Hydrostatic-pressure tuning of magnetic, nonmagnetic and superconducting states in annealed Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study demonstrates that hydrostatic pressure can tune the magnetic, nonmagnetic, and superconducting states in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing extreme sensitivity of phase transitions and the importance of fluctuations for superconductivity.

Contribution

It provides the first detailed pressure phase diagram for Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ showing access to multiple states in a single sample and highlights the role of transition fluctuations in superconductivity.

Findings

Hydrostatic pressure induces orthorhombic/antiferromagnetic, superconducting, and collapsed-tetragonal states.

Pressure coefficients for transition temperatures are approximately -1100 K/GPa for $T_{s,N}$ and -60 K/GPa for $T_c$.

No coexistence of bulk superconductivity with the o/afm state was observed.

Abstract

We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. We find that for properly heat-treated crystals with modest Co-concentration, $x$ = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for $T \leq$ $T_{s,N}$) and superconducting ($T \leq T_c$) states against variation of pressure, disclosing pressure coefficients of d$T_{s,N}$/d$P$ = -(1100 $\pm$ 50) K/GPa and d$T_{c}$/d$P$ = -(60 $\pm$ 3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly first-order character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.