We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi2Se3 single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first principle calculations, we conclude that the appearance of these modes is due to c-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 cm −1 are associated with Raman active A1g bulk phonon modes, the other two at 136 and 158 cm −1 are associated with infrared active bulk phonons with A2u symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from D 3d in the bulk to C3v on the crystal surface. In particular, the 158 cm −1 surface phonon mode shows a Fano lineshape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

Bi2Se3 and Bi2Te3 show features in the range ~ 50-160 cm -1 , which have been assigned alternatively to Raman-forbidden, bulk infrared modes arising from symmetry breaking at the surface or to surface phonons, which couple to the topologically protected electronic states. Here, we present temperature-and wavelength-dependent Raman studies showing additional modes we ascribe to surface phonons in both Bi2Se3 and Bi2Te3. Our assignment is supported by density functional theory calculations revealing surface phonons at frequencies close to those of the extra peaks in the Raman data. The theoretical results also indicate that these modes are not a consequence of spin-orbit coupling and, thus, that their occurrence is unrelated to the topological properties of these materials.1 | P a g e I. INTRODUCTIONTopological insulators (TIs) are a new class of materials that are insulating in the bulk but exhibit metallic surfaces, which arise from strong spin-orbit coupling and particular properties of their band structure. The electronic surface states of TIs consist of gapless bands characterized by a linear (Dirac) dispersion, which are protected from backscattering by time reversal symmetry [1,2]. In recent years, these novel materials have attracted significant interest, not only due to their unique electronic properties, but also because they hold promise for applications in quantum computing [2,3] Bi2Se3 and Bi2Te3 are layered compounds, which have been extensively studied in the past due to their exceptional thermoelectric properties [8]. They were also among the first compounds identified as three-dimensional TIs [9,10,11,12]. Because of its crucial relevance to their surface conductivity properties, the study of electron-phonon coupling [13] and, moreover, the search for vibrations localized at the surface have been the subject of many studies in recent years [14,15,16,17]. In particular, inelastic helium scattering [14,15], surface enhanced Raman scattering [16] and time-resolved photoemission measurements [17] in Bi2Se3 and Bi2Te3 show features that were attributed to surface modes as well as strong electron-phonon coupling at the surface.Also, weak features observed in Raman spectra were attributed to surface effects unrelated to the topological surface states [18].Here, we present experimental results on the temperature-and excitation-wavelength (λL−) dependence of Raman scattering, as well as first-principles phonon calculations for bulk and fewquintuple-layer Bi2Se3 and Bi2Te3. Other than the expected, and previously reported Raman-active bulk modes [19,20], we find weak peaks at low temperatures in both compounds, which we ascribe 2 | P a g e to surface vibrational modes. Density functional theory calculations, which do not consider spinorbit coupling, support such an assignment in that they reveal a pair of surface-modes, the lowerfrequency of which is very close in frequency with the peaks found in the Raman experiments.Arguments are also given suggesting that spin-orbit effects are not important in determi...

We have used rotational anisotropic polarized Raman spectroscopy to study the symmetries, the temperature and the doping dependence of the charge ordered state in metallic (Sr 1 -x La x ) 3 Ir 2 O 7 . Despite the fact that the Raman probe size is greater than the charge ordering length, we establish that the charge ordering breaks the fourfold rotational symmetry of the underlying tetragonal crystal lattice into twofold, as well as the translational symmetry, and forms short-range domains with 90 • rotated charge order wave vectors, as soon as the charge order sets in below T CO =∼ 200 K and across the doping-induced insulator metal transition. We observe that this charge order mode frequency remains nearly constant over a wide temperature range and up to the highest doping level. These features above are highly reminiscent of the ubiquitous unidirectional charge order in underdoped high-T C copper-oxide-based superconductors (cuprates). We further resolve that the charge order damping rate diverges when approaching T CO from below and increases significantly as increasing the La-doping level, which resembles the behaviors for a disorder-interrupted ordered phase and has not been observed for the charge order in cuprates. 1 arXiv:1810.09087v2 [cond-mat.str-el]

We investigate the interaction of the optical electric field of intense sub-picosecond infrared pulses, between 3.5 and 12 micrometers, with visible quantum dots materials. Significant visible luminescence is observed from upconversion of the infrared light.