High-quality MgB2 Research Articles

MgB[formula omitted] Josephson Junctions fabricated by Focused Helium Ion Beam with different irradiation doses on SiC and MgO substrates

Recently, high quality MgB2 single junctions and junction arrays fabricated by Focused Helium Ion Beam irradiation (He-FIB) have been reported. However, the effects of irradiation doses and substrates on junction properties have not been analysed systematically. In this work, 30 nm MgB2 thin films were deposited on SiC (0001) and MgO (111) substrates using Hybrid Physical–Chemical Vapour Deposition (HPCVD) technique and then etched into 2–4 μm wide microbridges. 2000–6000 ions/nm doses of He-FIB were irradiated across these microbridges, creating narrow junction barriers. R–T curves exhibit a clear foot-like structure that enlarges as irradiation dose increases. Temperature dependence of junction properties (critical current Ic, junction normal state resistance Rn, characteristic voltage Vc) shows that all junctions in our experiment exhibit superconductor–normal–superconductor (SNS) like behaviour and higher irradiation doses create thicker junction barriers. Furthermore, we analyse junction properties dependence on irradiation doses which yields an exponential change law. Finally, Fraunhofer diffraction-like patterns and Shapiro steps of junctions are also measured. Our results show MgB2 Josephson Junctions with adjustable properties can be fabricated by He-FIB on SiC and MgO substrates, which provides a solid foundation for future application devices research.

High performance superconducting joint for MgB2 films

Large-area high-quality MgB2 thin films were needed in many superconducting applications such as superconducting radio frequency (RF) cavities. However, fabricating large films with high quality can be difficult. Thus, a jointing method between MgB2 films can be crucial in a lot of superconducting electronic applications. In this article, we successfully fabricated superconducting joints for MgB2 films. The sol-gel of Mg(BH4)2·Et2O was used as the glue. Two types of MgB2 joints were shown in this work, a two-piece face-to-face joint, and a three-piece bridging joint. The sol-gel derived MgB2 joints have shown very good superconducting performance, with a high superconducting critical temperature (TC) of 40 K, a high critical current (IC) of 340 mA at 0 T, and very nice current capacity in magnetic fields up to 4 T.

Current-phase relation in a short clean josephson junction model: application to MgB2

Motivated by recent data on high-quality MgB2 thin films implying that the smaller energy gap has l = 6 (i-wave) symmetry, we consider a simple model for an all-MgB2 symmetric Josephson Junction (JJ). The model assumes an arbitrary-strength delta-function barrier and one-dimensional current conduction. It is shown that in this context a nodal energy gap with i-wave symmetry acts as an isotropic energy gap (s-wave) with an amplitude modified by the energy-gap misalignment-angle with respect to the crystal principal axes. The corresponding exact Green’s function in momentum space is derived employing a novel approach. The ensuing current-phase relations in the strong and weak barrier-strengths limits are calculated and found to confirm known results, e.g., the Ambegaokar-Baratoff current-phase relation. Inspired by an HTS experiment that established the d-wave energy-gap symmetry, we propose a JJ-related experiment with a MgB2 bicrystal to confirm our premise that the smaller energy has i-wave symmetry.

High-quality MgB2 nanocrystals synthesized by using modified amorphous nano-boron powders: Study of defect structures and superconductivity properties

Nano sized magnesium diboride (MgB2) samples were synthesized using various high-quality nano-B precursor powders. The microscopic defect structures of MgB2 samples were systematically investigated using X-ray powder diffraction, Raman, resistivity measurements and electron paramagnetic resonance spectroscopy. A significant deviation in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra. Scanning electron microscopy analysis demonstrate uniform and ultrafine morphology for the modified MgB2. Defect center in particular Mg vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications.

