NEWS &ES if antiferromagnetic, would be geometrically frustrated contain copper. Because its superconductivity can easily by the hexagonal configuration of the cobalts; Fig. 2), in be destroyed by removing the water molecules, and thus view of the significantelectron-electron repulsion decreasing the interlayer separation, Nao.35 CoO2.1.3H,O typically observed for octahedrally coordinated 3 uld also provide insight into the relationship between transition metals So if this view is correct, the cobalt dimensionality andsuperconductivity Contrasts and uperconductor could have some of the ingredients that similarities between it and the copper oxides may shed make the copper oxide behaviour so interesting: further light on the challenging issue of understanding proximity to a Mott insulator-to-metal transition igh-T superconductivity The electronic, magnetic, and coupled with spin-1/2 magnetism. The high structural properties of this new cobalt oxide are likely to thermoelectric power observed for the more bethe subject ofi sodium-rich Naos CoO2 compound, which is not typical of simple metals and makes it attractive for References thermoelectricenergy conversion applications, is 1. Sleight,A.W.sckn242,1519-1527(1988) in this class of materials.8. For all of these reasonsthe a further evidence that something unusual is happenin 2. Orenstein, L& Millis, A I Science 288, 468-174(2000). 3. Takada, K ct. Nature 422, 53-55(2003). sodium cobalt bronzes were predicted to be good 4. Fouassicr, C, Mate ka, G, Reau, J -M. Hagenmuller, P J. Solid State Chen. 6. candidates for investigation as superconductors%, lo. Whatever its relationship to the high-TC oxides 5. Terasaki, I, Sasago, Y. Uchinokura, K Phys. Rev. B56, 12685-12687(1997). may be, the discovery of the Naa3s CoO2 1.3H20 6. Balsys, R L& Davis, R L. Solid State lonic 93. 279-382( 1996). superconductor represents a major advance, because 8. Singh, D . Phrs. Ren. B6). 13397-13402(200) there are so few examples of layered transition metal 9. Ray.R, Ghoshray, A, Ghoshra, K.& Nakamura, S. Phys.Rev. B59 oxide superconductors and relatively few examples of oxide superconductors in general that do not O.Tanaka, T, Nakamura, S& lida, S Jpn JAppl. Phys. 233, 1581-1582(1994) NANOPARTICLE WAVEGUIDES Watching energy transfer There may be plenty of room at the bottom, but the size of conventional optical elements is restricted by the diffraction limit of light. Plasmon waveguides made from metal nanoparticle chains may allow a drastic reduction in the size of photonic devices JoACHIMR. KRenn is at the Institute for As they reporton page 229 of this issue, Stefan Maier and Experimental Physics and Erwin Schrodinger Institute for colleagues have now directly observed energy transfer Nanoscale Research, Karl-Franzens-University Graz along a nanoparticle plasmon waveguide. These metal A-8010 Graz, Austria. nanoparticle chains may becomea centralcomponent e-mait: joachim. krenneuni-grazat of nanoscale photonic devices. Near-field optics, proposed around 20 years ago, M odern technologyrelies heavilyon photonic provides a neat way to circumvent the diffraction limit2. and optoelectronic devices for signal Instead of imaging with a system of lenses, a glass fibre transmission androuting, chemical analysis with a submicroscopic tip can be used to probe the light and sensorapplications Asis thecase forelectronics, fields close to a sample surface. Because the distance increasing the speed and sensitivity of photonicdevices between tip and sample is much smaller than the light relies on further miniaturization of photonicelements. wavelength, non-propagating light fields that are But thereisa fundamentallimit to theminimum size of bound to the sample surface can be detected. These conventional opticalelements such as dielectric evanescent fields decay in intensity within a fraction of waveguides. This limit is set by diffraction to beabout ving away from the half the wavelength oflight. For light, this surface, and carry information about sample features translateinto aminimumelement size of a few hundred smaller than the value set by the diffraction limit. nanometres-too large for fabricating nano-and Owing to their non-propagating character, evanescent molecular-scale photonic devices. Near-fieldoptics, fields can only be detected by a local probe immersed whichexploits non-propagating(evanescent)fields into the near-field of the sample. The corresponding of experimental device is called a near-field scanning rcumventing the diffraction(and thereforesize)limit. optical microscope(NSOM) naturematerialsivol2iaPril2003iwww.natUre.com/naturematerials @2003 Nature Publishing GroupNEWS & VIEWS if antiferromagnetic,would be geometrically frustrated by the hexagonal configuration of the cobalts; Fig. 2),in view of the significant electron–electron