Pressure-enhanced superconductivity in ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$

The pressure effect on the newly discovered charge-transferred ${\mathrm{BiS}}_2$-based superconductor, ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$, with a $T_c$ of 1.5 K at ambient pressure, is investigated by transport and magnetic measurements. Accompanied with the enhancement of metallicity under pressures, the onset superconducting transition temperature increases abruptly around 1.0 GPa, reaching $\sim 10.0$ K at 2.26 GPa. Alternating current magnetic susceptibility measurements indicate that a new superconducting phase with a higher $T_c$ emerges and dominates at high pressures. In the broad pressure window of $0.68\text{GPa}\le p\le 2.00$ GPa, the high-$T_c$ phase coexists with the low-$T_c$ phase. Hall effect measurements reveal a significant difference in electronic structures between the two superconducting phases. Our work devotes the effort to establish the commonality of pressure effect on the ${\mathrm{BiS}}_2$-based superconductors, and also uncovers the importance of electron carrier density in the high-$T_c$ phase.

Figure 1

Temperature dependent resistivity of ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$ at various pressures. (a) shows data over the full temperature range; whereas, (b) displays an enlarged plot of $\rho \left(T\right)$ below 15 K. Also shown in (b) is the definition of the onset temperature $T_c^{\mathrm{on}}$.

Figure 2

Phase diagram of ${\mathrm{Eu}}_3{\mathrm{Bi}}_2{\mathrm{S}}_4{\mathrm{F}}_4$, contour-plotted with $\rho (p,T)$. The blue region on the bottom marks superconductivity. $T_c^{\mathrm{on}}$ characterizes the onset of SC transition in $\rho \left(T\right)$; $T_c^{1\%}$ represents the temperature below which the resistivity is smaller than 1% of that in the normal state; $T_{c1}^{\chi }$ and $T_{c2}^{\chi }$ stand for the transitions observed in ${\chi }^{\prime }\left(T\right)$ corresponding to SC1 and SC2 phases respectively, seeing Fig. 3; while $T_N$ is the temperature scale of the long-range AFM ordering of Eu(1) moments.

Figure 3

The real part of ac magnetic susceptibility ${\chi }^{\prime }\left(T\right)$ under pressures, measured at a frequency $f=137.37$ Hz. ${\chi }_{\mathrm{dc}}$ is the dc magnetic susceptibility measured with SQUID at atmosphere pressure. The inset is a zoom-in plot of the low temperature region, the data of which have been vertically shifted for clarity. (b) Diamagnetic volume fraction ($4\pi {\chi }^{\prime }$) as a function of $T$. Note that the demagnetizaion factor is not taken into account.

Figure 4

(a) Superconducting transition under various magnetic fields at the pressure $p=2.26$ GPa. (b) Enlarged plot of (a). (c) Superconducting $B_{c2}\text{−}T$ phase diagram at $p=2.26$ GPa.

Figure 5

Field dependent Hall conductivity ${\sigma }_{xy}\left(B\right)$ under various pressures, measured at 15 K. The data for $p=0$ are also shown in the inset for clarity.