M. Filippi

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.

Feshbach Shape Resonances in Multiband High Tc Superconductors

We describe particular nanoarchitectures (superlattices of superconducting wires and layers) where a mechanism to evade temperature decoherence effects in a quantum condensate is switched on by tuning the charge density. The superlattice structure determines the subbands and the corresponding Bloch wavefunctions of charge carriers at the Fermi level with different parity and different spatial locations. The disparity and negligible overlap between electron wave-functions in different subbands suppress the single electron interband impurity scattering rate and allow the multiband superconductivity in the clean limit. The quantum trick that bestows to the system the property to resist to the decoherence attacks of high temperature is the Feshbach shape resonance in the interband off-diagonal exchange-like pairing term i.e., in the quantum configuration interaction between pairing channels in different subbands. It occurs by tuning the chemical potential at a particular point near a Van Hove singularity (vHs) in the electronic energy spectrum, or a Lifshitz electronic topological transition (ETT), associated with the change of the dimensionality of the Fermi surface topology of one of the subbands.

27Al and 11B NMR Study of the Two-Band Superconductor Mg1-xAlxB2

We have performed magnetization and 27 Al- and 11 B-NMR measurements of Mg 1- x Al x B 2 with x =0, 0.16, 0.22, 0.30, and 0.45 in both the normal and superconducting (SC) states. In the normal state, the temperature-independent values of 1/ T 1 T of 27 Al and 11 B decrease with increasing x . The 1/ T 1 of 27 Al decreases exponentially with temperature in the SC state followed by the appearance of a tiny coherence peak just below T c . The 1/ T 1 of 11 B for x ≥0.22 shows no gap behavior below T c , indicating the large reduction in superconducting gap with x at the B site. The temperature dependence of 1/ T 1 T in the SC state is quantitatively analyzed using a s wave SC model with two gaps of 2Δ/ k B T c ∼1.5 and 4. The present results show that Δ σ is markedlly deceases with increasing x , namely, with σ band filling by electron doping, providing convincing evidence for two SC gaps with spin singlet s -wave pairing in Mg 1- x Al x B 2 .

A study of the Nb3Ge system by Ge K-edge extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopy

The local structure of Nb3Ge intermetallic superconductor has been studied by Ge K-edge absorption spectroscopy. Extended x-ray absorption fine structure (EXAFS) experiments show two Ge–Nb distances. In addition to the crystallographic distance of ~2.87 A, there exists a second Ge–Nb distance, shorter than the first by ~0.2 A, assigned to a phase with short-range symmetry related to local displacements in the Nb–Nb chains. The x-ray absorption near-edge structure (XANES) spectrum has been simulated by full multiple-scattering calculations considering the local displacements determined by the EXAFS analysis. The XANES spectrum has been well reproduced by considering a cluster of 99 atoms within a radius of about 7 A from the central Ge atom and introducing determined local displacements.

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.