Review of the superconducting properties of MgB₂

Abstract

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 T c = 40 K for a binary compound. What makes MgB 2 so special? Its high T c , simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB 2 will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB 2 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 MgB 2 represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB 2 holds the record for the highest T c among simple binary compounds. However, the discovery of superconductivity in MgB 2 revived the interest in non-oxides and initiated a search for superconductivity in related materials; several compounds have since been announced to be superconductive: TaB 2 , BeB 2.75 , C-S composites, and the elemental B under pressure.

Correlated vortex pinning in Si-nanoparticle doped MgB2

“…4 compares the J c (B) variations of our samples for T ≥ 33 K. The undoped tape (Fig. 4a) shows approximately exponential J c (B) variation, which is typical for MgB 2 samples [2,10,14,20]. At low temperatures (high I c ), large self-field µ 0 H s (H s ≃ I c /c, where c is the circumference of the core) makes J c (B < µ 0 H s ) nearly constant, whereas at elevated fields (B → B irr ) J c rapidly decreases to zero.…”

“…However, the onset of resistance (hence T c (R → 0) = T c0 ) is not affected by sheathing [25], and the zero-field T c0 = 38.2 K for the undoped tape ( Fig. 2a) is typical for bulk MgB 2 samples [2,5,24]. A strong shift of its T c0 with magnetic field (i.e.…”

“…The discovery of superconductivity in MgB 2 compound [1] has aroused a great deal of interest in the scientific community [2]. Compared to high-temperature superconductors (HTS), MgB 2 has a lower transition temperature T c ≃ 39 K, but its simple composition, abundance of constituents and the absence of weak intergranular links [3,4,5] make the MgB 2 a promising material for applications at T ≥ 20 K, which is above T c s of conventional superconductors (LTS).…”

Carbon doping in MgB2: role of boron and carbon px(y) bands

“…The effects of doping MgB 2 with various elements such as s-elements Be [1] and Li [2], sp-elements Al [3,4] and Si [5], and d-elements F e, Co, Ni [6] and others [7] have been studied experimentally. Most of these substitutions take place in the Mg plane of MgB 2 , leaving the B plane in tact.…”

“…For some dopants at particular concentrations, it is also found that the dopants prefer to form a separate layer [3,4] rather than mix with the Mg layer or the B layer. For dilute amount of dopants the changes in the superconducting properties, in particular the superconducting transition temperature, T c , depends on the alloying element [1,2,3,4,5,6,7]. For larger concentrations, with only Mg 1−x Al x B 2 [3,4] being studied extensively, the T c is found to decrease due to the gradual filling of the B p x(y) -holes.…”