Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

Deposition and Properties of Superconducting MgB2 Thin Films

The recently discovered superconductor MgB₂ with T _c at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB₂ thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB₂ thin films with bulk superconducting properties.     

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.     

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.     

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.     

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.     

TES用超导薄膜制备及特性研究

超导转变边沿传感器(TES)是光强单位坎[德拉]量子化所需的单光子探测器,金属超导薄膜物理特性的研究是TES研究的基础.采用不同电源功率磁控溅射制备高纯Al、Ti超导纳米薄膜,分析其薄膜生长速率随气压和功率的变化.利用表面应力仪及电学测量仪,分析薄膜表面应力及电学特征.最后将薄膜放入商用稀释制冷机中,进行超低温测试,获...     

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.     

Fabrication and Properties of MgB2 Coated Superconducting Tapes

We present the formation of MgB₂ coatings by simple and novel aerosol deposition technique which has a potential to escalate towards the fabrication of long superconducting tapes. The thin MgB₂ coatings were produced by using pre-synthesized MgB₂ powder. The ability of this technique to form a precursor powder in a thin film form has greatly reduced the intricacies involved in the synthesis of MgB₂ by other techniques like hybrid physical chemical vapor deposition etc. The as-synthesized thin films were characterized by the x-ray diffraction technique to study the structural properties. The thin films were found to be x-ray amorphous in nature depicting the formation of frustrated structure which showed a superconducting transition onset at around 36 K.     

Anisotropic Superconducting Properties of MgB2 and Related Compounds

Anisotropic superconducting properties of inter-metallic compounds MgB₂, Y₂PdGe₃, and CaAlSi with AlB₂ structure are studied by detailed angular dependent transport measurements. MgB₂ and CaAlSi shows appreciable anisotropy of the upper critical field, H _c2, with mass anisotropy parameter =3 and 2, respectively, while Y₂PdGe₃ is almost isotropic. In-plane anisotropy of H _c2 in these hexagonal superconductors is very small, consistent with the prediction based on GL theory.     

Effect of Substrates on Superconducting Properties of MgB2 Thick Films Fabricated by Spraying Method

MgB₂ thick films have been prepared on Al₂O₃, MgO and SrTiO₃ ceramic substrates using the spray method with two different heat-treatment cycles, 925???C with 10?min (Group?A) and 610???C with 24?h (Group?B). The structural/microstructural (XRD, SEM) and transport (R?CT, MR?CT) properties of the films prepared were investigated. XRD analysis showed that the films in Groups?A and?B consisted mainly of MgB₂, but the peaks originating from the substrates were also observed in films fabricated on the Al₂O₃ substrate. From scanning electron microscopy, it was seen that the surface of MgB₂ thick films prepared is not sufficiently homogeneous. Some cracks and heaps with different sizes were observed. The best electrical results were obtained for films in Group?A on Al₂O₃ substrate. The T _c value for films in Group?A was found to be 36.1?K, 36?K and 35?K, for Al₂O₃, MgO and SrTiO₃ substrates, respectively. No superconducting state was reached for films in Group?B on the SrTiO₃ substrate. In all the films which showed a superconducting state, the magnetic field strongly affected T _c . A?tail in the resistance curves was observed with increasing magnetic field for films in Group?A on the MgO and SrTiO₃ substrates. At magnetic fields above 1?T for films in Group?A on the SrTiO₃ substrate and above 2?T for films in Group?A on the MgO substrate, the zero-resistance temperature, T ₀, was not obtained.     

Pinning Properties of Cold-Processed MgB2 Composite Superconducting Tapes

Superconducting tapes from MgB₂ and powders of magnetic Ni, Co, and non-magnetic Ti were fabricated by cold-processed powder-in-tube (PIT) method. The cupronickel was used as sheath material and no heat treatment was applied. The increasing of critical current was observed in the samples with additions of nickel and titanium. The microstructure and phase analysis of tape core was performed by X-ray diffraction spectroscopy. A direct correlation between type and amount of additives, and the critical current of the tapes has been observed.     

Observation of Hall Scaling in MgB2 Thin Films

We have measured the Hall resistivity, _xy , and the longitudinal resistivity, _xx , in superconducting MgB₂ thin films in a mixed-state while changing the magnetic field and the current density. A Hall scaling behavior without the anomalous Hall effect was observed with a exponent of 2.0±0.1 in _xy =A _xx . This exponent is observed to be constant, i.e., independent of magnetic field, temperature, and current density.     

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.     

两种二硼化镁超导薄膜布图布线的湿法刻蚀技术(英文)

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。     

Degradation of Superconducting Properties in MgB2 by Formation of the MgB4 Phase

Superconducting MgB₂ polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB₄ phase. A series of samples was placed in an α-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an α-alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB₂ phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB₄ phase were formed and the T _zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J _cmag for the first series of samples was 1.9 × 10⁵ A/cm² at 10 K and 0 T, and for the second series of samples was 0.7 × 10⁴ A/cm² at 10 K and 0 T.