The ternary iron arsenide ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at ${T}_{c}=38\text{ }\text{ }\mathrm{K}$ in $({\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $x\ensuremath{\approx}0.4$. The parent compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ crystallizes in the tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure, which consists of $(\mathrm{FeAs}{)}^{\ensuremath{\delta}\ensuremath{-}}$ iron arsenide layers separated by ${\mathrm{Ba}}^{2+}$ ions. ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting ${\mathrm{Ba}}^{2+}$ for ${\mathrm{K}}^{+}$ ions we have introduced holes in the $(\mathrm{FeAs}{)}^{\ensuremath{-}}$ layers, which suppress the anomaly and induce superconductivity. The ${T}_{c}$ of 38 K in $({\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure.

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2.

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 ternary iron arsenide ${\text{BaFe}}_{2}{\text{As}}_{2}$, with the tetragonal ${\text{ThCr}}_{2}{\text{Si}}_{2}$-type structure, exhibits a spin-density-wave (SDW) anomaly at 140 K, very similar to LaFeAsO, which is the parent compound of the iron arsenide superconductors. ${\text{BaFe}}_{2}{\text{As}}_{2}$ is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance, and magnetic susceptibility. In the course of this transition, the space-group symmetry changes from tetragonal $(I4/mmm)$ to orthorhombic $(Fmmm)$. $^{57}\text{F}\text{e}$ M\"ossbauer spectroscopy experiments show a single signal at room temperature and full hyperfine field splitting below the phase-transition temperature (5.2 T at 77 K). Our results suggest that ${\text{BaFe}}_{2}{\text{As}}_{2}$ can serve as a parent compound for oxygen-free iron arsenide superconductors.

Spin-density-wave anomaly at 140 K in the ternary iron arsenide BaFe 2 As 2

THE attainment of unprecedentedly high transition temperatures (Tcs) in the copper oxide superconductors illustrates how working with more complex chemical systems allows greater opportunity to balance opposing forces within a single chemical compound, leading to a better optimization of physical properties. For many desired properties, materials with optimal chemical complexity have undoubtedly not yet been found. This appears to be the case for the intermetallic superconductors, whose study has languished in recent years, and which almost never show Tcs above 15 K. These are almost all binary compounds with substitution-type additives, or, rarely, true ternary compounds such as LuRh4B4 (Tc = 11.7 K; refs 1, 2). If, as some argue (refs 3, 4), materials such as AXC60 (ref. 5) and Ba0.6K0.4BiO3 (refs 6, 7) are conventional electron–phonon superconductors with Jcs of ~30 K, then the absence of higher Tcs in intermetallic compounds may mean only that more complex materials have not been sufficiently explored. We have recently found superconductivity at 23 K (a Tc equal to that of the previous intermetallic record holder, Nb3Ge; ref. 9) in the quaternary intermetallic system yttrium–palladium–boron–carbon8, but we were unable to identify the superconducting phase. Here we report superconductivity at temperatures up to 16.6 K for the single-phase quaternary intermetallic compounds LnNi2B2C (where Ln stands for Y, Tm, Er, Ho or Lu). The presence of the 3d transition metal nickel, and the layered crystal structure10 raise intriguing questions about the origin of the superconductivity, and the relatively high Tcs of these and the Y–Pd–B–C superconductor suggest that there may yet be more surprises in store.

Superconductivity in the quaternary intermetallic compounds LnNi 2 B 2 C

SUPERCONDUCTING intermetallic compounds with relatively high transition temperature (Tc) have been found in the systems Ln–Tr–B–C, where Ln is a lanthanide element (Y, Ho–Lu) and Tr a transition metal (Pd or Ni)1,2. In the nickel-bearing system, the superconducting phase has the formula LnNi2B2C (ref. 2). Although many structures are known for ternary carbides and borides, only a few have been reported for quaternary boro-carbides3. Here we describe two new phases that form in the Lu–Ni–B–C system: LuNi2B2C, a superconductor with a Tc of 16.6 K (ref. 2), and LuNiBC, a closely related non-superconducting phase. The superconductor is a variant on the layered ThCr2Si2-type structure4, with additional carbon in the Lu plane. LuNiBC is derived from LuNi2B2C by adding another layer of Lu–C, producing a NaCl-type intergrowth. A new homologous series of inter-metallic phases may be derivable from the simple building principles of these two structures.

H. W. Zandbergen

Papers

Superconductivity in the quaternary intermetallic compounds LnNi 2 B 2 C

The crystal structure of superconducting LuNi 2 B 2 C and the related phase LuNiBC

LaCuO25+x and YCuO2.5+x Delafossites: Materials with Triangular Cu2+δ Planes

La6BaYCu8O20: A New Oxygen Deficient Perovskite