MgB2 Josephson junctions produced by focused helium ion beam irradiation

Planar magnesium diboride Josephson junctions are fabricated using focused helium ion beam irradiation. A single track of ion irradiation with a 30 kV He+ beam with nominal beam diameter < 0.5 nm is used to create a normal-metal barrier on a MgB2 film deposited by hybrid physical-chemical vapor deposition. Josephson coupling is observed below the critical temperature of the electrodes for a He+ doses between 8x1015/cm2 to 4x1016/cm2. Analysis of the temperature dependence of the normal resistance and critical voltage of the junctions shows highly uniform barriers with nearly ideal resistively-shunted junction behavior for higher-dose junctions, while nonequilibrium effects dominate the properties of lower-dose junctions over most of the temperature range. These results demonstrate that focused helium ion beam irradiation can produce high-quality proximity-coupled MgB2 Josephson junctions with tailorable properties, promising for use in superconducting devices and circuits.

Growth of magnesium diboride thin films on boron buffered Si and silicon-on-insulator substrates by hybrid physical chemical vapor deposition

We report on the growth of high quality MgB2 thin films on silicon and silicon-on-insulator substrates by hybrid physical chemical vapor deposition. A boron buffer layer was deposited on all sides of the Si substrate to prevent the reaction of Mg vapor and Si. Ar ion milling at a low angle of 1° was used to reduce the roughness of the boron buffer layer before the MgB2 growth. An Ar ion milling at low angle of 1° was also applied to the MgB2 surface to reduce its roughness. The resultant MgB2 films showed excellent superconducting properties and a smooth surface. The process produces thin MgB2 films suitable for waveguide-based superconducting hot electron bolometers and other MgB2-based electronic devices.

The preparation of cylindrical MgB2 films on stainless steel by HPCVD for magnetic shielding

HTC SQUID has a great potential in ultra-weak magnetic signal detections due to its advantages of energy saving and volume reduction of the overall system, including the cryogenics. However, the application of HTC SQUID is limited because lack of proper magnetic shielding. Superconducting magnetic shielding is an efficient way for HTC SQUID. In this work, we fabricate MgB2 thick film on a stainless steel cylinder using HPCVD to make a preliminary attempt for SMS with MgB2 films. High quality MgB2 film is deposited on the inner side of a hollowed cylinder. The transition temperature of the film is 37.5 K, and the critical current density is 7.6 × 106 A⋅cm−2 at 10 K and 4.6 × 106 A⋅cm−2 at 20 K, which is comparable with the planar films. The shielding effectiveness of the cylindrical MgB2 film is numerical simulated. Height and thickness of the cylindrical magnetic shielding designed according to the simulation result was calculated and a corresponding shielding effectiveness of 65 dB is obtained. The numerical result shows the feasibility of using MgB2 film as magnetic shielding.

Revisiting optical properties of MgB2 with a high-quality sample prepared by a HPCVD method

We investigated a high-quality MgB2 thin film with a thickness of ~1000 nm on an Al2O3 substrate using optical spectroscopy. We measured the reflectance spectra of the film at various temperatures both below, and above, the superconducting transition temperature, Tc simeq 40 K. An earlier study showed that when the sample surface is exposed to air the optical properties of the surface change immediately, however, the saturated change is negligibly small in the far-infrared region. The optical conductivity spectrum in the normal state shows two (narrow and broad) Drude modes, with the narrow Drude mode being dominant in the low frequency region below 1000 cm−1. Our study, which uses a good-quality sample, provides more reliable data on the optical properties of MgB2, in a similar spectral range. The optical data is analyzed further using an extended Drude model, and the electron-phonon spectral density function, α2F(ω), is extracted. The spectral density function α2F(ω) features two peaks: a small one near 114 cm−1, and a strong peak around the 550 cm−1 where the B-B bond stretching phonon exists. Our data in the superconducting state does not show the expected energy shift of the onset of scattering associated with the α2F(ω) peaks.

