High Critical Current Density of c-Axis-Oriented MgB2 Thin Films

From the magnetic hysteresis loops (M-H) and the current-voltage (I-V) relations, we obtained the critical current density (J _c) for c-axis-oriented MgB₂ thin films with a transition temperature of 39 K. The temperature dependence of J _c obtained from the M-H and the I-V curves coincide, which indicates the strongly linked nature of the intergrains in these thin films. And, the high value of J _c was estimated to be 4×10⁷ A/cm² at 5 K and 1×10⁵ A/cm² at 37 K under a self field, indicating a promising candidate for technological applications.     

Microwave and Terahertz Surface Resistance of MgB2 Thin Films

The knowledge of the surface resistance R _s of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the R _s of MgB₂ films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between R _s and normal-state resistivity right above T _c, ρ₀, i.e., R _s decreases almost linearly with the decrease of ρ₀. (2) A low residual R _s, less than 50 μΩ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB₂ has the lowest R _s compared with two other superconductors Nb₃Sn and the high-temperature superconductor YBa₂Cu₃O_7−δ(YBCO). (3) From THz measurement it is found that the R _s of MgB₂ up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of R _s follows ω ⁿ , where ω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB₂ thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.     

Fabrication and Transport Properties of MgB2 Thin Films and Tunnel Junctions

We report on fabrication and characterization of MgB₂ thin films and tunnel junction structures. The MgB₂ films were prepared on Al₂O₃, Si, glass, and plastic foil substrates by either vacuum codeposition of boron and magnesium, or high-temperature magnesium annealing of boron films. The crystalline structure of our films depended directly on the method of preparation. The films prepared by codeposition and postannealed in Ar atmosphere were amorphous with nanocrystal inclusions and were characterized by very smooth surfaces. On the other hand, the boron-precursor films annealed in magnesium vapor were rough, polycrystalline with approximately 1-μm-diameter single-crystal blocks. Because of their surface quality, the amorphous films were used for preparation of point contact junctions and for optical characterization. The point-contact spectra of tested junctions exhibited a two-gap structure. The MgB₂ polycrystalline films was used for bulk transport studies. The best films were characterized by the critical temperature T _c of up to 39 K and the current density j _c at 4.2 K of about 10⁷ A/cm².     

MgB2 Thin Films on Metal Substrates for Superconducting RF Cavity Applications

Magnesium diboride is a promising material for superconducting RF (SRF) cavity applications. Compared to the currently used superconductor for SRF cavities Nb, MgB₂ has the potential to achieve lower RF loss and higher acceleration field due to its higher critical temperature and thermodynamic critical magnetic field. Since the RF field only penetrates a few penetration depths into a superconductor, a superconducting coating of several hundred nanometers on a metal cavity is sufficient for superb SRF cavity performances. In this work, we report the properties of MgB₂ thin films deposited by the hybrid physical–chemical vapor deposition (HPCVD) technique on different metal substrates including Nb, Mo, Ta, and stainless steel. All the films were polycrystalline, as indicated by X-ray diffractometry and scanning electron microscopy, and showed T _c ～39 K, determined by resistance versus temperature, magnetic susceptibility, and dielectric resonator measurements. MgB₂ films deposited on Nb substrates polished to various degrees of smoothness exhibit similar T _c . The result is a promising step in the investigation of using MgB₂ as an alternative to Nb for SRF cavities.     

Artificial Pinning Centers on MgB2 Superconducting Thin Films Coated by FeO Nanoparticles

