Takeshi Egami Professor and Chair 223 LRSM 215.898.5138 egami@seas.upenn.edu

Ph.D., University of Pennsylvania, 1971
B.Eng., University of Tokyo, 1968

Research Interests
Structure and properties of transition metal oxides, superconductivity, neutron scattering, synchrotron radiation, disordered and amorphous state.

Current Research Projects
Research is focused on the structure and properties of electronic ceramics, in particular on the effect of local structure. He and his students found that the local atomic structure of a large number of functional oxides is significantly deviated from the average crystal structure in important ways and these deviations play critical roles in determining their properties. Their findings are advancing the understanding of the mechanism of high temperature superconductivity, the colossal magnetoresistance (CMR) phenomena, oxygen transport in catalytic support oxides and relaxor ferroelectricity.

His group uses neutron scattering and synchrotron x-ray scattering as main tools. Experiments are carried out mainly at the Intense Pulsed Neutron Source of Argonne National Laboratory, the National Synchrotron Light Source of Brookhaven National Laboratory and the High-Flux Isotope Reactor of Oak Ridge National Laboratory. Both elastic and inelastic measurements are carried out, and the data are often Fourier-transformed into real space to obtain the atomic pair-density correlation function (PDF). The PDF method has been used for glasses and liquids but rarely for crystals. The experimentally obtained PDF is modeled in real space to determine the local structure. This novel approach is practiced only by him and his former students at this moment. His group also carries out surface x-ray scattering measurement in the energy-dispersive mode to study the oxide surfaces in various environments.

The local structural deviations are usually due to lattice defects, but they are not the main focus of the group. They are zooming in to the local deviations caused by strong electron-lattice coupling. In CMR manganites it causes polaronic localization of charges, as observed by their PDF study. In superconducting cuprates they observed a very anomalous behavior of phonons at low temperatures which must be caused by the strong electron-phonon coupling as they have shown by several theoretical studies. It is suspected that such a strong electron-phonon coupling may even be playing a major role in the mechanism of high temperature superconductivity.

Selected Publications

• "Anomalous Dispersion of LO Phonons in La_1.85Sr_0.15CuO₄ at Low Temperatures", R. J. McQueeney, Y. Petrov, T. Egami, M. Yethiraj, G. Shirane and Y. Endoh, Phys. Rev. Lett., 82, 628 (1999).
• "Electron-Lattice Coupling in Manganites and Cuprates", T. Egami and Despina Louca, J. Superconductivity, 12, 23 (1999).
• "Local Lattice Distortions in La_1-xSr_xMnO₃ Studied by Pulsed Neutron Scattering", Despina Louca and T. Egami, Phys. Rev. B, 59, 6193 (1999).
• "Atomic Structure of PbZrO₃ Determined by Pulsed Neutron Diffraction", S. Teslic and T. Egami, Acta Cryst. B 54, 750 (1998).
• "Exact-Diagonalization Study of Electron-Lattice Coupling in the Effective Two-Band t-J Model", Y. Petrov and T. Egami, Phys. Rev. B, 58, 9485 (1998).
• "Electron-Lattice Interaction in Cuprates: Effect of Electron Correlation", S. Ishihara, T. Egami, and M. Tachiki, Phys. Rev. B, 55, 3163 (1997).
• "Universal Criterion for Metallic Glass Formation", T. Egami, Mater. Sci. Eng., A226-228, 261 (1997).
• "Lattice Effects in High-T_C Superconductors", T. Egami and S. J. L. Billinge, in Physical Properties of High Temperature Superconductors V, ed. D. M. Ginsberg (World Scientific, 1996) p. 265.
• T. Egami, S. Ishihara and M. Tachiki, "Lattice Effect of Strong Electron Correlation: Implication for Ferroelectricity and Superconductivity" Science, 261, 1307 (1993).