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Scientists Reveal Structural Dynamics and Microstructure Features in Quantum Hidden States of 2D-CDW Materials
 

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.

Publication web links:
Science Advances (Sci. Adv. 4, eaas9660 (2018)):
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Nanscale ((Nanscale 10, 7465 (2018)): http://pubs.rsc.org/en/content/articlepdf/2018/nr/c8nr00432c
PRB (Phys. Rev. B 96, 174203 (2017)) :
https://journals.aps.org/prb/pdf/10.1103/PhysRevB.96.174203
Structural Dynamics (Struct. Dyn. 4, 044012 (2017)) :
https://aca.scitation.org/doi/pdf/10.1063/1.4979643
PRL (Phys. Rev. Lett. 120, 167204 (2018) ):
https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.120.167204

Fig. 1. Schematic diagram of the experimental setup for 4D-ultrafast TEM (UTEM), as used with an in-situ photoexcitation measurements. (Image by Institute of Physics)
Fig. 2. Alterations of CDW modulation after the pulsed femtosecond-laser excitations. (Image by Institute of Physics)
Fig.3 Structural models and theoretical simulations for typical polaron ordered states under different photoexcitation conditions. (Image by Institute of Physics)
Fig.4. Temperature dependence of resistivities and photoexcited drops for1T-TaS2-xSex (x=0, 0.5) crystals. (Image by Institute of Physics)

Contact:
Institute of Physics
LI Jianqi
Email:ljq@aphy.iphy.ac.cn

Key word:
Quantum hidden state; ultrafast CDW transition; structural dynamics; insulator-metal transition;

Abstract:
In-situ TEM observations with a single fs-laser pumping, reveal a fluence-dependent transition from a commensurate CDW phase to a space-anomoly hidden CDW (H-CDW) order and an orientation-anomoly H-CDW phase. Photoexcited resistivity drops associated with the appearance of the H-CDW states depend on laser fluence and temperature, indicating the photodoping mechanism for the emergence of H-CDW states in the 1T-TaS2-xSex family.

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