Pressure-induced superconductivity beyond magnetic quantum criticality in a Kondo ferromagnet

Abstract

Quantum phase transitions are an established setting for emergent phenomena driven by strong electronic correlations, including strange metals and unconventional superconductivity. These have been explored extensively in Kondo lattice materials tuned to an antiferromagnetic quantum critical point (QCP), but superconductivity emerging near ferromagnetic quantum criticality is not yet observed, and the conditions under which it occurs in proximity to ferromagnetism are undetermined. Here, we report a new setting for superconductivity in the ferromagnetic Kondo-lattice material Ce5CoGe2, where there is a ferromagnetic ground state at ambient pressure, which evolves to antiferromagnetism under applied pressures. The antiferromagnetic transition is suppressed to a zero-temperature QCP, which is accompanied by strange-metal behavior. Superconductivity does not occur at the QCP, but instead appears at pressures beyond the magnetic instability. These findings suggest that Ce5CoGe2 represents a distinct class of correlated materials exhibiting a unique scenario for the emergence of superconductivity, likely associated with unconventional pairing mechanisms beyond spin-fluctuations.

Figures (5)

• Figure 1: Schematic phase diagrams of superconductivity and magnetism in different quantum materials.a, Representative phase diagram of CeCu$_2$Si$_2$, where the first superconducting dome (SC1) emerges near the antiferromagnetic (AFM) quantum critical point (QCP). As the tuning parameter $\delta$ increases, a second superconducting dome (SC2) appears, likely driven by valence fluctuations Yuan2003Yuan2006. b, Schematic phase diagram of uranium-based ferromagnetic superconductors, where superconductivity (SC) coexists with ferromagnetic (FM) order. In these systems, the FM transition is typically first-order, avoiding a QCP Saxena2000. c, Schematic phase diagram of a continuous FM QCP, at which superconductivity is not yet observed steppke2013shen2020N. d, Schematic phase diagram of Ce$_5$CoGe$_2$ under pressure. With increasing pressure, FM order first gives way to AFM order, which is subsequently continuously suppressed to zero temperature at an AFM QCP. Superconductivity emerges at higher pressures beyond the AFM QCP.
• Figure 2: Pressure-induced FM to AFM transition in Ce$_{5}$CoGe$_{2}$.a, Temperature dependence of the real part of the ac susceptibility $\chi^{\prime}(T)$ of Ce$_{5}$CoGe$_{2}$ under various pressures from 0.3 to 2.1 GPa. $\chi^{\prime}(T)$ of Ce$_{5}$CoGe$_{2}$ is displayed for b, 0.7 GPa, c, 1.2 GPa, and d, 1.8 GPa, under various applied magnetic fields up to 0.3 T. The black and red arrows indicate FM and AFM transitions, respectively. Note that the curves are vertically shifted for clarity.
• Figure 3: Quantum critical behavior in Ce$_{5}$CoGe$_{2}$.a, Low temperature dependence of resistivity $\rho(T)$ of Ce$_{5}$CoGe$_{2}$ measured at pressures between 0.6 and 3.5 GPa. The red and black arrows indicate AFM and FM transitions, respectively. b, Temperature dependence of the ac heat capacity coefficient $C_{\rm ac}(T)/T$ of Ce$_{5}$CoGe$_{2}$ measured at pressures between 1.7 and 3.2 GPa, where the red arrows indicate AFM transitions. c,$\rho(T)$ under various applied magnetic fields at 3.2 GPa. The red dashed line marks the $T$-linear resistivity, corresponding to strange-metal behavior. The blue dashed lines show fits to a $T^{\rm 2}$ dependence, corresponding to Fermi liquid behavior.
• Figure 4: Superconductivity of Ce$_{5}$CoGe$_{2}$.a, Low temperature $\rho$($T$) of Ce$_{5}$CoGe$_{2}$ between 5 and 15 GPa. b,$\chi^{\prime}(T)$ of Ce$_{5}$CoGe$_{2}$ at different pressures, where superconducting transitions corresponding to the sample and a Pb piece for reference are marked by black and red arrows, respectively. c,$\rho(T)$ under various applied magnetic fields at 9.6 GPa. d, Temperature dependence of the upper critical fields ($B_{\mathrm{c2}}$) at different pressures, with black dashed lines representing the WHH model fits.
• Figure 5: Phase diagram of Ce$_{5}$CoGe$_{2}$ under pressure.a, Temperature–pressure phase diagram of Ce$_{5}$CoGe$_{2}$ based on resistivity, ac heat capacity, and ac susceptibility measurements. The pink, blue, and orange symbols represent $T_{\rm C}$, $T_{\rm N}$, and $T_{\rm sc}$, respectively. The shaded regions correspond to the different labeled phases. The inset shows the pressure dependence of $(B'_{\mathrm{c2}}/T_{\rm sc})^{0.5}$, which qualitatively describes the evolution of the effective carrier mass. b, Pressure dependence of the low-temperature resistivity $\rho_{0}$, where $\rho_0$ is defined as the resistivity at 0.3 K. For pressures at which superconductivity occurs, $\rho_0$ corresponds to the resistivity of the normal state just above the superconducting transition.