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

Abstract

Temperature-dependent Raman scattering studies in polycrystalline MgB2 (10 < T < 300 K) reveal that the E2g phonon does not experience any self-energy renormalization effect across the superconducting critical temperature TC ≈ 39 K, in contrast with most of the current theoretical models. In the presence of our results, those models must be reviewed. The analysis of the temperature dependence of the E2g phonon frequency yields an isobaric Grüneisen parameter of | γE 2g | 1, smaller than the value of 3.9 obtained from isothermal Raman experiments under pressure. It is suggested that this apparent disagreement can be explained in terms of pressureinduced changes of the topology of the Fermi surface. MgB 2 has attracted much recent interest due to its remarkable physical properties such as: i) relatively high superconducting transition, T C = 39 K, for a binary compound with a simple crystal structure, ii) large and anisotropic coherence lengths, critical fields and current densities, and iii) critical currents that are not limited by grain boundaries (absence of weak link effects). [1] MgB 2 forms in a hexagonal structure with space group P6/mmm (D 1 6h ). The B atoms are located on a primitive honeycomb lattice consisting of graphite-type sheets. The B 2 -layers are intercalated with Mg-layers that also form a honeycomb lattice with a Mg atom in the center. For this space group, factor-group analysis predicts four modes at the Γ point: E u + A 2u + E 2g + B 1g , where only the E 2g mode is Raman-active and the B 1g mode is silent. The E 2g phonon is a doubly degenerate in-plane B-B bond-stretching mode[2] with nonvanishing Raman tensor elements (α xx − α yy ) and α xy . First principles lattice dynamics calculations indicate that these modes would be observed at 327 cm −1 (E u ), 405 cm −1 (A 2u ), 572 cm −1 (E 2g ) and 702 cm −1 (B 1g ). [3] In analogy with high T C superconductors (HTS), Hall effect measurements [4] indicate that the charge carriers in MgB 2 are holes with a hole density at 300 K of 1.7 − 2.8×10 23 holes/cm 3 . In fact, hole-mediated su- * Electronic address: hercules@ifi.unicamp.br; URL: http://www.ifi.unicamp.br/gpoms perconductivity has been proposed by An and Pickett for MgB 2 .[5] These authors attribute the relatively high value of T C to the strong coupling between holes and the in-plane boron phonon, E 2g modes. According to this model, the holes originate in the σ (sp 2 orbitals) bands due to charge transfer from the σ bands to the π (p z orbitals) bands. 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 the...

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