Photoinduced Ultrafast Symmetry Switch in SnSe.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Layered tin selenide (SnSe) is a promising thermoelectric material, with peak performance linked to its high-temperature Cmcm phase.
• Current methods to access the Cmcm phase rely on thermal equilibrium, limiting studies to ground-state conditions.
• Understanding dynamic phase transitions is crucial for optimizing thermoelectric materials.

Purpose of the Study:

• To investigate the ultrafast carrier and phononic dynamics in SnSe.
• To explore the possibility of achieving the Cmcm phase under nonequilibrium conditions.
• To understand the mechanisms driving transient phase transitions in SnSe.

Main Methods:

• Ultrafast optical spectroscopy to probe carrier and phononic dynamics.
• Analysis of crystal symmetry changes induced by optical excitation.
• Investigation of phonon coherence and lattice temperature dynamics.

Main Results:

• Optical excitation can induce a transient switch from the Pnma to the Cmcm phase in SnSe at room temperature within hundreds of femtoseconds.
• This transition occurs at an ultralow excitation carrier density threshold.
• The nonequilibrium Cmcm phase is driven by coherent Ag phonon excitation, creating a 'cold lattice with hot carriers' state.

Conclusions:

• Nonequilibrium optical excitation provides a novel pathway to access the high-performance Cmcm phase of SnSe at room temperature.
• The findings reveal a new mechanism for dynamic phase control in thermoelectric materials.
• This research offers critical insights into the nonequilibrium thermoelectric properties of SnSe.