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The Crumpling Transition of Dynamically Triangulated Random Surfaces
Christian Münkel, Dieter Heermann
To cite this version:
Christian Münkel, Dieter Heermann. The Crumpling Transition of Dynamically Triangulated Random Surfaces. Journal de Physique I, EDP Sciences, 1993, 3 (6), pp.1359-1370. �10.1051/jp1:1993184�.
�jpa-00246799�
Oassification Physics Abstracts
05.90 64.60A 64.60F 68.10 87.20C 12.90
The Crumpling Transition of Dynamically Wiangulated Random Surfaces
Christian Mfinkel and Dieter W. Heermann
(~) Institut fur theoretische Physik, Universitit Heidelberg, Philosophenweg 19, 6900 Heidelberg, Germany
(~) Interdisziplinires Zentrum fur wissenschaftliches Rechnen der Universitit Heidelberg, 6900
Heidelberg, Germany
(Received
2 February J993, accepted 9 February1993)
Abstract. We present the crumpling transition in three-dimensional Euclidian space of dynamically triangulated random surfaces with edge extrinsic curvature and fixed topology of
a sphere as well as simulations of a dynamically triangulated torus. We used longer runs than previous simulations and give new and more accurate estimates of critical exponents. Our data indicate a cusp singularity in the specific heat. The transition temperature, as well as the exponents are topology dependent.
1. Introduction.
In this paper we are concerned with the statistical mechanics of surfaces. A
possible approach
in space dimensions D > I is to discretize the surfaceusing
atriangulation [1-4].
For thiswe need to
specify
the Hamiltonian 7i. The surface S isreplaced by
asimplical triangulation T, specified by
the number of nodes N, of links andtriangles,
and the X-coordinate fieldby
the coordinates X of the nodes, The metrical fluctuations of the manTold are modeled
by summing
overtriangulations
inducedby link-flips [5-7].
The Hamiltonian is nowchoosen,
suchthat the
partition
function is not dominatedby configurations
withspikes.
In order to surpress thesespikes
one adds a term with extrinsic curvature. As a function of the extrinsic curvature the model shows a transition(crumpling transition)
at finiterigidity
of the surface[8-18].
In aprevious
paper, we discussed the extrinsic and intrinsicgeometrical properties
ofdynamically triangulated
random surfaces(for example
theHausdorff-dimension, spectral
andspreading dimension) above,
below and at the transition[32].
The
partition
function of the above model can be written asZN "
jd~Xo j ~~ fl d~X;e~'~ (1)
where the translational mode is
integrated
out. The Hamiltonian 7i is defined a8N
~ =
p, (
(xi xj)2
+ >£ (i
na,nb>)
"~
~°~ " ~~~
ii,)) ~
,
.>~J
~ fi
7i~ ~
The Gaussian part of the Hamiltonian
zig
is a sum over thepositions
X inembedding
Euclidian space of all nearestneighbour nodes,
I-e- all links of thetriangulation.
We shall usefl
= Ibecause of the
rescaling
invariance.7ie is an
edge
extrinsic curvature term[19-26]. £~
~ denotes a summation over all
adjacent triangles
which share anedge
and na, na, isthe"scilar product
of thevectors normal to a
triangle.
The third part of the Hamiltonian 7im is the discretization of the square root of the metric.
a; denotes the number of nearest
neighbours
of node I. adepends
on the measure. We useda =
D/2
for the simulations in D = 3 dimensionalembedding
space.Here we
investigate
such a modelnumerically (Monte
Carlosimulations), applying
finite-sizescaling
ideas. These ideas have proven successful in other contexts for theanalysis
ofphase
transitions. We expect that the above model shows aphase
transition at some critical value of thecoupling
I. There are severalquestions
which we want to address: What is the order of the transition? If the transition is of secondorder,
what are theexponents?
Are the exponentstopology dependent?
Is the transition temperaturetopology dependent?
2. Simulation Method and Autocorrelations.
First let us
study
the effect of the extrinsic curvature on the autc-correlation time of severalobservables,
I-e-, we are interested in thedynamics
of a random surface inducedby
a Monte Carlo process. Such astudy
is of course aprerequisite
for a detailed numericalstudy
of theapparent transition which such surfaces
undergo
as a function of abending rigidity.