MgB₂ thin films were fabricated on MgO (100) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850?°C in magnesium vapor. In order to investigate the effect of FeO nanoparticles on magnetic properties of MgB₂ thin films, the films were coated with different concentrations of FeO nanoparticles by spin coating process. The magnetic field dependence of the critical current density $J_{\mathrm{c}}$ was calculated from the M?CH loops and also magnetic field dependence of the pinning force density $f_{\mathrm{p}}(b)$ was determined for the films containing different concentrations of FeO nanoparticles. The values of the critical current density $J_{\mathrm{c}}$ in zero field at 5?K was found to be around 1×10⁶?A/cm² for pure MgB₂ film, 1.4×10⁶ for MgB₂ film coated with 25?%, 7.2×10⁵ for MgB₂ film coated with 33?%, 9.1×10⁵ for MgB₂ film coated with 50?% and 1.1×10⁶?A/cm² for MgB₂ film coated with 100?%. It?was?found that the film coated with 25?% FeO nanoparticles has slightly enhanced critical current density and it can be noted that especially the film coated with 25?% FeO became stronger in the magnetic field. The films coated with FeO were successfully produced and they indicated the presence of artificial pinning centers created by FeO nanoparticles. The superconducting transition temperature of the film coated with 25?% FeO nanoparticles was determined by moment?Ctemperature (M?CT) measurement to be 34?K which is 4?K higher than that of the pure film.     

Crystal Structure and Superconductivity of MgB2 Thin Films Grown on Various Oxide Substrates

The crystal structure and superconductivity of MgB₂ thin films grown on various oxide substrates were investigated by X-ray diffraction and resistance measurement. The films were prepared by a two-step method, in which precursors B films were annealed in Mg vapor at 900^○C. The X-ray diffraction shows that the MgB₂ films grown on C–AL₂O₃, R–AL₂O₃, and MgO (001) are c-axis oriented while the films grown on SrTiO₃ (001), LaAlO₃ (001), and ZrO₂ (001) are aligned with the (101) direction normal to the substrate planes. All the grown films show superconductivity and their transition temperature varies with the substrates in the range of 34–39 K. We think that the transition temperature variation is probably due to the lattice matching between the film and the substrate, as well as the interdiffusion at the film/substrate interface. The experimental results suggest that if there is no severe interdiffusion at the film/substrate interface in the high temperature annealing process, more substrates could be used for the growth of MgB₂ films using the two-step method.     

Scaling Behavior in Aluminum Doped MgB2 Superconductor

Magnetization measurements have been performed on Mg_1?x Al _x B₂ (0≤x≤0.1) at various temperatures below T _c and in fields up to 9 T. The reversible magnetization has been used to calculate the thermodynamic critical field (H _c ) which is then used to obtain a new universal behavior for both the critical current density (J _c ) and the pinning force (P _f ). This approach is based on vortex core energy ( ${\sim}H^{2}_{c}$ ) and clearly exhibits the deviation from single vortex pinning with increasing temperature and higher concentration of Al-doping.     

Effects of Small Fields on the Magnetic Response of Superconducting Thin Films of MgB2

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB₂ submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.     

Two-Dimensional Superconducting Fluctuations in MgB2 Films

The origin of the resistive transition broadening for superconducting MgB₂ films is investigated experimentally. The crucial role of two-dimensional weak and critical fluctuations is demonstrated.     

Mixed State Hall Resistivity in YBCO c-Axis-Oriented Thin Films

Hall voltage measurements on YBCO c-axis-oriented thin films in the mixed state were carried out. The films were grown on MgO substrates utilising evaporation technique followed by relatively low heating temperature. Hall voltage was measured in low magnetic fields perpendicular to the film surface, in which sign reversal near T_c was observed. Relatively large transverse voltage was found to be insensitive to the magnetic field inversion (even Hall effect). This effect is discussed in terms of the vortices guided motion model which is argued to give a reasonable explanation for the even Hall effect in type II superconductors.     

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB₂ thin films were deposited on MgO (100) substrate and r-plane Al₂O₃ $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB₂ films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J _c was calculated from M–H loops and also the magnetic field dependence of F _p was compared for the different temperature ranges from 5 to 25 K. The critical current density J _c was found to be around 1.0×10⁶ A/cm² and 1.7×10⁶ A/cm² in zero field at 5 K for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al₂O₃ in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.     

Fabrication of Superconducting MgB2 Thin Films by Magnetron co-Sputtering on (001) MgO Substrates

We fabricated superconducting MgB₂ thin films on (001) MgO substrates by magnetron rf and dc co-sputtering on heated substrates. We annealed the samples ex-situ and in-situ at temperatures between 450 ^°C and 750 ^°C. The substrate temperature during the sputtering process and the post annealing temperatures play a crucial role in forming MgB₂ superconducting thin films. We achieved a critical onset temperature of up to 27.1 K for the ex-situ and 25.6 K for the in-situ annealed samples at a film thickness of 30 nm. The samples shows an out of plane (0002)-Peak which was determined by x-ray diffraction.     

