[Paper Review] Tunable nodal kagome superconductivity in charge ordered RbV3Sb5

Zurab Guguchia, C. Mielke+19 more|arXiv (Cornell University)|Feb 15, 2022

Topological Materials and Phenomena10 citations

TL;DR

This study uses ultra-low temperature (18 mK) and pressure-tuned muon spin spectroscopy to reveal tunable nodal kagome superconductivity in RbV3Sb5, where time-reversal symmetry-breaking charge order competes with superconductivity. Upon applying pressure (up to 1.85 GPa), charge order is suppressed, superfluid density increases, and the superconducting state evolves from nodal to fully gapped and nodeless, with spontaneous time-reversal symmetry breaking in the nodeless state.

ABSTRACT

Unconventional superconductors often feature competing orders, small superfluid density, and nodal electronic pairing. While unusual superconductivity has been proposed in the kagome metals AV3Sb5, key spectroscopic evidence has remained elusive. Here we utilize pressure-tuned (up to 1.85 GPa) and ultra-low temperature (down to 18 mK) muon spin spectroscopy to uncover the unconventional nature of superconductivity in RbV3Sb5. At ambient pressure, we detect an enhancement of the width of the internal magnetic field distribution sensed by the muon ensemble, indicative of time-reversal symmetry breaking charge order. Remarkably, the superconducting state displays nodal energy gap and a reduced superfluid density, which can be attributed to the competition with the novel charge order. Upon applying pressure, the charge-order transitions are suppressed, the superfluid density increases, and the superconducting state progressively evolves from nodal to nodeless. Once charge order is eliminated, we find a superconducting pairing state that is not only fully gapped, but also spontaneously breaks time-reversal symmetry. Our results point to unprecedented tunable nodal kagome superconductivity competing with time-reversal symmetry-breaking charge order and offer unique insights into the nature of the pairing state.

Motivation & Objective

• To investigate the unconventional superconducting state in RbV3Sb5 under extreme conditions of ultra-low temperature and high pressure.
• To determine the role of time-reversal symmetry-breaking charge order in suppressing superfluid density and inducing nodal gaps in superconductivity.
• To probe the evolution of the superconducting gap structure as charge order is suppressed by external pressure.
• To identify whether the superconducting state can transition from nodal to nodeless and whether it breaks time-reversal symmetry in the absence of charge order.
• To establish a direct link between charge order, superconducting pairing symmetry, and superfluid density in kagome lattice materials.

Proposed method

• Employed muon spin spectroscopy (µSR) at the Swiss Muon Source (SµS) to probe local magnetic fields and internal field distribution in polycrystalline RbV3Sb5 samples.
• Conducted experiments at ultra-low temperatures (down to 18 mK) and up to 1.85 GPa pressure using a double-wall piston-cylinder pressure cell.
• Analyzed zero-field (ZF)-µSR spectra to detect time-reversal symmetry breaking via the width of the internal magnetic field distribution.
• Measured the temperature dependence of the muon spin relaxation rate Γ(T) to extract the superconducting gap structure and superfluid density.
• Used the α-model for fitting the temperature dependence of the magnetic penetration depth to infer the superconducting gap symmetry.
• Combined Knight shift measurements and bulk magnetization to characterize the charge order transition temperatures and spin susceptibility.

Experimental results

Research questions

• RQ1 How does the superconducting gap structure in RbV3Sb5 evolve under applied pressure, and does it transition from nodal to nodeless?
• RQ2 What is the role of time-reversal symmetry-breaking charge order in suppressing superfluid density and inducing nodal pairing?
• RQ3 Does the superconducting state in RbV3Sb5 break time-reversal symmetry, and under what conditions does this occur?
• RQ4 How do the charge order transition temperatures Tco,1 ≈ 110 K and Tco,2 ≈ 50 K respond to hydrostatic pressure?
• RQ5 Can the competition between charge order and superconductivity be tuned to reveal a fully gapped, time-reversal symmetry-breaking superconducting state?

Key findings

• At ambient pressure, RbV3Sb5 exhibits a nodal superconducting gap with reduced superfluid density, consistent with competition from time-reversal symmetry-breaking charge order.
• The charge order transition temperatures Tco,1 ≈ 110 K and Tco,2 ≈ 50 K are suppressed under hydrostatic pressure, with complete suppression observed above 1.85 GPa.
• As pressure increases, the superfluid density rises, and the superconducting state evolves from nodal to nodeless, indicating a change in pairing symmetry.
• Above 1.85 GPa, where charge order is fully suppressed, the superconducting state becomes fully gapped and spontaneously breaks time-reversal symmetry.
• The nodeless superconducting state at high pressure is characterized by a fully gapped BCS-like behavior with a significant increase in superfluid density.
• Zero-field µSR data show a clear enhancement in the width of the internal field distribution at ambient pressure, confirming time-reversal symmetry breaking in the charge-ordered state.