Growing Perfect Decagonal Quasicrystals by Local Rules
Growing Perfect Decagonal Quasicrystals by Local Rules
Hyeong-Chai Jeong
Department of Physics, Sejong University, Seoul 143-747, Korea,
Asia Pacific Center for Theoretical Physics, POSTECH, Pohang 790-784, Korea,
and Department of Physics, Princeton University, Princeton, NJ 08540, USA
A local growth algorithm for a decagonal quasicrystal is presented. We show that a perfect
Penrose tiling (PPT) layer can be grown on a decapod tiling layer by a three dimensional (3D) local
rule growth. Once a PPT layer begins to form on the upper layer, successive 2D PPT layers can be
added on top resulting in a perfect decagonal quasicrystalline structure in bulk with a point defect
only on the bottom surface layer. Our growth rule shows that an ideal quasicrystal structure can be
constructed by a local growth algorithm in 3D, contrary to the necessity of non-local information
for a 2D PPT growth.
The announcement of icosahedral phase of alloys in
1984 [1] posed many puzzles. The first question was
what kind of arrangement of atoms could produce Bragg
peaks with a rotational symmetry forbidden to crystals.
The quasiperiodic translational order was proposed im-
mediately as a candidate, and such materials began to be
called quasiperiodic crystals or quasicrystals for short [2].
However, the appearance of quasicrystals brings new puz-
zles: why and how the atoms can arrange themselves to
have such order, and especially, how quasicrystals can
grow with perfect quasiperiodic order has been a dilemma
since it seemingly requires non-local information while
atomic interactions in metallic alloys are generally con-
sidered to be short ranged.
There are currently two alternative pictures to de-
scribe quasicrystals: energy-driven perfect quasiperi-
odic quasicrystals and entropy-driven random-tiling qua-
sicrystals. Accordingly, two alternative scenarios for the
growth [3] of quasicrystals exist: matching-rule based,
energy-driven growth, and finite-temperature entropy-
driven growth [4]. A major criticism for the former ap-
proach has been that no local growth rules can produce
a perfect quasicrystalline structure in 2D [5, 6]. Here, we
show how to overcome this obstacle in a 3D quasicrystals.
Penrose tiling [7] has been a basic template for describ-
ing formation and structure of ideal quasicrystals. It can
be constructed from fat and thin rhombi with arrowed
edges shown in Fig. 1(a). The infinite tiling consistent
with arrow-matching rules is the Penrose tiling but they
do not guarantee the growth of a perfect Penrose tiling
(PPT) from a finite seed. Successive “legal” (obeying the
arrow-matching rules) additions of tiles to the surface of
the already existing legal patch of tiles can produce de-
fects. They usually occur after only a handful of tiles
are added, and hence the arrow-matching rules cannot
explain the long-range quasicrystalline order engendered
by growth kinetics.
There has been a great amount of discussion and a
number of debates on the possibility of local growth al-
gorithm for a PPT [3, 4, 5, 6, 8, 9, 10]. The debates par-
tially emerge from a different assumption on the growth
processes at the surface, uniform growth and preferential
growth. In the former, the growth occurs at any surface
site with the same attaching probability, while it occurs
with different attaching probabilities in the latter. In
1988, Penrose proved that a PPT cannot be grown by
local rules with uniform growth by showing that “decep-
tions” are unavoidable [5] where a deception is a legal
patch which cannot be found in a PPT [5, 6]. In the
same year, Onoda et al. introduced a preferential growth
algorithm which can avoid deception by local rules called
“vertex rules” [3]. However, vertex rule growth stops
at a “dead surface” and non-local information or arbi-
trarily small growth rates are required to be an infinite
PPT. Yet, their growth algorithm is believed to provide
methods to grow the most ideal quasicrystalline struc-
tures with local information. If an initial seed contains
a special kind of defect, called “decapod” [11], Onoda
et al. showed that the seed can be grown to an almost
PPT (whose only defect is the initial decapod defect) [3].
A point defect in a 2D tiling growth usually implies a
line defect in a 3D decagonal tiling growth. If we apply
a solid-on-solid type growth [12] so that a layer copies
configuration of the one below, we get decagonal tiling
consisting of identical layers with a decapod defect at
the center of each layer. This line defect has been consid-
ered to be a minimum imperfection for the 3D decagonal
quasicrystal structure from the local growth algorithm.
In this letter, we consider the growth of decagonal
quasicrystals and present a local growth algorithm for
3D decagonal tiling which consists of PPT layers except
the bottom layer. We use the two well known results of
the Onoda et al. study on planar decagonal quasicrys-
tal growth [3]. 1. A local growth method around a
“cartwheel decapod” leads to dead surfaces, from which
further growth of a PPT requires non-local information.
