Little-Parks oscillations with half-quantum fluxoid features in Sr2RuO4 micro rings

摘要

In a micro ring of a superconductor with a spin-triplet equal-spin pairingstate, a fluxoid, a combined object of magnetic flux and circulatingsupercurrent, can penetrate as half-integer multiples of the flux quantum. Acandidate material to investigate such half-quantum fluxoids isSr$_\mathsf{2}$RuO$_\mathsf{4}$. We fabricated Sr$_\mathsf{2}$RuO$_\mathsf{4}$micro rings using single crystals and measured their resistance behavior undermagnetic fields controlled with a three-axes vector magnet. Proper Little-Parksoscillations in the magnetovoltage as a function of an axially applied field,associated with fluxoid quantization are clearly observed, for the first timeusing bulk single crystalline superconductors. We then performed magnetovoltagemeasurements with additional in-plane magnetic fields. By carefully analyzingboth the voltages $V_+$ ($V_-$) measured at positive (negative) current, wefind that, above an in-plane threshold field of about 10 mT, the magnetovoltagemaxima convert to minima. We interpret this behavior as the peak splittingexpected for the half-quantum fluxoid states.