Recently, the research group led by Prof. SUN Yuping and Prof. LU Wenjian in the Functional Materials Laboratory at Institute of Solid State Physics, Hefei Institutes of Physical Science made a new progress in the study of manipulation of charge density wave order in monolayer 1T-TiSe₂ by strain and charge doping. Their work was published in Physical Review B.

The charge density wave is one of the most intensively studied collective quantum states in condensed matter physics. Below the CDW transition temperature, the lattice distorts simultaneously accompanied with the redistribution of charge and the abrupt change of electronic transport properties, which might open new potential applications in optoelectronic and quantum information processing devices.

Recent experimental studies showed that reducing the thicknesses of 1T-TiSe₂ can effectively increase the CDW transition temperature from 200 K in the bulk to 230 K in the monolayer. The CDW transition temperature of monolayer 1T-TiSe₂ is comparatively closer to the room temperature than that of other monolayer TMD materials.

Hence, How to further enhance the CDW transition temperature of monolayer 1T-TiSe₂ (even higher than room temperature) is a highly focused research topic.

The joint team used first-principles calculations to investigate the electronic band structures, phonon dispersion curves and electron-phonon coupling of monolayer 1T-TiSe₂.

The effects of the substrate-induced strain on CDW in experiment can be simulated by applying the in-plane biaxial compressive and tensile strains.

They found that the tensile strain can effectively enhance the CDW order, and then the charge density wave transition temperature can be remarkably enhanced.

However, the compressive strain can suppress the CDW order.

The effects of the light or an electric field on CDW in experiment can be simulated by charge-carrier doping. The research team found that both electron and hole doping can suppress the CDW instability, and then can introduce potential superconductivity.

The above research shows that controllable electronic phase transition from the CDW state to the metallic state or even the superconducting state can be realized in monolayer 1T-TiSe₂.

It is an important guidance for future experiments to obtain new electronic devices based on monolayer 1T-TiSe₂.

The research works were financed by National Key Research and Development Program and National Natural Science Foundation of China.

Link to the paper: Manipulating charge density wave order in monolayer 1T-TiSe₂  by strain and charge doping: A first-principles investigation.

Figure 1 Evolution of CDW formation energy and average displacements of Ti atoms in the CDW phase (left), and the phonon dispersion curves of monolayer 1T-TiSe₂ in the normal phase under the biaxial strain (right). (Image by WEI Mengjun)

Figure 2 Evolution of phonon dispersion curves and the average displacements of Ti atoms (left), and electron-phonon coupling constant and superconductivity transition temperature TC of monolayer 1T-TiSe₂ under charge doping (right). (Image by WEI Mengjun)