Unconventional Behavior of Field-induced Quasiparticle Excitation in Ca(Al_0.5Si_0.5)₂

Abstract

The temperature (T ) and magnetic field (H) dependence of the magnetic penetration depth, λ(T, H), in Ca(Al0.5Si0.5)2 exhibits significant deviation from that expected for conventional BCS superconductors. In particular, it is inferred from a field dependence of λ(H) (∝ H) at 2.0 K that the quasiparticle excitation is strongly enhanced by the Doppler shift. This suggests that the superconducting order parameter in Ca(Al0.5Si0.5)2 is characterized by a small energy scale ∆S/kB ≤ 2 K originating either from anisotropy or multi-gap structure.KEYWORDS: Ca(Al0.5Si0.5)2, superconductivity, penetration depth, flux line lattice, µSR, Since the discovery of binary intermetallic compound superconductor MgB 2 (T c =39 K), 1 the origin of pair correlation leading to such high critical temperature has been drawing much interest. Up to now, a majority of experiments as well as theories suggests that the pair correlation is mediated by a strong electron-phonon interaction, where the relatively high T c is explained by the light mass of the two-dimensional honeycomb layer formed by B atoms. The two-dimensional feature is also preferable for the strong electron-phonon interaction, and thereby the AlB 2 -type crystal structure common to MgB 2 is currently attracting attention as a possible basis for developing new superconductors.In binary silicides, ThSi 2 , 2 USi 2 , 3 and several rareearth metal disilicides have the AlB 2 -type structure. β-ThSi 2 is known to be a superconductor with a critical temperature T c =2.41 K.4 Above 16 GPa, CaSi 2 takes a AlB 2 -like structure, and it becomes superconducting with T c =14 K.5 Recently, a new ternary silicide, Sr(Ga,Si) 2 , which also has the AlB 2 -type structure, was reported to be a superconductor with T c =3.4 K, 6 stimulating active investigation of analogous compounds. In this class of materials, ternary silicide Ca(Al 0.5 Si 0.5 ) 2 has the highest critical temperature T c =7.7 K.7 It is reported on this compound that the behavior of electron-heat capacity deviates from that of the BCS-type, and that the effect of hydrostatic pressure on T c is positive.8 Unfortunately, despite various experiments so far, there is very little known on the structure of superconducting order parameter in Ca(Al 0.5 Si 0.5 ) 2 .In this paper, we report the quasiparticle excitation in the flux line lattice (FLL) state of Ca(Al 0.5 Si 0.5 ) 2 studied by muon spin rotation (µSR). The magnetic penetration depth λ, which reflects the quasiparticle excitation, can be determined by µSR due to the inhomogeneity of the magnetic field distribution in the FLL state. We show the temperature (T ) and magnetic field (H) dependence of λ in polycrystalline sample of Ca(Al 0.5 Si 0.5 ) 2 . Our result indicates that λ is proportional to T 3 at low temperature, and that it exhibits a steep increase with almost linear dependence on the applied magnetic field. These findings strongly suggest that Ca(Al 0.5 Si 0.5 ) 2 has an anisotropic order parameter or a multi-gap structure with a band having a small gap energy, which...

Magnetic penetration depth in unconventional superconductors

“…On the other hand, μSR studies have been interpreted as evidence of either one highly anisotropic or two distinct energy gaps. [120] Furthermore, 5-fold and 6-fold stacking sequence of (Al,Si) layers corresponding to two different values of T c were found [182]. Therefore, an experimental study of in-and out-of-plane superfluid density was needed to understand anisotropic superconducting gap structure and mechanism of superconductivity in AlB 2 type compounds.…”

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

“…3 Superconductivity has been detected in the whole composition range; the critical temperature T c (x) strongly depends on composition, showing a sharp peak at x = 1 (where T c = 7.8 K), and dropping off rapidly as the Al/Si ratio departs from unity. 3 Several experiments 4,5,6 and ab initio calculations 7,8,9,10 have concentrated on the x = 1 case (i.e. CaAlSi).…”

Anomalous phonon properties in the silicide superconductors CaAlSi and SrAlSi

“…20 Therefore, this silicide system is very interesting not only for the comparison with superconducting properties of MgB 2 but also for the investigation of its own lattice-dynamical properties. However, in spite of various investigations so far, [20][21][22][23][24][25] there is very little experimental knowledge on the lattice-dynamical properties and EPC mechanism in 1H-CaAlSi and SrAlSi.…”