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.


For website front pageAuthors: 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.


fig1aAuthors: 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.


fig1aAuthors: 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.


FigS2Authors: Daixiang Mou, Aashish Sapkota, H.-H. Kung, Viktor Krapivin, Yun Wu, A. Kreyssig, Xingjiang Zhou, A. I. Goldman, G. Blumberg, Rebecca Flint, Adam Kaminski

Abstract: We use Angle Resolved Photoemission Spectroscopy (ARPES), Raman spectroscopy, Low Energy Electron Diffraction (LEED) and x-ray scattering to reveal an unusual electronically mediated charge density wave (CDW) in K0.9Mo6O17. Not only does K0.9Mo6O17 lack signatures of electron-phonon coupling, but it also hosts an extraordinary surface CDW, with TS_CDW=220 K nearly twice that of the bulk CDW, TB_CDW=115 K. While the bulk CDW has a BCS-like gap of 12 meV, the surface gap is ten times larger and well in the strong coupling regime. Strong coupling behavior combined with the absence of signatures of strong electron-phonon coupling indicates that the CDW is likely mediated by electronic interactions enhanced by low dimensionality.


VernerPaperImageAuthors: V. K. Thorsmølle, M. Khodas, Z. P. Yin, Chenglin Zhang, S. V. Carr, Pengcheng Dai, and G. Blumberg

Abstract: The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin-density-wave transition both being in close proximity to the superconducting phase. A key question is the nature of high-Tc superconductivity and its relation to orbital ordering and magnetism. Here we study the NaFe1-xCoxAs superconductor using polarization-resolved Raman spectroscopy. The Raman susceptibility displays critical enhancement of nonsymmetric charge fluctuations across the entire phase diagram, which are precursors to a d-wave Pomeranchuk instability at temperature θ(x). The charge fluctuations are interpreted in terms of quadrupole interorbital excitations in which the electron and hole Fermi surfaces breathe in-phase. Below Tc, the critical fluctuations acquire coherence and undergo a metamorphosis into a coherent in-gap mode of extraordinary strength.


150212154625_1_540x360 Authors: H.-H. Kung, R. E. Baumbach, E. D. Bauer, V. K. Thorsmølle, W.-L. Zhang, K. Haule, J. A. Mydosh, G. Blumberg

Abstract: A second-order phase transition in a physical system is associated with the emergence of an “order parameter” and a spontaneous symmetry breaking. The heavy fermion superconductor URu2Si2 has a “hidden order” (HO) phase below the temperature of 17.5 kelvin; the symmetry of the associated order parameter has remained ambiguous. Here we use polarization-resolved Raman spectroscopy to specify the symmetry of the low-energy excitations above and below the HO transition. We determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state.

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Nature Article Picture Authors: B. S. Dennis, M. I. Haftel, D. A. Czaplewski, D. Lopez, G. Blumberg and V. A. Aksyuk

Abstract: Highly confined optical energy in plasmonic devices is advancing miniaturization in photonics. However, for mode sizes approaching ≈10 nm, the energy increasingly shifts into the metal, raising losses and hindering active phase modulation. Here, we propose a nanoelectromechanical phase-modulation principle exploiting the extraordinarily strong dependence of the phase velocity of metal–insulator–metal gap plasmons on dynamically variable gap size. We experimentally demonstrate a 23-μm-long non-resonant modulator having a 1.5π rad range, with 1.7 dB excess loss at 780 nm. Analysis shows that by simultaneously decreasing the gap, length and width, an ultracompact-footprint π rad phase modulator can be realized. This is achieved without incurring the extra loss expected for plasmons confined in a decreasing gap, because the increasing phase-modulation strength from a narrowing gap offsets rising propagation losses. Such small, high-density electrically controllable components may find applications in optical switch fabrics and reconfigurable plasmonic optics.