Abstract
It is a well-established fact that the physical properties of compounds follow their crystal symmetries. This has especially pronounced implications on emergent collective quantum states in materials. Specifically, the effect of crystal symmetries on the properties of superconductors is widely appreciated, although the clarification of this relationship is a core effort of on-going research. Emergent phenomena on honeycomb lattices are of special interest, as they can give rise to spectacular phenomenology, as manifested by the recent discovery of correlated states in magic-angle graphene, or by the high-temperature superconductivity in MgB$_2$. Here, we report on the structural and microscopic superconducting properties of a class of ternary superconductors with Al/Si honeycomb layers, i.e. Ca$_{1-x}$Sr$_{x}$AlSi. We show that this solid solution is a remarkable model system with a highly tunable two-gap to single-gap superconducting system on a honeycomb lattice, where the superconductivity is enhanced by a subtle structural instability, i.e. the buckling of the Al/Si layers.
Highlights
The increase of the superconducting transition temperature at a structural phase boundary is a widely observed behavior in various superconductors [1–4]
We show that the Ca1−xSrxAlSi solid solution is a highly tunable two-gap to single-gap superconducting system on a honeycomb lattice, where the superconductivity is strongly connected to a structural instability, i.e., the buckling of the Al/Si layers
The highest superconducting transition is observed for the parent compound of the solid solution x = 0 (CaAlSi), with a critical temperature of Tc ≈ 7.8 K
Summary
The increase of the superconducting transition temperature at a structural phase boundary is a widely observed behavior in various superconductors [1–4]. The relationships between structural instabilities and superconductivity has been a recurring theme in superconductivity research. These structural phase transitions are associated with a phonon softening, which influences the phonon-mediated Cooper pairing and may lead to a enhancement of the superconductivity [5,6]. The most prominent bulk superconductor with (boron) honeycomb layers sandwiched between Mg(II) layers, is MgB2 with a critical temperature of Tc = 39 K [11]. This remains the highest critical temperature at ambient pressure to-date, among materials, in which Cooper pairing is believed to be mediated by phonons, and described by the BCS (Bardeen-Cooper-Schrieffer) theory [12]
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