Superstructures, Miscibility Gaps and Superconductivity in Two-Band Electronic Systems

超导是一种材料失去电阻率的现象，是一种有趣且技术上重要的现象。例如，医疗诊断系统，核磁共振成像，使用超导体来产生所需的高磁场，输电线路可以使用超导体来无损耗地传输电力。然而，超导体对磁场很敏感，如果磁场超过特定的材料阈值，超导性就会被抑制。这个阈值通常与材料变成超导的温度有关，代表了一个上限。对大大超过这一极限的材料的研究表明，这类材料的超导性可能是由于与经典系统不同的影响。该项目由美国国家科学基金会材料研究部的固态和材料化学项目支持，将允许佛罗里达州立大学的研究人员探索这种效应的可能起源，并有望为改善超导体性能的方法提供见解。一种由铌、钯和硫或硒组成的材料是本研究的重点，其晶体结构和电阻率取决于钯的含量，磁场阈值超过期望值四倍以上，使得该化合物非常适合研究这种效应。在这种特殊的化合物中，携带电流的电子会经历额外的相互作用，从而影响超导性，从而影响磁场阈值。这项研究进一步发展了未来技术需求的劳动力，并推进了对外来超导性的理解。它结合了具有新颖结构特征的材料的发现和生长，以及对其特性的深入表征，这是一项跨学科的活动，需要适用于许多领域的各种技能。在本科、研究生和研究生阶段对学生进行晶体生长和材料表征的艺术和科学培训，有助于培养活跃在该领域的下一代科学家和工程师。技术综述：在具有可变钯化学计量的三元铌钯和钽钯硫属化合物中观察到非常规的多带超导性。电荷、自旋和晶格之间的相互作用是这些效应的核心，产生了非常规的物理行为。该项目由美国国家科学基金会（NSF）材料研究部支持，重点研究Nb2PdxX5 （X=chalcogen）和相关体系中嵌入层、混相间隙、结构顺序、上层结构形成和电子行为的相互作用，其中钯原子可以被认为是嵌入原子。这些相的超导性与最高临界场Hc2与超导转变温度Tc的高比值有关。在这些体系中，钯原子在不同钯含量的远端不相称的上层结构中排列，并在某些钯浓度没有发现的地方产生混相间隙。超导性与钯的化学计量学和这些超结构的发展有关，其中推导出的超导相干长度与超结构的周期性具有相同的顺序，表明这两种效应密切耦合。利用x射线衍射对Nb2PdxX5单晶进行生长和表征，研究其超结构的细节，以及它们与超导转变温度和上临界场Hc2的关系。美国国家科学基金会支持的国家设施对这项研究至关重要，同步加速器源的x射线衍射和高磁场测量将在这里进行。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical SummarySuperconductivity, where a material loses its resistivity, is an interesting and technologically important phenomenon. For instance, medical diagnostic systems, MRIs, use superconductors to generate the high magnetic fields needed, and power transmission lines may use superconductors to transmit electrical power without losses. However, superconductors are sensitive to magnetic fields, with superconductivity suppressed if the magnetic field exceeds a material specific threshold. This threshold is usually related to the temperature where a material becomes superconducting, and represents an upper limit. Research into materials that substantially exceed this limit indicates that the superconductivity in this class of materials may be due to different effects than in the classic systems. This project, supported by the Solid State and Materials Chemistry Program in NSF’s Division of Materials Research, will allow researchers at Florida State University to explore possible origins of this effect, and is expected to provide insights into ways of improving the performance of superconductors. A material consisting of niobium, palladium and sulfur or selenium is the focus of this research, where its crystal structure and electrical resistivity depends on the palladium content, and the magnetic field threshold exceeds the expected value more than four-fold, making this compound well suited to study this effect. In this particular compound, the electrons carrying the current experience additional interactions that affect the superconductivity, and thus, the magnetic field threshold. This research further develops the work force for tomorrow’s technology needs, and advances the understanding of exotic superconductivity. It combines the discovery and growth of materials with novel structural features with an in-depth characterization of their properties, an interdisciplinary activity that requires a variety of skills that are applicable in many fields. Training students in the art and science of crystal growth and characterization of materials at undergraduate, graduate and postgraduate levels is instrumental to the next generation of scientists and engineers that will be active in this field. Technical SummaryUnconventional multi-band superconductivity has been observed in ternary niobium-palladium and tantalum-palladium chalcogenides, phases with variable palladium stoichiometry. The interactions between charge, spin and lattice are at the core of these effects, giving rise to unconventional physical behavior. This project, supported by the NSF’s Division of Materials Research, focuses on the interplay of intercalation, miscibility gaps, structural order, superstructure formation, and electronic behavior in Nb2PdxX5 (X=chalcogen) and related systems, where the palladium atoms can be considered the intercalating atoms. Superconductivity in these phases is associated with a record high ratio of the upper critical field Hc2 to the superconducting transition temperature Tc. In these systems, the palladium atoms order in long-range incommensurate superstructures for different palladium content, and induce miscibility gaps, where certain palladium concentrations are not found. The superconductivity is linked to the Pd stoichiometry and the development of these superstructures, where the derived superconducting coherence length is of the same order as the superstructure periodicity, suggesting an intimate coupling of the two effects. Single crystals of Nb2PdxX5 will be grown and characterized using X-ray diffraction to investigate the details of the superstructures, and their correlation with the superconducting transition temperature and the upper critical field Hc2. NSF supported National Facilities are crucial to this research, where X-ray diffraction at synchrotron sources and high magnetic field measurements will be carried out.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.