Phonon density - of - states in the new superconductor MgB 2

We provide here experimental data on the phonon density-of-states of MgB2 obtained by the inelastic neutron scattering technique. The measurements were performed for the natural boron-based magnesium diboride with use of a time-of-flight neutron spectrometer. Several phonon bands were observed in the phonon spectrum at energies of about 33, 55, 82 and 99 meV. We show that the cutoff energy of the density-of-states occurs at around 105 meV which is much higher than expected so far from heat-capacity data and partially explains the high Tc value observed for MgB2. The characteristic phonon energies are indicative of an intermediate coupling regime in this compound. We conclude that a much needed neutron experiment aimed at the study of the isotopic effect in the phonon density-of-states of MgB2 is conceivable. Recently, superconductivity at T c ≈ 39 K was observed in magnesium diboride MgB 2 by Nagamatsu et al. [1]. The transition temperature is surprisingly high for a non-cuprate material and much above the limit expected for a classical superconducting compound. The observation of a boron isotope effect on T c suggests a phonon-mediated pairing mechanism [2]. In the Bardeen-Cooper-Schrieffer (BCS) theory [3] T c scales with the Debye energy when the effective attractive interaction between electrons is due to phonons. A detailed picture of the phonon density-of-states is therefore highly desirable. This was the motivation for our experimental study. Inelastic neutron scattering (INS) is a most powerful tool to probe the characteristic energies of lattice vibrations. MgB 2 crystallizes in the hexagonal AlB 2-type structure [4] in which the B ions constitute graphite-like sheets in the form of primitive honeycomb lattices separated by hexagonal layers of Mg as illustrated in Figure 1. It was concluded from band structure calculations [5] that the charge carriers reside in the boron planes which are metallic due to the covalent B-B bonds. Thus there is a structural similarity to the high-T c cuprate superconductors in which the charge carriers are also located in the CuO 2 planes. It is well known that the phonon energies of atoms in a two-dimensional network are enhanced compared to a three-dimensional one. For example, the phonon density-of-states of (two-dimensional) graphite extends to a Fig. 1. Crystal structure of MgB2. Magnesium and boron are drawn as big and small spheres, respectively. The structure can be considered as a sequence of alternating graphite-like layers of hexagonal symmetry. The boron atoms are six-fold …