MgB2超导膜制备技术

介绍了自MgB2超导电性被发现以来,用于制备MgB2薄膜/厚膜的各种基片、主要生长方法以及退火工艺,简单概述了薄膜制作超导隧道结方面的研究状况.     

金属-绝缘体颗粒膜的巨霍尔效应研究进展

介绍了金属-绝缘体颗粒膜巨霍尔效应的研究背景及样品的制备与测量，总结了近年来该领域的研究进展和应用前景，最后对研发应用中存在的问题和趋势提出了自己的看法。     

Enhancement in High-Field J c Properties and the Flux Pinning Mechanism of MgB2 Thin Films on Crystalline SiC Buffer Layers

We have studied the influence of crystalline SiC buffer layers on the critical current density and on the flux pinning mechanism in MgB₂ thin films. Crystalline SiC buffer layers were deposited on the Al₂O₃ (0001) substrates by using a pulsed laser deposition method, and then MgB₂ thin films were grown on the SiC-buffered layer by using a hybrid physical-chemical vapor deposition technique. MgB₂ thin films with crystalline SiC-buffered layers showed a significant critical current density’s enhancement in the high magnetic field region. An uncommon plateau-like behavior was also observed when the normalized flux pinning force density was scaled with the reduced magnetic field. Based on the analyses of the scaling behavior of the flux pinning force, grain boundary pinning is likely to be a dominant pinning mechanism in the SiC-buffered MgB₂ thin films.     

Cu2O薄膜的制备方法

氧化亚铜（Cu2O）薄膜在太阳能电池及其它光电器件上有重要应用,它具有无毒、制备成本低、材料广泛易得等优点。制备Cu2O薄膜的方法主要有热蒸发、溅射、化学气相沉积和电化学沉积等,本文对Cu2O薄膜的制备方法进行了综述与展望。     

Size Effects on the Critical Scaling Laws in a-axis and c-axis Oriented 123 Thin Films

a-axis (CuO₂planes perpendicular to the substrate) and c-axis oriented high T_cfilms of the 123 family have been grown on (100) SrTiO₃substrates by magnetron sputtering from stoichiometric targets. Critical scaling near the melting transition has been analyzed from transport measurements. Critical exponents and the dimensionality of the systems are discussed in terms of structure related size effects and the characteristic superconducting lengths. The interplay of these parameters govern a change from 3D (c-axis) to 2D (a-axis) behavior.     

两种二硼化镁超导薄膜布图布线的湿法刻蚀技术

超导薄膜实现布图布线工艺是制备超导电子元件的必要步骤.报道了两种二硼化镁超导薄膜布图布线的湿法刻蚀技术:一种是先利用双氧水( H2O2)刻蚀前驱体硼薄膜,然后将刻蚀的样品放入钽坩埚中在镁蒸气下高温退火,实现了对超导薄膜二硼化镁(MgB2)布图布线的刻蚀;另一种是选用氢氟酸(HF)和硝酸(HNO3)的混合溶液直接在二硼化镁超导薄膜上进行图形刻蚀.通过上述两种方法刻蚀出的MgB2薄膜图形精确度高,超导转变温度Tc都在38K以上,临界电流Ic约为1×106 A/cm2.     

通过氧化Cu膜制备Cu_2O薄膜

通过热蒸镀Cu膜并在空气中退火制备Cu2O薄膜,利用X射线衍射(XRD)、能量分散X射线谱(EDX)和原子力显微镜(AFM)研究了已沉积和不同温度退火薄膜的晶体结构、成份和表面形貌。结果表明,Cu膜在200℃退火30分钟可以得到具有单一成份的Cu2O薄膜。四探针测量得到所制备的Cu2O薄膜电阻率为0.22Ωcm。用紫外可见光分光光度计(UV-vis)研究了Cu2O薄膜的光学特性,得出其光学带隙为2.4eV。