V. Palmisano

Isotope effect on the E 2 g phonon and mesoscopic phase separation near the electronic topological transition in Mg 1 − x Al x B 2

We report the boron isotope effect on the E2g phonon mode by micro-Raman spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes 10B and 11B. The isotope coefficient on the phonon frequency is near 0.5 in the full range decreasing near x = 0. The intraband electron-phonon (e-ph) coupling, for the electrons in the sigma band, has been extracted from the E2g line-width and frequency softening. Tuning the Fermi energy near the electronic topological transition (ETT), where the sigma Fermi surface changes from 2D to 3D topology the E2g mode, shows the known Kohn anomaly on the 2D side of the ETT and a splitting of the E2g phonon frequency into a hard and soft component from x = 0 to x = 0.28. The results suggest a minor role of the intraband phonon mediated pairing in the control of the high critical temperature in Mg1-xAlxB2. The common physical features of diborides with the novel multigap FeAs-based superconductors and cuprates is discussed.

Controlling mesoscopic phase separation near electronic topological transitions via quenched disorder in ternary diborides

A phase separation driven by the negative compressibility of the electron gas, near electronic topological transitions (ETT), could drive the system at the verge of a catastrophe. We show here that the metastable phases very close to the ETT transition are observed in a mesoscopic phase separation (MePhS) driven by the quenched lattice disorder. By using high resolution synchrotron radiation x-ray powder diffraction we have identified the MePhS for the intermetallic ternary Mg1−x Alx B2 in the proximity of two ETTs: the first at x1 = 0. 1a nd the second at x2 = 0.3. We have identified the competition between a first ‘relaxed’ (R) hole poor and a second ‘tense’ (T) hole rich phase, and by micro-Raman we observe the splitting of the in-plane phonon E2g mode in the proximity of the first ETT at x = 0.1. The anisotropic quenched disorder due to a random distribution of Al 3+ and Mg 2+ ions both in the axial (c axis) direction and planar (ab plane) direction, probed by x-ray diffraction and Raman data, is proposed to be the physical variable that allows the formation of metastable phases near the critical points of electronic topological transitions, where Feshbach shape resonances in interband pairing amplifies the superconducting critical temperature. (Some figures in this article are in colour only in the electronic version) Q.1

II.1 Magnesium diboride and the two-band scenario

The Feshbach shape resonance in the interband pairing is shown to provide the mechanism for evading temperature de-coherence effects in macroscopic quantum condensates. This mechanism provides the Tc amplification in particular nanoscale material architectures like superlattices of metallic layers. The Feshbach resonance is reached by tuning the chemical potential to a Van Hove-Lifshitz singularity (vHs) in the electronic energy spectrum associated with the change of the Fermi surface dimensionality of one of the superlattice subbands. The case of light element diborides where Mg is substituted for Al in AlB2 and its similarity with intercalated graphite compounds is discussed.