Ultrafast photoinduced dynamics of low-dimensional excitonic materials at room temperature

Abstract

One challenge of condensed matter physics is to explain the complex behaviour of the material world. Doing so requires determining the significant degrees of freedom, which can then be controlled, varied, measured experimentally, or modelled theoretically. Here, we use the frameworks of elementary excitations and symmetry breaking as starting points to understand two low-dimensional materials. With ultrafast spectroscopic measurements, we probe these materials at room temperature and in non-idealised regimes, building on where these frameworks fall short. We first investigate monolayer MoS₂ and WS₂, which exhibit strong light-matter interaction with strongly bound excitons in the visible range. While knowledge of the physics of these materials has grown significantly in the past decade, atomic-scale intrinsic defects remain experimentally and theoretically challenging to probe and control. Using a range of spectroscopic measurements supported by ab initio calculations, we build a picture of how defects and how the well-known chemical treatment TFSI interact with these materials. We then propose a generalisable chemical treatment for both passivating defects and tuning optoelectronic properties. We next turn our attention to the quasi-1D excitonic insulator candidate Ta₂NiSe₅. At 328 K, this material is proposed to undergo a spontaneous symmetry breaking, leading to the Bose-Einstein condensation of excitons in the ground state. Yet, experiment and theory continue to wrestle over whether an excitonic insulator phase exists in this system, where discrete crystal symmetries and structural order may compete with electron-electron interactions. Using broadband, temperature-dependent pump-probe measurements, we identify a spectral range in the near infrared that maps an order-parameter behaviour. Using this spectral region to track the out-of-equilibrium state, we measure a sub-50 fs melting timescale for the order, which suggests strong electron correlations dominate the material’s broken symmetry. To further understand how an excitonic insulator phase would behave in a real material, we turn to ultrafast microscopy measurements, in which we detect the propagation of coherent modes oscillating at the frequency of optical phonons but moving at electron-like velocities. We propose that this behaviour can be explained by hybridisation between the lattice degrees of freedom with the phase mode of an excitonic condensate hosted by Ta₂NiSe₅ at room temperature.