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

TL;DR

This study investigates the interplay of Kondo resonance, ferromagnetism, and superconductivity in Fe$_3$GeTe$_2$, revealing unconventional conductance suppression and high spin polarization at mesoscopic interfaces with Nb.

Contribution

It demonstrates the coexistence of Kondo resonance and spin-polarized superconducting proximity effects in a van der Waals ferromagnetic Kondo lattice, revealing novel quantum phenomena.

Findings

Observation of zero-bias anomaly due to Kondo resonance above T_c

Unconventional suppression of conductance below T_c

Detection of anomalously high spin polarization

Abstract

In the new van der Waals Kondo-lattice Fe$_3$GeTe$_2$, itinerant ferromagnetism and heavy fermionic behaviour 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 Fe$_3$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.