In 1972, Sir Mott, a Nobel laureate in physics, proposed that when the mean free path of a quasiparticle approaches the Fermi wavelength, the quasiparticle will lose their coherence and can only transport by hopping under the Anderson localization picture. As a result, the system undergoes a phase transition from metal to insulator, which is known as the Mott-Ioffe-Regel (MIR) limit. However, the unconventional metallicity observed in some systems beyond the MIR limit challenges this conventional notion. For example, linear-in-temperature resistivity beyond the MIR limit has been observed in cuprates. Owing to its intimate connection with unconventional superconductivity, Planck dissipation, and quantum criticality, the metallicity beyond the MIR limit has been intensively investigated. For many years, the study of unconventional metallic phases (such as strange metals) has mainly focused on the temperature dependence of resistivity. However, the electronic coherent behavior near the MIR limit is difficult to observe due to the short mean free path.
Recently, the team of Prof. Cheng Zhang and Prof. Faxian Xiu from Fudan University and Prof. Xiang Yuan from East China Normal University, in collaborated with the Steady State High Magnetic Field Laboratory of Chinese Academy of Sciences, the Pulse High Magnetic Field Center of Huazhong University of Science and Technology, the National High Magnetic Field Laboratory of the United States, the Laboratoire National des Champs Magnétiques Intenses of France, Nanjing University, Shanghai Jiao Tong University, Peking University, Shanghai University of Science and Technology, and the Indian Institute of Science, discover that CaAs3 presents anomalous strong electron coherence when approaching the MIR limit. Taking advantage of multiple national high magnetic field facilities, the research team has realized quantum transport and magneto-infrared spectroscopy under extremely low temperature and high magnetic field, and revealed the Landau quantization and quantum oscillations of CaAs3 when approaching the MIR limit. The results were published in the National Science Review, issue 12, 2024, Entitled "Observation of quantum oscillations near the Mott-Ioffe-Regel limit in CaAs3", The first authors of the study are Ph.D. students Yuxiang Wang and Minhao Zhao from Fudan University and Prof. Jinglei Zhang from the High Magnetic Field Laboratory of Chinese Academy of Sciences. The corresponding authors are Prof. Cheng Zhang and Prof. Faxian Xiu from Fudan University and Prof. Xiang Yuan from East China Normal University.
The research team grew high-quality CaAs3 single crystals and carried out quantum transport experiments under a strong steady-state magnetic field of up to 45.22T. The resistivity of CaAs3 presents an insulator-like temperature dependence and is close to the MIR limit at around 2K. Despite such a highly insulating state, the Shubnikov-de Haas oscillation from the bulk had been observed in magnetoresistance, indicating that the carriers in CaAs3 form highly coherent band transport rather than hopping transport predicted by traditional theory. In the meantime, the Hall and Seebeck coefficients of CaAs3 also show anomalous sign reversal behavior at low temperatures. Furthermore, the research team carried out a magneto-infrared spectroscopy study on CaAs3, and further confirmed the Landau quantization by the interband-Landau-level transitions. By comparing the effective mass and Fermi velocity obtained from the transport and magneto-infrared spectroscopy, a strong band renormalization is found near the Fermi level. The team uses a two-fluid-like model to analyze the above experimental results and points out that these unconventional behaviors are due to the interplay between the mobility edge and the van Hove singularity. The electronic coherence behaviors such as quantum oscillations are mainly contributed by mobile electrons above the mobility edges, while the electrical resistance and Hall coefficient of the insulating state are dominated by localized electrons below the mobility edges. This finding challenges the conventional theory of the electron hopping transport behavior near the MIR limit, and opens up a new perspective of quasiparticle coherence for the study of metallicity near the MIR limit.
