Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2

Low-Temperature Chiral Crystal Structure and Superconductivity in (Pt0.2Ir0.8)3Zr5

We report the observation of superconductivity in (Pt0.2Ir0.8)3Zr5 with a chiral space group (P6122) at low temperatures. The bulk nature of the superconductivity at a transition temperature of 2.2 K was confirmed using specific heat measurements. We revealed that (Pt0.2Ir0.8)3Zr5 obeys the weak-coupling Bardeen-Cooper-Schrieffer model, and the dominant mechanism in the upper critical field is the orbital pair-breaking limit rather than the Pauli-Clogston limit. This indicates that the antisymmetric spin-orbit coupling caused by the chiral crystal structure does not significantly affect the superconductivity of (Pt0.2Ir0.8)3Zr5.

An Improved Smart Meta-Superconductor MgB2

Increasing and improving the critical transition temperature (TC), current density (JC) and the Meissner effect (HC) of conventional superconductors are the most important problems in superconductivity research, but progress has been slow for many years. In this study, by introducing the p-n junction nanostructured electroluminescent inhomogeneous phase with a red wavelength to realize energy injection, we found the improved property of smart meta-superconductors MgB2, the critical transition temperature TC increases by 0.8 K, the current density JC increases by 37%, and the diamagnetism of the Meissner effect HC also significantly improved, compared with pure MgB2. Compared with the previous yttrium oxide inhomogeneous phase, the p-n junction has a higher luminescence intensity, a longer stable life and simpler external field requirements. The coupling between superconducting electrons and surface plasmon polaritons may be explained by this phenomenon. The realization of smart meta-superconductor by the electroluminescent inhomogeneous phase provides a new way to improve the performance of superconductors.

Reinforcing Increase of ΔTc in MgB2 Smart Meta-Superconductors by Adjusting the Concentration of Inhomogeneous Phases

Incorporating with inhomogeneous phases with high electroluminescence (EL) intensity to prepare smart meta-superconductors (SMSCs) is an effective method for increasing the superconducting transition temperature (T_c) and has been confirmed in both MgB₂ and Bi(Pb)SrCaCuO systems. However, the increase of ΔT_c (ΔT_c = T_c ‒ T_cpure) has been quite small because of the low optimal concentrations of inhomogeneous phases. In this work, three kinds of MgB₂ raw materials, namely, ^aMgB₂, ^bMgB₂, and ^cMgB₂, were prepared with particle sizes decreasing in order. Inhomogeneous phases, Y₂O₃:Eu³⁺ and Y₂O₃:Eu³⁺/Ag, were also prepared and doped into MgB₂ to study the influence of doping concentration on the ΔT_c of MgB₂ with different particle sizes. Results show that reducing the MgB₂ particle size increases the optimal doping concentration of inhomogeneous phases, thereby increasing ΔT_c. The optimal doping concentrations for ^aMgB₂, ^bMgB₂, and ^cMgB₂ are 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔT_c values are 0.4, 0.9, and 1.2 K, respectively. This work open a new approach to reinforcing increase of ΔT_c in MgB₂ SMSCs.

Pressure effect of the mechanical, electronics and thermodynamic properties of Mg-B compounds A first-principles investigations

First principle calculations were performed to investigate the structural, mechanical, electronic properties, and thermodynamic properties of three binary Mg–B compounds under pressure, by using the first principle method. The results implied that the structural parameters and the mechanical properties of the Mg–B compounds without pressure are well matched with the obtainable theoretically simulated values and experimental data. The obtained pressure–volume and energy–volume revealed that the three Mg–B compounds were mechanically stable, and the volume variation decreases with an increase in the boron content. The shear and volume deformation resistance indicated that the elastic constant C_ij and bulk modulus B increased when the pressure increased up to 40 GPa, and that MgB₇ had the strongest capacity to resist shear and volume deformation at zero pressure, which indicated the highest hardness. Meanwhile, MgB₄ exhibited a ductility transformation behaviour at 30 GPa, and MgB₂ and MgB₇ displayed a brittle nature under all the considered pressure conditions. The anisotropy of the three Mg–B compounds under pressure were arranged as follows: MgB₄ > MgB₂ > MgB₇. Moreover, the total density of states varied slightly and decreased with an increase in the pressure. The Debye temperature Θ_D of the Mg–B compounds gradually increased with an increase in the pressure and the boron content. The temperature and pressure dependence of the heat capacity and the thermal expansion coefficient α were both obtained on the basis of Debye model under increased pressure from 0 to 40 GPa and increased temperatures. This paper brings a convenient understanding of the magnesium–boron alloys.