From our results which we present below we must conclude that extensive computer resources must beapplied
because of the verylong
relaxation times which even for moderate surface sizes canreach several tenthousand sweeps.
We want to look at the closed
dynamically triangulated
random surfaces without self- avoidance. As models for such surfaces we take the first twotopologically
closed surfaces:The
sphere
and the torus(cf. Fig. I).
The torus isspecified by identifying edges
of the pa-rameter space P whereas the
sphere,
because of its genus needs aslightly
morecomplicated
set-up
procedure.
What we are interested in is to calculate the autc-correlation function of an observable O as
a function of time
~°~~'~~
~~~~~i~~< I$(~~
~ ~~~and its
dependence
on the number of nodes N of the surface. From the autc-correlation function we are then able to extract the autc-correlation time To(N).
This is donecalculating
the
integrated
autc-correlationfunction,
I-e-,+OJ tint,O "
j j
d~ PO (~i
N) (4)
-OJ
We also
compared
theintegrated
autc-correlation time with theexponential
correlation time and the statisticalinefficiency,
but found nodisagreement
within the errors. If thesuccessively
Fig-I-
Shown are two examples of configurations of dynamically triangulated random surfaces.The left picture shows a sphere and the right part a torus.
generated configurations
can be considered asnon-interacting
beads as is done in the Rousetheory [27,
28] we will find forexample
for the radius ofgyration
rR» c~ N
(5)
syr
and in
general
we expectTo cc N~
(6)
To calculate observables such as the
gaussian
partzig
of the Hamiltonian or the radius ofgyration Rgyr,
we use the standard Monte Carloalgorithm [29-31].
Such analgorithm
induces a stochastic
dynamics
and all our resultsapply only
to such an induceddynamics.
One Monte Carlo sweep is
completed
when eachtriangulation point
wasgiven
the chance for adisplacement
from itsprevious position
and theedges
of thetriangulation
weregiven
the chance to re-connected orflip
to anorthogonal position (cf. Fig.
2. Theflip operation implements
thesum over all
possible triangulation).
Thiscorresponds
to one newconfiguration.
Fig.2.
This figure demonstrates the elementary moves which are made during one update of atriangulated random surfaces.
Table I summarizes our results for the exponent a for the
sphere.
The exponents where extracted from the data of system sizes from 36 up to 288. At thispoint
we should mention that theexpected
transition is atlc
cS I-S- Out data was collected above and below thetransition.
Above and below the transition the radius of
gyration
shows the Rouse behaviour. The Gaussian part of the Hamiltonian showsclearly
adependence
on thebending rigidity.
Above the transitionpoint
the autc-correlation exponentchanges
to one and below it is almost zero.The same holds for the
edge
part of the Hamiltonian.Table I. Table
of
the resultsfor
the exponent a on the auto-correlation timesof dynamically triangulated
randomsphere.
I is the measureof
thebending rigidity. Skiff
andflips give
the acceptanceprobability for
adisplacement of
a node andflip for
anedge change.
r rate
0.00 0.2+0.3 0.2+0.2 1.1+0.2 0.51 0.76
1.75 1.0 + 0.1 0.8 + 0.2 1.1 + 0.2 0.47
+
Almost all of the observations for the
spherical
case carry over to the case of the torus.3. Results of the Simulations.
Finite size
scaling
assumes, that there isonly
one relevant linearlength scale,
which is com-pared
to the correlationlength.
Toapply
finite sizescaling
to thecrumpling
transition ofdynamically triangulated
random surfaces(DTRS)
one must therefore assume asingle length
scale determined
by
the number of nodes N and the internal dimension d of the surfaceL cc
N"~ (7)
This internal dimension d also
depends
on the externalproperties
of the surface and 1[32].
So let us first look at the
specific
heat. If the transition is of second order we would haveC(I, L)
=L°'"i~ ((I lc)L~"] (8)
where
t~
isa
scaling function,
whichdepends
on how oneimplements
the surface. At the critical I thescaling
function isregular
and thescaling hypothese
leads toCj$~
c~AN°'"~
+(9)
for the
scaling
of thepeak
in thespecific
heat. If we assume a first order transition then Cj$~~diverges
asL~
because of the b-distribution ofCcc (T) [33, 35].