The performance improvement of MgB2 prepared by the Mg diffusion method with the MgB4 addition

The Mg diffusion method with the MgB4 addition is carried out to improve the MgB2 application performance. The precursor B powders and MgB4 powders are uniformly mixed as the ratio of (1- 4x) : x mol %, (x = 0, 1, 2, 5, 10 and 20). The mixed powders are pressed into bulks and the excess of Mg powders are added into the iron pipe. Then the heat treatment is carried out under the pure argon atmosphere. Finally, the high-quality MgB2 bulks are achieved with high density. The superconducting critical temperature of the MgB2 bulks decreases slightly with the increase of MgB4 addition content. However, the critical current density increases significantly at the high-fields. It is perhaps due to the small-sized MgB4 impurities in MgB2 bulk function as effective pinning centers and generates significant lattice distortion in the MgB2 lattice to yield improved Jc performance in the applied field. It suggests that further improvement in the superconducting properties of the MgB2 added with the MgB4 will be possible via a better control of the composition and processing parameters. This also provides a reference for improving the preparation of MgB2 wire.

Fabrication and radio frequency test of large-area MgB2 films on niobium substrates

Magnesium diboride (MgB2) is a promising candidate material for superconducting radio frequency (RF) cavities because of its higher transition temperature and critical field compared with niobium. To meet the demand of RF test devices, the fabrication of large-area MgB2 films on metal substrates is needed. In this work, high quality MgB2 films with 50 mm diameter were fabricated on niobium by using an improved HPCVD system at Peking University, and RF tests were carried out at SLAC National Accelerator Laboratory. The transition temperature is approximately 39.6 K and the RF surface resistance is about 120 μΩ at 4 K and 11.4 GHz. The fabrication processes, surface morphology, DC superconducting properties and RF tests of these large-area MgB2 films are presented.

Thickness dependence of Jc (0) in MgB2 films

MgB2 superconducting films, whose thicknesses range from 10nm to 8µm, have been fabricated on SiC substrates by hybrid physical–chemical vapor deposition (HPCVD) method. It is the first time that the Tc and the Jc of MgB2 films are studied on such a large scale. It is found that with the increasing of thickness, Tc elevates first and then keeps roughly stable except for some slight fluctuations, while Jc (5K, 0T) experiences a sharp increase followed by a relatively slow fall. The maximum Jc (5K, 0T)=2.3×108Acm−2 is obtained for 100nm films, which is the experimental evidence for preparing high-quality MgB2 films by HPCVD method. Thus, this work may provide guidance on choosing the suitable thickness for applications. Meanwhile, the films prepared by us cover ultrathin films, thin films and thick films, so the study on them will bring a comprehensive understanding of MgB2 films.

High Magnetic Field Generated by Bulk MgB2 Prepared by Spark Plasma Sintering

From the applications' point of view, the advantage of low density given by MgB2 material must be taken into consideration, and the generation of strong magnetic flux densities using MgB2 should be investigated. In this paper, we have studied the magnetic properties of samples processed by a fast spark plasma sintering machine that is able to produce dense and high-quality MgB2 samples. Experiments were carried out both on small pieces of the samples and on large-sized samples performing trapped field measurements in a field-cooling process. For temperatures between 10 and 30 K, the results show a strong dependence of the magnetic behavior of the large-sized samples to the applied magnetic field sweep rate, whereas nothing particular appeared on the small samples. As the magnetic flux density produced by MgB2 bulks is directly linked to the potential of the applications, we report on the field produced at the surface of one single MgB2 sample of 30-mm diameter and the field produced inside a stack of two MgB2 samples of 20-mm diameter. A generation of magnetic flux density (magnetic polarization $\mu_{0}M$ ) up to 4.80 T at 10 K and 3.92 T at 20 K inside the stack of the two MgB2 samples was observed under a negative supporting field of −1.47 and −1.95 T, respectively. According to these values to their very low density and to their ease of manufacturing, MgB2 bulks are promising materials for the applications of superconductors.

