Low-Energy Charge-Density Excitations in ${\mathrm{MgB}}_{2}$: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

Low-Energy Charge-Density Excitations in ${MgB}_{2}$ : Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

Y. Q. Cai, P. C. Chow, O. D. Restrepo, Y. Takano, K. Togano, H. Kito, H. Ishii, C. C. Chen, K. S. Liang, C. T. Chen, S. Tsuda, S. Shin, C. C. Kao, W. Ku, and A. G. Eguiluz

Phys. Rev. Lett. 97, 176402 – Published 24 October 2006

Abstract

A sharp feature in the charge-density excitation spectra of single-crystal ${MgB}_{2}$ , displaying a remarkable cosinelike, periodic energy dispersion with momentum transfer ( $q$ ) along the $c^{*}$ axis, has been observed for the first time by high-resolution nonresonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small- $q$ collective mode residing in the single-particle excitation gap of the $B$ $π$ bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of ${MgB}_{2}$ .

Received 7 May 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.176402

Authors & Affiliations

Y. Q. Cai^1,*, P. C. Chow^1,†, O. D. Restrepo^2,3, Y. Takano⁴, K. Togano⁴, H. Kito⁵, H. Ishii¹, C. C. Chen¹, K. S. Liang¹, C. T. Chen¹, S. Tsuda⁶, S. Shin^6,7, C. C. Kao⁸, W. Ku⁹, and A. G. Eguiluz^2,3

¹National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
²Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA
³Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6030, USA
⁴National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
⁵National Institute of Advanced Industrial Science and Technology, 1-1-4 Umezono, Tsukuba 305-8586, Japan
⁶Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
⁷Institute of Physical and Chemical Research (RIKEN), Sayo-gun, Hyogo 679-5198, Japan
⁸National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973-5000, USA
⁹Department of Physics, Brookhaven National Laboratory, Upton, New York 11973-5000, USA

^*Electronic address: cai@nsrrc.org.tw
^†Present address: HP-CAT, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.

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Vol. 97, Iss. 17 — 27 October 2006

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Physical Review Letters

Low-Energy Charge-Density Excitations in MgB2: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

Y. Q. Cai, P. C. Chow, O. D. Restrepo, Y. Takano, K. Togano, H. Kito, H. Ishii, C. C. Chen, K. S. Liang, C. T. Chen, S. Tsuda, S. Shin, C. C. Kao, W. Ku, and A. G. Eguiluz

Phys. Rev. Lett. 97, 176402 – Published 24 October 2006

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Low-Energy Charge-Density Excitations in ${MgB}_{2}$ : Striking Interplay between Single-Particle and Collective Behavior for Large Momenta