Photoinduced dynamics of flat bands in the kagome metal CoSn

Letter

Photoinduced dynamics of flat bands in the kagome metal CoSn

D. Puntel, W. Bronsch, M. Tuniz, M. Kang, P. M. Neves, S. Fang, E. Kaxiras, J. G. Checkelsky, R. Comin, F. Parmigiani, and F. Cilento

Phys. Rev. B 109, L081104 – Published 8 February 2024

Abstract

CoSn is a prototypical kagome compound showing lattice-born flat bands with suppressed bandwidth over large parts of the Brillouin zone. Here, by means of time- and angle-resolved photoelectron spectroscopy, we provide direct evidence of the response to photoexcitation of the flat bands, which underlie information about localization in real space. In particular, we detect a sudden shift and broadening of the flat bands, while after one picosecond only a broadening survives. We ascribe both these effects to an ultrafast disruption of electron localization, which renormalizes the effect of electron-electron interaction and affects the flat band dispersion. Since both variations are in the order of a few meV for an excitation fluence of a few hundreds of $μ J / {cm}^{2}$ , our measurements suggest that the flat bands are resilient to near-infrared photoexcitation in a moderate fluence reégime.

Received 31 May 2023
Accepted 17 January 2024

DOI:https://doi.org/10.1103/PhysRevB.109.L081104

Physics Subject Headings (PhySH)

Flat bands

Kagome lattice

Angle-resolved photoemission spectroscopy Density functional theory Time & angle resolved photoemission spectroscopy Ultrafast femtosecond pump probe

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

D. Puntel ¹, W. Bronsch², M. Tuniz¹, M. Kang³, P. M. Neves³, S. Fang³, E. Kaxiras^4,5, J. G. Checkelsky³, R. Comin ³, F. Parmigiani^2,6, and F. Cilento ^2,*

¹Dipartimento di Fisica, Università degli Studi di Trieste, 34127 Trieste, Italy
²Elettra - Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, 34149 Trieste, Italy
³Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
⁴Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
⁵John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
⁶International Faculty, University of Cologne, Albertus-Magnus-Platz, 50923 Cologne, Germany

^*federico.cilento@elettra.eu

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Vol. 109, Iss. 8 — 15 February 2024

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Images

Figure 1
(a) Scheme of the experimental geometry (top view). Inset: Relative orientation of the crystal Brillouin zone and the direction of the $p$ polarization in the geometry of the experiment. (b) Schematic lattice of a CoSn kagome layer. Colored circles denote the adjacent sites with opposite-sign phase of the wave function (green and red), causing hopping to the nearest-neighbor (gray) to vanish. The gray-shaded area indicates the real-space region of electron confinement. Inset: First Brillouin zone of CoSn. (c) Band structure of CoSn along the $Γ − K$ direction measured with 92 eV photons. The blue (brown) arrow indicates the upper (lower) flat band. Data from measurements reported in Ref. [30]. (d) Dispersion measured with $p$ -polarized photons at 10.8 eV and 115 K along $Γ − K$ ( $A − H$ ). In the right panel, EDCs extracted in the momentum regions of corresponding color: orange for small momentum values, green for large momentum values. The best fit is overlaid as a black line. A and B identify two Lorentzian peaks used to fit the EDCs. (e) DFT calculations of the band structure of CoSn along the $Γ − K$ direction in the vicinity of the Fermi level, already reported in Ref. [30]. The upper and lower flat bands are indicated with blue and brown lines respectively, the Dirac band in red and a non-kagome derived band in black. The degrees of freedom of the $3 d$ orbitals from which the two bands originate are also reported. The square markers indicate the position of the two peaks resulting from the fit of the EDCs extracted from the measurement at 10.8 eV. The EDCs are integrated in the momentum region centered at the marker position, with the resulting Lorentzian width indicated as a shaded area. (f) $DFT + U$ [32] calculations for $U = 0$ eV [solid lines, same as reported in (e)] and $U = 2$ eV (dashed-dotted lines). The side panel displays the EDCs obtained using the peak positions from the $U = 0$ eV case and applying the same functional form and parameters (area, width, Gaussian broadening, and temperature) obtained from the fits of the experimental data.
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Figure 2
(a) Difference between the map at time zero and the average before time zero. (b) Differential EDCs integrated in the whole momentum region region at 0 and 5 ps time delay. The black superimposed line is the fit, described in the main text. (c) Differential EDCs plotted in a color map as a function of time delay. (d) Time traces extracted in the energy region marked by the dotted lines in (c). The light-blue trace is the Gaussian fit to the dark-blue curve.
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Figure 3
Variation with respect to equilibrium of the parameters extracted by fitting the dEDCs, as a function of time delay. Bars indicate the uncertainty of the fitting procedure on the parameters. (a) Electronic temperature. The solid line is the Gaussian fit reported from Fig. 2. (b) Lorentzian width of peak A (brown) and B (light blue). (c) Position (binding energy) of peaks A and B. The solid line is the best fit of the A position dynamics to a Gaussian. (d) Area under each Lorentzian peak.
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