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tive plant ecosystems would have substantial OPTICS
impact on the behavior of rivers and the con-
According to the results of McMahon and Fermi arcs connect
struction of their floodplain deposits.
Davies, mudrock in river deposits continued
to increase through the Devonian-Carbonif- topological degeneracies
erous interval (about 370 to 300 million years
ago) during the evolution of deeply rooted Surface or bulk Fermi arcs are engineered in photonic
plants and true forests. If the timing of the
initial increase in mudrock is correct, it sug- structures to connect ideal Weyl points or exceptional points
gests that the earliest plants made a substan-
tial impression on the landscape.
What was it about these early plant- By S¸ahin K. Özdemir Weyl points differ in critical ways. For ex-
bearing ecosystems that generated so much ample, any perturbation that breaks parity
mudrock in their deposits? There are many n condensed matter and photonics, to- (P) or time-reversal (T) symmetry lifts the
ideas, and little is certain. McMahon and pology is defined with respect to the en- 2D Dirac point degeneracy and opens a
Davies point to processes by which plants ergy bands in momentum space (1). The band gap; the Dirac point splits and loses its
may have increased mud production during boundary between two topologically topological protection. However, creation of
erosion and weathering. With that in mind, different phases (for example, support- topologically protected Weyl points actually
soil
hori-
there are preserved Precambrian I ing right- versus left-handed particles) requires the breaking of P- or T-symmetry,
zons composed of mudrocks and plenty of appears as degeneracies where two linear or both (see the figure) (11, 12). Hermitian
Precambrian marine mudrocks indicating dispersion bands intersect. Fundamental perturbations only shift Weyl points but
that substantial fluxes of mud passed from point degeneracies in two-dimensional (2D) cannot lift their degeneracy. A Weyl point
the continents to marine basins before plants and 3D Hermitian systems—known as Di- is annihilated only by bringing together two Downloaded from
evolved. Our planet, it seems, has always had rac and Weyl points, respectively—have points of opposite chirality (for example,
enough mud to go around. It is therefore been observed in photonic structures (2–6) ones with chirality of +1 or –1).
likely that early plants affected the mechan- but not the ideal Weyl points and the heli- The minimum number of Weyl points
ics of floodplain construction. For example, coidal dispersion, which leads to the open in systems respecting P-symmetry (system
the presence of plants on the landscape de- Fermi arcs connecting points of opposite breaks T-symmetry) is two, whereas in sys-
creases erosion rates; and thus it was long chirality. On page 1013 of this tems respecting T-symmetry
hypothesized that erosion—in particular by issue, Yang et al. (7) demon- (system breaks P-symmetry),
wind—removed sediment from prevegetated strate an ideal Weyl system it is four. Weyl points with http://science.sciencemag.org/
landscapes (11, 12). Even if mud was depos- with four Weyl points (8, 9) “...the reported nonzero chirality have been
ited on prevegetated floodplains, its removal and helicoidal surface Fermi observations... previously reported in systems
by erosion might have been efficient. arcs interconnecting them in a with broken P- and T-symme-
In addition to inhibiting erosion, plants 3D photonic crystal composed will surely try. However, Weyl points in
also interact with river flows and promote of metallic inclusions. On page help to explore those studies were not ideal
the deposition of fine-grained sediment. This 1009 of this issue, Zhou et al. because they were at differ-
can help armor riverbanks and slow their lat- (10) explore non-Hermitian to- photonics in ent frequencies and were not
eral migration; such a process might also aid pological photonics in which new regimes...” symmetry-related. on March 1, 2018
in preserving muddy floodplain deposits. radiative losses come into play, Yang et al. succeeded to cre-
Whatever the exact causes, the muddy sig- and demonstrate the emer- ate four ideal Weyl points with
nal that plants left in the geological record gence of bulk Fermi arc and half topological different topological chirality (two with +1
is prodigious. That such seemingly simple charges in a 2D-periodic photonic crystal. and two with –1) in the microwave regime
early-appearing plant ecosystems had such The observation of Weyl points at mi- by using a specially designed meta-crystal
sedimentary impact means that there is still crowave (2, 3) and optical frequencies (4) that breaks P-symmetry but leaves T-sym-
much to learn about the nature of the inter- serves as a fingerprint of the topological na- metry untouched. They characterized the
actions and coevolution of terrestrial ecosys- ture of the corresponding photonic system band structure of their meta-crystal using
tems and their underlying landscapes. j and guarantees the existence of Fermi arcs, angle-resolved transmission spectroscopy
which act as pipelines connecting Weyl by using a near-field antenna as the station-
REFERENCES
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7. J. P. Grotzinger et al., Science 350, aac7575 (2015). Despite both being degeneracies of a Her- states at each angle, confirming the linear
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(2017). In order to characterize the surface arcs,
11. S. A. Schumm, Geol. Soc. Am. Bull. 79, 1573 (1968).
12. R. H. Dott Jr., J. Geol. 111, 387 (2003). Yang et al. probed their meta-crystal using
Department of Engineering Science and Mechanics, The
Pennsylvania State University, University Park, PA 16802, USA. two different configurations of transmis-
10.1126/science.aas9886 Email: sko9@psu.edu sion near-field scanning system in which
SCIENCE sciencemag.org 2 MARCH 2018 • VOL 359 ISSUE 6379 995
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