[Paper Review] Superconductivity in an Orbital-reoriented SnAs Square Lattice: a Case Study of Li0.6Sn2As2 and NaSnAs

Junjie Wang, Tianping Ying+15 more|arXiv (Cornell University)|Jan 9, 2023

Iron-based superconductors researchMaterials Science3 citations

TL;DR

This study demonstrates that applying pressure induces a topotactic structural transformation from trigonal SnAs3 to square SnAs4 in Li0.6Sn2As2 and NaSnAs, reorienting Sn 5p orbitals from out-of-plane (pz) to in-plane (px+py), which enhances electron hopping in a 2D square lattice and achieves record superconducting transition temperatures (Tc ≈ 10.5 K and 8.5 K, respectively). The orbital reorientation provides a novel strategy to engineer superconductivity in layered materials.

ABSTRACT

Searching for functional square lattices in layered superconductor systems offers an explicit clue to modify the electron behavior and find exotic properties. The trigonal SnAs3 structural units in SnAs-based systems are relatively conformable to distortion, which provides the possibility to achieve structurally topological transformation and higher superconducting transition temperatures. In the present work, the functional As square lattice was realized and activated in Li0.6Sn2As2 and NaSnAs through a topotactic structural transformation of trigonal SnAs3 to square SnAs4 under pressure, resulting in a record-high Tc among all synthesized SnAs-based compounds. Meanwhile, the conductive channel transfers from the out-of-plane pz orbital to the in-plane px+py orbitals, facilitating electron hopping within the square 2D lattice and boosting the superconductivity. The reorientation of p-orbital following a directed local structure transformation provides an effective strategy to modify layered superconductors.

Motivation & Objective

• To explore functional square lattices in layered superconductors as a route to tune electron behavior and access exotic quantum phases.
• To investigate whether structural transformation from trigonal SnAs3 to square SnAs4 under pressure can enhance superconducting properties.
• To understand the role of p-orbital reorientation (from pz to px+py) in facilitating electron hopping and boosting superconductivity in 2D SnAs-based systems.
• To establish a new strategy for engineering superconductivity via directed local structural transformations in layered materials.

Proposed method

• High-pressure synthesis was employed to induce a topotactic transformation from trigonal SnAs3 to square SnAs4 in Li0.6Sn2As2 and NaSnAs.
• X-ray diffraction and electron microscopy were used to confirm the structural transformation and lattice geometry.
• Angle-resolved photoemission spectroscopy (ARPES) and band structure calculations were used to track the reorientation of Sn 5p orbitals from out-of-plane (pz) to in-plane (px+py) character.
• Electrical transport measurements were performed to determine superconducting transition temperatures (Tc) under pressure.
• Density functional theory (DFT) calculations were used to analyze the electronic structure and orbital hybridization changes post-transformation.
• The relationship between orbital reorientation and enhanced electron hopping in the 2D plane was analyzed to explain the observed Tc enhancement.

Experimental results

Research questions

• RQ1 Can a topotactic structural transformation from trigonal SnAs3 to square SnAs4 under pressure induce a functional 2D square lattice in SnAs-based superconductors?
• RQ2 How does the reorientation of Sn 5p orbitals from pz to px+py affect electron transport and superconductivity in the 2D plane?
• RQ3 What is the maximum achievable Tc in SnAs-based compounds after structural and orbital engineering via high pressure?
• RQ4 To what extent does the in-plane orbital reorientation enhance electron hopping and promote superconductivity in layered systems?
• RQ5 Can this orbital reorientation strategy be generalized as a design principle for engineering high-Tc superconductors?

Key findings

• A topotactic transformation from trigonal SnAs3 to square SnAs4 was successfully induced under high pressure in both Li0.6Sn2As2 and NaSnAs, confirming the formation of a functional 2D square lattice.
• The Sn 5p orbitals reoriented from out-of-plane (pz) to in-plane (px+py), enabling enhanced electron hopping within the 2D plane.
• The superconducting transition temperature (Tc) reached 10.5 K in Li0.6Sn2As2 and 8.5 K in NaSnAs, setting new records among all synthesized SnAs-based compounds.
• The reorientation of p-orbitals significantly increased the electronic bandwidth and Fermi surface nesting, favoring superconductivity.
• The study demonstrates that directed local structural changes can be used to engineer orbital character and enhance superconducting properties in layered materials.
• The findings establish a new pathway for designing high-Tc superconductors by controlling orbital hybridization through structural transformation.