Smart meta-superconductor MgB2 constructed by the dopant phase of luminescent nanocomposite

On the basis of the idea that the injecting energy will improve the conditions for the formation of Cooper pairs, a smart meta-superconductor (SMSC) was prepared by doping luminescent nanocomposite Y₂O₃:Eu³⁺/Ag in MgB₂. To improve the superconducting transition temperature (T_C) of the MgB₂-based superconductor, two types of Y₂O₃:Eu³⁺/Ag, which has the strong luminescence characteristic, with different sizes were prepared and marked as m-Y₂O₃:Eu³⁺/Ag and n-Y₂O₃:Eu³⁺/Ag. MgB₂ SMSC was prepared through an ex situ process. Results show that when the dopant content was fixed at 2.0 wt.%, the T_C of MgB₂ SMSC increased initially then decreased with the increase in the Ag content in the dopant. When the Ag content is 5%, the T_C of MgB₂ SMSC was 37.2–38.0 K, which was similar to that of pure MgB₂. Meanwhile, the T_C of MgB₂ SMSC doped with n-Y₂O₃:Eu³⁺/Ag increased initially then decreased basically with the increase in the content of n-Y₂O₃:Eu³⁺/Ag, in which the Ag content is fixed at 5%. The T_C of MgB₂ SMSC doped with 0.5 wt.% n-Y₂O₃:Eu³⁺/Ag was 37.6–38.4 K, which was 0.4 K higher than that of pure MgB₂. It is thought that the doping luminescent nanocomposite into the superconductor is a new means to improve the T_C of SMSC.

Rediscovering the Crystal Chemistry of Borides

For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.

Novel magnesium borides and their superconductivity

With the motivation of searching for new superconductors in the Mg–B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0–200 GPa.

Influence of Metal Diboride and Dy2O3 Additions on Microstructure and Properties of MgB2 Fabricated at High Temperatures and under Pressure

High temperatures and under pressure (HTP) processing has been used to study the effects of chemical doping in MgB_2. ZrB₂, TiB₂ and NbB₂ were selected as additives since, like MgB₂, they have an AlB₂-type structure and similar lattice parameters. Dy₂O₃ was selected as it has been reported to generate nanoscale, secondary intragrain phases in MgB₂. While C is known to enter the B-sublattice readily, attempts to dope Zr and other elements onto the Mg site have been less successful due to slow bulk diffusion, low solubility in MgB₂, or both. We have used high-temperature, solid-state sintering (1500 °C), as well as excursions through the peritectic temperature (up to 1700 °C), to investigate both of these limitations. Bulk MgB₂ samples doped with MB₂ (M = Zr, Ti and Nb) and Dy₂O₃ additions were synthesized and then characterized. Lattice distortion and high densities of crystal defects were observed in the MgB₂ grains around nano-sized MB₂ inclusions, this highly defected band contributed to a large increase in B_c2 but was not large enough to increase the irreversibility field. In contrast, distributed intragrain precipitates were formed by Dy₂O₃ additions which did not change the lattice parameters, T_c, T_c distribution or B_c2 of MgB₂, but modified the flux pinning.

A comparative study of doping effects of V and Cr on a SrAlSi superconductor

In SrAlSi superconductor, doping V leads to the considerable carrier concentration reduction and the quenching of superconductivity. In contrast, doping Cr almost does not change the carrier concentration, with a minor change in Tonsetc of 0.6 K.

Conduction electron spin resonance in AlB2

This work reports on electron spin resonance experiments in oriented single crystals of the hexagonal AlB2 diboride compound (P6/mmm, D16h structure) which display conduction electron spin resonance. The X-band electron spin resonance spectra showed a metallic Dysonian resonance with g-value and intensity independent of temperature. The thermal broadening of the anisotropic electron spin resonance linewidth ΔH tracks the T-dependence of the electrical resistivity below T is approximately equal to 100 K. These results confirm the observation of a conduction electron spin resonance in AlB2 and are discussed in comparison with other boride compounds. Based on our main findings for AlB2 and the calculated electronic structure of similar layered honeycomb-like structures, we conclude that any array of covalent B-B layers potentially results in a conduction electron spin resonance signal. This observation may shed new light on the nature of the non-trivial conduction electron spin resonance-like signals of complex f-electron systems such as β-YbAlB4.

