Co-doped BaFe_(2)As_(2) Josephson junction fabricated with a focused helium ion beam

Josephson junction plays a key role not only in studying the basic physics of unconventional iron-based superconductors but also in realizing practical application of thin-film based devices,therefore the preparation of high-quality iron pnictide Josephson junctions is of great importance.In this work,we have successfully fabricated Josephson junctions from Co-doped BaFe_(2)As_(2)thin films using a direct junction fabrication technique which utilizes high energy focused helium ion beam(FHIB).The electrical transport properties were investigated for junctions fabricated with various He^(+)irradiation doses.The junctions show sharp superconducting transition around 24 K with a narrow transition width of 2.5 K,and a dose correlated foot-structure resistance which corresponds to the effective tuning of junction properties by He^(+)irradiation.Significant J_c suppression by more than two orders of magnitude can be achieved by increasing the He^(+)irradiation dose,which is advantageous for the realization of low noise ion pnictide thin film devices.Clear Shapiro steps are observed under 10 GHz microwave irradiation.The above results demonstrate the successful fabrication of high quality and controllable Co-doped BaFe_(2)As_(2)Josephson junction with high reproducibility using the FHIB technique,laying the foundation for future investigating the mechanism of iron-based superconductors,and also the further implementation in various superconducting electronic devices.

A new magneto-cardiogram study using a vector model with a virtual heart and the boundary element method

A cardiac vector model is presented and verified, and then the forward problem for cardiac magnetic fields and electric potential are discussed based on this model and the realistic human torso volume conductor model, including lungs. A torso-cardiac vector model is used for a 12-lead electrocardiographic （ECG） and magneto-cardiogram （MCG） simulation study by using the boundary element method （BEM）. Also, we obtain the MCG wave picture using a compound four-channel HTc.SQUID system in a magnetically shielded room. By comparing the simulated results and experimental results, we verify the cardiac vector model and then do a preliminary study of the forward problem of MCG and ECG. Therefore, the results show that the vector model is reasonable in cardiac electrophysiology.

Fabrication and properties of high performance YBa_2Cu_3O_(7-δ) radio frequency SQUIDs with step-edge Josephson junctions

We describe the fabrication of high performance YBa2Cu3O7-δ （YBCO） radio frequency （RF） superconducting quantum interference devices （SQUIDs）, which were prepared on 5 mm×5 mm LaAlO3 （LAO） substrates by employing stepedge junctions （SEJs） and in flip-chip configuration with 12 mm×12 mm resonators. The step in the substrate was produced by Ar ion etching with step angles ranging from 47° to 61°, which is steep enough to ensure the formation of grain boundaries （GBs） at the step edges. The YBCO film was deposited using the pulsed laser deposition （PLD） technique with a film thickness half of the height of the substrate step. The inductance of the SQUID washer was designed to be about 157 pH. Under these circumstances, high performance YBCO RF SQUIDs were successfully fabricated with a typical flux-voltage transfer ratio of 83 mV/φ0, a white flux noise of 29 μφ0/√Hz, and the magnetic field sensitivity as high as 80 fT/√Hz. These devices have been applied in magnetocardiography and geological surveys.

Measurements,characteristics,and origin of new electromagnetic interference on magnetocardiographic measurements

In order to eliminate the influence of the large-amplitude magnetic field noise that has complicated magnetocardio- graphic studies since October 2009, we have performed high-accuracy measurement of the environmental magnetic field noise in the vicinity of Beijing Subway Line 4 using a three-component height Tc radio frequency （rf） superconducting quantum interference device （SQUID）. By analysing the spatial form and other characteristics of the time and the fre- quency domains and by calculating the circumferential magnetic field distribution based on a duel-end feeding system model, we reach the following conclusions： （i） the main source of magnetic field noise is the magnetic field generated by thel subway trains, and （ii） the magnetic field interference results mainly from the imbalance between traction current and return rail current that is caused by the leakage current.

High-Temperature Superconducting YBa_(2)Cu_(3)O_(7-δ)Josephson Junction Fabricated with a Focused Helium Ion Beam

As a newly developed method for fabricating Josephson junctions,a focused helium ion beam has the advantage of producing reliable and reproducible junctions.We fabricated Josephson junctions with a focused helium ion beam on our 50 nm YBa_(2)Cu_(3)O_(7-δ)(YBCO)thin films.We focused on the junction with irradiation doses ranging from 100 to 300 ions/nm and demonstrated that the junction barrier can be modulated by the ion dose and that within this dose range,the junctions behave like superconductor–normal conductor–superconductor junctions.The measurements of the I–V characteristics,Fraunhofer diffraction pattern,and Shapiro steps of the junctions clearly show AC and DC Josephson effects.Our findings demonstrate high reproducibility of junction fabrication using a focused helium ion beam and suggest that commercial devices based on this nanotechnology could operate at liquid nitrogen temperatures.

Influence of Hot Rolling on Cube Texture of Ni Substrate for Coated Conductor

Pure Ni and Ni alloy tapes with sharp cube texture and low-angle grain boundaries prepared by thermomechanical process were extensively used as substrates for coated conductor. The thermomechanical process usually includes hot forging and cold rolling. In this study, a hot-rolling process between hot forging and cold rolling was induced. The influence of hot rolling on the cube texture of pure Ni was discussed. Sharp cube texture on pure Ni was obtained by suitable hot rolling, cold rolling, and recrystallization treatment. This deformation texture of tape was studied using orientation distribution function （ODF）. Orientation mapping, content of the cube texture, and grain boundary distribution were performed using an EBSD system mounted on LEO-1450 SEM. The results show that the substrates that are hot rolled have a sharp cube texture and low-angle grain boundaries.

Mechanical Properties of （Bi,Pb）-2223 Multifilament Tapes with Ag-Alloy Sheath

Ag-Mg-V alloy is prepared and investigated to develop a new sheath alloy used for BSCCO tapes. Bi-2223 Ag/AgMgV and Bi-2223 AgMgV/AgMgV tapes are studied with the help of stress-strain measurement, optical microstructure and critical current Ic. The value of Ic at 77K and at magnetic field B = 0 is obtained to be 90A/cm^2 for the samples of Bi-2223 Ag/AgMgV tapes, which is higher than that of Bi-2223 AgMgV/AgMgV tapes （72A/cm^2）. The resistance of AgMgV alloy, up to 0.37μΩ·cm at 77K, is higher than that of pure Ag （0.28μΩ·cm） after annealed at 840℃ for 4012, which is studied for reducing the ac loss. The values of the fracture strength and the maximum strain are 86 MPa and 0.50% for Bi-2223 Ag/AgMgV tapes and 108 MPa and 0.33% for Bi-2223 AgMgV/AgMgV tapes, respectively.