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The step-edge type high temperature superconductor (HTS) radio frequency (rf) superconducting quantum interference devices (SQUIDs) were fabricated from YBa2Cu3O7−δ thin films with a field sensitivity of 66 fT/Hz1/2. Even though the sensitivity of the rf SQUID sensor used in the experiments was degraded to 77 fT/Hz1/2 in the magnetic shielded room (MSR), the performance of the rf SQUID sensor was good enough for the ultra-low field (ULF) nuclear magnetic resonance (NMR) experiments at a measurement field of 61.5 μT. With the help of a strictly controlled fast switching circuit and a coupled LC circuit which acted as flux transformer, the signal-to-noise ratio (SNR) of the rf SQUID-based ULF NMR system with different operating points was measured. The optimum SNR was 40 dB which increased significantly by coupling an input coil closely beneath the rf SQUID sensor. The extracted 1/T1 value for pure water sample at 23 ∘C showed the rf SQUID based ULF NMR was an effective system for ULF NMR researches. All above, the direct coupled method and the control circuit show a great potential in the future rf SQUID-based ULF NMR &MRI system.

The BiFeO3/YBCO heterostructures were fabricated on (001)-oriented SrTiO3 substrates by pulsed laser deposition. The superconducting transition temperature of YBCO was found to decline with increasing the c-axis parameter. The microstructure and dislocations in BiFeO3/YBCO heterostructure were detected by transmission electron microscopy (TEM) proving the epitaxial growth of BiFeO3/YBCO heterostructures on SrTiO3. The edge dislocation with a Burgers vector [100] was observed in BiFeO3, which contributed to the relaxation of BiFeO3. The results of magnetization hysteresis cycles indicated that the saturation magnetization increased from 14 emu/cm3 of BiFeO3/SrTiO3 to 42 emu/cm3 of BiFeO3/YBCO/SrTiO3. The second phase Fe2O3 on the surface of BiFeO3 was found to be α-Fe2O3 with weak magnetization at room temperature. Therefore, the increased magnetization could be accounted for by the induced ferromagnetism at the interface of BiFeO3/YBCO.

We report on improvements of mode-locked soliton pulse energy and peak power in a thulium-doped fiber laser. At the pump power of 370 mW, conventional soliton can be first generated using nonlinear polarization rotation technology. By increasing the pump power to 550 mW and appropriately adjusting cavity polarization state, the conventional soliton pulse can be converted into the Gaussian pulse. Due to dispersion management, excess accumulation of nonlinear phase shifts in the cavity can be avoided. In this case, the pulse energy and peak power can be increased from 77 pJ to 1.38 nJ and from 106.9 W to 2.67 kW, respectively. These improvements indicate that the Gaussian pulse operation can effectively enhance the pulse energy and peak power of the thulium-doped fiber laser.

We propose and demonstrate a high repetition-rate passively mode-locking Tm-doped fiber laser. Mode-locking operation is achieved by using a conventional nonlinear polarization rotation (NPR) structure. Fundamental frequency soliton laser is obtained with the repetition-rate of 5.05 MHz and the pulse width of 54.7 ps. 3-dB bandwidth and signal and noise ratio (SNR) are 3.1 nm and 53.03 dB, respectively. In order to improve repetition-rate of the mode-locking pulse, a narrow band filter is adopted. In this case, maximum repetition-rate of the output pulse can be up to 7.75 GHz (1534 orders). Pulse width and 3-dB bandwidth of mode-locking laser are compressed to 67.2 ps and 0.2 nm, respectively, and SNR is 19.71 dB. In addition, the laser center wavelength can be tuned from 1882 nm to 1925 nm by adjusting the filter.

