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Progress in theoretical research and experimental detection of hybridization kinetics in heavy fermion systems
[ Instrument Network Instrument R & D ] For a long time, the understanding of heavy fermion physics is mainly based on the static hybrid image provided by the average field method. The image believes that the f electrons will hybridize with the conduction band near the Fermi surface under the coherence temperature T *, thereby forming a heavy electron energy band and generating a direct and indirect hybrid band gap, causing a local-cruising transition of f electrons .
In a group of systems composed of identical particles, if only one particle is allowed to be contained in a quantum state (ie, a microscopic state determined by a set of quantum numbers), such particles are called fermions. In other words, particles whose spins are semi-odd (1/2, 3/2 ...) are collectively called fermions and obey the Fermi-Dirac statistics. Fermions satisfy the Pauli exclusion principle, that is, two or more fermions cannot appear in the same quantum state. The lepton, nucleon, and superson all have a spin of 1/2, and are therefore fermions. Resonant particles with spins of 3/2, 5/2, 7/2, etc. are also fermions. Neutrons and protons are composed of three quarks with a spin of 1/2. A nucleus consisting of an odd number of nuclei. Because neutrons and protons are fermions, the spin of an atomic nucleus consisting of an odd number of nucleons is a half integer.
But in recent years, there is more and more experimental evidence that the real understanding of the local-cruising transition physics of heavy fermions must go beyond the simplified image of the average field theory. Yang Yifeng, researcher of the EX9 team of the Institute of Physics of the Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics, and his collaborators have conducted a long-term exploration of this issue, developed the phenomenological theory of heavy fermion two-fluids, and proposed the power of hybridization Based on the study of fluctuations, the basic theoretical image of heavy fermion physics was re-established. In 2017, Fudan University's Feng Donglai's group measured the angular-resolved photoelectron spectroscopy (ARPES) of the heavy fermion material CeCoIn5 and found that band bending caused by hybridization is far more coherent than other experiments such as resistance measurement. It has appeared above the temperature T *, which is different from the expectation of the average average field image, which leads to the contradiction and confusion in understanding.
Recently, Yang Yifeng, in collaboration with Professor Qi Jingbo from the University of Electronic Science and Technology of China, conducted a systematic study of CeCoIn5 using ultrafast spectroscopy and found that there are actually two characteristic temperature scales in the system: T \ (\ dagger \) and T *. Among them, the high temperature temperature scale T \ (\ dagger \) corresponds to the temperature at which bending can start in ARPES measurement. Above T \ (\ dagger \), the relaxation rate of high-energy quasiparticles detected by ultrafast experiments is almost unchanged. Under T \ (\ dagger \), it began to decrease rapidly with the decrease of temperature, indicating that the hybridization changed the electronic structure near the Fermi surface, resulting in a direct band gap and inhibiting the energy relaxation of high-energy quasi-particles. The low temperature temperature scale T * corresponds to the coherence temperature of the traditional resistance measurement. Below T *, the relaxation rate exhibits a non-linear effect, and the size depends on the intensity of the irradiated light. This phenomenon means that a narrow energy appears in the density of states. Gap (indirect band gap), leading to the bottleneck effect of the relaxation process. Ultrafast spectroscopy detected both of these phenomena at the same time, with different responses to two different band gaps, while ARPES only detected the direct band gap below T \ (\ dagger \), which cannot be seen due to the energy resolution The small indirect band gap appears only at *, and the resistance changes significantly only when the indirect band gap appears below T *.
The above results show that the direct and indirect band gaps do not appear simultaneously as predicted by the average field theory, but are a two-stage process that gradually develops with decreasing temperature: the hybrid effect begins to appear at high temperature T \ (\ dagger \), which first leads to The energy band bending and direct band gap near the Fermi surface gradually develops with decreasing temperature, and a long-range correlation occurs at a low temperature T *, forming an indirect band gap. Thereafter, the heavy electron state is truly established and protected by the indirect band gap. In order to prove this image, Yang Yifeng supervised doctoral students Hu Danqing, Dong Jianjun, etc., carried out Monte Carlo numerical simulation (DQMC) of the periodic Anderson model, carefully analyzed the evolution of the hybridization relationship with temperature, and found that the indirectness predicted in the average place Before the band gap is opened, there does exist a high-temperature transition region, which has a low-energy hybrid fluctuation behavior and results in the direct band gap at the Fermi surface. This phenomenon is absent from the average field theory.
Monte Carlo method is also called statistical simulation method and statistical test method. It is a numerical simulation method that takes probability phenomena as research objects. It is a calculation method to estimate the unknown characteristic by obtaining the statistical value according to the sampling survey method. Monte Carlo is a famous gambling city in Monaco, the law is named to show the nature of its random sampling. Therefore, it is suitable for performing simulation tests on discrete systems. In computational simulation, by constructing a probability model similar to the performance of the system and performing random experiments on a digital computer, the random characteristics of the system can be simulated.
The above research shows the importance of dynamic hybrid fluctuations from both experimental and theoretical aspects, so it is necessary to re-understand heavy fermion physics on the basis of hybrid dynamics. But to overcome the obstacles of traditional images, constructing a new heavy fermion microscopic theory will still be a long process.
Source: Institute of Physics, Encyclopedia