Exploring new quantum states has always been one of the frontier topics in condensed matter physics. Ultrafast spectroscopy has revealed many mysteries in this area due to its unique capability in achieving extremely high temporal resolution. Layered charge density wave (CDW) materials provide an ideal research platform for revealing the emerging physical phenomena. 

As a typical CDW material, 1T-TaS₂ undergoes a continuous CDW phase transition during the heating and cooling processes. Many investigations mainly focus on the CCDW-NCCDW phase transition, and very few investigations have been conducted within the commensurate CDW phase below 180K. To date only very few investigations have reported the behavior of 1T-TaS₂ in the temperature range of CCDW phase, suggesting possible existence of a quantum spin liquid state.

In a study published in PNAS, Prof. ZHAO Jimin's group from the Institute of Physics of the Chinese Academy of Sciences (CAS)/Beijing National Research Center for Condensed Matter Physics, along with the collaborators from the University of Science and Technology of China of CAS, and Prof ZHENG Liu from the Tsinghua University, conducted the ultrafast spectroscopy investigation of 1T-TaS₂, and discovered a novel quantum state in 1T-TaS₂.

Prof. ZHAO's group used ultrafast spectroscopy to detect the low temperature ultrafast dynamics of 1T-TaS₂, and detected six time-domain coherent oscillation modes in the temperature domain for the commensurate CDW, among which the 2.47 THz mode is the amplitude mode (AM) of the CDW. 

Using the high resolution of ultrafast spectroscopy, it was found that the temperature dependence of the frequency, full width at half maximum (FWHM) and integrated amplitude the AM has a crossover at 65 K: each physical quantity satisfies the conventional T² relation at 65-200K. However, a completely different T^3.56 relation was found at temperatures below 65K.

Based on the analysis of the experimental results, the researchers identified the state below 65K as a new emerging quantum state owing to the couplings among the multi-degree-of-freedom. The finding of this study deepens our understanding of CDW materials and provides new perspectives on exploring the cutting-edge areas such as quantum spin liquid.