Atomic Dynamics via meV-Resolution X-Ray Scattering: New Results on High-Temperature Superconductors

Condensed Matter Seminars

Speaker: Alfred Baron, Materials Dynamics Laboratory, RIKEN SPring-8 Center
Date & Time: February 25, 2016, 2:00 - 3:00
Location: AMPEL 311
Local Contact: George Sawatzky
Intended Audience: Graduate

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 speaker has been responsible for the design, construction, comm­issioning and operation of two IXS beamlines [1] at SPring-8 – introducing high-resolution IXS within Japan. A review of IXS may be found in [2].

The talk will give an overview of IXS capabilities, highlighting notable points, and briefly mentioning the range of science to which IXS has been applied. The talk will then focus on high-temperature superconductors. For an iron-arsenide material, SrFe2As2, 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 YBa2Cu3O7-δ were obtained. This phonon mode shows a line-width increase below Tc 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.

[1] A.Q.R. Baron, et al., J. Phys. Chem. Solids 61, 461 (2000) and A.Q.R. Baron, SPring-8 Inf. Newsl. 15, 14 (2010), http://user.spring8.or.jp/sp8info/?p=3138

[2] 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

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

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