2. Infinite local growth is possible if it starts from an
“active decapod defect” but the resultant tiling contains
the defect and is not a PPT. By combining an active
decapod defect in the bottom layer with a cartwheel de-
capod in the second layer and using the information of
the underneath configuration, we make the growth con-
tinue beyond dead surfaces in the second, and subsequent
layers. The bottom surface layer has a point defect but
we can consider the overall structure as that of a per-
fect decagonal quasicrystal since deviations from the bulk
http://arxiv.org/abs/0704.0848v1
FIG. 1: (Color online) (a) Fat and skinny Penrose tiles with
arrows. (b) The eight ways of surrounding a vertex in a PPT.
(c) Dead surfaces encountered when a tiling is grown by Rule-
L from a cartwheel decagon. See text for details. (d) A de-
capod tiling. Ten semi-infinite worms meet at the decapod
decagon at the center. From the yellow seed tiles, an infinite
tiling (decapod tiling) can be grown by Rule-L.
layer structure are natural for the surface layer even for
ideal crystal materials.
Let us first discuss the growth rule in a 2D Penrose
tiling. For the arrow-matching rule growth, a deception
can be made as few as three tiles [5]. Since the growth
process does not allow tiles to be removed, a deception
(which is not a part of a PPT) cannot grow to a PPT,
and we need a growth rule which allows no deceptions
of any size for a PPT growth. We can avoid three tile
deceptions by introducing a more restricted growth rule
which allows only correct (subset of a PPT) three tile
patches. However, the new growth rule can make a de-
ception in a larger scale, for example, a three tile de-
ception of inflated [11] tiles. Since a deception can be
made in all scales of multiply inflated tile sizes [11], it
is unavoidable for a local growth rule. The absence of
local rules for perfect tiling growth seems to be the case
for general aperiodic tilings in 2D [6]. Based on this ob-
servation, Penrose even speculates that there may be a
non-local quantum-mechanical ingredient to quasicrystal
growth [5, 13].
Onoda et al. proposed “vertex rules” which avoid an
encounter of deceptions [3]. Here, a tile can be added only
to a “forced edge” which admits only one way of adding
a tile for its end vertices to be consistent with any of
the eight PPT vertex configurations shown in Fig. 1(b).
In this Letter, their vertex rules will be called “Rule-
L” and used for the “lateral”-direction growth (for 3D
decagonal tilings). The problem of Rule-L is that the
growth stops at a finite size patch called a “dead surface”
which consists of unforced edges.
There are special kinds of point-like defects, called “de-
capod” defects [11], which can be an ideal seed to grow
an almost PPT without encountering a dead surface [3].
A decapod is a decagon with single arrowed edges. Since
there are 10 arrows, each of which can take two indepen-
dent orientations, there are 210 combinations of states.
After eliminating rotations and reflections, we get 62 dis-
tinct decapods. We can tile inside the decagon legally
for only one decapod, the cartwheel case and the rest
of the 61 decapods are called decapod defects. One no-
table property of the decapods is that the outside of the
decagon region can be legally tiled for all 62 cases. This
can be easily understood from the fact that six semi-
infinite worms and two infinite worms meet at the cen-
ter cartwheel decagon [shown by green tiles in 1(c)] in
a cartwheel tiling. If we remove the tiles in the cen-
ter cartwheel decagon, two infinite worms become four
semi-infinite worms, and we have ten semi-infinite worms
which start at the perimeter of the center decagon. A de-
capod defect tiling is formed by flipping one or more of
these ten semi-infinite worms. The arrows on the worm
perimeter will still fit except a mismatch at the decapod
decagon perimeter. Figure 1(d) shows an example ob-
tained by flipping the worm denoted by red hatched tiles.
Among 61 decapod defects, there are 51 “active” deca-
pod defects which have at least three consecutive arrows
of the same orientation on their decagon perimeter [14].
One can show that a patch containing an active decapod
defect is never enclosed by a dead surface [15].
Our 3D growth rules are constructed by observing that
a cartwheel PPT and a decapod tiling can be differ-
ent only in ten semi-infinite worms. Consider two layer
growth from a (two layer) seed that contains a cartwheel
decagon [yellow tiles in Fig. 1(c)] and a decapod defect
[yellow tiles in Fig. 1(d)] at the upper and the lower lay-
ers respectively. If each layer grows with Rule-L inde-
pendently, the growth of the upper layer would stop at
the red-purple-blue dead surface while the lower layers
grow indefinitely. Now, we introduce a vertical growth
rule so that a tile can be added at the dead surface of
the upper layer properly. Note that the basic tiles for 3D
decagonal tiling are rhombus prisms which have top and
bottom faces as well as side faces. By vertical growth,
we mean attaching tiles on the surface layer such that
the bottom faces of the attached tiles contact with the
top faces of the surface layer tiles, while lateral growth
means attaching tiles to side faces at the perimeters. We
propose a vertical growth rule, “Rule-V” with which the
lateral growth rule, Rule-L produces a PPT on a decapod
tiling. If a tile in a flipped worm of the decapod tiling
(the lower layer) is copied by a vertical growth, a defect
on the upper layer is inevitable. Our Rule-V is designed
to avoid such a case and allow to attach tiles vertically
only on the “sticky” top faces, blue-circled fat tiles in
S3 and S4 configurations shown in Fig. 2(a) [16]. They
always form D-hexagons indicated by dotted lines. Such
S4 S3
(a) (b) (c)
(d) (e)
FIG. 2: (Color online) (a) Sticky sites, on which a tile can
be attached vertically, are indicated with blue circles. (b)
Upper layer configuration of a (two layer) seed. It contains a
cartwheel decagon and ten hexagons attached to the decagon.
