REPORTS functions technique) of a quantum-mechanical n- and p-regions are equal (rh = re), charge 6. T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, analysis of oscillations of dj around the mi- carriers injected into graphene from the contact Science 313, 951 (2006). A-B 7. V. V. Cheianov, V. I. Fal’ko, Phys. Rev. B 74, 041403 rage image of a bilayer island formed on the S shown in Fig. 4A would meet again in the (2006). other side of symmetric PNJ in the monolayer focus at the distance 2w from the source 8. M. Katsnelson, K. Novoselov, A. Geim, Nat. Phys. 2, 620 sheet. To compare Fig. 2D shows the calculated (contact D3 in Fig. 4A). Varying the gate volt- (2006). mirage image of a spike of electrostatic potential age over the p-region changes the ratio n2 = 9. V. G. Veselago, Sov. Phys. Usp. 10, 509 (1968). r r 10. J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000). (smooth at the scale of the lattice constant in h/ e. This enables one to transform the focus 11. J. B. Pendry, Nature 423, 22 (2003). graphene), which induces LDOS oscillations into a cusp displaced by about 2(|n| −1)w along 12. D. R. Smith, J. B. Pendry, M. Wiltshire, Science 305, 788 equal on the two sublattices. The difference be- the x axis and, thus, to shift the strong coupling (2004). 13. M. S. Dresselhaus, G. Dresselhaus, Adv. Phys. 51, tween these two images is caused by the lack of from the pair of leads SD3 to either SD1 (for backscattering off A-B symmetric scatterers r < r )orSD (for r > r ). Fig. 4, B and C, 1 (2002). h e 5 h e 14. M. Berry, Adv. Phys. 25, 1 (1976). specific to graphene (21). illustrate another graphene-based device in 15. V. I. Arnold, Singularities of Caustics and Wave Fronts; Unlike the ideal left-handed metamaterial which a prism-shaped top-gate may be used Mathematics and Its Applications Series, Vol. 62 (Kluwer, (10), focusing in the PNJ is not perfect. In sym- as a focusing beam splitter. For example, Dordrecht, 1990). metric junctions, it occurs only for electrons ex- electrons emitted from contact B (Fig. 4B) are 16. H. C. Manoharan, C. P. Lutz, D. M. Eigler, Nature 403, b 512 (2000). actly at the Fermi level, and it is spread into distributed between the contacts b and , 17. J. Friedel, Philos. Mag. 43, 153 (1952). caustics for electrons excited to higher energies. whereas the signal sent from contact A (Fig. 18. E. McCann, V. I. Fal’ko, Phys. Rev. Lett. 96, 086805 Therefore, the sharpness of electron focusing 4C) is replicated into the pair of contacts a and (2006). decreases with temperature. If the focused elec- a. Graphene has recently been brought into con- 19. J. D. Bernal, Proc. Phys. Soc. London A 106, 749 (1924). 20. S. Hembacher et al., Phys. Rev. Lett. 94, 056101 tron flow is detected by a contact of size d >> tact with a superconducting metal, and the (2005). lF, a pronounced signal in the focus will per- Josephson proximity effect through graphene has 21. V. V. Cheianov, V. I. Fal’ko, Phys. Rev. Lett. 97, 226801 sist up to T ~ ħvkc d/a. For example, in a bal- been observed (22). Consequently, a beam (2006). listic structure with a ~1mm, d ~ 0.1 mm, and splitter (Fig. 4, B and C) can be used to ex- 22. H. B. Heersche, P. Jarillo-Herrero, J. B. Oostinga, r r 12 −2 L. Vandersypen, A. F. Morpurgo, http://arxiv.org/ftp/ e = p ~3×10 cm , focusing may persist periment with Einstein-Podolsky-Rosen (23) cond-mat/papers/0612/0612121.pdf. up to the nitrogen temperature. The interference pairs of particles. 23. A. Einstein, B. Podolsky, N. Rosen, Phys. Rev. 47, 777 effectsshowninFigs.2and3arewashedoutat (1935). References and Notes on March 1, 2007 a much smaller temperature scale, T ~ ħv/a. 24. This project started during the Max Planck Institute für 1. K. Novoselov et al., Science 306, 666 (2004). “ Focusing of electrons by a sharp p-n junc- Physik Komplexer Systeme workshop Dynamics and 2. R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Relaxation in Complex Quantum and Classical Systems tion in graphene can be used to turn the n-p-n Properties of Carbon Nanotubes (Imperial College Press, and Nanostructures” and was funded by the Engineering junction into a Veselago lens for electrons. In London, 1998). and Physical Sciences Research Council, Lancaster 3. K. Novoselov et al., Nature 438, 197 (2005). Portfolio Partnership grant EP/C511743. such a device (Fig. 4A), the density of charge 4. Y. Zhang, Y.-W. Tan, H. L. Stormer, P. Kim, Nature 438, carriers in the p-region (with width w) can be 201 (2005). 27 November 2006; accepted 31 January 2007 controlled by the top gate. If the densities in the 5. K. Novoselov et al., Nat. Phys. 2, 177 (2006). 