Authors: H.-H. Kung, S. Maiti, X. Wang, S.-W. Cheong, D.L. Maslov, G. Blumberg
Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, Bi2Se3. We observe a sharp peak at 150 meV in the pseudovector A2 symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.
We are proud to announce that graduate student Mai Ye has been awarded the David C. Langreth Graduate Development Award. This awarded is awarded to "promising early-stage graduate student, typically in the second year of graduate study, providing travel and living expenses for use in attending a scientific summer school or tutorial workshop in condensed matter or related fields." For more information please see here.
Also congradulations to undergraduate students Viktor Krapivin for winning the Mary Wheeler Wigner Memorial Scholarship for "demonstrated outstanding academic excellence" (more here) and William Cheng for winning the Robert L. Sells Scholarship for "demonstrated outstanding academic excellence" (more here).
Authors: H.-H. Kung, M. Salehi, I. Boulares, A.F. Kemper, N. Koirala, M. Brahlek, P. Lostak, C. Uher, R. Merlin, X. Wang, S.-W. Cheong, S. Oh, G. Blumberg
We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi2Se3 single crystal and thin films. Besides the 4 Raman active bulk phonons, we observed 4 additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. By symmetry analysis and comparison with theoretical calculation, we assigned these additional modes to out-of-plane surface phonons, where the frequency is slightly modified from the bulk phonon due to out-of-plane lattice distortion near the crystal surface. In particular, 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 restriction of crystalline symmetry from D3d in the bulk to C3v at the surface of Bi2Se3. The 158 cm-1 surface phonon mode show a Fano line shape, suggesting interaction with an electronic continuum at the crystal surface. In addition, we observed two weak features at 67 and 126 cm-1 likely corresponding to in-plane surface vibrational modes.