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We report the preparation method of, and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature Tc( 10 B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in Tc between Mg 11 B2 and Mg 10 B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.PACS numbers: 74.70. Ad, 74.62.Bf The discovery of superconductivity with T c ≈ 39K in magnesium diboride (MgB 2 ) was announced in January 20011 . It caused excitement in the solid state physics community because it introduced a new, simple (3 atoms per unit cell) binary intermetallic superconductor with a record high (by almost a factor of two) superconducting transition temperature for a non-oxide and non-C 60 -based compound. The reported value of T c seems to be either above or at the limit suggested theoretically several decades ago for BCS, phonon-mediated superconductivity 2,3 . An immediate question raised by this discovery is whether this remarkably high T c is due to some form of exotic coupling. Therefore, any experimental data that can shed light on the mechanism of superconductivity in this material are of keen interest.One probe of the extent to which phonons mediate superconductivity is the isotope effect 4,5 . In the classical form of BCS theory 6 , the isotope coefficient α, defined by the relation T c ∝ M −α , where M is the mass of the element, is equal to 1/2. For simple metals like Hg, Pb, Sn, and Zn, the isotope coefficient is found experimentally to be close to 1/2. More detailed and realistic theories predict slight deviations from α = 1/2 2,7 . In this Letter, we describe how to prepare high-quality powders of MgB 2 and, more importantly, present data on the boron isotope effect, which is consistent with phonon mediated coupling within the framework of the BCS model.MgB 2 crystallizes in the hexagonal AlB 2 type structure, which consists of alternating hexagonal layers of Mg atoms and graphite-like honeycomb layers of B atoms. * On leave from Commissariat a l'Energie Atomique, DRFMC-SPSMS, 38054 Grenoble, France † On leave from Dept. of Physics, Grinnell College, Grinnell, IA 50112 This material, along with other 3d − 5d transition metal diborides, has been studied for several decades, mainly as a promising technological material 8 . The B -Mg binary phase diagram 9 is shown in Fig. 1. As can be seen, MgB 2 decomposes peritectically and has no exposed liquid-solidus line. Whereas the growth of single crystals of this compound promises to be a difficult problem, high quality powders can be formed in the following manner. Elemental Mg (99.9 % pure in lump form) and isotopically pure boron (99.5 + % pure, < 100 mesh) are combined in a sealed Ta tube in a stoichiometric ratio. The Ta tube is then sealed in a quartz ampoule, placed in a 950• C box furnace for two hours, and ...

Transport and thermodynamic properties of a sintered pellet of the newly discovered M gB2 superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, Hc2, with magnetization data giving dHc2/dT = 0.44 T /K at the transition temperature of Tc = 40.2 K. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of one Tesla. The value of dHc/dT at Tc is estimated to be about 12 mT /K, a value similar to classical superconductors like Sn. Hence, the Ginsburg-Landau parameter κ ∼ 26. Estimates of the critical supercurrent density, Jc, using hysteresis loops and the Bean model give critical current densities on the order of 10 5 A/cm 2 . Hence the supercurrent coupling through the grain boundaries is comparable to intermetallics like N b3Sn.74.25. Bt, 74.25.Fy, 74.25.Ha, 74.60.Ge, 74.60.Jg