News Resource: School of Physics

Editor: News Agency of BIT

Translator: Xiang Tong, News Agency of BIT

Beijing Institute of Technology, November 16th, 2021: Recently, Professor Yang Fan, together with his collaborators has made important progress in high-temperature superconductivity mechanism field. The associated research result “Possible Superconductivity with a Bogoliubov Fermi Surface in a Lightly Doped Kagome U(1) Spin Liquid” was published in international top l publications Phys. Rev. Lett.

The dead Nobel winner P.W.Anderson, who is also one of the founders of Condensed Matter Physics once has a very famous theory: when a quantum spin liquid state is doped with free carriers, it will become an unconventional superconductor. Quantum spin liquid is a kind of special spin system. These systems do not enter the magnetic order and other spontaneous symmetry breaking phases when the temperature drops to zero, thus exceeding Landau's general framework of the state of matter. Anderson points out that in this kind of states, the electrons with opposite spins have naturally formed a pairing, turning into Mott insulator due to the lack of free carriers. At this time, arbitrary doping will lead to the emergence of superconducting states. Although his theory is profound and reasonable, it has never been realized in a real actual material or a simple model system. In 1973 and 1987 Anderson proposed the spin liquid ground state of Triangular Lattice and the Square Lattice Heisenberg Model. The latter is believed to cause high temperature superconductivity after doping. Later, the two-model system was judged to be a magnetically ordered state by a variety of numerical calculation results, so that the spin liquid could not be realized. However, ZnCu3(OH)6Cl2, a new spin system that has attracted much attention emerged in recent years. This system has a Kagome lattice , as shown in the figure (1) below. Experiments in every aspect show that this system might be a spin liquid. The different numerical results of many international research groups also support the ground state of quantum spin liquid in theory. The most likely state can be approximatively described by the projected π-flux state in figure (1). But there has not been a systematic and in-depth study of what kind of superconducting state the system will form after doping.

FIG.1. Kagome Lattice and Two Kinds of Spin Liquids

FIG.2. Phase Diagram of the Doped on Kagome Lattice

In order to study the quantum state of the Kagome lattice spin liquid after doping, Yang Fan and his collaborators use the variational Monte Carlo method to calculate the corresponding t-J model. In this variational study, a new wave function is used: the so-called SU(2)-gauge rotation is performed on the projected average field state at the original half-full state, and the corresponding gauge rotation angle is used as the variational parameter as the energy optimization. Using this method can obtain the lowest energy projected wave function after doping. The phase diagram is shown in the following figure (2). The π-and 0- flux states of the SU(2)-gauge rotation projection are obtained in the range of small doping and large doping concentration, respectively. These states break the lattice translation and space-time inversion symmetry. Moreover, the gauge rotation leads to the breaking of the gauge symmetry of the overall U(1), resulting in superconductivity. Interestingly, in this superconducting state, the Bogoliubov quasiparticle of the system has a Fermi surface, so the elementary excitation of the system is similar to that of a standard Fermi liquid. For example, this similarity can be reflected in specific heat, nuclear magnetic resonance, scanning tunneling spectroscopy, etc. But the system also has the Meissner effect and zero resistance that the usual Fermi liquid does not have. This is a novel and rare superconducting state, which originates from interband pairing caused by strong correlation.. This result also suggests that for a large class of so-called U(1) spin liquids, it is generally possible to obtain this superconducting state with Fermi surface after doping. On the one hand, this work found a possible practical system realization for Anderson’s theory, and on the other hand, obtained a novel superconducting state, which has important theoretical value.

This paper work is published in the top journals of physics Phys. Rev. Lett. 127,187003 (2021). Associate Professor Jiang Yifan of ShanghaiTech University is the first author of the paper. Professor Yang Fan and Professor Yao Hong from the Institute of Advanced Study of Tsinghua University are the co-corresponding authors. The paper is funded by the National Natural Science Foundation of China.

Paper link: DOI: 10.1103/PhysRevLett.127.187003

About the author: Yang Fan joined the physics department of BIT in 2004 and was hired as a professor in 2013. His research direction is strong correlation and superconductivity theory, and a series of research results have been made in the fields of copper oxide high temperature superconductivity, quantum spin liquid, iron-based superconductivity, magic angle graphene related electronic states, and quasicrystal line superconductivity etc. He has undertaken the National Natural Science Foundation of China five times and won the title of New Century Excellent Talents by the Ministry of Education. He has published more than 50 papers in international high-level journals, including seven papers published in the top journal Phys. Rev. Lett, five of which are first or corresponding authors. The total citation rate is more than 1000 times.