MgB2 superconductor : a review

Synthesis, structure and properties of the most intensively studied newly discovered intermetallic binary superconductor MgB 2 have been reviewed up to October, 2002. Though this material was known since 1950s, it was recently discovered to be superconductive with a record high T c ∼ 40 K. The detection of this binary superconductor by a Japanese research group in January, 2001 initiated the basic research to find out other binary and ternary intermetallic superconductors like TaB 2 . BeB 2 . 7 5 , MgCNi 3 etc. It has a hexagonal unit cell with cell parameters a ∼ 3.1432 A and c ∼ 3.5193 A. MgB 2 bulk samples synthesized under high pressure (∼3.5 GPa) and high temperature (∼1000 °C) has density ∼2.63 g/cm 3 . The normal state carriers of MgB 2 are holes which have been established from the positive thermoelectric power and Hall coefficient measurements. The external pressure decreases the critical temperature (T c ) with dT c /dP in the range of -I to -2 K/GPa. The T c decreases rapidly by the doping of Mn, Li, Co, C, Al, Ni and Fe but increases slightly by Zn doping. However, no significant change of T c is observed by the doping of Si and Be. It is further noticed that the anisotropic ratio γ (= H c 2 ab /H c 2 c) -1- 5 with lower critical field (H c 1 ) ∼ 25-48 mT and upper critical field, H c 2 (0) ∼ 40 T. The critical current density (J c ) of it is as high as 1.3×10 6 Acm - 2 in the self-field and 9.4×10 4 Acm - 2 in a magnetic field of 2 T at the temperature of 20 K. The most interesting feature of this superconductor is the appearance of two superconducting energy gaps one Δ 1 ∼ 1.92 meV and the other at Δ 2 ∼ 3.45 meV. From theoretical studies, it is found that MgB 2 superconductor has a minimum of the density of states (DOS) just above the Fermi level which can account for the decrease of T c with the increase of pressure This review mainly deals with experimental results obtained by different research groups working on this new superconductor. Since more new and interesting results are expected in near future, critical theoretical analysis of the existing experimental data has not been made. We hope to do this work in our next review on this subject.