Magnetic transitions in CaMn7O12 : a Raman observation of spin-phonon couplings

The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that exhibits a giant magnetically-induced ferroelectric polarization which makes it very interesting for magnetoelectric applications. Here, we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1 associated to Mn(B)O6 (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the (a,b) plane. At low energies, two large low energy excitations are observed at 25 and 47 cm-1. The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electro-magneto-active modes

Temperature dependence and resonance effects in Raman scattering of phonons in NdFeAsOx_{x}F1−x_{1-x} single crystals

We report plane-polarized Raman scattering spectra of iron oxypnictide superconductor NdFeAsO$_{1-x}$F$_x$ single crystals with varying fluorine $x$ content. The spectra exhibit sharp and symmetrical phonon lines with a weak dependence on fluorine doping $x$. The temperature dependence does not show any phonon anomaly at the superconducting transition. The Fe related phonon intensity shows a strong resonant enhancement below 2 eV. We associate the resonant enhancement to the presence of an interband transition around 2 eV observed in optical conductivity. Our results point to a rather weak coupling between Raman-active phonons and electronic excitations in iron oxypnictides superconductors.Comment: 4 pages, 3 figures, to appear in Phys. Rev.

Electromagnon and phonon excitations in multiferroic TbMnO3

We have performed Raman measurements on TbMnO3 single crystal under magnetic field along the three crystallographic directions. The flip of the spin spiral plane creates an electromagnon excitation. In addition to the electromagnons induced by the Heisenberg coupling, we have detected the electromagnon created by the Dzyaloshinskii-Moriya interaction along the c axis. We have identified all the vibrational modes of TbMnO3. Their temperature dependences show that only one phonon observed along the polarization axis is sensitive to the ferroelectric transition. This mode is tied to the Tb3+ ion displacements that contribute to the ferroelectric polarization

Impact of the Spin Density Wave Order on the Superconducting Gap of Ba(Fe1−x_{1-x}Cox_x)2_2As2_2

We report a doping dependent electronic Raman scattering measurements on iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals. A strongly anisotropic gap is found at optimal doping for x=0.065 with $\Delta_{max}\sim 5\Delta_{min}$. Upon entering the coexistence region between superconducting (SC) and spin-density-wave (SDW) orders, the effective pairing energy scale is strongly reduced. Our results are interpreted in terms of a competition between SC and SDW orders for electronic state at the Fermi level. Our findings advocate for a strong connection between the SC and SDW gaps anisotropies which are both linked to interband interactions.Comment: 4 pages, 3 figure

Doping dependence of the lattice dynamics in Ba(Fe1−x_{1-x}Cox_x)2_2As2_2 studied by Raman spectroscopy

We report Raman scattering spectra of iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with varying cobalt $x$ content. Upon cooling through the tetragonal-to-orthorhombic transition, we observe a large splitting of the E$_g$ in-plane phonon modes involving Fe and As displacements. The splitting of the in-plane phonons at the transition is strongly reduced upon doping and disappears for $x=0.06$ qualitatively following the trend displayed by the Fe magnetic moment. The origin of the splitting is discussed in terms of magnetic frustration inherent to iron-pnictide systems and we argue that such enhanced splitting may be linked to strong spin-phonon coupling.Comment: 6 pages, 6 figure

Amplitude `Higgs' mode in 2H-NbSe2 Superconductor

We report experimental evidences for the observation of the superconducting amplitude mode, so-called `Higgs' mode in the charge density wave superconductor 2H-NbSe2 using Raman scattering. By comparing 2H-NbSe2 and its iso-structural partner 2H-NbS2 which shows superconductivity but lacks the charge density wave order, we demonstrate that the superconducting mode in 2H-NbSe2 owes its spectral weight to the presence of the coexisting charge density wave order. In addition, temperature dependent measurements in 2H-NbSe2 show a full spectral weight transfer from the charge density wave mode to the superconducting mode upon entering the superconducting phase. Both observations are fully consistent with a superconducting amplitude mode or Higgs mode.Comment: Accepted for publication in Phys. Rev. B Rapid Com. 5 pages with 3 figure