ScienceWeek

MATERIALS SCIENCE: ON INTERFACE ELECTRONIC RECONSTRUCTION

The following points are made by S. Okamoto and A.J. Millis (Nature 2004 428:630):

1) Surface science is an important and well-established branch of materials science involving the study of changes in material properties near a surface or interface. A fundamental issue has been atomic reconstruction: how the surface lattice symmetry differs from the bulk.

2) "Correlated-electron compounds" are materials in which strong electron-electron and electron-lattice interactions produce new electronic phases, including interaction-induced (Mott) insulators, many forms of spin, charge and orbital ordering, and (presumably) high-transition-temperature superconductivity(1,2}.

3) To assess the effects of a surface or interface on correlated-electron behavior it is necessary to understand the changes in the parameters governing bulk correlated-electron behavior. The three key factors are interaction strengths, bandwidths and electron densities(1,3), all of which may change near a surface or interface. In most cases, surface- or interface-induced changes in all three factors will contribute, but in developing a general understanding it is desirable first to study the different effects in isolation.

4) The authors focus on the effect of electron-density variation caused by the spreading of charge across an interface. A different charge distribution effect -- the compensation of a polar surface by electronic charge rearrangement -- has been proposed to change the behavior of C60 films(4). (Indeed, Hesper et al(4) coined the term "electronic reconstruction" in reference to this specific effect; the authors suggest that this useful phrase be applied more generally to denote electronic phase behavior that is fundamentally different at a surface from in bulk.)

5) Proximity to a surface or interface can also change the electron interaction parameters(5), the electron bandwidth, and level degeneracy. Experimental studies of surfaces and heterostructures have been interpreted along these lines.

6) In summary: The authors propose that the fundamental issue for the new field of correlated-electron surface/interface science is "electronic reconstruction": how does the surface/interface electronic phase differ from that in the bulk? As a step towards a general understanding of such phenomena, the authors present a theoretical study of an interface between a strongly correlated Mott insulator and a band insulator. The authors find dramatic interface-induced electronic reconstructions: in wide parameter ranges, the near-interface region is metallic and ferromagnetic, whereas the bulk phase on either side is insulating and antiferromagnetic. The authors suggest that extending the analysis to a wider range of interfaces and surfaces is a fundamental scientific challenge and may lead to new applications for correlated electron materials.

References (abridged):

1. Imada, M., Fujimori, A. & Tokura, Y. Metal-insulator transitions. Rev. Mod. Phys. 70, 1039-1263 (1998)

2. Tokura, Y. & Nagaosa, N. Orbital physics in transition-metal oxides. Science 288, 462-468 (2000)

3. Altieri, S., Tjeng, L. H. & Sawatzky, G. A. Ultrathin oxide films on metals: new physics and new chemistry? Thin Solid Films 400, 9-15 (2001)

4. Hesper, R., Tjeng, L. H., Heeres, A. & Sawatzky, G. A. Photoemission evidence of electronic stabilization of polar surface in K3C60. Phys. Rev. B 62, 16046-16055 (2000)

5. Duffy, D. M. & Stoneham, A. M. Conductivity and 'negative U' for ionic grain boundaries. J. Phys. C 16, 4087-4092 (1983)

Nature http://www.nature.com/nature

--------------------------------

Related Material:

ON THE SEMICONDUCTOR-ELECTROLYTE INTERFACE

The following points are made by Michael Graetzel (Nature 2001 414:340):

1) When a semiconductor is placed in contact with an electrolyte, electric current initially flows across the junction until electronic equilibrium is reached, the state where the Fermi energy of the electrons in the solid is equal to the redox potential of the electrolyte. The transfer of electric charge produces a region on each side of the junction where the charge distribution differs from the bulk material, and this is known as the "space charge layer".

2) On the electrolyte side, the space charge layer corresponds to the familiar "electrolytic double layer", i.e., the compact Helmholtz layer followed by the diffuse Gouy-Chapman layer. On the semiconductor side of the junction, the nature of the band bending depends on the position of the Fermi level in the solid. If the Fermi level of the electrode is equal to the flat bend potential, there is no excess charge on either side of the junction and the bands are flat. If electrons accumulate at the semiconductor side, one obtains an accumulation layer. If, however, the electrons deplete from the solid into the solution, a depletion layer is formed, leaving behind a positive excess charge formed by the immobile ionized donor states. Finally, electron depletion can go so far that the concentration of electrons at the interface falls below the intrinsic level. As a consequence, the semiconductor is p-type at the surface and n-type in the bulk, corresponding to an inversion layer.

Nature http://www.nature.com/nature

--------------------------------

Related Material:

OPTICS: ON SURFACE PLASMONS

The following points are made by W.L. Barnes et al (Nature 2003 424:824):

1) Surface plasmons (SP) are waves that propagate along the surface of a conductor, and they are of interest to a wide spectrum of scientists, ranging from physicists, chemists and materials scientists to biologists. Renewed interest in SPs comes from recent advances that allow metals to be structured and characterized on the nanometer scale. This in turn has enabled us to control SP properties to reveal new aspects of their underlying science and to tailor them for specific applications. For instance, SPs are being explored for their potential in optics, magneto-optic data storage, microscopy, and solar cells, as well as being used to construct sensors for detecting biologically interesting molecules.

2) SPs were widely recognized in the field of surface science following the pioneering work of Ritchie in the 1950s (1). SPs are waves that propagate along the surface of a conductor, usually a metal, and are essentially light waves that are trapped on the surface because of their interaction with the free electrons of the conductor (strictly speaking, they should be called surface plasmon polaritons to reflect this hybrid nature(2)). In this interaction, the free electrons respond collectively by oscillating in resonance with the light wave. The resonant interaction between the surface charge oscillation and the electromagnetic field of the light constitutes the SP and gives rise to its unique properties.

3) For researchers in the field of optics, one of the most attractive aspects of SPs is the way in which they help us to concentrate and channel light using subwavelength structures. This could lead to miniaturized photonic circuits with length scales much smaller than those currently achieved(3,4). Such a circuit would first convert light into SPs, which would then propagate and be processed by logic elements, before being converted back into light. To build such a circuit one would require a variety of components: waveguides, switches, couplers and so on. Currently much effort is being devoted to developing such SP devices; one example is a 40 nm thick gold stripe that acts as a waveguide for SPs An appealing feature is that, when embedded in dielectric materials, the circuitry used to propagate SPs can also be used to carry electrical signals. Developments such as this raise the prospect of a new branch of photonics using SPs, sometimes called "plasmonics".

4) In summary: By altering the structure of a metal's surface, the properties of surface plasmons -- in particular their interaction with light -- can be tailored, which offers the potential for developing new types of photonic devices. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.(5)

References (abridged):

1. Ritchie, R. H. Plasma losses by fast electrons in thin films. Phys. Rev. 106, 874-881 (1957)

2. Burstein, E. in Polaritons (eds Burstein, E. & De Martini, F.) 1-4 (Pergamon, New York, 1974)

3. Hecht, B., Bielefeldt, H., Novotny, L., Inouye, Y. & Pohl, D. W. Local excitation, scattering, and interference of surface plasmons. Phys. Rev. Lett. 77, 1889-1892 (1996)

4. Pendry, J. Playing tricks with light. Science 285, 1687-1688 (1999)

5. Kneipp, K. et al. Single molecule detection using surface-enhanced Raman scattering (SERS). Phys. Rev. Lett. 78, 1667-1670 (1997)

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com