Preparation and DC characteristics of MgB2/B/MgB2 Josephson junctions

Since the discovery of its superconductivity, magnesium diboride (MgB2) has been identified as a promising superconductor to be used in Josephson junction devices due to its high transition temperature, large energy gap, long coherence length, and expected easier fabrication of Josephson junctions as compared with high temperature superconductors. The high-quality MgB2 films and excellent tunnel barrier materials are the core elements for a Josephson junction. Here in this paper, all MgB2 thin film tunnel junctions with B tunnel barriers are fabricated in situ on sapphire substrates and their tunneling characteristics re investigated. The experimental results indicate that the MgB2/B/MgB2 junctions exhibit good tunneling characteristics. The deposition of the MgB2/B/MgB2 trilayer is carried out in a completely in situ process. The bottom and top MgB2 layers are grown to a thickness of 100 nm by hybrid physical-chemical vapor deposition (HPCVD) technique at about 973 K and in 102 Pa Ar atmosphere on a single crystal Al2O3 (0001) substrate. The 35-nm-thick amorphous B insulator layer is deposited using chemical vapor deposition method at 723 K and in 103 Pa pure Ar. In the process of the top MgB2 layer deposition, the amorphous B reacts with Mg in Mg vapor, leading to its thickness decreasing to 10 nm. Square-shaped junctions each with a size of 4 mm5 mm are determined by the metallic mask method. The resistivity temperature (R-T) curves and the DC current-voltage (I-V) curves of the MgB2/B/MgB2 junctions at different temperatures are measured by the four-point probe method in the physical property measurement system (PPMS). The experimental results show excellent superconducting properties of the top and bottom superconductor with high Tc (above 39.5 K), appreciable Jc values (107-108 A/cm2). In the I-V characteristics of junction at temperatures ranging from 4.2 K to 39.2 K, the junctions exhibit clear Josephson tunneling characteristics with jc～0.52 A/cm2 at 4.2 K, which remains nonzero up to 31.3 K. The hysteresis is pronounced at 4.2 K, becoming smaller as temperature increases, and eventually disappearing at around 19.2 K. By using the differential I-V curves, only gap is observed in differential conductance vs. voltage characteristics (dI/dV-V) curves, because MgB2 layer grown using HPCVD technique is always c-axis oriented and more than 99% contribution to the conduction is from band charge carriers.

Superconducting MgB2 film prepared by chemical vapor deposition at atmospheric pressure of N2

A simple and effective chemical vapor deposition equipment was developed for deposition of superconducting MgB2 thin films. The pure precursor Boron films were prepared in base pressure of low vacuum and deposited in atmospheric pressure. After the precursor film annealed in Mg vapor, the superconducting MgB2 film was fabricated. During the precursor Boron films preparation, N2 and Ar were used as carrier gas. Compared to Ar gas, the films show better crystallization, surface morphology and superconducting performance when N2 is adopted as carrier gas. With flow rate of 200 sccm of N2 gas, the fabricated MgB2 films exhibit the highest superconducting transition temperature of 39.5K, which is among the best results of MgB2 thin films. This method provides a suitable method to realize high quality MgB2 Josephson junctions and industrial manufacture of MgB2 superconducting thin films on a large scale.

Frequency Dependence of Optical Conductivity in MgB<sub>2</sub> Superconductor

Using Green’s function method, the frequency dependence of optical conductivities of high-quality MgB2 film is calculated in the framework of the single- and two-band model. By comparing the numerical and experimental results, it is shown that the single-gap isotropic model is insufficient to understand consistently optical behaviors. Also, it is concluded that the two-band model consistently describes the optical behaviors. In the two-gap model, we consider that the both components of optical conductivity are a weighted sum of the contribution from σ and π bonds and hybridization between them is negligible.

