Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

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.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

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.     

(111)⇒(001) orientational phase transitions and nature of the insulating state in BiSrCaCuO (2212 and 2223) films

The influence of the growth temperature T _s on the structure, optical absorption, and electrical conductivity of BiSrCaCuO films has been studied. It was observed that nonmonotonic changes in the parameters of the films with T _s are caused by (111)⇒(001) phase transitions at T _s ≈550 °C for the 2212 phase and T _s ≈600 °C for the 2223 phase. These phase transitions stimulate the formation of metallic conductivity and are caused by a change in the system of preferential ordering planes of the atoms. Pis’ma Zh. Tekh. Fiz. 24, 13–20 (January 12, 1998)     

Structure and superconducting properties of pure and variously doped bulk MgB2 obtained by uniaxial and isostatic hot pressing

The preparation of dense MgB₂ bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB₂ powder. The superconducting characteristics of the obtained materials, namely critical temperature (T_c) and current (J_c), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB₂ matrix. Both the adopted hot pressing techniques gave rise to undoped MgB₂ pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB₂, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB₂ samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB₂ and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB₂ crystal.     

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB₂. MgB₂ thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB₂ thin layers, the presented data show that the superconducting critical temperature T _c exceeds the bulk value only when the MgB₂ films are grown on atomically flat (0001) SiC single crystals and on MgB₂ bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB₂ thin films.     

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.     

Effect of Firing Temperature on the Structure and Superconducting Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films

YBa₂Cu₃O_7−x (YBCO) films were prepared on LaAlO₃ single crystal substrate under various firing temperatures (750–800 °C) in the crystallization process by metalorganic deposition (MOD) method. The coating solution was made by mixing the fluorine-free precursor solution containing Y and Cu with Ba–fluorine precursor solution (Ba-TFA). The effect of firing temperature on the structure and superconducting properties of YBCO films was systematically investigated. The results indicated that YBCO-films were smooth, crack-free, exhibited good textures and retain high oxygen content according to the XRD and SEM images. Sample of YBCO-film fired at 780 °C showed highest superconducting properties including high critical transition temperature T _c=89 K, sharp transition temperature ΔT _c<1 K, and critical current density J _c=2.8 MA cm⁻², which are attributable to excellent in-plane textures and dense microstructures with good connectivity between the grains.     

Enhanced Critical Current Density in MgB2 Superconductor via Si and C Coadditions

In this study, nanosize Silicon and Carbon (Si+C) were reacted with MgB₂ in order to enhance the critical current density. The polycrystalline bulks were synthesized by the direct in situ reaction method and their phase formation, crystal structure, and superconducting properties were evaluated. The enhanced relative peak intensity of Mg₂Si and MgB₄ indicates the formation of a large volume fraction of these two phases with increasing (Si+C) additions. The a-axis lattice parameter shrinks significantly while c-axis increases slightly. The estimated C doping level at B site increases, leading to a degradation of the superconducting transition temperature with increasing (Si+C) additions. By a reaction with (Si+C), the field dependence of critical current density is shown to enhance at both 5 K and 20 K.     

Possible Multigap Superconductivity in SmFeAsO0.8F0.2: A Point-contact Andreev-reflection Spectroscopy Study

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO_0.8F_0.2 state-of-the-art polycrystals, with critical temperature T _c≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB₂ and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to T _c, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ₁=6.15±0.45 meV and Δ₂=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same T _c. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.     

Photoinduced Quasiparticle Relaxation Dynamics in Near-optimally Doped SmFeAsO0.8F0.2 Single Crystals

We investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor with T _c=49.5 K by means of femtosecond spectroscopy. Multiple relaxation processes are evident, with distinct superconducting state dynamics and a clear “pseudogap”-like feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude Δ _PG=61±9 meV above and below T _c. The fluence and temperature dependence of the superconducting state dynamics shows similar behaviour to the cuprate superconductors with the superconducting-condensate vaporization energy of E _va/k _B≈1.5 K per Fe ion.     

Electroless deposition of superconducting MgB2 films on various substrates

The superconducting properties of magnesium diboride (MgB₂) films prepared by electroless deposition on various substrates including silver, gold and silicon are reported. In this study, MgB₂ films were fabricated on silver, gold, and silicon using an electroless plating technique, while controlling the redox potential to improve the deposition quality. The structure, morphology, and superconducting properties of the samples were investigated using X-ray diffraction, magnetometry, scanning electron microscopy, and Raman spectroscopy. X-ray diffraction and Raman spectroscopy confirmed that the films are polycrystalline MgB₂ but also contain some impurity phases. All the MgB₂ films show superconducting transitions near 39 K, the value for bulk MgB₂, with the superconducting volume fraction ranging from approximately 1 to 2%. We find a strong dependence of film quality with the oxidation potential of the bath.     

