Revealing the Order Parameter Dynamics of 1T-TiSe2 Following Optical Excitation

Result of the Month

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dynamics of the electronic band structure | © Scienta Omicron

The dynamics of the electronic band structure along the high symmetry direction Γ-M-Γ for a temperature of 80 K

Author: Prof. Alessandra Lanzara Institute: Berkeley Lab Materials Sciences Division Nature Logo | © Nature Scientific Reports
Date: 12/2022
Instruments: R4000

The formation of a charge density wave state is characterized by an order parameter. The way it is established provides unique information on both the role that correlation plays in driving the charge density wave formation and the mechanism behind its formation. Here we use time and angle resolved photoelectron spectroscopy to optically perturb the charge-density phase in 1T-TiSe2 and follow the recovery of its order parameter as a function of energy, momentum and excitation density. Our results reveal that two distinct orders contribute to the gap formation, a CDW order and pseudogap-like order, manifested by an overall robustness to optical excitation. A detailed analysis of the magnitude of the gap as a function of excitation density and delay time reveals the excitonic long-range nature of the CDW gap and the short-range Jahn–Teller character of the pseudogap order. In contrast to the gap, the intensity of the folded Se4p* band can only give access to the excitonic order. These results provide new information into the long-standing debate on the origin of the gap in TiSe2 and place it in the same context of other quantum materials where a pseudogap phase appears to be a precursor of long-range order. 

The key to study this intriguing interplay of long- and short-range order and to understand the emergence of such correlated behavior is the order parameter. Indeed, having direct access to the quenching and re-formation dynamics of the order parameter during a photoinduced phase transition retains unique information on the mechanism behind the establishment of the new order. In this regard, time- and angle-resolved photoemission spectroscopy (tr-ARPES) yields a powerful way to access the formation and dynamics of the gap, given that it is the only technique that can directly monitor the onset of the order parameter with simultaneous momentum and time resolution. While signatures of the gap in the optical conductivity as well as the dynamics of the valence band at the Γ point have been studied before, in this study we directly follow the actual gap dynamics while also providing a direct comparison with all other relevant quantities, namely excited carriers and backfolded Se4p* band. We find that following photoexcitation, the gap undergoes only a partial quenching, up to 30%, of its equilibrium value (∼ 130 meV). This points to the existence of a pseudogap and to the presence of multiple coexisting mechanisms contributing to the CDW formation in TiSe2. Finally, we reveal that the folded Se4p* is mostly connected to only one of the components cautioning the common assumption that spectral weight dynamics is directly related to order parameter dynamics. 

Figure Description: (a) First BZ in the high symmetry (orange) and CDW phase (black); the grey bar illustrates the cuts measured in the ARPES spectra in Γ-M-Γ direction. (b) Schematic band structure of the CDW state (adapted from Ref.15). (c) and (d) ARPES spectra of the M-point in equilibrium (c) and after excitation with 780 nm pump pulses at 80 μJ/cm2 fluence (d). For clarity the spectra are mirrored at the M point. The Fermi level is indicated by the dashed orange line. (e) EDCs taken at M corresponding to the spectra in panel (c) and (d). The region in momentum space over which the EDCs were integrated is shown by the white box in panel (c). Markers indicate the fitted peak positions of the Ti3d conduction band (red triangle) as well as the Se4p−1 (blue diamonds) and Se4p−2 valence bands (brown circles). Circles represents raw data and solid black lines represent the smoothed raw data using the Gaussian method (10 meV window). 

Method 

Time-resolved ARPES measurements were conducted at the Lawrence-Berkeley National Laboratory with 22.3 eV extreme-ultraviolet (XUV) femtosecond pulses. Photoelectrons are detected with a hemispherical electron analyzer (Scienta Omicron R4000).

In conclusion, this work places TiSe2 in the same context of other quantum materials where a pseudogap phase appears to precede long-range order. We find strong indications that the gap in TiSe2 is governed by two contributions, one caused by an excitonic condensate with long coherence length and one of Jahn–Teller character with short coherence length. The latter contribution can still give a well defined gap even after long range order was destroyed. In contrast, the well studied intensity of the folded Se4p* band gives access to predominantly only one of these contributions and originates mostly from the excitonic order. Thus, in summary our work exemplifies how valuable mechanistic insight can be gained by studying the order parameter and shines new light onto the complicated interplay of long and short range order in TiSe2, which is eventually the key for the understanding of doping or pressure induced superconductivity in this and other materials. 

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Authors

Maximilian Huber, Yi Lin, Nicholas Dale, Renee Sailus, Sefaattin Tongay, Robert A. Kaindl & Alessandra Lanzara 

Institutes

Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA 

Maximilian Huber, Yi Lin, Nicholas Dale, Robert A. Kaindl & Alessandra Lanzara 

Physics Department, University of California Berkeley, Berkeley, CA, 94720, USA 

Nicholas Dale & Alessandra Lanzara 

Materials Science and Engineering Department, Arizona State University, Phoenix, AZ, 85281, USA 

Renee Sailus & Sefaattin Tongay 

Department of Physics and CXFEL Labs, Arizona State University, Phoenix, AZ, 85287, USA 

Robert A. Kaindl 

Corresponding Author

Alessandra Lanzara 

alanzara@lbl.gov  

Publication

Huber, M., Lin, Y., Dale, N. et al. Revealing the order parameter dynamics of 1T-TiSe22 following optical excitation. Sci Rep 12, 15860 (2022). https://doi.org/10.1038/s41598-022-19319-w