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: 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.
Authors: 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.
Authors: 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.
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.
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.