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

非技术摘要超导，完全没有电阻，是一些元素和化合物令人惊讶的低温特性。固体的这一性质于1911年被发现，并被认为在1957年被Bardeen，Cooper，Schrieffer(BCS)理论完全理解。1987年，一些铜氧化物(铜酸盐)被发现具有出人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，目前还不清楚。对高温超导电性的全面认识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导致室温(300Kelvin)超导体的发现，这是一项具有非常重要的实际应用的发展。该项目对一类铜酸盐化合物进行了实验研究，在这种化合物中，超导电性可以通过施加小磁场来完全消除。这使得可以在从600K到远低于1K的广泛温度范围内研究非超导或正常的金属状态。铜酸盐的正常状态与铜或铅等著名金属的物理性质有很大的不同，因此铜酸盐被称为奇怪的金属。对这种奇怪的金属状态的理解被认为是理解超导起源的关键。在这个项目中，在专门准备的薄膜上进行了各种输运和其他实验。磁场、温度和电子掺杂的变化可以改变正常态的性质，有望深入了解这种奇怪金属态的性质和超导电性。该项目支持马里兰大学的博士和本科生的教育-这是一所人口多样化的城市大学-先进的真空沉积和电子表征技术。这些技术已被证明是对学术和技术环境中富有成效的科学生涯的极好培训。技术摘要了解超导的机制和强关联氧化物中的非费米液体(奇异金属)正常态是凝聚态物理学中最重要的悬而未决的问题之一。最近，PI(和其他人)报道了一些引人注目的实验结果，这些结果为铜氧化物(Cuprate)的物理提供了新的见解，并报告了一个相关的镍酸盐系统--掺锶的NdNiO2的超导电性的发现。本项目在这些突破的基础上，对电子掺杂铜酸盐和掺杂镍酸盐的奇异正常态有了更详细的了解。这个项目提供了一套全面的实验来研究超导电性被磁场、无序或临界电流抑制时正常状态的性质。电子掺杂的铜酸盐对这一研究特别有利，因为在适度的直流磁场(H10T)下就可以达到低温正常态(0TTC)。对铜酸盐正常态的性质的了解被认为是理解铜酸盐中高温超导电性的关键。所用的实验技术有：电阻率、霍尔效应、磁阻、能斯特效应、热电势、比热和应变(压力)。一些输运实验是在塔拉哈西和洛斯阿拉莫斯的NSF支持的国家高磁场实验室(NHMFL)进行的，磁场非常高。这项研究包括深入研究PI于2019年在n型铜氧化物的高掺杂区域发现的巡回铁磁性，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这一非常重要的相关物理材料研究领域取得进展至关重要。该项目包括对高中生、理科专业本科生、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。同以往一样，首席调查员将鼓励妇女和代表性不足的群体参与这一项目。该项目的外部合作为学生在不同的实验室环境中体验研究提供了一个独特的工具，即大学、行业和政府。马里兰大学研究生资源促进多样性(GRADMAP)计划的持续参与将让本科生参与旨在吸引更广泛受众进行研究生学习的研究暴露计划。将继续努力通过现有的物理系推广计划，鼓励马里兰大学附近以少数族裔为主的公立学校的学生进行更广泛的科学教育。该奖项反映了NSF的法定使命，通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为是值得支持的奖项。

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.