Switchable half-quantum flux states in a ring of the kagome superconductor CsV$_3$Sb$_5$

TL;DR

This study reports the discovery of switchable half-quantum flux states in CsV$_3$Sb$_5$ rings, revealing potential unconventional pairing and an electrically tunable landscape of fractional flux states in a kagome superconductor.

Contribution

It demonstrates reversible switching between $rac{ ext{half-quantum}}{ ext{full-quantum}}$ flux states in CsV$_3$Sb$_5$ rings, indicating unconventional superconductivity and multicomponent order parameters.

Findings

Observation of $rac{ ext{pi}}{ ext{phase}}$-shifted Little-Parks oscillations at zero bias.

Reversible tuning of flux states with applied bias current.

Appearance of $h/4e$ oscillations between $rac{ ext{pi}}{ ext{phase}}$ and 0-phase regimes.

Abstract

Magnetic flux quantization in units of $\Phi_0 = h/2e$ is a defining feature of superconductivity, rooted in the charge-2e nature of Cooper pairs. In a ring geometry, the flux quantization leads to oscillations in the critical temperature with magnetic flux, known as the Little-Parks effect. While the maximal critical temperature is conventionally at zero flux, departures from this rule, for instance shifts by a half-quantum flux $\Phi_0/2$, clearly signal unconventional superconducting states and require sign-changing order parameters. Historically, such $\pi$-phase shifts in Little-Parks oscillations have been found in tricrystals or engineered ring structures that intentionally incorporate a $\pi$-phase shift. Here we report the discovery of switchable half-quantum flux states in rings made from single crystals of the kagome superconductor CsV$_3$Sb$_5$. We observe Little-Parks oscillations with a $\pi$-phase shift at zero bias current, which can be reversibly tuned to conventional Little-Parks oscillations upon applying a bias current. Between the $\pi$-phase and 0-phase regimes, $h/4e$ periodic oscillations appear. Our observations suggest unconventional pairing, potentially in the form of a multicomponent order parameter in the kagome superconductor CsV$_3$Sb$_5$, and reveal an electrically tunable landscape of competing superconducting condensates and fractional flux states.