Resonant Raman scattering in mercurate single crystals

We report resonant electronic Raman scattering in optimally doped single layer HgBa$_2$CuO$_{4+\delta}$ (Hg-1201) and trilayer HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg-1223) single crystals. Analysis of the $B_{1g}$ and B$_{2g}$ channels in the superconducting state of Hg-1201 advocates for a gap having d-wave symmetry. In addition a resonant study $B_{1g}$ pair-breaking peak and the $A_{1g}$ peak suggests that the $A_{1g}$ peak is not directly related to the d-wave superconducting gap amplitude. Comparison with trilayer Hg-1223 demonstrates the universal behavior of this two energy scales in optimally doped cuprates.Comment: 4 pages, 4 figures, M2S-Rio (invited paper). to appear in Physica

Evidence for two distinct energy scales in the Raman spectra of YBa2(CuNi)O6.95

We report low energy electronic Raman scattering from Ni-substituted YBa2Cu3O6.95 single crystals with Tc ranging from 92.5 K to 78 K. The fully symmetrical A1g channel and the B1g channel which is sensitive to the dx2-y2 gap maximum have been explored. The energy of the B1g pair-breaking peak remains constant under Ni doping while the energy of the A1g peak scales with Tc (EA1g/Tc=5). Our data show that the A1g peak tracks the magnetic resonance peak observed in inelastic neutron scattering yielding a key explanation to the long-standing problem of the origin the A1g peak.Comment: 10 pages, 4 figures and 1 tabl

Crystal growth and in-plane optical properties of Tl2_2Ba2_2Can−1_{n-1}Cun_nOx_x (n=1,2,3) superconductors

Single crystals of thallium-based cuprates with the general formula Tl$_{2}$Ba$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{x}$(n=1,2,3) have been grown by the flux method. The superconducting transition temperatures determined by the ac magnetic susceptibility are 92 K, 109 K, and 119 K for n=1,2,3 respectively. X-ray diffraction measurements and EDX compositional analysis were described. We measured in-plane optical reflectance from room temperature down to 10 K, placing emphasis on Tl-2223. The reflectance roughly has a linear-frequency dependence above superconducting transition temperature, but displays a pronounced knee structure together with a dip-like feature at higher frequency below T$_c$. Correspondingly, the ratio of the reflectances below and above T$_{c}$ displays a maximum and a minimum near those feature frequencies. In particular, those features in Tl2223 appear at higher energy scale than Tl2212, and Tl2201. The optical data are analyzed in terms of spectral function. We discussed the physical consequences of the data in terms of both clean and dirty limit.Comment: 8 pages, 13 figures, to be published in Phys. Rev.

η\eta collective mode as A1g_{1g} Raman resonance in cuprate superconductors

We discuss the possible existence a spin singlet excitation with charge $\pm2$ ($\eta$-mode) originating the $A_{1g}$ Raman resonance in cuprate superconductors. This $\eta$-mode relates the $d$-wave superconducting singlet pairing channel to a $d$-wave charge channel. We show that the $\eta$ boson forms a particle-particle bound state below the $2\Delta$ threshold of the particle-hole continuum where $\Delta$ is the maximum $d$-wave gap. Within a generalized random phase approximation and Bethe-Salpether approximation study, we find that this mode has energies similar to the resonance observed by Inelastic Neutron Scattering (INS) below the superconducting (SC) coherent peak at $2\Delta$ in various SC cuprates compounds. We show that it is a very good candidate for the resonance observed in Raman scattering below the $2\Delta$ peak in the $A_{1g}$ symmetry. Since the $\eta$-mode sits in the $S=0$ channel, it may be observable via Raman, X -ray or Electron Energy Loss Spectroscopy probes

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.

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

Coupling between quasiparticles and a bosonic mode in the normal state of HgBa2_2CuO4+δ_{4+\delta}

We report a doping dependent study of the quasiparticles dynamics in HgBa$_2$CuO$_{4+\delta}$ via Electronic Raman Scattering. A well-defined energy scale is found in the normal state dynamics of the quasiparticles over a broad doping range. It is interpreted as evidence for coupling between the quasiparticles and a collective bosonic mode whose energy scale depend only weakly with doping. We contrast this behavior with that of the superconducting gap whose amplitude near the node continuously decreases towards the underdoped regime. We discuss the implications of our findings on the nature of the collective mode and argue that electron-phonon coupling is the most natural explanation.Comment: 5 pages, 4 figure