Phonon modes of MgB₂: super-lattice structures and spectral response

Abstract

Micrometre-sized MgB2 crystals of varying quality, synthesized at low temperature and autogenous pressure, are compared using a combination of Raman and infra-red (IR) spectroscopy. These data, which include new peak positions in both spectroscopies for high quality MgB2, are interpreted using DFT calculations on phonon behaviour for symmetry-related structures. Raman and IR activity additional to that predicted by point group analyses of the P6/mmm symmetry are detected. These additional peaks, as well as the overall shapes of calculated phonon dispersion (PD) models are explained by assuming a double super-lattice, consistent with a lower symmetry structure for MgB2. A 2× super-lattice in the c-direction allows a simple correlation of the pair breaking energy and the superconducting gap by activation of corresponding acoustic frequencies. A consistent physical interpretation of these spectra is obtained when the position of a phonon anomaly defines a super-lattice modulation in the a-b plane.

Coherent phonon decay and the boron isotope effect for MgB2

“…If this mode is essential for superconducting transport, a coherent decay path to the acoustic frequency cannot be established in the same way that we have shown 22 for MgB 2 . Fig.…”

“…the conventional P6/mmm symmetry and the 2Â super-lattice symmetry) on MgB 2 and on AlB 2 , clearly show that the PDOS is insensitive to changes in imposed symmetry, even when additional frequencies occur. Note also that the phonon anomaly identified in earlier work 22 is present in the PDs for pure isotopic forms of MgB 2 . This result indicates that the phonon anomaly does not originate from the natural mixture of isotopes which may be a potential source of local asymmetry.…”

“…This mechanism is described in terms of the important E 2g mode which is strongly implicated in the BCS model for superconductivity in MgB 2 . 16 This analysis of experimental data and ab initio calculations shows that major acoustic energies for a 2Â super-lattice define a linear trend with integer multiples corresponding to allowed frequency modes 22 in MgB 2 . Table 2).…”

“…The approach described above and in earlier work 22 provides an improved explanation for dynamic perturbations on the MgB 2 structure that are introduced by the phonons themselves and thus, result in a strongly-coupled electronic response. 22 …”

“…These calculations show that additional Raman and IR peaks for MgB 2 can be explained by super-lattice modes that approximate a dynamic, phonon-distorted crystal structure. 22 In addition, this work identifies a correspondence of phonon energies to superconducting energy gaps, suggesting that coherent phonon decay modes, and phonon excitation from the E 2g mode, are important mechanisms for superconductivity in this structure type.…”