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.


From left to right: Weilu, Shangfei, Girsh, Viktor, William, Sean.

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).

An undergraduate student member of this lab, Viktor Krapivin, has been awarded the Goldwater Scholarship!

Congratulations to Viktor for winning this prestigious national award!


You can read more about Viktor and this award 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.



Authors: S.-F. Wu, P. Richard, H. Ding, H.-H. Wen, Guotai Tan, Meng Wang, Chenglin Zhang, Pengcheng Dai, G. Blumberg


Using polarization-resolved electronic Raman scattering we study underdoped, optimally doped, and overdoped Ba1−xKxFe2As2 samples in the normal and superconducting states. We show that low-energy nematic fluctuations are universal for all studied doping ranges. In the superconducting state, we observe two distinct superconducting pair-breaking peaks corresponding to one large and one small superconducting gap. In addition, we detect a
collective mode below the superconducting transition in the B2g channel and determine the evolution of its binding energy with doping. Possible scenarios are proposed to explain the origin of the in-gap collective mode. In the superconducting state of the underdoped regime, we detect a reentrance transition below which the spectral background changes and the collective mode vanishes.