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

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.

7 pages 5 figures

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• Robust Superconductivity in Quasi-One-Dimensional Multiband Materials
• Coexistence of Eu-antiferromagnetism and pressure-induced superconductivity in EuFe2As2 single crystal
• Magnetic and superconducting phase diagram of Eu(Fe${}_{1-x}$Ni${}_x$)As${}_2$
• Bulk domain Meissner state in the ferromagnetic superconductor EuFe${}_2$(As${}_{0.8}$P${}_{0.2}$)${}_2$: Consequence of compromise between ferromagnetism and superconductivity
• Role of Vanadium-Oxide Layer in Electronic State of Sr${}_2$VFeAsO${}_{3-\delta }$ with Oxygen Deficiency