role of LDOPS in enhancing superconducting critical currents gets at the heart of random versus commensurate pinning. Our findings highlight the importance of random pinscapes in enhancing the superconducting critical currents of applied superconductors.
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