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

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 $T\le$ $T_{s,N}$) and superconducting ($T\le T_c$) states against variation of pressure, disclosing pressure coefficients of $d{T}_{s,N}/dP=-(1100\pm 50)$ K/GPa and $d{T}_c/dP=-(60\pm 3)$ 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.

Figure 1

Magnetic susceptibility $\chi$($T$) [(a)–(c)] and in-plane electrical resistance, normalized to the room-temperature value $R$($T$)$/R_{300\text{K}}$ [(d),(e)], of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ with $x=0.028/T_{\mathrm{anneal}}=350{}^{\circ }$C at $P=0$ MPa [(a),(d)], 60 MPa [(b),(e)], and 230 MPa (c). $\chi$ data were taken in $B$ $=$ 1 T [(a), (c)] and 1 mT (b) after ZFC. Small step in $\chi$($P=60$ MPa) (b) around 8 K results from the solidification of ${}^4$He, while step at 3 K marks $T_c$ of a small In sample used as a manometer.

Figure 2

$T$-$P$ phase diagram of single crystalline Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ with $x=0.028/T_{\mathrm{anneal}}=350{}^{\circ }$C inferred from $\chi$($T$) data. Filled (open) up triangles correspond to the transition into the low-$T$ o/afm phase at $T_{s,N}$. Filled squares represent transition into the sc phase at $T_c$ inferred from ZFC measurements. For those $T_c$ values determined below the solidification line of ${}^4$He (black solid line), the $P$ values have been corrected by a factor 0.78 to account for the $P$ drop accompanying solidification. Closed diamonds indicate the size of the diamagnetic signal (demagnetization effects can be neglected for the platelike crystals and the chosen field geometry with $B$ aligned along the plate) (right scale) in units of $-{\left(4\pi \right)}^{-1}$, where $-{\left(4\pi \right)}^{-1}$ corresponds to 100$\%$ shielding volume. Filled (open) circles correspond to transition into the low-$T$ cT phase.

Figure 3

$T$-$P$ phase diagram of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ with $x$ $=$ 0.029 and $T_{\mathrm{anneal}}=400{}^{\circ }$C inferred from $\chi$($T$) measurements. Filled squares represent $T_c$ values (left scale) inferred from ZFC measurements and filled diamonds represent the corresponding diamagnetic shielding volume (right scale). Filled (open) circles correspond to $T_{\text{cT}}$ as inferred from measurements upon warming (cooling). Note that in a cooling and subsequent warming run at $P=160$ MPa a measurable $\Delta \chi$ at $T_{\text{cT}}$ was revealed only upon warming. The black solid line indicates the solidification of ${}^4$He.