We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor Bi2Se3. Far-infrared and mid-infrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal c axis of this layered material, and supplemented with UV-visible ellipsometry to obtain the optical conductivity σ1(ω). With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as the increase in the absorption edge due to direct electronic transitions. Magnetic fields H c dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum freecarrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field. arXiv:0912.2769v2 [cond-mat.str-el]

We report an infrared optical study of the pnictide high-temperature superconductor BaFe(1.84)Co(0.16)As(2) and its parent compound BaFe(2)As(2). We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund's coupling J is the primary mechanism that gives rise to correlations.

We present a comprehensive infrared spectroscopic study of lattice dynamics in the pnictide parent compound BaFe2As2. In the tetragonal structural phase, we observe the two degenerate symmetry-allowed in-plane infrared active phonon modes. Following the structural transition from the tetragonal to orthorhombic phase, we observe splitting into four non-degenerate phonon modes and a significant phonon strength enhancement. These detailed data allow us to provide a physical explanation for the anomalous phonon strength enhancement as the result of anisotropic conductivity due to Hund's coupling.The pnictide high temperature superconductors display a rich phase diagram including temperature and doping dependent structural and magnetic phase transitions in proximity to the superconducting phase [1][2][3]. Such complexity has myriad observational consequences. Phonon behavior provides a unique window into the phase diagram of the pnictides and as we will show, reveals a fascinating interplay between structure, charge and magnetism. Infrared (IR) spectroscopic studies consistently showed an anomalous phonon strength enhancement at low temperatures in the 122 and 1111 families [4][5][6][7], yet these studies did not observe all of the phonon modes predicted by group theory. In order to understand the origin of the phonon strength enhancement, it is necessary to experimentally distinguish each phonon mode in the orthorhombic phase. In this work, by observing all of the predicted phonon modes at low temperature, we are able to comment on the origins of the phonon strength enhancement.The samples in this study were square platelet single crystals of BaFe 2 As 2 (Ba122) approximately 2 x 2 x 0.1 mm in size [8]. In Ba122, two phase transitions have been observed as a function of temperature; a structural phase transition from high temperature tetragonal (HTT, TrCr 2 Si 2 type) to low temperature orthorhombic (LTO) at T ST R ≈140K, and a magnetic phase transition from paramagnetic (PM) order to spin density wave (SDW) order below T SDW ≈ T ST R ≈ 140 K [8,12,13]. Fabrication and characterization are described elsewhere [8].We measured near-normal incidence reflectance R(ω) of the ab face, over a frequency range of ω ≈ 20 to 12,000 cm −1 . The reflectance measurements were performed for a variety of temperatures ranging from T = 6.5 K to 295 K. Additionally, we performed variable-angle spectroscopic ellipsometry from ω ≈ 5,500 to 45,000 cm −1 . In order to extract the optical constants, we performed a Kramers-Kronig constrained variational analysis using refFIT software, as detailed in the references [9,10].The optical conductivity σ 1 (ω) is plotted in Figure 1. At low energy, a conspicuous Drude response is present for T > T SDW , becoming mostly gapped at low temperature. For temperatures T < T SDW , a significant loss of conductivity below ≈ 700 cm −1 results in the development of a large optical transition centered at ≈ 1,000 cm −1 where σ 1 is greater than the PM state value extending to 2000 cm −1 . Additionally, there ...

We report a novel aspect of the competition and coexistence between magnetism and superconductivity in the high-T(c) cuprate La(2-x)Sr(x)CuO4 (La214). With a modest magnetic field applied H parallel c axis, we monitored the infrared signature of pair tunneling between the CuO2 planes and discovered the complete suppression of interlayer coupling in a series of underdoped La214 single crystals. We find that the in-plane superconducting properties remain intact, in spite of enhanced magnetism in the planes.