Enhancing Superconducting Properties of MgB2 by Addition of Magnetic Particles

Main results of ongoing comprehensive study of the influence of magnetic nanoparticles (MNP) on superconducting properties (transition temperature Tc, critical fields and critical current density, Jc, etc.) of high-quality Fe-sheathed MgB2 wires are briefly reviewed. We prepared over 60 Fe-sheathed MgB2 wires doped with 12 different types and quantities of MNPs, such as elemental magnets, magnetic borides, ferrites, and rare earth borides. Generally, doping with MNPs suppresses Tc and higher MNP contents cause deterioration of superconducting properties of wires. However, light doping (≤ 2.5 wt%) with some types of MNPs (e.g., Ni) improved flux pinning and in-field Jc of wires. Particularly promising seems co-doping with core-shell MNPs, where both critical fields and Jc were simultaneously enhanced. Possible origins of these improvements (such as magnetic flux pinning) are briefly discussed.

Magnesium Diboride Superconducting Films Synthesized by Different Electron-Beam Annealing Currents

Considerable progress to synthesize magnesium diboride (MgB2) films by electron-beam annealing has been made. A series of MgB2 films with a Tc higher than 35 K had been fabricated. MgB2 film superconductivity is affected by electron-beam accelerating voltage, beam current, annealing duration, beam spot size and Mg/B ratio. In order to fabricate better MgB2 films, these influence factors must be intensively studied. In this paper, the influence of e-beam current on superconductivity was investigated with an accelerating voltage of 32 kV, an annealing time of 0.26 s and different annealing beam currents of 9.9 mA, 10.7 mA, 12.8 mA, 13.3 mA and 14.0 mA. The results show the MgB2 film with 12.8 mA has the highest Tc and the densest structure. Its zero-field Jc at 15K has reached 3.2×106 A/cm2. The variation of the film superconducting properties with the beam currents was grasped, which will lay the foundation for the fabrication of high quality MgB2 thin films

Quantitative imaging of flux vortices in the type-II superconductor MgB2using cryo-Lorentz transmission electron microscopy

Imaging of flux vortices in high quality MgB$_2$ single crystals has been successfully performed in a commercial Field Emission Gun-based Transmission Electron Microscope. In Cryo-Lorentz Microscopy, the sample quality and the vortex lattice can be monitored simultaneously, allowing one to relate microscopically the surface quality and the vortex dynamics. Such a vortex motion ultimately determines the flow resistivity, $\rho_{f}$, the knowledge of which is indispensable for practical applications such as superconducting magnets or wires for Magnetic Resonance Imaging. The observed patterns have been analyzed and compared with other studies by Cryo-Lorentz Microscopy or Bitter decoration. We find that the vortex lattice arrangement depends strongly on the surface quality obtained during the specimen preparation, and tends to form an hexagonal Abrikosov lattice at a relatively low magnetic field. Stripes or gossamer-like patterns, recently suggested as potential signatures of an unconventional behavior of MgB$_2$, were not observed.

Perfect Domain-Lattice Matching between MgB2and Al2O3: Single-Crystal MgB2Thin Films Grown on Sapphire

We have found that single-crystal films can be grown on (0001) Al2O3 substrates through the golden relation of a perfect lattice-matching ratio (8:3√3) between the a-axis lattice constants of MgB2 and Al2O3. Selected-area electron diffraction patterns evidently indicate hexagonal MgB2 film with a 30° in-plane rotation with respect to the Al2O3 substrate. The critical current density at zero field is comparable to the depairing critical current density and rapidly decreases with increasing applied field owing to the lack of pinning sites, as observed for high-quality MgB2 single crystals.

Perfect Domain-Lattice Matching between MgB2 and Al2O3: Single-Crystal MgB2 Thin Films Grown on Sapphire

We have found that single-crystal films can be grown on (0001) Al2O3 substrates through the golden relation of a perfect lattice-matching ratio (8:3√3) between the a-axis lattice constants of MgB2 and Al2O3. Selected-area electron diffraction patterns evidently indicate hexagonal MgB2 film with a 30° in-plane rotation with respect to the Al2O3 substrate. The critical current density at zero field is comparable to the depairing critical current density and rapidly decreases with increasing applied field owing to the lack of pinning sites, as observed for high-quality MgB2 single crystals.