repulsions typically observed for octahedrally coordinated 3d transition metals8 .So if this view is correct,the cobalt superconductor could have some of the ingredients that make the copper oxide behaviour so interesting: proximity to a Mott insulator-to-metal transition coupled with spin-1/2 magnetism.The high thermoelectric power observed for the more sodium-rich Na0.5CoO2 compound,which is not typical of simple metals and makes it attractive for thermoelectric energy conversion applications,is further evidence that something unusual is happening in this class of materials5,8.For all of these reasons the sodium cobalt bronzes were predicted to be good candidates for investigation as superconductors5,9,10. Whatever its relationship to the high-Tc oxides may be, the discovery of the Na0.35CoO2⋅1.3H2O superconductor represents a major advance, because there are so few examples of layered transition metal oxide superconductors and relatively few examples of oxide superconductors in general that do not contain copper. Because its superconductivity can easily be destroyed by removing the water molecules,and thus decreasing the interlayer separation,Na0.35CoO2⋅1.3H2O could also provide insight into the relationship between dimensionality and superconductivity. Contrasts and similarities between it and the copper oxides may shed further light on the challenging issue of understanding high-Tc superconductivity.The electronic,magnetic,and structural properties of this new cobalt oxide are likely to be the subject of intense investigation for some time. References 1. Sleight, A. W. Science 242, 1519–1527 (1988). 2. Orenstein, J. & Millis, A. J. Science 288, 468–474 (2000). 3. Takada, K.et al. Nature 422, 53–55 (2003). 4. Fouassier, C., Matejka, G., Reau, J.-M. & Hagenmuller, P.J. Solid State Chem. 6, 532–537 (1973). 5. Terasaki, I., Sasago, Y. & Uchinokura, K. Phys. Rev. B 56, 12685–12687 (1997). 6. Balsys, R. J. & Davis, R. L. Solid State Ionics 93, 279–382 (1996). 7. Choy, J. H., Kwon, S. J. & Park, G. S. Science 280, 1589–1592 (1998). 8. Singh, D. J. Phys. Rev. B 61, 13397–13402 (2000). 9. Ray, R., Ghoshray, A., Ghoshray, K. & Nakamura, S. Phys. Rev. B 59, 9454–9461 (1999). 10.Tanaka, T., Nakamura, S. & Iida, S.Jpn. J. Appl. Phys. 2 33, L581–L582 (1994). 210 nature materials | VOL 2 | APRIL 2003 | www.nature.com/naturematerials NANOPARTICLE WAVEGUIDES Watching energy transfer There may be plenty of room at the bottom, but the size of conventional optical elements is restricted by the diffraction limit of light. Plasmon waveguides made from metal nanoparticle chains may allow a drastic reduction in the size of photonic devices. JOACHIM R. KRENN is at the Institute for Experimental Physics and Erwin Schrödinger Institute for Nanoscale Research,Karl-Franzens-University Graz, A-8010 Graz,Austria. e-mail: joachim.krenn@uni-graz.at Modern technology relies heavily on photonic and optoelectronic devices for signal transmission and routing,chemical analysis and sensor applications.As is the case for electronics, increasing the speed and sensitivity of photonic devices relies on further miniaturization of photonic elements. But there is a fundamental limit to the minimum size of conventional optical elements such as dielectric waveguides.This limit is set by diffraction to be about half the wavelength of light.For visible light,this translates into a minimum element size of a few hundred nanometres — too large for fabricating nano- and molecular-scale photonic devices.Near-field optics, which exploits non-propagating (evanescent) fields rather than propagating light waves,provides a way of circumventing the diffraction (and therefore size) limit. As they report on page 229 of this issue,Stefan Maier and colleagues1have now directly observed energy transfer along a nanoparticle plasmon ‘waveguide’.These metal nanoparticle chains may become a central component of nanoscale photonic devices. Near-field optics,proposed around 20 years ago, provides a neat way to circumvent the diffraction limit2 . Instead of imaging with a system of lenses,a glass fibre with a submicroscopic tip can be used to probe the light fields close to a sample surface.Because the distance between tip and sample is much smaller than the light wavelength,non-propagating light fields that are bound to the sample surface can be detected.These ‘evanescent’fields decay in intensity within a fraction of the light wavelength when moving away from the surface,and carry information about sample features smaller than the value set by the diffraction limit. Owing to their non-propagating character,evanescent fields can only be detected by a local probe immersed into the near-field of the sample.The corresponding experimental device is called a near-field scanning optical microscope (NSOM). © 2003 Nature Publishing Group