This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since the early 1950s but only recently discovered to be superconductive at a remarkably high critical temperature Tc = 40 K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB2 will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and wires. The largest critical current densities, greater than 10 MA cm−2, and critical fields, 40 T, are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is yet to be resolved as a wide range of values have been reported, γ = 1.2–9. Also, there is no consensus on the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB2 holds the record for the highest Tc among simple binary compounds. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materialss; several compounds have since been announced to be superconductive: TaB2, BeB2.75, C–S composites, and the elemental B under pressure.

https://iopscience.iop.org/article/10.1088/0953-2048/14/11/201/pdf

Review of the superconducting properties of MgB2

This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since early 1950's, but recently discovered to be superconductive at a remarkably high critical temperature Tc=40K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, transparency of grain boundaries to current promises that MgB2 will be a good material for both large scale applications and electronic devices. During the last seven month, MgB2 has been fabricated in various forms, bulk, single crystals, thin films, tapes and wires. The largest critical current densities >10MA/cm2 and critical fields 40T are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is still to be resolved, a wide range of values being reported, between 1.2 and 9. Also there is no consensus about the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, being the tip of an iceberg who awaits to be discovered. Up to date MgB2 holds the record of the highest Tc in its class. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materials, several compounds being already announced to become superconductive: TaB2, BeB2.75, C-S composites, and the elemental B under pressure.

Review of superconducting properties of MgB2

The growth and properties of electronic oxide thin films are reviewed. In particular, the synthesis and properties of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures are discussed. Crystal structures and functional properties common to many oxide materials are briefly reviewed. A description of film-growth techniques follows. Finally, an extensive overview of the epitaxial growth for specific oxide material systems is given. This includes the epitaxial growth of electronic oxide thin films on oxide and non-oxide substrates.

Synthesis and properties of epitaxial electronic oxide thin-film materials

A comprehensive review of structure work on high-Tc oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba1−x,Kx)BiO3 (Tc=30 K),II. (La2−x, Srx)CuO4 (Tc=40 K),III. (Nd, Ce, Sr)2CuO4 (Tc=28 K),IV. (Nd2−x, Cex)CuO4 (Tc=24 K),V. YBa2Cu3O7 (Tc=90 K),VI. YBa2Cu4O8 (Tc=80 K),VII. Y2Ba4Cu7O14 (Tc=40 K),VIII. Pb2Sr2NdCu3O8 (Tc=70 K),IX. TlBa2CaCu2O7 (Tc=103 K),X. TlBa2Ca2Cu3O9 (Tc=120 K),XI. Tl2Ba2CuO6 (Tc=90 K),XII. Tl2Ba2CaCu2O8 (Tc=112 K),XIII. Tl2Ba2Ca2Cu3O10 (Tc=125 K). Except forI (perovskite type),II (K2NiF4 type) andIV (Nd2CuO4 type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.

Crystal structures of high- T c oxides

A brief summary is given of the current status of chemical substitution experiments in MgB2. Analysis of the data in the literature shows that only in a handful of cases has partial substitution of an atom for Mg or B been substantiated. These cases are Al and Mn substitution for Mg, and C substitution for B. C and Al substitution have received the most attention. Oxygen incorporation is an interesting chemical issue. A summary of our work on the Mg1−xAlxB2 system is presented.

The substitutional chemistry of MgB2

We report the preparation of Mg 1− x Li x B 2 compounds. Nearly single phase samples were obtained for x ⩽0.3. The in-plane lattice parameter a decreases with Li doping, while the lattice parameter c does not show obvious change. The superconducting transition temperature of Mg 1− x Li x B 2 decreases with Li doping and loss of superconductivity occurs for x =0.5 sample. The results were discussed in terms of the T c dependence on hole doping and contraction of the lattice parameter a .

Effect of Li doping on structure and superconducting transition temperature of Mg1-xLixB2

We report the preparation of samples with the nominal composition Mg1−xB2 (0⩽x⩽0.5). Nearly single-phase MgB2 was obtained for x=0 with a minor amount of MgO. For 0<x⩽0.5, MgB4 coexists with MgB2 and some minor impurity phases. The amount of MgB4 increases with x. With increasing x, the lattice parameter c of MgB2 increases, the lattice parameter a decreases, and correspondingly Tc decreases. The results are discussed in terms of the presence of Mg vacancies or B interstitials in the MgB2 structure. This work is helpful to understand MgB2 films with different Tcs, as well as the Mg site doping effect for MgB2.

Influence of the starting composition Mg1-xB2 on the structural and superconducting properties of the MgB2 phase

In this paper, single phase AICNi(3) compound was prepared and its physical properties were investigated. It was found that AICNi(3) is not superconducting down to 4 K and a paramagnetic into weak ferromagnetic transition takes place at temperature near 300 K. (c) 2005 Elsevier B.V. All rights reserved.

Synthesis and physical properties of AlCNi3

The structural change and superconductivity in the nominal composition MgCxNi3 (x = 0.5-1.55) and MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) were investigated and the phase diagram of Ni-rich region in Mg-C-Ni ternary system was determined. It was found that in MgCxNi3 system there are two phases: alpha-MgCxNi3 and beta-MgCNi3. alpha-MgCxNi3 phase exists in the MgCxNi3 samples with x 1.0 prepared at the temperature of 900985 degreesC and in the samples with x < 1.0 prepared at the temperature higher than 965 degreesC. It has a large and constant lattice parameter of 0.3810 run. The beta-MgCNi3 phase is superconducting. The investigation results indicate that a homogeneous region with changeable content of Mg does not exist in MgxCyNi3 (x = 0.75-1.55, y = 0.85, 1.0 and 1.45) systems. (C) 2002 Elsevier Science B.V. All rights reserved.

The structural change and superconductivity in MgCxNi3 (x=0.5–1.55) and MgxCyNi3 (x=0.75–1.55, y=0.85, 1.0 and 1.45) and the phase diagram of Ni-rich region in Mg–C–Ni ternary system

We found a series of superconducting phases in the Tl-Ba-Ca-Cu-O system and determined the crystal structures of these superconducting phases by means of the X-ray powder diffraction method. All of these superconducting phases belong to the tetragonal symmetry. They have the same lattice constant a and different lattice constant c . These superconducting phases can be divided into two types. The space group of type I is P4/mmm. The space group of type II is 14/mmm. The chemical formulae of type I and type II can be generalized as TlBa 2 Ca n −1 Cu n O 2 n +2.5 ( n =2, 3, 4) and Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1, 2, 3), respectively. T c increases as n increases. On the condition that n is the same, type II has a higher T c than type I. The relations among the crystal structures of the superconducting phases are discussed. We proposed a method which can deduce the crystal structure of the superconducting phase from the d 1 -value of the first diffraction line (00 l ) or lattice parameter c and which is confirmed by experimental data.

Shunlian Jia

Papers

Effect of Li doping on structure and superconducting transition temperature of Mg1-xLixB2

Influence of the starting composition Mg1-xB2 on the structural and superconducting properties of the MgB2 phase

Synthesis and physical properties of AlCNi3

The structural change and superconductivity in MgCxNi3 (x=0.5–1.55) and MgxCyNi3 (x=0.75–1.55, y=0.85, 1.0 and 1.45) and the phase diagram of Ni-rich region in Mg–C–Ni ternary system

Crystal-structures and superconductivity of superconducting phases in the tl-ba-ca-cu-o system