Evidence for Bose liquid from anomalous shot noise in nanojunctions of bad metal beta-Ta

We report anomalous shot noise in nanojunctions of beta-tantalum, a ``bad" metal whose electronic properties are inconsistent with the Fermi liquid theory. Fano factors cluster around even multiples of the values expected for Fermi liquids, suggesting that beta-Ta may host a correlated charge liquid of Cooper pair-like electron groups. Further evidence for correlations is provided by the effects of magnetic impurities, as well as reduced density of states near the Fermi level indicated by point contact spectroscopy and first principles calculations. Our results open new avenues for studies and applications of electron correlations.

💡 Research Summary

This paper presents experimental evidence for a correlated electron state, interpreted as a “Bose liquid,” in the bad metal beta-tantalum (β-Ta), obtained through detailed shot noise measurements in nanoscale junctions.

The study focuses on β-Ta, an archetypal “bad metal” whose resistivity has a small negative temperature coefficient, defying explanations by both Fermi liquid theory and single-particle localization. To probe the fundamental charge carriers in this anomalous metal, the authors employed shot noise spectroscopy. Shot noise, the white noise arising from the discrete nature of charge carriers, is characterized by the Fano factor (F). For tunneling junctions (TJs) between normal Fermi liquids, F=1 (single-electron charge, q=e). For short metallic junctions (MJs) in the diffusive regime, F=1/3.

The researchers developed a method to create stable β-Ta-based junctions by incorporating a small amount of nitrogen (forming TaN_x), which preserved the bad metal properties. They fabricated both TJs (with an MgO barrier) and MJs (formed via barrier breakdown). Careful characterization ensured the proper transport regime for each type.

The central finding is a dramatic and systematic enhancement of the Fano factor. The extracted F values did not scatter randomly but clustered around even multiples of the Fermi liquid expectations. Specifically, TJs showed F values around 2, 4, and 8, while MJs showed values around 2/3, 6/3 (i.e., 2), and 8/3. This “even-multiple” enhancement was observed consistently across multiple devices and over a temperature range (4.5K to 15K) far above β-Ta’s very low superconducting transition temperature (<1 K).

The authors interpret these results as strong evidence for a correlated charge liquid where the elementary transport excitations carry charge q = 2e, 4e, 6e, or 8e. The most natural explanation is the formation of electron pairs (2e) or larger groups, akin to local, incoherent Cooper pairs that lack long-range phase coherence—a state often termed a “Bose liquid.” This distinguishes it from superconductivity, as the phase coherence necessary for zero resistance is absent at these temperatures.

To bolster this interpretation, the paper provides two key supporting pieces of evidence:

1. Magnetic Impurity Effects: Introducing magnetic impurities (Fe layers) significantly altered the temperature dependence of resistivity and Hall coefficient in TaN_x/Fe multilayers. This sensitivity to spin suggests the underlying correlated state involves spin-singlet pairing or is otherwise strongly coupled to spin degrees of freedom.
2. Suppressed Density of States (DOS): Point-contact spectroscopy (PCS) measurements on TaN_x showed a conductance dip near zero bias, indicating a reduced DOS at the Fermi level. This was corroborated by advanced first-principles calculations (using the r2SCAN functional), which also predicted a pronounced dip in the DOS for β-Ta, a hallmark of strong electron correlations renormalizing the electronic structure.

In conclusion, this work demonstrates that shot noise is a powerful tool for identifying non-Fermi liquid charge carriers. The results suggest that the anomalous “bad metal” behavior in β-Ta originates from a robust, correlated electron liquid of pair-like entities, persisting well above any superconducting transition. This finding implies that such correlated states may be relevant in a broader class of materials than previously thought, opening new avenues for exploring correlation-driven phenomena and their potential applications in electronics and spintronics.