Reversible Tuning of the Collapsed Tetragonal Phase Transition in CaFe2As2 by Separate Control of Chemical Pressure and Electron Doping

TL;DR

This study demonstrates how chemical pressure and electron doping can reversibly control the phase transition in CaFe2As2, revealing a new way to tune its structural and electronic properties, including inducing superconductivity.

Contribution

It introduces a method to reversibly tune the collapsed tetragonal phase in CaFe2As2 through separate control of chemical pressure and electron doping.

Findings

Ru substitution suppresses magnetic and structural transitions.

Electron doping destabilizes the collapsed tetragonal phase.

Superconductivity with Tc of 41 K is observed at high doping levels.

Abstract

Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.