Electrons and phonons in the ternary alloyCaAl2−xSixas a function of composition

Abstract

We report a detailed first-principles study of the structural, electronic and vibrational properties of the superconducting C32 phase of the ternary alloy CaAl2−xSix, both in the experimental range 0.6 ≤ x ≤ 1.2, for which the alloy has been synthesised, and in the theoretical limits of high aluminium and high silicon concentration. Our results indicate that, in the experimental range, the dependence of the electronic bands on composition is well described by a rigid-band model, which breaks down outside this range. Such a breakdown, in the (theoretical) limit of high aluminium concentration, is connected to the appearance of vibrational instabilities, and results in important differences between CaAl2 and MgB2. Unlike MgB2, the interlayer band and the out-of-plane phonons play a major role on the stability and superconductivity of CaAlSi and related C32 intermetallic compounds.

Pressure effects on the superconducting transition innH-CaAlSi

“…At present, we cannot conclusively explain the different phonon softening behavior of the systems. However, by comparing our (partial) phDOS's obtained for different nH stacking variants with those of previous VCA calculations, 12,17 we observe that phonon softening mainly involves Al out-of-plane vibrations. Phonon softening appears if a block of three or more like planes is found in the sequence.…”

“…13,14 The interplay of these two ingredients gives rise to intriguing effects both on the normal and the superconducting properties. The presence of an ultrasoft phonon mode at ∼7 meV was initially evidenced by first-principles calculations 11,12,15,16,17 and recently confirmed by neutron-scattering experiments. 17 The ultrasoft phonon modes are believed to induce strong eph coupling and to lead to an enhanced specific heat anomaly at T c as well as to a positive pressure dependence of T c in CaAlSi.…”

“…The computational details are the same as in Ref. 12, unless otherwise specified. 27 Besides the identified 1H, 5H and 6H stacking variants, we also considered hypothetical 2H and 3H-CaAlSi, characterized by an |AB|AB| and |AAB|AAB| stacking of the Al-Si planes, respectively.…”

“…In CaC 6 and CaAlSi, the so-called "interlayer" bands are filled and experience significant electron-phonon (e-ph) interaction to the out-of-plane buckling vibrations of the honeycomb layers. 8,9,10,11,12 CaAlSi is of particular interest because it exhibits an ultrasoft phonon mode and crystalizes with several stacking variants. 13,14 The interplay of these two ingredients gives rise to intriguing effects both on the normal and the superconducting properties.…”

Elasticity of (K,Na)AlSi3O8 hollandite from lattice dynamics calculations

“…For each of the disordered sites we construct a new pseudopotential called alchemical, which is obtained from mixing the atoms that participate in the solid solution in the appropriate amounts (Giantomassi et al, 2005,). In practice the alchemical pseudopotentials are obtained as follows: the local potentials of the original pseudopotentials are linearly mixed, the form factors of the non-local projectors are all preserved, and all considered to generate the alchemical potential, the scalar coefficients of the non-local projectors are multiplied proportionally and the core charge cutoff radius and functions are linear combinations of respectively the radii and the functions of the original pseudopotentials.…”