Electrically Controlled Superconductor-Insulator Transition Observed in Kagome Metal
The electronic correlations (e.g. unconventional superconductivity (SC), chiral charge order and nematic order) and giant anomalous Hall effect (AHE) in topological kagome metals AV3Sb5 (A = K, Rb, and Cs) have attracted great interest. Electrical control of those correlated electronic states and AHE allows us to resolve their own nature and origin and to discover new quantum phenomena. Here, we show that electrically controlled proton intercalation has significant impacts on striking quantum phenomena in CsV3Sb5 nanodevices mainly through inducing disorders in thinner nanoflakes and carrier density modulation in thicker ones. Specifically, in disordered thin nanoflakes (below 25 nm), we achieve a quantum phase transition from a superconductor to a “failed insulator” with a large saturated sheet resistance for T → 0 K. Meanwhile, the carrier density modulation in thicker nanoflakes shifts the Fermi level across the charge density wave (CDW) gap and gives rise to an extrinsic-intrinsic transition of AHE. With the first-principles calculations, the extrinsic skew scattering of holes in the nearly flat bands with finite Berry curvature by multiple impurities would account for the giant AHE. Our work uncovers a distinct disorder-driven bosonic superconductor-insulator transition (SIT), outlines a global picture of the giant AHE and reveals its correlation with the unconventional CDW in the AV3Sb5 family.
Electrically Controlled Superconductor
A new international collaboration, led by RMIT, has discovered a distinct disorder-driven bosonic superconductor-insulator transition, which is published in February. This discovery sheds light on the anomalous Hall effect and its correlation with the unconventional charge density wave in the AV3Sb5 kagome metal family. This has potential applications in future ultra-low energy electronics.
Superconductors are promising for low-energy electronics because they can transmit electricity without energy dissipation. They are already used in fields such as hover trains and high-strength magnets like medical MRIs. However, understanding how superconductivity forms and works in many materials remains an unsolved issue, limiting its applications.
The kagome superconductor family AV3Sb5 has recently attracted intensive interest because of their unique properties. These materials have an unusual lattice, named after a Japanese basket-weave pattern with corner-sharing triangles. They provide ideal platforms for physics studies such as topology and strong correlations. However, the origin of the material’s giant anomalous Hall effect and superconductivity is still a topic of debate.
Researchers from RMIT University and the High Magnetic Field Laboratory in China confirm for the first time the electric control of superconductivity and AHE in a van der Waals kagome metal CsV3Sb5.
The layered kagome metals AV3Sb5 possess topological electron bands and geometrical frustration of vanadium lattices, which have attracted significant interest in condensed matter physics due to the various quantum phenomena they support, including unconventional, novel nematic order, chiral charge density order, giant anomalous Hall effect (AHE), and the interplay between two-gap superconductivity and charge density wave (CDW) in AV3Sb5.
Despite several recently proposed mechanisms, including the extrinsic skew scattering of Dirac quasiparticles with frustrated magnetic sublattice, the orbital currents of novel chiral charge order, and the chiral flux phase in the CDW phase, the origin of giant AHE in AV3Sb5 and its correlation with chiral CDW remain unclear.
Dr. Guolin Zheng (RMIT), the first author of the study, stated that they have obtained many intriguing results with proton gate technique in vdW spintronic devices. They would like to apply this technique on AV3Sb5, which harbors a similar carrier density level, as the technique can effectively modulate the carrier density up to 1021 cm-3.
The ability to tune the carrier density and the corresponding Fermi surfaces can play a crucial role in understanding and manipulating these novel quantum states and potentially realizing some exotic quantum phase transitions.
The team chose to test this theory on CsV3Sb5, which potentially has the largest spare atom space for proton intercalation. The devices were easily designed and fabricated based on the team’s rich experience in this field.
The team found that their subsequent results with CsV3Sb5 depended strongly on material thickness. Co-first author, Dr. Cheng Tan (RMIT), stated that it was difficult to effectively modulate the “thicker” nanoflakes (more than 100 nm). However, when the thickness decreased to around 40 nm, the injection of the proton became more manageable, and the injection was highly reversible.
Interestingly, with the evolving proton intercalation, the carrier type (or the “sign” of the Hall effect) could be modulated to either hole or electron type, and the amplitude of the AHEs achieved were effectively tuned as well.
Further experimental and theoretical investigations indicate that this dramatic modulation of giant AHE originates from the Fermi level shift in the reconstructed band structures.
Dr. Guolin Zheng explained that the results of the gated AHE also revealed that the most likely origin of the AHE is skew scattering, which further improves their understanding of the kagome metal. However, they have not yet observed superconductor-insulator transition in 40 nm nanoflakes, and they must further try thinner CsV3Sb5 nanoflakes to explore this possibility.
The researchers chose CsV3Sb5 for their experiment as it has the largest free space for proton intercalation. They used their experience to easily design and make the devices for the experiment.
The results obtained from CsV3Sb5 were highly dependent on its thickness. It was difficult to intercalate protons in thicker nanoflakes but much easier in thinner ones. The material was also found to be highly reversible.
With the proton intercalation, the type of carrier (whether it was hole or electron) and the amplitude of AHEs could be effectively modulated. The modulation of the giant AHEs was found to be caused by the shift in the Fermi level in the reconstructed band structures.
The experiments showed that the most probable cause of the AHEs was the skew scattering mechanism. This helped improve the understanding of kagome metals. However, the researchers have not yet observed a superconductor-insulator transition in 40 nm nanoflakes, and they plan to explore thinner CsV3Sb5 nanoflakes to investigate this further.
The coexistence of electronic correlations and band topology in AV3Sb5 allows for investigating transitions of correlated states, such as the superconductor-insulator transition.
The team decreased the number of atomic layers to further explore quantum phase transitions in CsV3Sb5.
Initially, ultrathin nanoflakes (<10 nm) were used, and the critical temperatures of the superconductivity phase decreased with increasing proton intercalation, but it was uncertain whether the superconductivity disappeared completely.
Cheng changed strategies and dealt with thicker nanoflakes (10-20 nm) and tried different electrode materials to improve electrical contact.
The new strategy was successful, and the team observed a superconductor-to-insulator transition in the temperature-dependent resistance curves and a decrease in the critical temperature of the CDW phase with increasing proton injection.
Proton intercalation caused disorder and suppressed both CDW and superconducting phase coherence, resulting in a superconductor-insulator transition associated with localized Cooper pairs and a saturated sheet resistance of up to 106 Ω at temperatures approaching zero, which was called a “failed insulator.”
The research conducted by the team led to the discovery of a disorder-driven bosonic superconductor-insulator transition, which is a significant finding. They also provided a comprehensive understanding of the giant anomalous Hall effect and its correlation with the unconventional charge density wave in the AV3Sb5 family. The discovery of an electrically-controlled superconductor-insulator transition and anomalous Hall effect in kagome metals is a breakthrough, and it has the potential to inspire further investigations in this area. This discovery could lead to the development of energy-saving nanoelectronic devices. The team’s research paper titled “Electrically controlled superconductor-to-failed insulator transition and giant anomalous Hall effect in kagome metal CsV3Sb5 nanoflakes” was published in February 2023 in the journal Nature Communications.
More information: Guolin Zheng et al, Electrically controlled superconductor-to-failed insulator transition and giant anomalous Hall effect in kagome metal CsV3Sb5 nanoflakes, Nature Communications (2023). DOI: 10.1038/s41467-023-36208-6