A team of scientists led by Dr. Binbin Yue from HPSTAR and Dr. Fang Hong from Institute of Physics, CAS, has revealed the concurrence of both superconductivity and ferromagnetism in one single system of high-entropy carbide. Published in Acta Materialia, the work has shown the ability of high-entropy carbides as an exotic platform to study the correlation between superconductivity and magnetism, and further benefit the understanding of superconductivity mechanism.

Magnetism and superconductivity (SC) are originally thought to be mutually exclusive, while both phenomena in fact show interesting coexistence in some exotic materials, such as cuprates, iron-based superconductors, and the recently-discovered nickelate. The interplay of superconductivity and magnetism is one of the most striking features of the quantum mechanical description of solids. For topological superconductors which can serve as building blocks of fault-tolerant quantum computers, ferromagnetism (FM) and superconductivity are also two key ingredients. However, a single material platform with concurrent ferromagnetism and superconductivity is still rare.

Recently, high-entropy materials have attracted attention not only due to their remarkable resistance to degradation and superior mechanical properties. Besides, high-entropy materials have also been known to exhibit novel attributes, including superconducting or magnetic properties. Based on the expansion of the compositional space by multicomponent and the unpredictable properties indicated by the cocktail effect that differs from single elements, it might be possible to achieve the coexistence of ferromagnetism and superconductivity in one high-entropy material.

Caption: Superconductivity and ferromagnetism observed in (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)C_0.9  

In this work, we report the observation of these two quantum orderings in high-entropy carbide ceramics (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)C_0.9 and (Ta_0.25Ti_0.25Nb_0.25Zr_0.25)C, synthesized via spark plasma high-pressure sintering. A clear magnetic hysteresis loop was observed in these high-entropy carbides, indicating a FM ground state, with such an FM ordering even existing above room temperature. A sharp superconducting transition is observed in (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)C_0.9 with a T_c of 3.4 K and upper critical field of ~3.37 T. The specific heat measurements further confirmed the bulk nature of the superconductivity. Meanwhile, superconductivity is suppressed to some extent, and the zero-resistance state disappears in (Ta_0.25Ti_0.25Nb_0.25Zr_0.25)C, in which stronger ferromagnetism is presented. The upper critical field of (Ta_0.25Ti_0.25Nb_0.25Zr_0.25)C is only ~1.5 T, though they show a higher transition temperature near 5.7 K. Combining electron paramagnetic resonance (EPR) spectroscopy and electron energy loss spectroscopy tests, it’s found that ferromagnetism stems from carbon vacancy-induced magnetism, which has been proposed in previous work on carbon-based compounds including graphite and metal carbides.

The concurrence of SC and FM ordering within the same system, whether through coexistence or phase separation at the microscopic level, provides an exotic platform to study the correlation between superconductivity and magnetism, along with other emerging quantum phenomena, significantly benefitting the design of multifunctional electronic devices.

早期超导研究认为超导和磁性是互斥的关系。但是，这种认知逐步被非常规超导体的发现打破，如铜氧化物超导体，铁基超导体，以及最新发现的镍基超导体等，这类材料中的反铁磁性及其涨落行为对于高温超导的出现至关重要。因此，超导与磁性之间的关系对于超导机理的理解十分重要。但是，在同一材料体系中同时存在超导和铁磁性，还比较罕见。另一方面，近年来高熵化合物的发展引起了巨大的关注，在具有优异力学性能的同时，一些高熵化合物，包括高熵合金和高熵陶瓷，也表现出了超导、磁性等优异物理性能。高熵化合物的鸡尾酒效应以及元素可调性，也为发现超导和磁性共存的材料体系带来了可能。北京高压科学研究中心的岳彬彬研究员等，与中国科学院物理所的洪芳副研究员等团队合作，通过放电等离子高压烧结，合成了两种高熵碳化物，(Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)C_0.9和 (Ta_0.25Ti_0.25Nb_0.25Zr_0.25)C，并在这两种化合物中同时观测到了超导现象以及铁磁性，同时发现强的铁磁性会抑制超导。结合电子顺磁共振谱以及电子能量损失谱，推测铁磁性的产生来源于样品中存在的碳空位。超导和铁磁序在同一材料体系中的同时存在，不论是基于两者共存还是微观尺度的相分离，都为研究超导和磁性之间的关系提供了独特的研究平台，也有利于未来的多功能电子设备设计。相关工作以《Observation of superconductivity and ferromagnetism in high-entropy carbide ceramics》为题发表在Acta Materialia期刊上。