**Date/Time**

Date(s) - Feb 23 2016

*11:15 AM - 12:15 PM*

**Location**

Sycamore Room, Building 40, room 195

**Category(ies)**

**Investigations of high-temperature superconductors via ****non-resonant inelastic x-ray scattering**

**Alfred Q. R. Baron**

Materials Dynamics Laboratory, RIKEN SPring-8 Center, Hyogo, Japan

*baron@spring8.or.jp*

High-resolution inelastic x-ray scattering (IXS) measures atomic motions at THz frequencies over angstrom-scale correlation lengths. While the required instrumentation is difficult to set up, dedicated facilities make the method available to the broader scientific community, and potentially of interest for anyone studying lattice dynamics, electron-phonon coupling, superconductivity, ferro-electricity/multi-ferroicity, phase transformations, magneto-elastic coupling, elasticity, or the dynamics of liquids or glasses. In particular, IXS allows access to tiny (~0.01 mm) samples so phonon measurements are possible on newly discovered materials, or in extreme conditions (e.g. P > 100 GPa, T > 2000K).

The talk will provide some introduction to IXS (see also 1) and then will then focus on high-temperature superconductors. For an iron-arsenide material, SrFe_{2}As_{2}, IXS was used to investigate *magneto-elastic coupling*, permitting first observation phonon splitting in these materials at finite momentum transfers, and allowing construction, finally, of a reasonable model for phonon dispersion incorporating the effects of magnetic fluctuations. Meanwhile, using the full capability of a new beamline, *first clear measurements of the bond-stretching mode in YBa _{2}Cu_{3}O_{7-}*

*. This phonon mode shows a*

_{δ}were obtained*line-width increase below T*increasing (to ~20 meV) as T is reduced. The momentum transfer where the line-width increases coincides with that where charge density wave (CDW) order appears in under-doped materials. This suggests a common origin that evolves, with doping, from static competition with superconductivity into dynamical synergy.

_{c}

Time permitting, I will mention recent Compton scattering results on *YBa _{2}Cu_{3}O_{6+y} *, y~ 0.3, 0.6, 0.95. A smooth evolution for the electron momentum number density n(

**k**) is seen with doping, into something very reasonable for optimal doping. However, in the highly under-doped material, y~0.3, p~0.035, we see electron density peaks at the nodal locations, as was completely unexpected. However, they are reminiscent of the electron Fermi-surface pockets recently observed in extremely high field quantum oscillation experiments. This suggests the pockets may evolve from a number density peak, and highlights a disconnect between number density and Fermi-surface structure in correlated materials.

[1] A.Q.R. Baron, in *Synchrotron Light Sources and Free Electron Lasers,* edited by E. Jaeschke, *et al.* (Springer Publishing, 2015) http://arxiv.org/abs/1504.01098

[2] N. Murai, *et al.*, Phys. Rev. B **93**, 20301 (2016). http://arxiv.org/abs/1510.05487

[3] A. Q.R. Baron, D. Ishikawa, H. Uchiyama, T. Fukuda, T. Masui, R. Heid, K.-P. Bohnen, S. Miyasaka and S. Tajima, In Preparation.

[4] T.-H. Chuang, R. Heid, M. Itou, K.-P. Bohnen, K. Lee, K. Kamiya, Y. Sakurai, S. Miyasaka, T. Tohyama, S. Tajima, and A. Q.R. Baron, to be resubmitted.