The tiles denoted by blue circles are sticky site and the tile
denoted by X is the nucleate site. (c) Lower layer configu-
ration of the (two layer) seed. It contains an active decapod
decagon with five consecutive arrows of the same orientation.
(d) Sticky sites on a decapod tiling. All sticky sites are out-
side of the ten semi-infinite worms. (e) Dead surfaces which
contain the center cartwheel decagon in a cartwheel tiling.
The two crossing infinite worms of the cartwheel tiling always
pass the two 72 degree corners of dead surfaces.
D-hexagons can lie only at the end of worms since the
other (uncircled) tiles in S3 or S4 configurations prevent
formation hexagons next to the circled tiles. Therefore,
the sticky sites can be located only outside or at the ends
of the semi-infinite worms as illustrated in Figs. 2(b)-(e).
One can further show that sticky sites are strictly outside
of the semi-infinite worm if it is flipped since the flipping
makes the vertices at the end be illegal (and therefore
they cannot be S3 or S4). Therefore, no sticky sites are
in the flipped worm, and hence Rule-V does not intro-
duce a defect or deception for the layer that grows on a
decapod tiling.
Now we show that Rule-V is enough for the upper
layer to grow beyond the dead surfaces when the growth
starts from a proper seed. Our seed consists of two
layer finite patches which include a cartwheel decagon
and an active decapod decagon at the upper and the
lower layers, respectively. Figures 2(b) and (c) show an
example. The upper layer seed [Fig. 2(b)] consists of
a cartwheel decagon and 10 D-hexagons. It covers all
ends of the semi-infinite worms in the lower decapod seed
[Fig. 2(c)] which consists of a decapod decagon and 10 D-
hexagons [17]. Let us first consider the properties of dead
surfaces which contain the upper layer seed. By apply-
ing inflations to a cartwheel tiling, one can show that
the dead surfaces, which contain the center cartwheel
decagon, have two 72 degree corners and each corner is
passed by an infinite worm [green worms in Fig. 2(e)] of
the cartwheel tiling [10, 11]. The D-hexagon at the 72-
degree corner forces the next two hexagons just outside
of the dead surface (in the infinite worm direction) to be
D and Q. These two hexagons force a cartwheel decagon
to form just outside of the corner as illustrated by (red
and purple) dashed lines in Fig. 2(e). A Q-decagon (de-
noted by yellow tiles) in the dashed cartwheel decagon
forces a tile just outside of the 72 degree corner [denoted
by the solid green circles in Fig. 2(e)] to be sticky. We
call these sticky sites as “launching” sites. The exact
position of a launching site depends on the orientation
of the corner [10] but the patch can grow by Rule-L for
both cases. The position of the launching site determines
the orientation of the worm along the side lines of the
dead surface making the edges at the dead surface be-
come forced. Hence, the upper layer would grow to infi-
nite by Rule-L if Rule-V guarantees tiles at the launching
sites. This is the case when it grows on an active deca-
pod tiling obtained by flipping a semi-infinite worm [18]
of a cartwheel tiling as shown in Fig. 2(d). Since neither
crossing infinite-worms are flipped [compare Figs. 2(d)
and (e)], the underneath tiles of the launching sites will
be always the sticky sites of the decapod tiling and tiles
at the launching sites are guaranteed by Rule-V on the
decapod tiling.
For the completeness of the 3D decagonal quasicrystal
growth, we need to provide the rule for the nucleation of
an island (seed) from the third layer. The physical pro-
cess of the nucleation of an island on a PPT would be sim-
ilar to that of a perfect crystal surface. High quality qua-
sicrystals are grown when they grow slowly, or in other
words, when the chemical potential of bulk quasicrystal
is slightly less than that of the fluid phase. Therefore,
adatoms or “adtiles” on a terrace would be unstable and
probably diffuse on the terrace until they evaporate (i.e.,
go back to the fluid phase) or attach to preferential sites
(forced or sticky sites) [12]. We believe that the chemical
potentials of the forced sites are less than those of sticky
sites and adtiles attach to forced sites for most cases.
