Pressure Tuned Quantum Phase Transitions in Model Itinerant Magnets

技术摘要对于复杂且无序的材料（稀土铜酸盐、重费米子材料、过渡金属氧化物和硫化物）的量子相变研究一直非常活跃。理想情况下，人们希望将一种简单的化学计量材料调整到其量子临界点，并直接测量其有序参数的消失，从而分离出关键的物理机制。铬，元素反铁磁体，提供了一个非凡的机会来研究一个基本问题：当金属的自旋和电荷顺序被压力抑制时，磁性是如何在金属中出现的。在NiS2中调整Mott-Hubbard金属-绝缘体跃迁与压力类似地允许实验接近裸量子奇点。最后，直接测量CeFe2（一种处于铁磁性和反铁磁性之间的材料）的有序参数，应该可以得出关于不同磁性基态可能共存的硬结论，并具有中尺度上自组织的潜力。这些模型系统将为磁性、量子相变和相关材料的研究提供信息。广泛的技术，从x射线散射到磁输运，以及物理，化学和材料科学概念的混合，应该培养学生在工业，国家实验室或学术界的职业生涯。从计算机存储到发电，磁铁构成了现代技术的基础，但磁性起源的重要问题在很大程度上仍然没有答案。通过在接近绝对零度的温度下将元素磁铁铬压缩到100,000个大气压，就有可能揭示和研究磁有序首次从量子无序中出现的那个点。同样，探索不同类型磁序之间的竞争也是可能的，其中自旋的趋势——电子的量子特性，使其像材料内部的小条形磁铁一样——可以在从纳米到毫米的长度尺度上进行平行或反平行的操作。最后，本提案将探讨磁性如何与材料导电或不导电的能力相结合。从x射线到导电的广泛技术，以及将物理、化学和材料科学的概念结合起来的必要性，应该能很好地培养学生在工业、国家实验室或学术界的职业生涯。将研究视角引入教育是另一个重点，对私家侦探和实验室里的研究生都是如此。这包括公开讲座，为科学和非科学专业学生开设课程，为当地学校的K-8学生提供指导，以及作为全市项目一部分的外展活动。

Technical AbstractThere has been enormous activity devoted to quantum phase transitions in complicated and often disordered materials: rare earth cuprates, heavy fermion materials, transition metal oxides and sulfides Ideally, one would like to tune a simple, stoichiometric material to its quantum critical point and directly measure the disappearance of its order parameter, thereby isolating the key physical mechanisms. Cr, the elemental antiferromagnet, offers an extraordinary opportunity to investigate the fundamental question of how magnetism emerges in metals as its spin and charge order is suppressed by pressure. Tuning the Mott-Hubbard metal-insulator transition with pressure in NiS2 similarly permits experimental access to the naked quantum singularity. Finally, direct measurements of the order parameters in CeFe2, a material sitting on the knife edge between ferro- and antiferromagnetism, should permit hard conclusions to be drawn about the possible coexistence of different magnetic ground states, with the potential for self-organization on the mesoscale. These model systems will inform the study of magnetism, quantum phase transitions, and correlated materials in general. The wide array of techniques, from x-ray scattering to magnetotransport, and the mix of physics, chemistry and materials science concepts, should train students well for careers in industry, the national laboratories, or academia. NON-TECHNICAL ABSTRACTMagnets form the basis of modern technology, from computer storage to power generation, but the important question of how magnetism originates is still largely unanswered. By squeezing the elemental magnet chromium to 100,000 atmospheres at nearly absolute zero temperature it is possible to reveal and study the very point where magnetic order first emerges from quantum disorder. Similarly, it is possible to explore the competition between different types of magnetic order, where the tendency of spins - the quantum property of an electron that makes it act like a little bar magnet inside a material - to line up parallel or antiparallel can be manipulated on length scales from nanometers to millimeters. Finally, this proposal will probe how magnetism can be coupled to the ability of materials to conduct or fail to conduct electricity. The wide array of techniques, from x-rays to electrical conduction, and the necessity to combine concepts from physics, chemistry and materials science, should train students well for careers in industry, the national laboratories, or academia. Bringing research perspectives to education is another emphasis, both for the P.I. and the graduate students in the laboratory. This includes public lectures, course development for both science and non-science majors, mentoring for K-8 students in the local schools, and outreach activities as part of a citywide program.