Evidence of Spontaneous Vortex Ground State in An Iron-Based Ferromagnetic Superconductor

TL;DR

This study provides experimental evidence of a spontaneous vortex phase in an iron-based ferromagnetic superconductor, confirming a nearly 40-year-old theoretical prediction and revealing a unique quantum state without external magnetic fields.

Contribution

It presents the first rigorous experimental verification of spontaneous vortex phase in Eu(Fe$_{0.91}$Rh$_{0.09}$)$_{2}$As$_{2}$, demonstrating intrinsic vortex ground state in this material.

Findings

Observation of a first-order transition to SVP below magnetic transition temperature

Confirmation of intrinsic spontaneous-vortex ground state through magnetization measurements

Establishment of a new platform for studying quantum vortex matter

Abstract

Spontaneous vortex phase (SVP) is an exotic quantum matter in which quantized superconducting vortices form in the absence of external magnetic field. Although being predicted theoretically nearly 40 years ago, its rigorous experimental verification still appears to be lacking. Here we present low-field magnetic measurements on single crystals of the iron-based ferromagnetic superconductor Eu(Fe$_{0.91}$Rh$_{0.09}$)$_{2}$As$_{2}$ which undergoes a superconducting transition at $T_\mathrm{sc}$ = 19.6 K followed by a magnetic transition at $T_\mathrm{m}$ = 16.8 K. We observe a characteristic first-order transition from a Meissner state within $T_\mathrm{m}<T<T_\mathrm{sc}$ to an SVP below $T_\mathrm{m}$, under a magnetic field approaching zero. Additional isothermal magnetization and ac magnetization measurements at $T\ll T_\mathrm{sc}$ confirm that the system is intrinsically in a spontaneous-vortex ground state. The unambiguous demonstration of SVP in the title material lays a solid foundation for future imaging and spectroscopic studies on this intriguing quantum matter.