Figure 3 presents the electronic band structure of the BiSe thin film32 along the Γ-M direction measured using ARPES with an incident photon SURFACES, INTERFACES AND NANOSYSTEMS energy of 60 eV. Panel (a) shows the spectra taken at 300 K, while panel SYNCHROTRON SOLEIL HIGHLIGHTS 2020 (b) shows the spectra at 100 K. The Dirac point at room temperature is observed at a binding energy of 0.30 eV, while the Dirac point shifted to the higher binding energy of 0.42 eV at 100 K (the energies are measured TEMPO BEAMLINE with respect to the Fermi energy). At lower temperatures, the shifting of the Dirac point by 0.12 eV towards a higher binding energy shows that Associated publication the electronic structure of the BiSe film is changing [5].32 Correlated plasmons in the topological FIGURE 3 insulator BiSe induced by long-range 2 3 electron correlations. T. J. Whitcher, M. G. Silly, M. Yang, P. K. Das, D. Peyrot, X. Chi, M. Eddrief, J. Moon, S. Oh, A. H. Castro-Neto, M. B. H. Breese, A. T. S. Wee, F. Silly & A. Rusydi. NPG Asia Mat., 12: art.n° 37 (2020). References [1] L. Fu & C. L. Kane, Phys. Rev. B, 76, 045302 (2007). [2] T. C. Asmara et al., J. Appl. Phys. 115, 213706 (2014). [3] E. G. C. P. Van Loon et al., Phys. Rev. Lett. 113, 246407 (2014). [4] D. van der Marel, In Strongly Correlated Systems (269-296). Springer Berlin Heidelberg (2015). [5] M. Brahlek et al., Solid State Commun. 215, 54 (2015). Corresponding author Andrivo Rusydi Department of Physics, CONCLUSIONS National University of Singapore, In summary, by simultaneously determining the complex dielectric 6 Science Drive 2, function, loss function and electronic structure and dispersion of BiSe 117576, Singapore as a function of temperature using a combination of spectroscopic2 3 phyandri@nus.edu.sg ellipsometry, and ARPES, we observe unusual spectral weight transfer Captions yielding a significant shift in the Fermi level and two correlated plasmons, a new type of plasmon seen in correlated systems. From the spectral FIGURE 1: a The real (e1) and b imaginary (e2) parts of the weight transfer analysis, we find that upon cooling below 250 K, the dielectric function of Bi2Se3 as a function of temperature. c Comparison of e2(solid line), and the loss function decrease in electronic screening leads to the increase of long-range Im(-1/)e (dashed line) for all temperatures. electron correlations, which increase the potential energy of the system FIGURE 2: a The optical conductivity (σ1) of Bi2Se3 as and also results in the formation of the correlated plasmons which results a function of temperature divided into three spectral in a change in the electronic structure and subsequently, a reversible regions. b σ1 of Bi2Se3 integrated over the whole shift in the Fermi energy. Our result shows new correlated plasmons spectrum. c The change in integrated σ1 of the three spectral regions as the sample is cooled. in BiSe and the methodology introduced here can be used to probe32 these plasmons in topological insulators. FIGURE 3: a and b Electronic band dispersion of Bi2Se3 thin film measured using 60 eV photon energy along the Γ-M direction at 300 K, and 100 K, respectively. c and d show the energy dispersion curves of the spectra. 29