Giant Anharmonicity and Nonlinear Electron-Phonon Coupling inMgB2: A Combined First-Principles Calculation and Neutron Scattering Study

Abstract

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB2. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E2g symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B σ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to quantitatively explaining the observed high Tc and boron isotope effect in MgB2 PACS numbers: 63.20. Ry, 63.20.Kr, 74.25.Jb, 74.25.Kc The recent discovery of superconductivity at 40 K in the MgB 2 binary alloy system [1] has triggered enormous interest in the structural and electronic properties of this class of materials. The system has a very simple crystal structure [2], where the boron atoms form graphitelike sheets separated by hexagonal layers of Mg atoms (see inset to Fig. 1). Our pseudopotential plane wave band structure calculations show that the bands near the Fermi level arise mainly from the p x,y σ bonding orbitals of boron, while the Mg does not contribute appreciably to the conductivity, in good agreement with initial reports from other groups [3][4][5]. In the case of graphite these σ bands are full, but for MgB 2 they are partially unoccupied, creating a hole-type [6] conduction band like the high-T c cuprates. In contrast to the cuprates, however, the normal-state conductivity is three-dimensional in nature instead of being highly anisotropic, thus eliminating the "weak-link" problem that has plagued widespread commercialization of the cuprates. The normal-state conductivity [6][7][8] is also one to two orders-of-magnitude higher than either the Nbbased alloys or Bi-based cuprates used in present day wires, and this feature combined with low cost and easy fabrication [8,9] could make this class of materials quite attractive for applications.From a fundamental point of view the central question is whether the high T c in this new system can be understood within the framework of a conventional electronphonon mechanism, or a more exotic mechanism is responsible for the superconducting pairing. The observed boron isotope effect [10] argues for an electron-phonon mechanism, while the positive Hall coefficient [6] suggests similarities with the cuprates [11]. To answer this question, we have carried out inelastic neutron scattering measurements of the phonon density of states, and compare these results with detailed first-principles calculations of the lattice dynamics (and electronic) calculations for MgB 2 . Excellent agreement is found between theory and experiment. More importantly, the numerical results demonstrate that the in-plane boron phonons near the zone-center (with E 2g symmetry at Γ) are very anharmonic and strongly coupled to the partially occupied c...

Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study

“…8 With a MgB 2 -like phonon spectrum and a slightly higher N͑E F ͒ than that of MgB 2 as mentioned earlier, one may wonder if ReB 2 can exhibit superconductivity at a "high" temperature. Experimentally, in the Re-B system, Re 3 B and Re 7 B 3 were found to have a T c of 4.8 and 3.3 K, respectively.…”

“…We note that the crystal structure of ReB 2 is quite different from other metal diboride compounds, a majority of which assume MgB 2 type structure. In MgB 2 , boron atoms are on the same plane, with a graphene structure 7,8 while they are buckled in ReB 2 . In addition, Mg is located above the center of B hexagon in MgB 2 , while in ReB 2 , Re is located right on top of the B atom.…”

Role of the E2g phonon in the superconductivity of MgB2: a Raman scattering study

“…Based on this model, several papers have discussed the possibility of E 2g phonon being a frozen-in mode, strongly coupled to the σ electronic bands near the Fermi level. [2,3,6,7] It is claimed that the E 2g phonon, due to its rather strong coupling to the σ electronic bands, would be highly anharmonic, presenting a very large linewidth. However, Boeri et al [8] pointed out that neither the presence at the Fermi level of the σ bands nor their strong coupling to the E 2g phonon are sufficient to induce such anharmonic effects, and therefore, the strong anharmonicity would be mainly related to the small value of the σ conduction holes Fermi energy (≃ 0.45 eV) in the unperturbed crystal.…”

Effect of the Al content on the optical phonon spectrum inMg1−xAlx

“…1a reveals two weak shoulders on the low-and the high-frequency side of the 600 cm −1 band (ν 2 ), around 400 (ν 1 ) and 750 cm −1 (ν 3 ), respectively. Although the origin of the central ν 2 band is still questioned, 11,12 it is generally assigned to the Ramanactive mode E 2g , to which theoretical predictions attribute a peak frequency 6,7,9,12,17,18 ranging from 470 to 660 cm −1 . It is worth noticing that further spectral contributions observed in previous Raman experiments 10,12 have been ascribed to peaks in the phonon density of states (around 430, 620, 710 and 780 cm −1 ) derived from neutron scattering experiments.…”