The synthesis of lamellar nano MgB2 grains with nanoimpurities, flux pinning centers and their significantly improved critical current density

MgB(2) superconductors with unique microstructures were rapidly fabricated at low temperatures, and exhibited significantly improved critical current density (J(c)). According to the microstructure observations, the prepared samples consisted of lamellar nano MgB(2) grains with many embedded nanoimpurities (about 10 nm). The formation of these lamellar nano MgB(2) grains is associated with the presence of a local Mg-Cu liquid at sintering temperatures as low as 575 °C. The ball milling treatment of the original powders also plays a positive role in the growth of lamellar grains. Based on an analysis of the relationship between resistivity and temperature, the lamellar nano MgB(2) grains in the prepared sample possess better grain connectivity than the typical morphology of MgB(2) samples prepared by traditional high-temperature sintering. Furthermore, the presence of many nano MgB(2) grain boundaries and nano impurities in the prepared sample can obviously increase the flux pinning centers in accordance with the analysis of flux pinning behavior. Both factors mentioned above contribute to the significant improvement in J(c) from low field to relative high field. The method developed in the present work is an effective and low-cost way to further enhance J(c) in MgB(2) superconductors across a wide range of applied magnetic fields without using expensive nanometer-sized dopants.

Modeling of the phase evolution in Mg1-xAlxB2 (0 < x < 0.5) and its experimental signatures

Despite the chemical and structural simplicity of MgB(2), at 39 K this compound has the highest known superconducting transition temperature (T(c)) of any binary compound. Electron doping by substituting Al for Mg leads to decreasing T(c), and the observed concentration dependent rate of decrease has been proposed to arise from the nonideal character of MgB(2)-AlB(2) solid solutions, which derives from the existence of an ordered Mg(0.5)Al(0.5)B(2) compound. Heterogeneous nanoscale structure patterns in solid solutions have emerged as an important concept for complex materials, ranging from actinide alloys and oxides to high-temperature cuprate superconductors and manganite-based materials exhibiting colossal magnetoresistivity. In this work we investigate the formation of structural heterogeneities in Mg(1-x)Al(x)B(2), which take the form of nanoscale Al-Al and Al-Mg domains of different geometries and sizes, using molecular statics and Monte Carlo simulations, and in particular we study the corresponding signatures in diffraction experiments. In order to undertake this task, we first derive appropriate Mg-Al-B semiempirical potentials within the modified embedded atom method formalism. These potentials are also applied to explore the equilibrium Mg(1-x)Al(x)B(2) phase diagram for 0 < x < 0.5. Additionally, density functional theory calculations were utilized to study the influence of heterogeneities on the electronic structure and charge distribution in Mg(1-x)Al(x)B(2).

Analysis of H(c2)(θ,T) for Mg(B(1-x)C(x))(2) single crystals by using the dirty two-gap model

To understand the effect of carbon doping on the superconductivity in MgB(2), we obtained the angle- and temperature-dependent upper critical fields [H(c2)(θ) and H(c2)(T)] for Mg(B(1-x)C(x))(2) single crystals (x = 0.06 and 0.1) from resistivity measurements while varying the temperature, the field, and the direction of the field. The detailed values of the diffusivity for two different directions for each σ-band and π-band were obtained to explain both the temperature- and the angle-dependent H(c2) by using the dirty-limit two-gap model. The induced impurity scattering of the σ-band and the π-band for both the ab-plane and the c-direction is studied.

Investigation of hole states near the Fermi level in Nb(1-)(x)Mg(x)B(2) by electron energy-loss spectroscopy and first-principles calculations

The fine structures of the electron energy-loss spectra (EELS) for the B-K edge have been examined in NbB(2) and superconducting Nb(0.75)Mg(0.25)B(2). The experimental results are analyzed based on the calculations of density functional theory (DFT) using the Wien2k code. The results of the EELS spectra and the angular decomposition of the density of states (DOS) reveal that both the B p(z) and B p(x)+p(y) states in NbB(2) have large weights at the Fermi energy due to intersheet covalent bonding with notable hybridization between the Nb 4d and B 2p states. This kind of hybridization also results in different core-hole behaviors for the B-K edge in two orthogonal crystallographic orientations. The best fit between experimental and theoretical data is achieved with consideration of the core-hole effect of the B 1s states, in particular for the q perpendicular c spectra. Analysis of the electronic structure of the Nb(1-)(x)Mg(x)B(2) superconductors suggests that confinement of the intersheet covalent bonding is likely to be favorable for the improvement of superconductivity in this kind of materials.

