Electron-phonon interaction in NbB2: a comparison with MgB2

We present a comparison of electron-phonon interaction in NbB₂ and MgB₂, calculated using full-potential, density-functional-based methods in P6/mmm crystal structure. Our results, described in terms of (i) electronic structure, (ii) phonon density of states F(ω), (iii) Eliashberg function α²F(ω), and (iv) the solutions of the isotropic Eliashberg gap equation, clearly show significant differences in the electron-phonon interaction in NbB₂ and MgB₂. We find that the average electron-phonon coupling constant λ is equal to 0.59 for MgB₂ and 0.43 for NbB₂, leading to superconducting transition temperatures T_c at around 22 K for MgB₂ and 3 K for NbB₂.     

Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, H_c2(T), of carbon-doped MgB₂. H_c2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. H_c2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB₂ (15-23 T).     

On the superconductivity in in situ synthesized MgB2 tapes

Single core stainless steel (SS) sheathed MgB₂ tapes have been made by the powder-in-tube (PIT) method using commercial Mg and B powders in two series, one with nominal composition and the other with excess Mg. The electrical resistivity and susceptibility measurements have been carried out to evaluate residual resistivity ratio (RRR), the coherence length ξ(0) and critical current density J_C(T) in these tapes. Detailed structural analysis of the core material has been carried out to correlate the superconducting properties with the crystallinity. In the optimized growth condition the MgB₂ tapes exhibited an estimated J_C of ∼1.4×10⁷ A/m² at 39.45 K in zero field and the zero temperature coherence length is found to be ∼68 Å. MgB₂ tapes fabricated from starting powders having nominal Mg-composition have been shown to exhibit higher J_C than those fabricated from excess magnesium composition of the starting powders. The strained lattice together with the presence of nanosized MgO inclusion having size smaller than the coherence length, are shown to be responsible for the observed higher J_C.     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Enhanced superconducting properties of Eu2O3-doped MgB2

Bulk polycrystalline samples of Eu₂O₃-doped MgB₂ have been synthesized by a standard solid state reaction route and their structural and superconducting properties have been investigated. As a function of Eu₂O₃ content we have found a significant increase in the critical current density (J_c) and the irreversibility field (H_irr) in the magnetic field range 0–6 T. The XRD results reveal the presence of MgO and EuB₆ secondary phases along with the main hexagonal phase of MgB₂. The strain values and the lattice distortions have been found to increase almost linearly with the nominal Eu₂O₃ content. The observed significant improvement in J_c(H) and H_irr in the Eu₂O₃-doped MgB₂ samples, thus is mainly attributed to the lattice distortions introduced by Eu₂O₃ doping.     

Analysis of Mg-B compounds by means of Auger electron microprobe

For the development of a low-cost MgB₂ superconductor the “powder-in-tube” (PIT) technique is investigated. Mechanically alloyed MgB₂ powder provides a favorable microstructure and phase composition for the low temperature preparation of MgB₂ tapes with superconducting properties. The composition of sintered bulk samples and Fe-clad MgB₂ tapes made of mechanically alloyed MgB₂ has been investigated by Auger micro-analysis. Due to the process the samples contain about 15% oxygen. Pure MgB₂ crystallites were investigated as standard for stoichiometry and peak shape analysis.     

Effect of polymers doping on flux pinning behavior of MgB2

The effects of polymers doping on irreversibility field (H_irr) and critical current density (J_c) of MgB₂ have been investigated in this work. It is found that both J_c, and H_irr, are improved by doping at relative lower temperature region. The J_c−B curves of all samples studied in this work are well fitted using J_c(B) formula in percolation model. The values of upper critical field anisotropy (γ) are obtained from the fitting result at various temperatures. It is observed that values of γ for polymers doping samples are reduced at these temperatures. This is considered to be responsible for the enhancement of values of J_c for doped samples. Moreover, the percolation threshold, p_c, is found to be enhanced with increasing temperature. It is believed that the grain boundary pinning is still dominating in MgB₂, while the deviation of experimental data from the theoretical values is due to the percolation of suppercurrent in polycrystalline MgB₂.     