Phase formation sequences and mechanism of MgB<Subscript>2</Subscript> superconductor with minor Sn doping

Combined with thermal analysis and phase identification, the phase formation of Sn-doped MgB₂ superconductor during the sintering process were systematically investigated. As compared to the sintering of MgB₂, the first exothermal peak occurs at a lower temperature, which suggests the accelerated formation of MgB₂ after minor Sn doping. The sintering process of minor Sn-doped MgB₂ orderly underwent the melting of Sn, the reaction between Mg and Sn, the eutectic Mg–Sn reaction, the solid–solid Mg–B reaction, the melting of Mg, the liquid–solid Mg–B reaction and the Sn precipitation. Based on the phase formation mechanism, MgB₂ bulks was successfully synthesized by Sn-activated sintering at 600 °C for only 5 h, exhibiting a dramatic decrease in the sintering time compared to the sintering of undoped MgB₂.     

Structure and Superconductivity of (MgB<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.97</Subscript>Cu<Subscript>0.03</Subscript>

A series of samples of (MgB_2−x C _x )_0.97Cu_0.03 (x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25) and MgB₂ were synthesized by a solid state reaction method. The structure, superconducting transition temperature and transport properties of the samples were studied by means of X-ray diffraction (XRD) and resistivity measurements. It is found that the c-axis of the lattice remains unchanged with increasing C doping, while the a-axis shows a small decrease. The T _c of the samples steadily decreases with increasing C doping. It is suggested that the chemical pressure effect plays a more important role influencing the normal state transport and T _c than the change of carrier concentration.     

Fabrication and ferroelectric properties of sol–gel derived 1–1 intergrowth-superlattice-structured Bi<Subscript>3</Subscript>TiNbO<Subscript>9</Subscript>-Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films

1–1 intergrowth-superlattice-structured Bi₃TiNbO₉–Bi₄Ti₃O₁₂ (BTN–BIT) ferroelectric thin films have been prepared on p-Si substrates by sol-gel processing. The precursor films are crystallized in the desired intergrown BTN–BIT superlattice structures by optimizing the processing conditions. Synthesized BTN–BIT thin films annealed below 750 °C are polycrystalline, uniform and crack-free, no pyrochlore phase or other second phase, and exhibited good ferroelectric properties. As the annealing temperature increases from 600 to 700 °C, both remanent polarization P _r and coercive electric field E _c of BTN–BIT thin films increase, but the pyrochlore phase in BTN–BIT films annealed above 750 °C will impair the ferroelectric properties. The BTN–BIT thin films annealed at 700 °C have a P _r value ~19.1μC/cm² and an E _c value ~135 kV/cm.     

Enhancement of Phase Coherence Between Superconducting Grains in Strongly Underdoped Bi2Sr2CaCu2O8+δ

We report a new finding on the phase coherence between strongly underdoped superconducting Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) grains. Contrary to the expectation based on the linear suppression of the superfluid density at T=0 K with the decreasing hole concentration in the underdoped region, the phase coherence between superconducting grains was remarkably enhanced at a hole concentration (p≃0.08) in the underdoped regime. The likely candidate giving rise to our finding is the phase-disordered superconducting quasiparticles around the antinode of the Fermi surface.     

Superconducting Bands Stabilizing Superconductivity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> and MgB<Subscript>2</Subscript>

It is shown that the ceramic superconductor YBa₂Cu₃O₇ as well as the superconducting intermetallic compound MgB₂ possesses a narrow, partly filled “superconducting band” with Wannier functions of special symmetry in their band structures. This result corroborates previous observations about the band structures of numerous superconductors and non-superconductors showing that evidently superconductivity is always connected with such superconducting bands. These findings are interpreted in the framework of a nonadiabatic extension of the Heisenberg model. Within this new group-theoretical model of correlated systems, Cooper pairs are stabilized by a nonadiabatic mechanism of constraining forces effective in narrow superconducting bands. The formation of Cooper pairs in a superconducting band is mediated by the energetically lowest boson excitations in the considered material that carry the crystal-spin angular momentum 1⋅ℏ. These crystal-spin-1 bosons are proposed to determine whether the material is a conventional low-T _c or a high-T _c superconductor. This interpretation provides the electron–phonon mechanism that enters the BCS theory in conventional superconductors.