Correlated plasmons in the topological insulator BiSe induced by long-range2 3 electron correlations Using a combination of angle-dependent at 1.04 eV and 1.52 eV indicates the presence of a plasmon at spectroscopic ellipsometry and angle-resolved these energies below 250 K and they are closely linked to the photoemission spectroscopy as a function peaks in e at 1.00 eV and 1.48eV which are optical excitations.2 Correlated plasmons also have a significantly lower loss than of temperature and supported by first- conventional plasmons, and a positive real part of the dielectric principles calculations, we reveal a new pair function, e , both of which can be seen in Fig. 1 [2, 3].1 of correlated plasmons at 1.04eV and 1.52eV Figure 2a shows the optical conductivity of the BiSe thin film for and a significant Fermi level shift of 0.12eV, each of the temperatures measured, divided into three spectral2 3 accompanied by spectral weight transfer in regions. The total spectral weight, W, of the BiSe thin film32 the topological insulator, Bismuth Selenide. across the measured spectral range is shown in Fig. 2b as a Our results show the importance of electronic function of temperature. This shift in spectral weight cannot be correlations in determining the electronic explained by thermal activation as the energies associated with temperatures below 500 K are too small (<43 meV), therefore structure and appearance of correlated the extra energy gained or lost must come from the potential plasmons in topological insulators and their energy electron-electron correlations [2, 3] Both of the correlated potential applications in plasmonics. plasmons seen in Fig. 1c disappear at temperatures of 250 K and above, which also coincides with the drop in electron In recent years, three-dimensional topological insulators have conductivity and thus electron density, that is seen in Fig. 2b. attracted considerable interest due to their fascinating fundamental Figure 2c shows the change in W of each of the three spectral properties and potential applications of having an electrically regions from Fig. 2a as the sample is cooled. The increase in conducting surface, but an insulating bulk [1]. Bismuth Selenide electron energy of the order of several eV comes from the long- (BiSe) is one of the most commonly investigated topological range electron-electron correlations, which are now prominent insulators [1]. In particular, considerable effort has been made2 3 due to decreased electronic screening [2].This in turn gives to study various types of plasmons that have also been shown rise to plasmonic activity, which has been seen in conductive to occur in topological insulators, including surface, and Dirac materials [2, 4]. plasmons.The potential applications of topological insulators FIGURE 1 FIGURE 2 have now begun to be realized in the real world,and these include being used in quantum computing, and plasmonic, photonic and spintronic devices. RESULTS AND DISCUSSION Figures 1a and 1b show the real, e1, and imaginary, e , parts o2 f complex the dielectric function, respectively, measured using spectroscopic ellipsometry over a range of temperatures from 475 K down to 77 K. There is an edge that occurs in the e2 spectra at 1.0 eV for all temperatures that gets sharper as the sample is cooled. Since spectroscopic ellipsometry is a photon-in photon out technique, it can be used to search for plasmonic activity in correlated electron systems [2]. Figure 1c shows a comparison of e and the loss function for all temperatures in the2 spectral region 0.6 eV – 1.6 eV. The peaks in the loss function 28