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 ...

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The behavior of a type II superconductor in the presence of a magnetic field is governed by two characteristic length scales, the London penetration depth and the coherence length. We present magnetization measurements on MgB2 powder showing an anisotropy in the upper critical field and hence the coherence length of 6. Using the technique of small angle neutron scattering we show that this anisotropy is not mirrored in the London penetration depth, which is almost isotropic. This result can be explained by the superconductivity residing in two distinct electronic bands of the material, only one of which is highly anisotropic.