|A New Gap-Opening Mechanism in a Triple-Band Metal|
A "wire" of indium only one or a few atoms wide grown on a silicon surface comprises an ideal test laboratory for studying one-dimensional (1D) metals. A new example comes from a collaboration between researchers from Yonsei University in Korea, the ALS, and the University of Oregon, who have discovered that the phase transition from metal to insulator that occurs at low temperature in indium wires on the silicon (111) surface involves not only the expected shift in the electronic structure (band-gap opening) but also a band restructuring that gives rise to an energy gap in a second band.
Low-dimensional metals have attracted much attention because of their unique electronic properties, which often lead to exotic physics, including unconventional superconductivity, charge and spin density waves, and violations of the usual rules for interactions of electrons with either other electrons or lattice vibrations and other "excitations" (non-Fermi liquid behavior). Research on 1D metals has been enriched by the synthesis of quite novel materials, of which carbon nanotubes and metallic atomic wires on surfaces are recent examples.
To make metallic atomic wires, researchers deposit metal atoms on insulating substrates, where the atoms form self-organized 1D atomic chain structures that are often macroscopic in length but truly nanoscopic in width, spanning just one or a few atoms. Successful recent syntheses are the formation of indium wires on a flat silicon (111) surface [the Si(111)4×1-In surface] and gold wires on a series of regularly stepped (vicinal) silicon surfaces [Au/Si(557), Au/Si(5512), and Au/Si(553)].
These atomic wires have well-defined 1D metallic electronic structures, commonly with one or more bands that are partially filled by electrons at energies up to the Fermi energy EF and with nearly ideal band dispersions (continuous dependence of electron energy on momentum k). Moreover, metal-insulator transitions occur at transition temperatures ranging from about 100 to 300 K in which the continuous bands are broken by an energy gap around EF. Since they are accompanied by lattice-period-doubling lattice distortions, the transitions are assumed to be due to the Peierls instability that is inherent in a 1D metal, especially one with a half-filled band.
The Korean-American collaboration made its measurements of indium wires on silicon with the soft x-ray angle-resolved photoemission endstation (Electronic Structure Factory) at Beamline 7.0.1, which provides unprecedented flexibility in collecting data in a large volume of the Brillouin zone (unit cell in momentum space) over a wide temperature range and in preparing samples in situ, two features that were crucial in the present study. The indium wires have three metallic bands, one of which is half filled while the others are less than half filled.
The team found that in the insulating state below 125 K, the expected band-gap opening of the half-filled band was accompanied by a restructuring in which the band with the smallest filling disappeared, owing to an interband interaction in which the electrons of the disappearing band transferred into the third band, which became half filled. Thus, the ground-state band structure consisted of two bands with energy gaps at the same momentum kF, a condition giving an energy advantage to the period-doubling lattice distortion associated with a charge density wave (CDW) and a Fermi surface "nesting" vector qCDW = 2kF.
The researchers believe that this first-ever observation of the band restructuring due to a strong interband interaction introduces a new gap-opening mechanism in a multiband metal. The physical origin of the direct interband charge transfer needs to be clarified but may be related to the complex interactions of the lattice with defects and with the complex multiple 1D bands with varying band fillings.
Research conducted by J.R. Ahn, J.H. Byun, and H.W. Yeom (Yonsei University, Korea); H. Koh (Yonsei University, Korea, and ALS); E. Rotenberg (ALS); and S.D. Kevan (University of Oregon).
Research funding: Ministry of Science and Technology of Korea through the Creative Research Initiative Program. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publication about this research: J.R. Ahn, J.H. Byun, H. Koh, E. Rotenberg, S.D. Kevan, and H.W. Yeom, "Mechanism of gap opening in a triple-band Peierls system: In atomic wires on Si," Phys. Rev. Lett. 93, 106401 (2004).