An evaluation of the
specific
heat C of DTRS(neglecting
the metric contribution7im) gives
the
following expression
Call " +
~ (< 7il
> < 7ie>~) (lo)
The first part is related to the Gaussian Halniltonian
zig
and the second to thespecific
heat C of theedge
extrinsic curvature 7ie. Thespecific
heat for theedge
extrinsic curvature is shownin
figure
3 for the twotopologies
considered. Theinterpolation
was doneby
the method ofFerrenberg
and Swendsen [36, 37]using histograms
of 7ie6 6
36 -
~~
64 - s -
64 --
5 108 --
100 -- 144 --
144 -
288 - 4
196 -
4
432 -
256 -
400 -
~ 3
2 2
0 0
O-S I I-S 2 2,5 3 O-S I I-S 2 2.5 3
~ ~
Fig.3.
Specific heat c(edge
extrinsic curvaturepart)
of the sphere(left part)
and of the torus(right part),
6
5.5
~
# 5
~- ~-
~iZ 4.5
#
E
4 torus --
sphere --
3,5
3
50 100 150 200 300 400500
N
FigA.
Maximum of the specific heat cfl~~ of the torus(o)
and the sphere(D).
Using
the data of thespecfic
heat obtainedby applying
theextrapolation method,
we canget
a very accurate estimate of thepositions
of the maxima and of theheights. Figure
4 showsCJ/~~
of the torus and thesphere.
Achange
to L~ behaviour is veryunlikely
and for thatreason the data
strongly
suggest a continuousphase
transition inagreement
withprevious
work [8, 14-17,
38, 39].
From the data shown infigure
4 we can obtain thefollowing
upperboundaries of critical exponents
Sphere: ~
s 0.00 + 0.04 Torus:~
s 0.06 + 0.02(11)
Using
thissimple method,
we cannotdistinguish
adiverging specific
heat with very small butpositiv
a, alogarithmic divergence
a, = 0 and a power law cusp a, <0,
where a, denotes the exponent of thesingular
part of thespecific
heat. We will use the abbreviation a instead of a,.Following
Fisher [40], these three cases may bedistinguished
with a fit of the formC(Al)
= A(Al~~ I)
+ B,
Al =
[lf 1[ (12)
Figure
5 shows such fits for a= 0.I
(power law),
a = -0.01(near logarithmic)
and a=
-1.2,
-2.0(power
lawcusp).
The fitsclearly
favour a power law cusp of thespecific
heat witha
large negative
value of a,although
we were not able to estimate the exponent aprecisely.
6 6
5.5 36 - 5.5 36 - .--=1
~ 64 - 64 4"I
~ IDS -
~ lot
-
4.5 144 - 4.5 144 - _,
~ 28K -
~ 28K -
432 - 432
3 5 3 5
3 "
3
2.5 2.5
~ ~
l.5 1.5
l i
o.5 o.5
lo 5 o 5 lo lo 5 o 5 lo
x x
6 6
5.5 36 - 5.5 36
64 64
log -
5 lo~ -
4.5 144 - 4.5 144 -
~ 288 - 28K -
432 - 4 432 -
3.5 3.5
3 " 3
25 2.5
2 2
1.5 l.5
l j
o.5 05
08 -0.6 -DA -0.2 0 0.2 0A 0.6 0.8 0.6 DA 02 0 o.2 DA 0.6
x x
Fig-S-
Plot of the specific heat c against z=
(al~° -1)
with a= 0.1
(upper left),
-0.01(upper right),
-1.2(lower left)
and a a= -2.0
(lower right).
Our results are in contrast to the estimates of Renken and
Kogut
[39]a/vd
=0.14(3)
for thesphere. They
assumedscaling
above N = 72, but the more accurate data inFigure
4 shows that thisassumption
is not true. Thedecreasing slope
inFigure
4 is also present in theirFigure
5 [39]although
this isslightly
maskedby larger
error barrs and less datapoints.
