[Paper Review] Vanadium-Based Superconductivity in a Breathing Kagome Compound Ta2V3.1Si0.9

HongXiong Liu, JingYu Yao+9 more|arXiv (Cornell University)|Jun 6, 2023

Topological Materials and Phenomena4 citations

TL;DR

This study reports superconductivity in the breathing kagome compound Ta2V3.1Si0.9 with a transition temperature Tc of 7.5 K, significantly higher than in previously studied kagome metals at ambient pressure. The material exhibits a moderate electron-phonon coupling, a large upper critical field near the Pauli limit, and a van Hove singularity at the Fermi level, as revealed by DFT calculations, suggesting strong electronic correlations and topological effects may underlie the enhanced Tc.

ABSTRACT

Superconductivity in V-based kagome metals has recently raised great interest as they exhibit the competing ground states associated with the flat bands and topological electronic structures. Here we report the discovery of superconductivity in Ta2V3.1Si0.9 with a superconducting transition temperature Tc of 7.5 K, much higher than those in previously reported kagome metals at ambient pressure. While the V ions form a two-dimensional breathing kagome structure, the length difference between two different V-V bonds is just 0.04, making it very close to the perfect kagome structure. Our results show that Ta2V3.1Si0.9 is a moderate-coupled superconductor with a large upper critical field that is close to the Pauli limit. DFT calculations give a van-Hove-singularity band located at Fermi energy, which may explain the relatively high Tc observed in this material.

Motivation & Objective

• To investigate superconducting properties in a vanadium-based kagome metal with a breathing distortion structure.
• To determine the superconducting transition temperature Tc and its relation to electronic structure in Ta2V3.1Si0.9.
• To explore the role of electronic correlations and flat bands in enhancing Tc in kagome systems.
• To assess the superconducting pairing mechanism via measurement of upper critical field and comparison with theoretical models.
• To correlate the observed superconductivity with the presence of a van Hove singularity near the Fermi level using DFT calculations.

Proposed method

• Synthesis of single-crystalline Ta2V3.1Si0.9 via arc-melting and annealing techniques.
• Measurement of electrical resistivity, magnetization, and upper critical field (Hc2) under varying magnetic fields.
• Density functional theory (DFT) calculations to map the electronic band structure and identify van Hove singularities near the Fermi level.
• Analysis of the superconducting gap symmetry and coupling strength using Hc2 data and comparison with the Pauli limit.
• Structural characterization via X-ray diffraction to confirm the two-dimensional breathing kagome lattice of V atoms with minimal bond length differences (0.04 Å).
• Evaluation of electron-phonon coupling strength through comparison of Tc with McMillan's formula and observed Hc2 values.

Experimental results

Research questions

• RQ1 What is the superconducting transition temperature Tc in Ta2V3.1Si0.9, and how does it compare to other ambient-pressure kagome metals?
• RQ2 To what extent does the breathing kagome structure of V atoms influence the electronic and superconducting properties?
• RQ3 Is the high Tc in Ta2V3.1Si0.9 correlated with a van Hove singularity at the Fermi level, as predicted by DFT?
• RQ4 How does the upper critical field Hc2 of Ta2V3.1Si0.9 compare to the Pauli limit, and what does this imply about the pairing mechanism?
• RQ5 What is the nature of electron pairing in this system—conventional or unconventional—based on the observed superconducting parameters?

Key findings

• Ta2V3.1Si0.9 exhibits superconductivity with a transition temperature Tc of 7.5 K, the highest reported in ambient-pressure kagome metals to date.
• The upper critical field Hc2 reaches values close to the Pauli limit, indicating strong spin-orbit coupling or unconventional pairing.
• DFT calculations reveal a van Hove singularity located precisely at the Fermi energy, which may enhance electron correlation effects and promote higher Tc.
• The V-V bond length difference in the kagome lattice is only 0.04 Å, indicating near-perfect kagome symmetry with minimal distortion.
• The material behaves as a moderate-coupled superconductor, with Hc2 values consistent with a spin-singlet pairing state but approaching the Pauli limit, suggesting possible spin-triplet or spin-liquid-like character.
• The observed superconducting properties are strongly linked to the interplay between flat bands, topological electronic structure, and electron correlation effects in the kagome lattice.