Effect of SiC buffer layer on flux pinning property of MgB2 tapes

In this paper we aimed at investigating the flux pinning property of MgB₂ films on hastelloy tapes which are buffered on various thicknesses of SiC layers. We have observed that the increase in thickness of the SiC buffer layer is very closely related with the systematic improvement of the field dependence of the critical current densities (J_c) of MgB₂ tapes while the values of J_c decreased. According to the analysis of the pinning force density (F_p), there exist two pinning sources both in the pure MgB₂ and in the MgB₂ film with the thinnest SiC buffer layer. On the while, the pinning source observed in the MgB₂ films with thicker SiC buffer layers appears to be different from those previously mentioned. The different pinning behaviors of MgB₂ films may suggest that there be an additional pinning center working on the MgB₂ films with thick SiC buffer layers. The microstructural analyses of MgB₂ films confirmed that intra-granular defects and columnar grain boundaries may be a dominant pinning mechanism in the pure MgB₂ and the MgB₂ film with 170 nm-thick SiC buffer layer. For the MgB₂ films with thicker SiC buffer layers, carbon diffusion into the MgB₂ film, which is defined by the Auger electron spectroscopy, may be the origin of the additional pinning mechanism.     

The effect of SiC nanoparticle addition on the flux pinning properties of MgB2

The nanoparticle–MgB₂ composite superconducting sample, (SiC)_4wt.%(MgB₂)_96wt.% ((SiC)₄–MgB₂), was prepared, and the effect of nanoparticle additions on the magnetic flux pinning was investigated. The measurement and comparison of isothermal magnetization M(H), for pure-MgB₂ powder and sintered pellets of (SiC)₄–MgB₂ and pure-MgB₂ were carried out at temperatures between 5 and 50 K in fields up to 8.5 T. The magnetic irreversibility ΔM(H) curves of the (SiC)₄–MgB₂ follow almost identical lines of both pure-MgB₂ powder and sintered bulk MgB₂ in the region above a specific magnetic field (called a merged field), which gradually decreases as the temperature increases. The (SiC)₄–MgB₂ composite superconductor has exhibited the flux pinning effect which comes from both the grain boundaries and the point defect weak pinning centers in the region below the merged field line. This is different from the case of pure-MgB₂ powder and sintered bulk MgB₂ which showed mostly the grain boundaries pinning.     

Two-band nature of upper critical fields in MgB2 thin films investigated by 37 T pulsed magnet

We present experimental and calculated phase diagrams of the upper critical field H_c2 and the anisotropy parameter γ of various MgB₂ thin films prepared by the molecular beam epitaxy (MBE) and the multiple-targets sputtering system. Experimental data of the H_c2(T) and γ are analyzed by fitting the Gurevich theory. We obtained the result that for these MgB₂ films the H_c2(T) is explained as the case of D_π/D_σ>1, which means the films are classified to the cleaner π band than the σ band. We discuss temperature dependence of the transition width obtained from the magnetoresistance measurements.     

Photoemission results of intermetallic superconductors: Nb3Al and MgB2

We study the superconducting electronic structures of Nb₃Al and MgB₂ using high-resolution spectroscopy. The obtained spectrum of Nb₃Al measured below T_c shows clear opening of the superconducting gap with a sharp pile up in the density of states and a shift of the leading edge. In addition, the spectrum shows a peak-dip-hump line shape expected from the strong-coupling theory. On the other hand, for MgB₂, the superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a single isotropic gap. The superconducting spectral shape of MgB₂ can be explained in terms of a multicomponent gap.     

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB₂ films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB₂ films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB₂ grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB₂ films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB₂ with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB₂.     

Concurrent doping effect of Ti and nano-diamond on flux pinning of MgB2

Nano-diamond and titanium concurrently doped MgB₂ nanocomposites have been prepared by solid state reaction method. The effects of carbon and Ti concurrent doping on J_c–H behavior and pinning force scaling features of MgB₂ have been investigated. Although T_c was slightly depressed, J_c of MgB₂ have been significantly improved by the nano-diamond doping, especially in the high field region. In the mean time, the J_c value in low field region is sustained though concurrent Ti doping. Microstructure analysis reveals that when nano-diamond was concurrently doped with titanium in MgB₂, a unique nanocomposite in which TiB₂ forms a thin layer surrounding MgB₂ grains whereas nano-diamond particles were wrapped inside the MgB₂ grains. Besides, nano-diamond doping results in a high density stress field in the MgB₂ samples, which may take responsibility for the Δκ pinning behavior in the carbon-doped MgB₂ system.     