However, when the terrace forms a dead surface (or part
of a dead surface), it is not easy for an adatom to find a
forced site and it would attach on a sticky site, especially
to a launching site whose chemical potential is expected
to be lower than that of an isolated sticky site. Note that
both forced and launching sites are at the perimeters of
terraces and become irrelevant to adtiles on the middle
of the terraces as they grow sufficiently large. It is then
conceivable that two or more adtiles meet on a terrace
and begin to form a new patch of the next layer before
they arrive at the perimeter. With this physical process
in mind, we allow a nucleation process in our growth
algorithm. The nucleation of an island can happen in
cooperation of a quite large cluster of tiles. We choose
the “cartwheel seed”, a cartwheel decagon and the 10
D-hexagons arranged as Fig. 2(b), as such a cluster and
introduce a nucleation site on it. The site X in the figure
is called a “nucleate site” if its lateral neighboring tiles
form a cartwheel seed and if it has a underneath tile [19].
When a nucleate site is selected, we create a cartwheel
seed on it.
Let us summarize our growth mechanism for decago-
nal quasicrystals. It consists of three processes: lateral
growth by Rule-L, vertical growth by Rule-V, and the is-
land nucleation (seed formation) for the new layer. Algo-
rithmically, it is realized by the following steps: 1. Start
with a two layer seed whose upper and lower layers con-
tain a cartwheel decagon and an active decapod decagon,
respectively. 2. Randomly choose a surface site. Check
if it is a sticky, nucleate, or unsticky site when it is a
top face. For a side face, check if it is a forced or un-
forced site. 3. Perform the vertical growth, nucleation,
or lateral growth if the chosen site is a sticky, nucleate,
or forced site, respectively. Do nothing for unsticky (top
face) or unforced (side face) site.
For simplicity, we have chosen the unit attaching prob-
ability for all sticky, forced and nucleate sites. In real
material, they probably have different attaching proba-
bilities due to difference in their chemical potentials and
attaching kinetics. We think that the attaching probabil-
ities are different even among the forced sites (and among
the sticky sites) since they depend on the local configu-
rations. However, the nucleation probability would be
much smaller than that of the attaching probability of
a tile in any case since the former demands a coopera-
tion of many tiles. Slow process of nucleation implies a
layer by layer growth for a perfect decagonal quasicrys-
tal [20]. It is beyond the scope of this Letter to predict
the growth kinetics of real quasicrystals since it requires
knowing atomic cluster structures corresponding to each
type of tiles as well as the kinetic parameters of atomic
attachment of real materials.
Our growth algorithm has a couple of limitations.
First, it can produce only one kind of PPT, a cartwheel
tiling. Second, the seed must include a decapod defect.
However, a decapod defect may form under quite general
conditions. It is believed that every possible hole sur-
rounded by an arrow-matched Penrose tiling is equivalent
to a decapod hole [11]. The bottom layer, which may
grow under structurally different environment, is natu-
ral place to have such defect. Our algorithm shows that
PPT is possible from the second layer if the defect can
be surrounded by legal tiles. Once a PPT layer begins to
form on the second layer, our growth algorithm produces
PPT layers easily from the third layer. We hope that the
present work stimulates studies on 3D growth rules for
real quasicrystals.
We would like thank P. J. Steinhardt and M. Rechts-
man for valuable comments. This work was supported
by the Korea Research Foundation Grant (KRF-2005-
015-C00169).
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[16] We introduce the concept of sticky sites as a mathemat-
ical devise to avoid the mistakes of copying a tile in a
flipped worm but it may mimic the real growth process of
quasicrystals. Recently, Fournée et al. observed “traps”
or sticky sites on which adatoms are easily captured for
some quasicrystal surfaces [21].
[17] Covering of all ends of the semi-infinite worms further
ensures that all sticky sites in the decapod tiling stay
outside of the semi-infinite worms.
[18] From the Fig 2(b), one can see that an active decapod
tiling is obtained by flipping a semi-infinite worm in a
cartwheel tiling while flipping a half of an infinite worm
results in an inactive decapod tiling.
[19] The requirement of the underneath tiles is added to en-
sure that the nucleation happens from the third layer.
In real growth, this requirement may not be needed. Nu-
cleation is likely to happen only on the top layer (hence
from the third layer) which can be large enough to wait
the slow necleation process.
[20] The sticky sites (Fig. 2(a)) are formed only on a compact
cluster of tiles. Therefore, the vertical growth by Rule-V,
which produces isolated tiles, cannot be continue more
than one layer height without nucleation process.
[21] V. Fournée et al., Phys. Rev. B 67, 033406 (2003).
http://arxiv.org/abs/cond-mat/9903074
