Intrinsic paramagnetic meissner effect due to s-wave odd-frequency superconductivity

Abstract

Since its discovery in 1911, superconductivity has remained one of the most intriguing phase transitions in the field of condensed-matter physics. Superconductivity, which occurs in certain materials cooled below a critical temperature, involves the disappearance of electrical resistance and the expulsion of external magnetic flux. In 1933, Meissner and Ochsenfeld first reported the expulsion of magnetic flux from the interior of superconducting lead. Their discovery, now known as the Meissner effect, enables the levitation of magnetic objects (e.g., Maglev trains in Japan). For certain unconventional forms of superconductivity, an inverse paramagnetic Meissner effect has been predicted in which superconductivity attracts external flux at a superconductor interface with certain forms of magnetism. In such a system, levitation would not be possible; instead, external flux would be amplified. Here, we probe superconductivity in a thin film of gold coupled to an antiferromagnet-superconductor system using low-energy muons. Our experimental setup consists of an Au/Ho/Nb trilayer in which each layer has a thickness measured in nanometers. We apply an external field, and we conduct measurements below the superconducting transition of Nb (8.5 K). We find that muon particles implanted with a specific energy in gold experience a change in their precession frequency, which is directly related to the magnitude of the local magnetic flux they experience. Because of the extreme sensitivity of this magnetometry technique to magnetism—smaller than 0.1 G—we detect an enhancement of the local magnetic field in gold that exceeds the externally applied field in the superconducting regime. This result provides a direct observation of the paramagnetic Meissner effect and demonstrates that conventional Meissner screening is not a universal property of superconductivity. We expect that our findings will motivate future experiments for harnessing magnetic energy that is generated.