Nonharmonic phonons in MgB2at elevated temperatures

Abstract

Inelastic neutron scattering was used to measure phonon spectra on MgB2 and Mg0.75Al0.25B2 from 7 K to 750 K to investigate anharmonicity and adiabatic electron-phonon coupling. First principles calculations of phonons with a linear response method were performed at multiple unit cell volumes, and the Helmholtz free energy was minimized to obtain the lattice parameter and phonon dynamics at elevated temperature in the quasiharmonic approximation. Most of the temperaturedependence of the phonon DOS could be understood with the quasiharmonic approximation, although there was also significant thermal broadening of the phonon spectra. In comparison to Mg0.75Al0.25B2, in the energy range of 60 to 80 meV the experimental phonon spectra from MgB2 showed a non-monotonic change with temperature around 500 K. This may originate from a change with temperature of the adiabatic electron-phonon coupling.2

Phononic Weyl Nodal Straight Lines in High-Temperature Superconductor MgB$_2$

“…Phonon spectra.-Figure 2(c) presents the phonon spectrum of MgB 2 along high symmetry momentum paths, which are in nice agreement with previous calculations [43][44][45][46][47] and experimental characterizations [48][49][50]. At the highsymmetry K( 1 3 , 1 3 , 0) and H( 1 3 , 1 3 , 1 2 ) points, there are three double-degenerate points each.…”

“…Strucuture.-The optimized lattice constants are a = 3.074 Å and c = 3.513 Å. These values are in good agreements with previous experimental [40][41][42] and computational [43][44][45][46][47] results. The structure is shown in Fig.…”

Electron paramagnetic resonance and Raman spectroscopy studies on carbon-doped MgB2 superconductor nanomaterials

“…Such an effect has been observed recently for Al-doped MgB 2 materials. 32 The decrease (red shift) in phonon-mode energy in nanoboron MgB 2 sample can be mainly attributed to the decreased density of states. As depicted in Fig.…”

“…29,30 However, unclear issues such as phonon anomalies like the anomalously large linewidth of the E 2g phonon mode, the anharmonicity, and multi-phonon contributions may be closely relate with the defects. 31,32 The importance of phonon-defect interaction as an alternative cause for the anomalies of high-temperature superconductors have been emphasized in earlier reports. 33 Therefore, the correlation of Raman and EPR results would give plausible information about the defect related electron-phonon or phonon-defect interactions, which most likely contribute to the Raman and EPR spectra of undoped and C-doped MgB 2 .…”

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

“…10,15,19,24 While these studies have described many properties of MgB 2 , the spread of experimental results for phonon frequencies is wide and not always consistent. Anharmonicity, 1,2,18,22,23,25,26 phonon anomalies, 7,19,27 presence of impurities, 9,11,12,17 off stoichiometry from Mg deficiency, 19 defects, 19 or unreacted boron, 17 structural disorder or distortion 5,11,[14][15][16] and multi-phonon contributions 16,20,25 are among the cited manifestations or reasons for divergent results on MgB 2 property-structure relations. Anharmonicity, 1,2,18,22,23,25,26 phonon anomalies, 7,19,27 presence of impurities, 9,11,12,17 off stoichiometry from Mg deficiency, 19 defects, 19 or unreacted boron, 17 structural disorder or distortion …”