Spin-polarized supercurrent through the van der Waals Kondo-lattice ferromagnet ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$

In the new van der Waals Kondo-lattice ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$, itinerant ferromagnetism and heavy fermionic behavior coexist. Both the key properties of such a system, namely, a spin-polarized Fermi surface and a low Fermi momentum, are expected to significantly alter Andreev-reflection-dominated transport at a contact with a superconducting electrode and display unconventional proximity-induced superconductivity. We observed interplay between Andreev reflection and Kondo resonance at mesoscopic interfaces between superconducting Nb and ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. Above the critical temperature ($T_c$) of Nb, the recorded differential conductance ($dI/dV$) spectra display a robust zero-bias anomaly which is described well by a characteristic Fano line shape arising from Kondo resonance. Below $T_c$, the Fano line mixes with Andreev-reflection-dominated $dI/dV$, leading to a dramatic, unconventional suppression of conductance at zero bias. As a consequence, an analysis of the Andreev reflection spectra within a spin-polarized model yields an anomalously large spin polarization which is not explained by the density of states of the spin-split bands at the Fermi surface alone. The results open up the possibilities of fascinating interplay between various quantum phenomena that may potentially emerge at the mesoscopic superconducting interfaces involving Kondo-lattice systems hosting spin-polarized conduction electrons.

Figure 1

(a) Crystal structure of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. (b)–(f) Magnetic force microscopy (MFM) dual-pass-phase images of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$, at different temperatures, showing contrast of ferromagnetic domains. The scale bar is $10\mu \mathrm{m}$. The stripe domains vanishes as temperature is raised to 205 K. (g) Temperature dependence of ballistic spectra (shown by colored dots) and their corresponding BTK fits (shown by black line). All the spectra are normalized, and an equal vertical shift to spectra with respect to the conductance spectrum at 1.7 K is given for clarity. (h) Temperature dependence of the superconducting gap (shown by black dots). The expected variation of the gap from BCS theory is shown by a solid red line.

Figure 2

(a)–(c) Three fitted ballistic spectra with different values of barrier strength ($Z$) and spin polarization ($P_t$). (d) $P_t$ vs Z. The extrapolated value of $P_t$ (at $Z=0$) is around 61%. (e), (f) In-plane and out-of plane magnetic field-angle dependence of resistance taken at zero bias and 11 K. The ${cos}^2\left(\varphi \right)$ and ${cos}^2\left(\theta \right)$ fits are shown by solid black lines. All the resistance curves are normalized, and an equal vertical shift to resistance curves with respect to the curve at the lowest magnetic field is given for clarity.

Figure 3

(a), (b) Band structure and the electronic density of states (DOS) is calculated for the ferromagnetic state, respectively. (c) The temperature dependence of high-$Z$ point contact. The inset shows the Fano line-shape fitting (shown by black line) of the conductance spectrum (shown by colored dots) taken at 10 K. (d) Temperature dependence of the normal state along with the Fano line-shape fits. Spectra are vertically shifted for clarity. (e), (f) Three normalized conductance spectra (shown by colored dots) and their Fano line-shape fitting (shown by black line) taken at 12 K.