Anomalous phonon properties in the silicide superconductors CaAlSi and SrAlSi

Abstract

Lattice-dynamical properties of CaAlSi and SrAlSi with a similar layer structure to MgB 2 have been first investigated by both Raman-scattering and ab initio calculations. All Raman-active phonons with EЈ symmetry have been clearly observed for both compounds. Their line shapes are asymmetric but their linewidths are ϳ10 cm −1 , which is very narrower than that of MgB 2 . In addition to the Raman-active modes, several extra peaks have been observed below 160 cm −1 . These low-energy extra modes can be assigned to the out-of-plane vibrations of Al perpendicular to Al-Si basal plane. Since these peak intensities are strongly affected by the incident energy ͑resonance Raman process͒, the electronic state is important for them. Moreover, in both crystals of CaAlSi and SrAlSi, we point out the energy difference for the different propagation directions along the c axis and the c plane, in spite of the very close wave vector to the Brillouin-zone center. This energy difference cannot be explained by a usual Raman-scattering scenario at this stage.

Electronic band structure and volume-dependent lattice dynamical properties of superconductor Sr (AlGe)

“…Our calculated phonon structure of Sr (AlSi) is very similar to the calculated phonon-dispersion curves and total phonon DOS results of Ref. [20]. Here Eliashberg function follows the phonon DOS but it has a large peak around 50 cm À1 .…”

“…Same behavior observed for Sr (AlGe) too, but the appearance of imaginary phonon frequencies makes the situation more complicated [22]. The low-lying phonon branch is predominantly composed of out-of-plane displacement of Al atoms [20] and moreover, it is believed that the lowfrequency mode has a large contribution to k and the soft phonon mode plays an important role in the enchantment of T c in Ca (AlSi) [22]. Huang et al [22] mentioned that the strength of the EP coupling is relatively weaker in these compounds than MgB 2 and the strong EP coupling in MgB 2 arises from the in-plane bondstretching E2 g mode, while in these compounds, the electrons mainly couple with the low-frequency phonon modes.…”

Elastic, electronic, lattice dynamical properties and electron–phonon interaction in the ternary superconductors Ba(GaGe) and Ba(GaSi)

“…6 and 7, respectively. Both results are in well agreement with phonon dispersion calculations of Kuroiwa et al [2] for Sr(AlSi). Our data for Ba(GaSi) are observed to be similar with Ref.…”

“…Ternary germanide and silicide systems have intensive research interest in the last decade, not only for the comparison with superconducting (SC) properties of MgB 2 (T c % 39 K [1]), but also for the investigation of their own lattice dynamical properties [2]. Firstly, the critical superconducting transition temperature T c of Sr(Ga 0.37 Si 0.63 ) 2 is determined as 3.5 K [3].…”

First-principles investigation of the Al–Si–Sr ternary system: Ground state determination and mechanical properties

“…The vibrational properties of the AlSiSr phase has been studied in the past years [51,52] due to its superconducting behavior. We now discuss the formation thermodynamic properties of the AlSiSr, Al 2 Si 2 Sr 3 and Al 2 Si 4 Sr 3 phases.…”