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Science 14 December 2007:
Vol. 318. no. 5857, pp. 1750 - 1753
DOI: 10.1126/science.1150124
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Reports

Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging

M. M. Qazilbash,1* M. Brehm,2 Byung-Gyu Chae,3 P.-C. Ho,1 G. O. Andreev,1 Bong-Jun Kim,3 Sun Jin Yun,3 A. V. Balatsky,4 M. B. Maple,1 F. Keilmann,2 Hyun-Tak Kim,3 D. N. Basov1

Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

1 Physics Department, University of California–San Diego, La Jolla, CA 92093, USA.
2 Abt. Molekulare Strukturbiologie, Max-Planck-Institut für Biochemie and Center for NanoScience, 82152 Martinsried, München, Germany.
3 IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Korea.
4 Theoretical Division and Center for Integrated Nanotechnologies, MS B262, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

* To whom correspondence should be addressed. E-mail: mumtaz{at}physics.ucsd.edu

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