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.
Our manuscript 'Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy' is published online in Phys. Rev. B 95, 245406 (2017) (link; PDF), see more details here.
Our manuscript 'Superconductivity and electronic fluctuations in Ba1−xKxFe2As2 studied by Raman scattering' is published online in Phys. Rev. B 95, 085125 (2017) (link; PDF), see more details here.
Our manuscript 'Analogy Between the “Hidden Order” and the Orbital Antiferromagnetism in URu2−xFexSi2' is published online in Phys. Rev. Lett. (link), see more details 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.
Authors: H.-H. Kung, S. Ran, N. Kanchanavatee, V. Krapivin, A. Lee, J.A. Mydosh, K. Haule, M.B. Maple, and G. Blumberg
In URu2Si2 two types of staggered phases involving long range ordering of the uranium-5f electrons compete at low temperature when a critical parameter x is tuned, where x can be chemical substituent concentration, pressure or magnetic field. When cooled at below the critical value x, the non-magnetic `hidden order' (HO) phase with broken local chiral symmetry emerges, whereas above xc, unconventional antiferromagnetic (AF) phase with broken local time-reversal symmetry appears. The two phases show strikingly similar electronic properties. `Janus faces' nature of the HO and AF phases has been theorized before, but the experimental signatures of the interplay between them are still lacking. Here, we use polarized Raman scattering to study the dynamical fluctuations between the two competing ground states as a function of x. Albeit the distinct discrete symmetries are broken above and below xc, we detect a resonance continuously evolving with parameter x, providing evidence for a unified order parameter across the URu2Si2 phase diagram.
Authors: S.-F. Wu, W.-L. Zhang, D. Hu, H.-H. Kung, A. Lee, H.-C. Mao, P.-C. Dai, H. Ding, P. Richard and G. Blumberg
We use electronic Raman scattering to study the low-energy excitations in BaFe2(As0.5P0.5)2 (Tc≈16 K) samples. In addition to a superconducting pair breaking peak (2Δ=6.7 meV) in the A1g channel with a linear tail towards zero energy, suggesting a nodal gap structure, we detect spectral features associated to Pomeranchuk oscillations in the A1g, B1g and B2g channels. We argue that the small Fermi energy of the system is an essential condition for these Pomeranchuk oscillations to be underdamped. The Pomeranchuk oscillations have the same frequencies in the B1g and B2g channels, which we explain by the mixing of these symmetries resulting from the removal of the σv and σd symmetry planes due to a large As/P disorder. Interestingly, we show that the temperature at which the peaks corresponding to the Pomeranchuk oscillations get underdamped is consistent with the non-Fermi liquid to Fermi liquid crossover determined by transport, suggesting that the Pomeranchuk instability plays an important role in the low-energy physics of the Fe-based superconductors.
Authors: W.-L. Zhang, Athena S. Sefat, H. Ding, P. Richard, and G. Blumberg
We use polarized Raman scattering to study the structural phase transition in EuFe2As2, the parent compound of the 122-ferropnictide superconductors. The in-plane lattice anisotropy is characterized by measurements of the side surface with different strains induced by different preparation methods. We show that while a fine surface polishing leaves the samples free of residual internal strain, in which case the onset of the C4 symmetry breaking is observed at the nominal structural phase transition temperature TS, cutting the side surface induce a permanent fourfold rotational symmetry breaking spanning tens of degrees above TS.
Authors: W.-L. Zhang, Z. P. Yin, A. Ignatov, Z. Bukowski, Janusz Karpinski, Athena S. Sefat, H. Ding, P. Richard, and G. Blumberg
We present a polarization-resolved and temperature-dependent Raman scattering study of AFe2As2(A=Ca,Eu). In the spin-density-wave phase, spectral weight redistribution is observed in the fully symmetric and nonsymmetric scattering channels at different energies. An anisotropic Raman response is observed in the fully symmetric channel in spontaneously detwinned CaFe2As2 samples. We calculate the orbital-resolved electronic structures using a combination of density functional theory and dynamical mean field theory. We identify the electronic transitions corresponding to these two spectral features and find that the anisotropic Raman response originates from the lifted degeneracy of the dxz/yz orbitals in the broken-symmetry phase.