In the low-dimensional correlated systems, an interplay between the lattice, spin, charge and orbital degree of freedom could result in remarkable physical properties and structural transitions, such as charge/orbital ordering, superconductivity, and colossal magnetoresistance.
In recent years, the transient superconductivity and quantum hidden (H) states as induced by ultrafast laser excitations have become a novel research hotspot in condensed matter physics. The H states can be generated under nonequilibrium phase transition, that doesn’t exist in thermodynamic phase diagram. In the charge density wave (CDW) system of 1T-TaS2, it is shown that the H state appears at low temperatures following with an ultrafast quantum transition upon a femtosecond pulsed-laser excitation.
Recently, Prof. LI Jianqi’s group from the Institute of Physics, Chinese Academy of Sciences (IOP, CAS), made some significant progress in revealing the structural dynamics, ultrafast transitions and quantum hidden states by using ultrafast transmission electron microscope (UTEM).
In recent studies, Dr. K. SUN and S.S. SUN in Prof. LI Jianqi’s group discovered the microstructure of H state and its dynamic behaviors in 1T-TaS2-xSex crystals. They also developed a series of novel experimental technologies for characterizing ultrafast structural transitions and the transient states in layered crystals.
Under the femtosecond-laser excitation, the ultrafast switch appears from insulating Mott state to metallic H state in 1T-TaS2. In this works, researchers have extended their study on the H states in 1T-TaS2-xSex crystals. In-situ cooling TEM observations, initiated by a single fs-laser excitation, reveal a clear transition from a commensurate CDW state to a quantum hidden state in which the photodoping concentration is estimated to be 1/9, this H-state cannot exist in thermodynamic phase diagram.
Furthermore, electrical resistivity measurements show that the notable drops in x=0 and 0.5 samples associated with the appearance of H-CDW states depend on laser fluence and temperature.
These results potentially verify the photodoping mechanism for the resistivity switching in the Mott states of 1T-TaS2-xSex (x=0, 0.5). The large resistivity switching and remarkable stability of the H state in 1T-TaS2-xSex is expected to have technological applications in high-speed nonvolatile memory devices.
This study entitled “Hidden CDW states and insulator-to-metal transition after a pulsed fs-laser excitation in layered chalcogenide 1T-TaS2-xSex” was published in Science Advances.
In recent years, Prof. LI’s group has been devoting much effort to study the ultrafast-TEM techniques and relevant structural dynamics in functional materials.
These works were supported by the National Key Research and Development Program of China, the National Basic Research Program of China, the National Natural Science Foundation of China, the Strategic Priority Research Program (B) of CAS and the Scientific Instrument Developing Project of CAS.