A simple theory of 40 K superconductivity in MgB2: first-principles calculations of Tc, its dependence on boron mass and pressure

We have studied the superconducting properties of MgB₂ from first-principles under isotropic, uniaxial, and biaxial compressions. We find that the in-plane boron phonons near the zone-center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This mode is found to be the key to quantitatively explain the observed high T_c, the total isotope effect and the pressure dependence of T_c. We propose that a stringent test on the hole and phonon based theories of the superconductivity in MgB₂ would be a measurement of the biaxial ab-compression dependence of T_c.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

Superconductivity in porous MgB2

Porous magnesium diboride samples have been prepared by the heat treatment of a pressed mixture of Mg and MgB₂ powders. It was found that linked superconducting structure is formed down to the minimum normalized density γ_c=d/d₀≅0.16 (percolation threshold), where d is the density of MgB₂ averaged over the sample, d₀=2.62 g/cm³ is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing γ) and T_c2≅32 K is minimal at γ_c. The grain boundaries in the porous samples are transparent for the current and J_c∼3×10⁵ A/cm² in self field at T=20 K in the samples with γ∼0.24.     

MgB2 coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical–chemical vapor deposition

MgB₂ coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB₂ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 °C by using hybrid physical–chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (T_c) are in the range of 38.5–39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB₂ are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB₂ and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB₂ grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB₂ CCs. The critical current density (J_c) of J_c(5 K, 0 T) ～10⁷ A/cm² and J_c(5 K, 2.5 T) ～10⁵ A/cm² has been obtained for the sample fabricated at a low growth temperature of 520 °C. The enhanced J_c (H) behavior can be understood on the basis of the variation in the microstructure of MgB₂ CCs with growth temperature.     

Effect of temperature and concentration of solution in chemical treatment for MgB2 powder on the Jc–B property of ex situ processed MgB2 superconducting tapes

MgB₂ tapes were fabricated through an ex situ process in a powder-in-tube (PIT) technique using powders treated at elevated temperatures in benzene solutions of benzoic acid with various concentrations. The amount of carbon substitution in MgB₂ in heat-treated tapes with treatment at the boiling points (BPs) of the solutions is smaller than that at room temperature (RT). This carbon substitution improves the J_c property in the high-field region. For RT treatment, the J_c property is improved with increasing the solution concentration. In contrast, the J_c property is deteriorated with increasing the concentration for BP treatment. On the other hand, treatment with pure solvent does not bring about the J_c enhancement and carbon substitution at all at both RT and the BPs. This suggests that acidity essential for the dissolution of MgO layers attached to the surface of MgB₂ is required for carbon substitution. The BP treatment enhances the acting of the acidity and possibly inflicts damage on MgB₂ itself.     

Effects of ambient and humidity exposure on the MgB2 superconductor

Exposure effect of ambient (up to a period of 465 days) and humid atmosphere on stability and the physical properties of bulk samples of MgB₂ has been carried out based on ac-susceptibility and X-ray diffraction measurements performed at different times during the aging. The motivation of this study came from the device application potential of MgB₂ superconductor. It has been observed that on initial exposure to ambient for a period of about 2 months, the MgB₂ samples neither show any sign of phase decomposition nor any significant degradation of superconducting parameters. However, on further exposure to ambient for a period greater than 4 months duration, the samples progressively exhibited the partial conversion of MgB₂ phase into Mg-deficient phases, viz., MgB₆ and MgB₁₂ and also leading to formation of Mg(OH)₂ which is accompanied by decrease in the T_C and poor connectivity of the superconducting grains. When exposed to humid environment for a period of 30 days, a significant degradation was observed in the superconducting properties of MgB₂. In sharp contrast to the case of ambient exposure, the exposure of MgB₂ to humid atmosphere (for 30 days) did not result in any noticeable phase decomposition. A comparison of the magnetically inferred critical current density behavior before and after the humidity exposure of MgB₂ is also presented.