Investigation of the strange metal normal state of electron-doped oxide superconductors

摘要超导性，即完全没有电阻，是某些元素和化合物的一种惊人的低温特性。固体的这种性质是在1911年发现的，1957年，人们认为巴丁、库珀、施里弗（BCS）理论完全理解了固体的这种性质。1987年，一些氧化铜化合物（铜酸盐）被发现具有出乎意料的高超导转变温度（高达140开尔文）。这种高温超导性不能用传统的BCS机制来解释，目前还没有被理解。对高温超导性的完全理解是凝聚态物理尚未解决的主要问题之一。这个问题的解决可能会导致室温（300开尔文）超导体的发现，这是一个具有非常重要实际应用的发展。本项目对一类铜化合物进行实验研究，其中超导性可以通过应用小磁场完全消除。这使得非超导或正常的金属状态可以在600 K到远低于1K的广泛温度范围内进行研究。正常状态下的铜酸盐与众所周知的金属，如铜或铅，具有完全不同的物理性质，因此铜酸盐被称为奇怪的金属。人们认为，理解这种奇怪的金属状态对于理解超导性的起源至关重要。在这个项目中，在特殊制备的薄膜上进行了各种传输和其他实验。磁场、温度和电子掺杂的变化可以改变正常状态的性质，从而有望深入了解奇怪金属状态和超导性的本质。该项目支持马里兰大学（一所人口多样化的城市大学）的博士和本科生在先进真空沉积和电表征技术方面的教育。这些技术已被证明是在学术和技术环境中从事生产性科学事业的极好训练。技术摘要：了解强关联氧化物中超导机制和非费米液体（奇异金属）正态是凝聚态物理尚未解决的重要问题之一。最近，PI（和其他人）报告了一些引人注目的实验结果，这些结果为铜氧化物（铜酸盐）的物理学提供了新的见解，并且还报告了在相关的镍酸盐体系中发现了超导性，Sr掺杂NdNiO2。本项目在这些突破的基础上进行后续研究，以更详细地了解电子掺杂铜酸盐和掺杂镍酸盐的奇怪正常状态。该项目提供了一套全面的实验来研究超导性被磁场、无序或临界电流抑制时正常状态的性质。电子掺杂的铜酸盐在本研究中特别有利，因为在适度的直流磁场（h10 T）下可以达到低温正常状态（0 ttc）。了解铜酸盐正常状态的性质被认为是理解铜酸盐高温超导的原因的关键。使用的实验技术有电阻率、霍尔效应、磁阻、能思特效应、热功率、比热和应变（压力）。一些输运实验是在美国国家科学基金会支持的位于塔拉哈西和洛斯阿拉莫斯的国家高磁场实验室（NHMFL）的高磁场下进行的。该研究包括对PI于2019年在n型铜酸盐高掺杂区域发现的流动铁磁性进行深入研究，PI的材料科学专业知识和物理测量专业知识的结合对于在相关物理材料研究的这一非常重要的领域取得进展至关重要。本项目包括对高中生、理科生、研究生和博士后科学家在凝聚态物理研究的各个实验方面的培训。与过去一样，首席研究员将鼓励妇女和代表性不足的群体参与该项目。这个项目的外部合作为学生提供了一个独特的工具，让他们在不同的实验室环境中体验研究，即大学、工业和政府。马里兰大学正在进行的研究生资源促进多样性（GRADMAP）项目将让本科生参与旨在吸引更广泛受众的研究生学习的研究项目。通过现有的物理系外展项目，鼓励在马里兰大学附近以少数族裔为主的公立学校的学生接受更广泛的科学教育的努力将继续下去。这些活动包括：物理节、物理探索日和暑期女生项目。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 abstractSuperconductivity, the complete absence of electrical resistance, is an amazing low temperature property of some elements and compounds. This property of solids was discovered in 1911 and was believed to be fully understood in 1957 by the Bardeen, Cooper, Schrieffer (BCS) theory. In 1987 some copper oxide compounds (cuprates) were discovered with unexpectedly high superconducting transition temperatures (up to 140 Kelvin). This high-temperature superconductivity cannot be explained by the conventional BCS mechanism and it is not presently understood. A complete understanding of high-temperature superconductivity is one of the major unsolved problems of condensed matter physics. The solution to this problem could lead to the discovery of room temperature (300 Kelvin) superconductors, a development with very significant practical applications. This project performs experimental studies on one class of cuprate compounds where the superconductivity can be completely eliminated by the application of a small magnetic field. This enables the non-superconducting, or normal, metallic state to be studied over a wide range of temperature from 600 K to well below 1K. The normal state of the cuprates has quite different physical properties from those of well-known metals, such as copper or lead, and the cuprates have been called strange metals. An understanding of this strange metal state is believed to be crucial for an understanding of the origin of the superconductivity. In this project a variety of transport and other experiments are performed on specially prepared thin films. Variation of the magnetic field, the temperature and electron doping can change the normal state properties in ways that are expected to give deep insight into the nature of the strange metal state and the superconductivity. This project supports the education of PhD and undergraduate students at the University of Maryland---an urban university with a diverse population---in advanced vacuum deposition and electrical characterization techniques. These techniques have proven to be excellent training for productive scientific careers in academic and technology settings.Technical abstractUnderstanding the mechanism of superconductivity and the non-Fermi liquid (strange metal) normal state in strongly correlated oxides is one of the most significant unsolved problems of condensed matter physics. Recently some dramatic experimental results have been reported by the PI (and others) that give new insights into the physics of the copper oxides (cuprates) and that also report the discovery of superconductivity in a related nickelate system, Sr doped NdNiO2. This project follows up on these breakthroughs to gain a more detailed understanding of the strange normal state of the electron-doped cuprates and the doped nickelates. This project provides a comprehensive set of experiments to study the nature of the normal state when superconductivity is suppressed, either by magnetic field, disorder or critical current. The electron-doped cuprates are particularly advantageous for this research because the low temperature normal state (0 T Tc) can be reached with a modest dc magnetic field (H 10 T). An understanding of the nature of the normal state of the cuprates is believed to be crucial for understanding the cause of high-temperature superconductivity in the cuprates. The experimental techniques used are resistivity, Hall Effect, magnetoresistance, Nernst effect, thermopower, specific heat and strain (pressure). Some transport experiments are done at very high magnetic field at the NSF supported National High Magnetic Field Laboratory (NHMFL) in Tallahassee and Los Alamos. The research includes an in-depth study of the itinerant ferromagnetism discovered by the PI in 2019 in the highly doped region of the n-type cuprates The combination of materials science expertise and physics measurement expertise of the PI is essential for making progress in this very important area of correlated physics materials research. This project incorporates the training of high-school students, undergraduate science majors, graduate students, and postdoctoral scientists in various experimental aspects of condensed matter physics research. As in the past, the principal investigator will encourage women and underrepresented groups to participate in this project. The external collaborations in this project provide a unique vehicle for students to experience research in different laboratory environments, i.e., university, industry, and government. An ongoing participation in the Graduate Resources Advancing Diversity (GRADMAP) program at the University of Maryland will involve undergraduates in research exposure programs designed to attract a broader audience to graduate studies. Efforts to encourage a broader education in science via existing Physics department outreach programs with predominately minority public-school students near the University of Maryland will be continued. These include, Physics is Phun, Physics Discovery Days, and the Summer School Girl’s Program.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.