BiFeO3 (BFO)/YBCO heterostructures are fabricated on (001)-oriented SrTiO3 substrates. The transport properties of BFO/YBCO heterostructures with Au and YBCO top electrode are investigated. The Au/BFO/YBCO heterostructures show obvious rectification effect. The current through Au/BFO/YBCO heterostructures at positive bias is mainly bulk-limited space-charge-limited current (SCLC) at temperatures from 15 K to 280 K. As the applied voltage on Au electrode is negative, Au/BFO/YBCO heterostructures satisfy the interface-limited Schottky emission at temperatures from 220 K to 300 K, instead of interface-limited Fowler-Nordheim tunneling at temperatures from 15 K to 80 K. The leakage current density is reduced by two orders of magnitude through substituting the Au electrode by YBCO electrode. The transport mechanism in YBCO/BFO/YBCO heterostructures also changes to SCLC. The different leakage mechanisms result from the reduced oxygen vacancies because the fabrication of YBCO layer increased oxygen concentration in BFO films.

200 nm Ni film is coated on 25 μm thick Mo foil, and graphene is grown on the Ni–Mo system by CVD method. After the annealing process of CVD, the Ni/Mo bilayer transforms into Ni–Mo alloy, then we have successfully fabricated MgB 2 films on graphene/Ni–Mo alloy system via the hybrid physical–chemical vapor deposition (HPCVD) technique. The transition temperature T c onset is 38.25 K with a corresponding transition width of 0.75 K. The average thickness of MgB 2 films is 200 nm (25% concentration B 2 H 6 ). The critical current density derives from the magnetization measurement at 5 K is, j c (5 K, 0 T) = 9.6 × 10 6 A/cm 2 . We can easily deposite MgB 2 on graphene/Ni–Mo alloy system with a lower B 2 H 6 concentration and less gas flow, which lays a good foundation for depositing MgB 2 thick films. The graphene in this system is multilayer and with defects, it may act like an intermediary film for the growth of MgB 2 , or a carbon-doping source.

The experiment for preparing MgB 2 thin films by electron beam (e-beam) annealing of Mg–B multilayer is reported. The annealing process lasts less than 1 s, which effectively suppresses the volatility of Mg as well as the reaction of Mg and oxygen. The influence of annealing parameters on the film quality including exposure time, accelerating voltage and beam current is investigated. It is found that e-beam with lower accelerating voltage can acquire MgB 2 film with smaller annealing energy density. Also, double e-beam scan annealing process helps to obtain more uniform MgB 2 thin films than the single scan. This rapid annealing method synthesizes MgB 2 thin films with critical temperature T c of 35 K and critical current density J c at 15 K of 7.6 × 10 6 A cm −2 .

We experimentally studied a passive mode-locked square-wave fiber laser with net-normal dispersion by using a nonlinear amplifying loop mirror. The output pulse width of the square-wave pulse (SWP) can be extended with the increasing of the pump power as well as single pulse energy. The SWP can be amplified without shape distortion when the mode-locked laser is injected into an erbium-ytterbium codoped fiber amplifier. Then, by inputting the amplified SWPs into a single mode fiber (SMF), a supercontinuum (SC) can be observed with the spectral range of about 1550 to 1850 nm. Broader and flatter SC with the 20-dB spectral range of about 1550 to 1950 nm can also be generated using a segment of dispersion-shifted fiber instead of SMF.

High‐quality single phase Nd1+xBa2−xCu3O7−d thin films have been fabricated on SrTiO3 substrates by the laser ablation method. The resistivity of the deposited films are usually lower than that of the YBCO film. The Tc(onset) and Tc(R=0) in the optimized thin films are as high as 91 and 88 K, respectively. The crystallinity of our films are higher than the best YBCO films ever reported. The rocking curve FWHM of the (005) peaks in our thin films is typically 0.05°, which is superior to the best reported YBCO films.

The proximity effect in multilayer structures with dirty two band superconductors and ferromagnets is investigated here. With a homogeneous exchange field and interband scattering in the ferromagnetic layer, the condensate functions that penetrate the ferromagnet are coupled up. This coupling effect is intensively suppressed by the exchange field and modulated by the diffusion coefficients of the two bands. Such coupling also affects the superconductor where the condensate functions of the two bands are also coupled up through the proximity effect. An inhomogeneous exchange field raises the damped oscillation of the condensate functions as the spatial distance from the interface increases. There is a 0−π phase shift when the ferromagnetic layer thickness varies beyond the ferromagnetic coherence length.