Our colleague Dino Novko, in collaboration with scientist from Institute for Research in Fundamental Sciences, Iran, has published a paper in Journal of Physical Chemistry Letters, in which they explore novel plasmonic features in charge-density-wave material TiSe₂.

Plasmon Excitations across the Charge-Density-Wave Transition in Single-Layer TiSe₂

Zahra Torbatian, Dino Novko, J. Phys. Chem. Lett. 15, 6045 (2024).

DOI: 10.1021/acs.jpclett.4c01034

Tailoring and inducing new states of light-matter interaction in strongly correlated materials is the new and growing paradigm in modern condensed matter physics. In recent years there have been an increasing interest for exploring van der Waals heterostructures consisting of semimetallic and semiconducting transition metal dichalcogenides (TMDs) mostly due to their exceptional optical properties, such as strong light-matter coupling and exciton binding energies. Some of these TMDs are characterized in addition with rich phase diagram of charge ordered states, such as charge-density-wave (CDW), superconductivity, and excitonic-insulator orders, which makes them an ideal platform for exploring exotic optical properties in strongly-correlated phases. Two-dimensional (2D) single- or few-layer TMDs are even more promising in this context, since they could support low-loss 2D plasmon modes which can range from zero to infrared energies or even above, which opens many possibilities for direct coupling between 2D plasmon and low-energy excitation modes of charge ordered states, like CDW excitations. Despite this great potential, the theoretical studies that can conjoin these two compelling worlds, i.e., plasmonics of 2D materials and correlated physics, are rare or even not present in the literature.

Here we utilize our well-established ab-initio electromagnetic linear response formalism that is based on density functional theory and density functional perturbation theory, in order to investigate the dynamics of 2D plasmon mode across the CDW transition in single-layer TiSe₂. From experimental measurements it is known that the collective electronic response in bulk TiSe₂ is characterized with an abrupt increase of plasmon energy and linewidth at the CDW transition temperature T_CDW, however, microscopic scattering mechanisms behind these intriguing modifications are not well understood. Even less so when it comes to the 2D plasmon across the CDW transition in single-layer TiSe₂. We disentangle various scattering mechanisms, like CDW gap excitation and plasmon-phonon coupling, and uncover an intriguing unconventional temperature dependence of the plasmon broadening in 2D TiSe₂. Below T_CDW we found a highly tunable hybrid mode that comes from the coupling between the CDW gap excitations and 2D plasmon. In recent experiment, very similar optical features were found for the CDW bearing TaSe₂, for which we here provide a compelling microscopic explanation. Our study is also able to explain the plasmon dynamics of bulk plasmon in TiSe₂.

Fig. 1 Low-energy electron excitation spectra A(q,ω) of single-layer TiSe₂ for several temperatures around T_CDW: (a) T=1000K, (b) T=1050K, (c) T=1100K, and (d) T=1200K. The hybrid CDW-plasmon mode is indicated with the black arrows. (e) Spectral function A(q,ω) at T=1100K for several momenta q around the hybrid CDW-plasmon mode. (f) Total plasmon damping [i.e., FWHM of plasmon peaks in A(q,ω)], which consists of both interband Landau damping and plasmon-phonon contributions, as a function of plasmon energy and temperature across the CDW transition. (g) Total and interband parts of plasmon damping for fixed plasmon energy (i.e., momentum q) and as a function of temperature. Plasmon decay rates as obtained from the infrared optical measurements and electron energy loss spectroscopy are shown for comparison. (h) Real part of the interband optical conductivity σ₁(ω) calculated for several temperatures approaching the T_CDW. The low-energy (LE) and high-energy (HE) peaks are depicted with green and purple arrows. (i) The energy position of the LE and HE peaks as a function of temperature. The results are compared with infrared spectroscopy and RIXS.

With this theoretical study, we provide an answer to several unresolved issues regarding plasmon dynamics in CDW-bearing TMDs, and show a great potential of using the 2D correlated layered materials in plasmonic research.