Research: Ultrafast Transition to New Metastable Crystal Phase Discovered

Metal chalcogenides make up a group of quantum materials which shows many fascinating properties. These materials have an almost two dimensional structure and are characterised by a complex diversity of various electronic and structural phases, topological states, charge density waves and different types of superconductivity. In addition, these phases also interact with each other.

In an international cooperation scientists from USA (Boulder), Germany (Kiel and Würzburg) and Uppsala University, Peter Oppeneer, have shown that an entirely new structural phase arises in the quantum material tantalum di-selenium (TaSe₂) through excitation with ultra-short laser pulses. The scientists discovered that laser excitation with a 30-femtosecond pulse (30·10^-15 s) quickly transforms the configuration of the electrons, which in turn makes the atoms move to new lattice positions within 300 femtoseconds and the material forms a new crystal structure. Through femtosecond photo-emission spectroscopy the scientists could show that the new structure is unusually stable and exists during 300 picoseconds (300·10^-12 s). The remarkable aspect of the discovery is that the material cannot undergo a phase transition to the new phase just by equilibrium thermodynamic processes, that is, when the temperature or the pressure are changed slowly.

The discovery which was published in the journal Science Advances shows that it is possible to reach hidden metastable phases which may accommodate novel properties through non-equilibrium processes.

Article reference:

Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe₂ mediated by mode-selective electron-phonon coupling,
Xun Shi et al., Science Advances 5, eaav4449 (2019)

Contact: Prof. Peter Oppeneer, phone 018-4713748