10.1126/science.1138020 articulated fossils, notably in the small shelly www.sciencemag.org Halwaxiids and the Early assemblages. Burgess Shale–type faunas, how- ever, yield articulated material such as the Evolution of the Lophotrochozoans halkieriids, which are probably related to the siphogonuchitids [known only from disassoci- Simon Conway Morris1* and Jean-Bernard Caron2* ated sclerites (10)], and wiwaxiids. However, the wider relationships of these groups, the Halkieriids and wiwaxiids are cosmopolitan sclerite-bearing metazoans from the Lower and members of which are similar to armored slugs, Downloaded from Middle Cambrian. Although they have similar scleritomes, their phylogenetic position is contested. are uncertain. One hypothesis interprets the A new scleritomous fossil from the Burgess Shale has the prominent anterior shell of the halkieriids halkieriids as stem-group lophotrochozoans, but also bears wiwaxiid-like sclerites. This new fossil defines the monophyletic halwaxiids and closely linking them to the origin of annelids indicates that they have a key place in early lophotrochozoan history. and brachiopods (11). The evolutionary route to the annelids was urgess Shale–type faunas house numer- to either extant or extinct phyla implies that hypothesized to be via the related wiwaxiids ous taxa that are phylogenetically con- body plans arose by macroevolutionary mecha- (12), and the peculiar halkieriid arrangement Btroversial and open to widely different nisms. This is consistent with body plans having of a prominent anterior and posterior shell interpretations. One approach is to incorporate a seemingly abrupt appearance and potentially presaged the bivalved brachiopods (11). More these taxa into the stem groups of major phyla with claims of a protracted (albeit cryptic) his- recently, the micrinids and tannuolinids, known (1), but this often presupposes homologies of tory (5). In contrast, hypotheses based on the only as isolated shells, have been interpreted as disparate structures and typically depends on an construction of stem groups generally imply mi- key intermediates (13) between the halkieriids attenuated fossil record. Alternative views re- croevolutionary processes. The component taxa and brachiopods. Alternative hypotheses have gard such taxa as either belonging to extant would be initiated in the latest Neoproterozoic, assigned the halkieriids to the crown-group mol- phyla (2, 3) or representing extinct phyla (4). with body plans emerging by functional tran- lusks (3), questioned the relevance of the These differences have major evolutionary im- sitions that were connected to feeding, locomo- micrinids and tannuolinids (14), and rejected plications. For example, the assignment of taxa tion, and defense (6, 7). This view is consistent the wiwaxiids as stem-group annelids (15). One with an explosive diversification of metazoans substantial contribution to this debate is the 1Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. 2Department of Natural History, (8, 9). identification of Odontogriphus and Wiwaxia as Royal Ontario Museum, Toronto, M5S 2C6, Canada. Numerous Cambrian groups have multi- stem-group mollusks (16). Despite these *To whom correspondence should be addressed. E-mail: plated skeletons (or scleritomes) (10). Typically, conflicting hypotheses, the Cambrian fossil [email protected] (S.C.M.); [email protected] (J.B.C.) the scleritomes of these groups occur as dis- record, in principle, will be central to unravel- www.sciencemag.org SCIENCE VOL 315 2 MARCH 2007 1255 REPORTS panding posteriorly (Fig. 2, B, C, and F). Finely spaced growth lines indicate accretionary growth (Fig. 2, C and F). The shell also bears coarser ridges (Fig. 2, A to C and F); they may repre- sent either the internal surface of the shell or more probably metamerism, traces of which are also discernible more posteriorly. In the ventral view, the shell is largely obscured by soft tissue, but the anterior margin is fairly acute and bears a prominent doublure (Fig. 1E). A relatively prominent strand (that begins close to the mar- gin of the anterior shell and can be traced in- distinctly toward the posterior) may represent the gut. In life, this animal was evidently benthic (Fig.3),anditisassumedtohavemovedona muscular foot. Feeding habits are conjectural, but the convex central region of the organism may have housed a voluminous sediment-filled gut. The scleritome, especially the elongate spines, was presumably protective and/or sensory in function. The shape of the spines suggests that they extended outward and then upward. Given their relative position in the scleritome, the spines are less likely to have provided any snowshoe-like support on the surface of floccu- lent sediment. The function of the anterior shell on March 1, 2007 is conjectural, but, as hypothesized in the halkieriids (11), it could have provided a Fig. 1. O. reburrus from the Middle Cambrian Burgess Shale. (A to D) Dorsal view.