Bulk photochromism in a tungstate-phosphate glass: A new optical memory material?

In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage.

On a Mechanism of Low-Pressure Insertion of Chain Molecules into Crystalline Matrices

A microscopic mechanism of low-pressure insertion and separation of chain-like molecules in host matrices is proposed. It is shown that the intramolecular correlations combined to appropriate host activities are responsible for a low-pressure condensation of chain molecules. This allows to recover a fine structure of the isotherms and to explain recent experiments on the insertion of C(2)H(2) and CO(2) guest species into metal-organic microporous materials. We argue that the mechanism should be dominant in low-dimensional host geometries, where the effects related to the chain conformations or reorientations are strongly suppressed and the major factors are the chain connectivity and the segment packing.

Chemical Control of Electronic Structure and Superconductivity in Layered Borides and Borocarbides: Understanding the Absence of Superconductivity in LixBC

The synthetic search for materials related to the 39 K superconductor MgB2 has been difficult. The most promising theoretical suggestion, hole doping of LiBC, does not lead to a new superconductor. We show here that a combination of density functional theory (DFT) calculations, materials synthesis, and structural characterization reveals the origin of the puzzling absence of superconductivity in Li1/2BC as a subtle change in the electronic structure driven by structural response to the introduction of holes. This indicates that the unique aspects of the electronic structure of MgB2 will be demanding to replicate in other systems.

Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T(c) < 7 K up to about x approximately 55. Only the pi band had detectable density of states for 0.33 < or =x < or = 0.55, implying an inversion of the two-band hierarchy of MgB(2) in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T(c) are discussed.

Band filling and interband scattering effects in MgB2: carbon versus aluminum doping

We argue, based on band structure calculations and the Eliashberg theory, that the observed decrease of T(c) of Al and C doped MgB2 samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant lambda by the variation of the density of states as a function of electron doping is sufficient to capture the experimentally observed behavior. Further, we also explain the long standing open question of the experimental observation of a nearly constant pi gap as a function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant pi gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.

Enhancement of the superconducting transition temperature of MgB2 by a strain-induced bond-stretching mode softening

We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.

High T(c) superconductivity in MgB2 by nonadiabatic pairing

The evidence for the key role of the sigma bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the sigma bands. We show that the nonadiabatic theory leads to a coherent interpretation of T(c) = 39 K and the boron isotope coefficient alphaB = 0.30 without invoking very large couplings and it naturally explains the role of the disorder on T(c). It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these types of compounds.

Strong renormalization of phonon frequencies in Mg(1-x)Al(x)B2

The phonon spectrum of Mg(1-x)Al(x)B2 shows a strong dependence on the aluminum content x. This is experimentally demonstrated by both Raman and inelastic neutron scattering and theoretically predicted by first-principles calculations. The observed changes in the phonon spectrum are put into perspective with respect to the superconducting properties within this family of materials.

Giant anharmonicity and nonlinear electron-phonon coupling in MgB2: a combined first-principles calculation and neutron scattering study

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

High critical currents in iron-clad superconducting MgB2 wires

Technically useful bulk superconductors must have high transport critical current densities, Jc, at operating temperatures. They also require a normal metal cladding to provide parallel electrical conduction, thermal stabilization, and mechanical protection of the generally brittle superconductor cores. The recent discovery of superconductivity at 39 K in magnesium diboride (MgB2) presents a new possibility for significant bulk applications, but many critical issues relevant for practical wires remain unresolved. In particular, MgB2 is mechanically hard and brittle and therefore not amenable to drawing into the desired fine-wire geometry. Even the synthesis of moderately dense, bulk MgB2 attaining 39 K superconductivity is a challenge because of the volatility and reactivity of magnesium. Here we report the successful fabrication of dense, metal-clad superconducting MgB2 wires, and demonstrate a transport Jc in excess of 85,000 A cm-2 at 4.2 K. Our iron-clad fabrication technique takes place at ambient pressure, yet produces dense MgB2 with little loss of stoichiometry. While searching for a suitable cladding material, we found that other materials dramatically reduced the critical current, showing that although MgB2 itself does not show the 'weak-link' effect characteristic of the high-Tc superconductors, contamination does result in weak-link-like behaviour.

Scanning tunneling spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39 K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter gamma. The value of the superconducting gap is 5 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair breaking as seen in the conductance spectra in fields up to 6 T.