Enhancing the Superconducting Transition Temperature ofBaSi2by Structural Tuning

Abstract

We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi2. Compared with the layered AlB2 structure of graphite or diboride-like superconductors, in the hexagonal structure of binary silicides the sp 3 arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, obtaining the enhancement of the superconducting transition temperature from 4 K to 8.7 K when the silicon planes flatten out. By performing ab initio calculations based on density functional theory we explain how the electronic and phononic properties of the system are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters.Nowadays, an important part of the activity research on superconductivity is focused on intercalated layered crystal structures. In these systems, where the relevant features for superconductivity are intrinsic to the layers, the bond buckling of the atoms forming the layers is a structural parameter known to damage the superconducting properties, regardless of the nature of the pairing mechanism [1]. The case of flat boron sheets in MgB 2 is well understood: superconductivity arises from a strong coupling between the sp 2 σ-bonding intralayer electrons of boron and the in-plane bond stretching phonons [2][3][4]. Besides, a buckling of the boron honeycomb structure, as observed in ReB 2 [5], was proved to decrease the superconducting transition temperature T c . Nevertheless, recent studies on graphite intercalated superconductors, namely (Yb,Ca)C 6 [6], and the ternary silicide CaAlSi [7,8], characterized by large T c s, point to the importance in these systems of the electron-phonon (EP) coupling between the interlayer electrons and the outof-plane vibrational modes of the atoms composing the layer. In that case, the buckling phonon modes corresponding to the antiphase motion along the c-axis of the atoms in the sheets can lead to an enhancement of the EP coupling. Experimentally, the effect of buckling can either be explored by means of high-pressure or directly by chemistry. An excellent testbed for such studies is the trigonal phase of binary silicides, like CaSi 2 or BaSi 2 , which constitutes a family of layered intercalated superconductors closely related to the graphite/diboride intercalated systems. Unlike the so-called AlB 2 -like structure of graphite/diboride compounds, where the planes are flat, in the hexagonal structure of binary silicides the Si planes buckle.In this Letter we investigate superconductivity in BaSi 2 and, in particular, we focus on its layered structure (EuGe 2 -type structure, P -3m1 space group) [9,10].As it is sketched in the inset of Fig. 2, trigonal BaSi 2 is made of planes of Ba arranged in a triangular lattice, interspersed with buckled hexagonal planes of Si. This phase ...

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