Progress | Quantitative Change Leads to Qualitative Change: Universal Physical Laws of Exotic Metals and High Temperature Superconductivity

Recently, the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics has used material genetic engineering "cross-scale characterization technique of continuous component epitaxial films and matching" to obtain a universal physical law (A~Tc) between singular metal scattering (linear resistance slope A) and high-temperature superconducting transition temperature (Tc). This law reveals the common driving mechanism of the two major trans-century problems of unconventional superconductivity and singular metal state, and takes a key step out of the "quantitative change leads to qualitative change" of high-temperature superconductivity. The relevant results were published in the journal Nature on February 17, 2022 [602, 431, 2022].

I. Questions and Challenges: Are there quantitative physical laws between high-temperature superconductivity and its singular metal normal?

Superconductivity has been studied for 110 years since its inception, and early superconductivity research focused on conventional metals and alloys, and its superconducting transition temperature is usually low (

The copper oxide superconducting family discovered in 1986 has a superconducting transition temperature that exceeds the McMillan limit and reaches a maximum of 135 K at atmospheric pressure. The existing experimental results confirm that electron pairing also exists in high-temperature superconductors, but unlike the electrophononic pairing mechanism, high-temperature superconductors are widely believed to be derived from the associated interactions between electrons, known as unconventional superconductors. After more than 30 years of research, the mechanism of high-temperature superconductivity has not yet reached a consensus, which has become a cross-century problem in the study of condensed matter physics. The reason is that the complexity of the high-temperature superconducting system makes the researchers still have insufficient understanding of the important physical quantity experiments that determine their transition temperature, and they cannot inspire theoretical breakthroughs.

As research continues, there is growing evidence that the mystery of high-temperature superconductivity may exist in the normal state of producing superconductivity. For copper oxide superconductors, when the temperature rises above the superconducting transition temperature, their resistivity ρ exhibits a linearly dependent (i.e., Δρ=A T) "exotic metal" behavior with temperature T, which is contrary to the Fermi liquid square relationship (Δρ=A T) of conventional metals, becoming the most "abnormal" characteristic of the normality of high-temperature superconductors. Like the mechanism of high-temperature superconductivity, the microscopic origin of exotic metal behavior is a major unsolved mystery of condensed matter physics. The existing experimental results show that the singular metallic state complements high-temperature superconductivity, so revealing the intrinsic quantitative connection between them is expected to provide important clues for further revealing the mechanism of high-temperature superconductivity.

Jin Kui, a researcher at the Institute of Physics, led the team to play the technical characteristics of superconducting single crystal thin films and superconducting combined thin films, and long-term in-depth study of a key high-temperature superconductivity system La Ce CuO (LCCO, the only electronic high-temperature superconducting system covering the full superconducting doping region, but can only exist stably in the form of single crystal films). In 2011, Based on a series of high-quality one-component LCCO superconducting single-crystal films obtained over several years of hard work, Jin Kui and his collaborators obtained a complete electron-doped copper oxide superdoped regional phase diagram for the first time [Nature 476, 73, 2011], and found that the second quantum critical point of the system was different from the antiferromagnetic spin fluctuation [Proc. Natl. Acad. Sci. USA 109, 8440, 2012】。

By studying the normal transport characteristics of LCCO films as low as 20 mK (millikelvin), they found that the scattering rate of exotic metals A was positively correlated with the superconducting transition temperature Tc, suggesting that there is some intrinsic connection between the exotic metal state and high-temperature superconductivity. However, limited by component control accuracy (about one percent), it is difficult to obtain a sufficient amount of high-precision data using the traditional single-point research model, which makes it extremely challenging to obtain quantitative laws between the two.

Second, innovation and results: the development of a new generation of high-throughput experimental technology to successfully obtain universal physical laws

The team has creatively introduced the concept and core technology of material genetic engineering into superconductivity research for many years, continuously developed high-throughput preparation and cross-scale rapid characterization technology for the characteristics of high-temperature superconducting materials [see Chinese Review, Acta Physica Sinica 70, 017403, 2021], promoted the deep cross-integration of material genetic planning and superconductivity research, and created a unique high-throughput superconductivity research paradigm [see Supercond. Sci. Technol. 32, 123001, 2019；Chin. Phys. B 27, 127402, 2018】。

In 2017, the team successfully prepared a single-oriented La Ce CuO high-throughput film with a continuous chemical component gradient (0.10≤x≤0.19) along a single crystal substrate in one square centimeter using combined laser molecular beam epitaxy technique for the first time [Sci. China: Phys., Mech. Astron. 60, 087421, 2017, Cover Story], whose end components correspond to the best superconducting doping (x = 0.10) and Fermi liquid metals (x = 0.19), respectively.

On this basis, combined with the cross-scale structure and transport characterization technology developed by the team from millimeters to microns, the physical property resolution was increased by two orders of magnitude (to one in ten thousandths), so as to accurately determine the quantum critical component xc. Through international cooperation, the Synchrotron Radiation Source (Beamline 12.3.2 @Advanced Light Source) at the Lawrence Berkeley National Laboratory in the United States was completed in the form of X-ray structure analysis in the micron range.

Based on the new generation of full-process high-throughput experiments, the team successfully accumulated a sufficient amount of reliable data in a few months, and for the first time observed the quantification law between the superconducting transition temperature Tc, the relative doped component (x-x) and the singular metal scattering rate A. More importantly, the T ~A law obtained from LCCO can be generalized to unconventional superconducting systems such as hollow copper oxides, iron-based superconductors, and organic superconductors, which are universal, indicating that the singular metal state and the unconventional superconducting state have common drivers.

Third, the high-throughput superconductivity research paradigm

This work is a success story of the high-throughput superconductivity research paradigm resulting from the deep cross-fusion of the Materials Gene Program and superconducting research. Two international reviewers highly praised the team's "cross-scale characterization technique for continuous component epitaxial films and matching" to accelerate the exploration of physical laws of high-temperature superconductivity quantification as "tour de force". There are still more quantitative laws to be discovered in copper oxide high-temperature superconductors, and the research team plans to continue to develop and use a new generation of high-throughput experimental technology, systematically explore other key factors that produce high-temperature superconductivity, and promote this experimental technology to other quantum material systems at the Single Crystal Thin Film Experimental Station of the Soft Materials Gene Platform of the Institute of Physics.

Under the suggestion of Academician Zhao Zhongxian, the work was conceived by Researcher Jin Kui and led by researcher Jin Kui to tackle the key problems, Jiang Kun, Yang Yifeng, Hu Jiangping, and Academician Xiang Tao of the Institute of Physics provided theoretical support, Cheng Zhigang, a special researcher of the Institute of Physics, assisted in low temperature testing, a Tamura researcher at lawrence Berkeley National Laboratory in the United States, and Professor Takeuchi of the University of Maryland helped characterize the structure of synchrotron radiation. Jin Kui team backbone Yuan Jie chief engineer and Chen Qihong special researcher as the first author, a number of postdoctoral, graduate and graduated doctoral students participated in the contribution. Researcher Jiangping Hu, Professor Takeuchi and Researcher Kui Jin are co-corresponding authors.

The above work has been supported by the National Key Research and Development Program, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, the Guangdong Provincial Key Area R&D Program, as well as the Chinese Academy of Sciences Strategic Pilot Science and Technology Special Project (Category B), Cutting-edge Key Projects and Innovation Cross-team.

EDIT: Hidden Idiot