The quantum critical phase in the spine-reducing fermion system

[ Instrument Network Instrument Development ] When a secondary phase change is continuously suppressed to near absolute zero by a non-temperature controlled external parameter, the system undergoes a quantum phase transition. The critical point at which the quantum phase transition occurs, that is, the quantum critical point, is a point on the outer parametric axis under absolute zero conditions, which can usually be obtained by adjusting pressure, magnetic field, and the like. The quantum phase transition is different from the phase transition controlled by thermal fluctuation at finite temperature, and its physical nature is based on the quantum fluctuation behavior of Heisenberg's uncertainty principle. Quantum phase transitions have been extensively studied in different systems such as heavy fermions, unconventional superconductors, quantum spins, and cold atoms, and are an important way to generate singular collective excitation patterns and new physical properties. It is especially important that the quantum critical point is generated near absolute zero, but its associated quantum fluctuations can profoundly affect the physical behavior at finite temperatures. Many unconventional physical properties, including high temperature superconductivity, may be closely related to quantum critical fluctuations.

The heavy fermion material is an ideal system for quantum critical behavior research. In this type of material, the conduction of electrons and local f/d electrons can produce a Kondo effect, thereby shielding the local magnetic moment. At the same time, there is an indirect exchange interaction between the local magnetic moments, which causes the magnetic moment to stabilize and tend to be magnetically ordered. These two contradictory physical processes compete with each other in the heavy fermion material, producing quantum critical phenomena and even unconventional superconductivity. On the other hand, the research on the spin-resistance effect mainly focuses on the insulating quantum spin system, which is another important research direction of condensed matter physics. In this type of system, quantum fluctuations caused by spin frustration can destroy long-range magnetic order and may lead to novel phenomena such as spin liquid at absolute zero. The introduction of spin frustration in the heavy fermion system with metallic behavior will enhance the quantum fluctuations at low temperatures and compete with the long-range RKKY exchange interaction of the conducting electron media. At this time, how the quantum critical behavior of the system will evolve is an important basic physical problem.

Recently, Nature Physics published a researcher from the Institute of Physics of the Chinese Academy of Sciences/Beijing National Center for Condensed Matter Physics, EX9 Group Researcher, Sun Peijie, Ph.D. students, Zhao Hengcan, Zhang Jiahao, etc. and Rice University. Professor Q. Si and Max Planck Professor F. Steglich's collaborative research results. They found that when the Kondo lattice of the heavy fermion system is located in the frustrated kagome lattice, the system produces a stable, stable quantum critical phase in the pressure magnetic field phase diagram through magnetic field and pressure regulation. Measurements of resistivity, magnetic susceptibility, and specific heat under pressure indicate that quantum fluctuations caused by spin retardation are the main cause of the quantum critical phase. Unlike the quantum critical points commonly found in other types of materials, the discovery of a wide-area quantum critical phase in the phase diagram space indicates a stable new state of matter caused by quantum fluctuations. At the same time, the research team also found that the quantum critical phase has singular physical properties such as non-Fermi liquid.

The heavy fermion material CePdAl of this study has a deformed kagome frustration structure. The spin retardation greatly reduces the antiferromagnetic order temperature of the material and forms a strong short-range spin correlation above the magnetic order temperature, which is characterized by a broad peak of magnetic susceptibility. By constructing a three-dimensional phase diagram of the magnetic field, pressure and temperature of the material at very low temperatures, the research team found that the material did not enter the non-magnetic heavy Fermi state after the antiferromagnetic order was pressed by the magnetic field or pressure. A quantum paramagnetic state is formed over a wide range of pressures and magnetic fields. The state of matter has a local magnetic moment, but does not form a long-range magnetic order near absolute zero, and has the characteristics of a metal spin liquid. Different from the conventional quantum critical point, this region caused by quantum fluctuations at absolute zero is called the quantum critical phase. Continue to increase the pressure or magnetic field, the quantum critical phase can be suppressed, the system undergoes a local parade transition, and enters the heavy Fermi state. This discovery has important implications for exploring and understanding the unconventional metal behavior caused by quantum fluctuations.

"Nature-Physics" also published a review article entitled "Frustration can be critical" written by Aline Ramires, Ph.D., of the Max Planck Institute for Complex Systems Physics, Germany, and gave a detailed review of the physical meaning of the research work.

The above research was supported by the Ministry of Science and Technology (2017YFA0303100, 2015CB921303, 2018YFA0305702), the National Natural Science Foundation of China (11774404, 11474332, 11574377, 11874400) and the Chinese Academy of Sciences (XDB07020200). Participating in this collaborative research also includes the P. Gegenwart team of professors at the University of Augsburg, Germany, Y.Isikawa, professor at Toyama University, Japan, Yang Yifeng, Chen Genfu, Cheng Jinguang, and research associate Zhang Shuai.