IT House February 7 news, according to the official website of the University of Science and Technology of China, the University of Science and Technology of China Pan Jianwei, Yao Xingcan, Chen Yuao, etc., in cooperation with Australian scientist Hu Hui, for the first time observed the critical divergence behavior of entropy wave decay in the Fermi superfluid at the limit of strong interaction (monogram), revealing that the system has a considerable phase transition critical zone, and obtained important transport coefficients such as thermal conductivity and viscosity coefficient. This work provides important experimental information for understanding the quantum transport phenomena of strongly interacting Fermi systems, and is an example of using quantum simulation to solve important physical problems.
IT House learned that on February 4, the results were published in the form of a long article (research article) in the form of an international authoritative academic journal Science.
According to reports, more than 80 years ago, Landau established a two-fluid theory, successfully explained the phenomenon of superflow of helium-4 liquid (strong interaction Bose system), and predicted that entropy or temperature would propagate in the form of waves in the superflow. The nature of entropy waves is similar to that of traditional sound waves, which gradually decay during propagation, so Landau named it second sound. The propagation and attenuation of the second sound, directly coupled to the superflow sequence parameter, is a unique quantum transport phenomenon that exists only in superfluids. Studying the attenuation behavior of the second sound in the Fermi superflow can not only answer the long-standing problem of "whether the two-fluid theory can describe the low-energy physics of the strong interaction Fermi superflow", but also characterize the critical transport phenomenon of the strong interaction Fermi system at the superluid phase transition.
The superfluid formed by ultracold Fermi atoms under the strong interaction (monogram) limit has excellent purity and controllability, which brings a new opportunity to study the attenuation of the second sound, which is also an important goal in the field of ultracooled atom quantum simulation. Observing the attenuation of the second sound requires not only the preparation of high-quality density uniform Fermi superflows, but also the development of methods to detect weak temperature fluctuations. Although Fermi superflow has been implemented for nearly 20 years, the above two key technologies have not been breakthroughs, so it is impossible to study the attenuation of the second sound.
In this work, after more than 4 years of hard work, the research team of the University of Science and Technology of China built a new ultra-cold lithium-dysprosium atom quantum simulation platform, integrated the development of gray sticky groups and algorithm cooling, box-type photostatic well and other advanced ultra-cold atomic regulation technology, and finally successfully achieved the preparation of the world's leading uniform Fermi gas; at the same time, the research team also based on low-noise traveling wave optical lattice and high-resolution in situ imaging technology. The experiments implemented and theoretically interpreted the Bragg spectroscopic methods of low momentum transfer (about five percent of Fermi momentum) and high energy resolution (better than one thousandth of Fermi energy), and used them to achieve high-resolution measurements of the density response of the system. Based on the above two key technological breakthroughs, the research team successfully observed the signal of the second sound (as shown in Figure 1(D)) in the density response of the monoframic superfluid, and obtained a complete density response spectrum of the monofum Fermi superfluid, and the experimental results were highly consistent with the description based on the dissipative two-fluid theory.
Further, the research team obtained the attenuation rate (acoustic diffusion coefficient) of the second sound, and accurately determined the thermal conductivity and viscosity coefficient of the system. The results show that the transport coefficients of monoframic Fermi superfluids reach the universal quantum mechanical limit, for example, the second sound diffusion coefficient is about /m, and the thermal conductivity is about n kB/m. These limit values are determined only by the reduced Planck ( ) and Boltzmann constants ( kB ) , particle mass m , and density n. In addition, they observed the critical divergence behavior of the above transport near the superluid phase transition and found that the monofumer superfluid has a considerable critical region (about 100 times more than the liquid helium superfluid critical region). This finding lays the foundation for further quantum simulation studies using the system to understand anomalous transport phenomena in the strongly correlated Fermi system.
▲ Schematic diagram of the (A) device. (B) Schematic diagram of the detection scheme. (C) The first sound signal. (D) The second sound signal | Source: The official website of the University of Science and Technology of China
Original link:
https://www.science.org/doi/10.1126/science.abi4480