Catteral et all. [38]
reported
an estimate ofa/vd
=
0.185(50),
butthey
used a different discretization basedon
#~ graphs
with aflip
acceptance rate ofonly O(I$l)
atlc
cS I-S andlarger
correlation times of theedge
extrinsic curvature than in our discretization.Another
possibility
to determine the order of the transition is the cumulant VN of theedge
extrinsic curvature
VN := 1-
~~~~ )
,
(13) ie~)~g
which behaves
quite differently
at temperature driven first- and second-order transitions[33,
34]:1, and 2.order: VN)ruin
'~I" T#Tcfixed (14)
2,order: VN)ruin
'~1"
~T =
Tc(N) (15)
1-order: VN)mjn
'~I"
~~~~
~~~ f
T=
Tc(N) (16)
3(E(
+Et
E+
and E- are theenergies
of the system above and below the transition. For a very weak first-order transition(E+
cSE-)
we also have VN)~n;n *2/3.
We
computed
VN definedby equation (13) using again
the method ofFerrenberg
and Swend-sen
[36, 37].
Theresulting figures
show thepredicted single peak
minima withwings following equation (14).
The finite sizedependence
of the minima of VN)~n;n shown inFigure
6distinctly
favours the
asymptotic
behaviour inequation (15)
and therefore a continuousphase
transitionor a very weak first order transition.
0.67
0.65
0.64 a
)
~
~
0.63
-.
~ ~
0.62 0.61 0.6
0.59
0 0.005 0.01 0.015 0.02 0.025 0.03
IN
Fig.6.
Finite size dependence of the minimum VN )nfin of the reduced cumulant of the edge extrinsic curvature.(o)
denotes data of the sphere,(D)
those of the torus and the arrow the large N limit2/3
of a continuous transition.In
general,
finite sizescaling predicts
also a shift Al=
If -16f'
of the effective transition'temperature' If proportional
toN~~(= L~~)
for a first- andproportional
toN~Q"~(=
L~Q")
for a second-order transition.Unfortunatly I~°
ofdynanfically triangulated
random surfaces is not known. For that reason, we have to use a twc-parameter fit with unknownsIT
and vd. But we can
improve
thereliability
of thisfit,
because we know more.First,
the fithas to be a
straight
line(neglecting
corrections toscaling)
and we havelimN-cc Al(N)
= 0.Second,
the shiftAl(N)
is different for thespecific
heat and the cumulant ingeneral.
Therefore theslope
of the fitted lines will be different ingeneral,
but we stiff havefimN-co Al(N)
= 0 for both observables.0.25 0.16
0.14 C -
V -
02 C -
V +- 012
O-1 0.15
~~g
~ o-t
~
l'~
~~~
0 .j
i
o ~o.02
0 o.05 o 0.15 o.2 o.25 o.3 o o.05 0. t o.15 o.2
x x
Fig.?.
Best fit of y = Al against z = N~~'"~ for the sphere(left part)
and the torus(right part).
(o)
denotes the specific heat c data,(D)
those of the cumulant V.Figure
7 shows the best fits for thesphere
and the torus. The estimates of the parametersare
lm
= 1.51 + 0.04,
ud = 3.2 + 0.5
(17)
for the
sphere
andlm
= 1.47 + 0.02,
vd = 2.5 + 0.5
(18)
for the torus.
We turn now to the order parameter itself. Common
practise
is to take(
=R/L (R
is thetypical radius,
L the linear size of themembrane)
to be a suitable order parameter[41-47].
Initially
[47] it was also definedas
Rg(L)
=(L (L
-oJ),
with the linear size L of thehexagon
and the radius of
gyration R(c~ £;; IX; X; [~).
A suitable choice for the order parameter therefore is(
:= R~IN (19)
with
N
~l~
"
j~r(j~r
i) £'i'i l'~i -'~I)~ (~°)
;,j
a; denotes the number of nearest
neighbours,
I-e- the number of links connected to a node I.The associated
susceptibility
isxR» := L~
(1(~) l()~)
=(
(lR~) lR~)~) (21)
Figure
8 shows the orderparameter (
andFigure
9 thesusceptibilty
of thesphere.
The results for the torus are similar.0.1
~
0.09
~
wP °
~
0.08 o° ~$ i
0.07
~~
°
j~~ ((
.~~
~ +
~
~
~~
0.04
o
~
° x
432
*0.03
~
~
~
0.02
~
~ ~
0.01
0
0 0.5 1.5
22.5
3I
Fig.8.
Order parameter (=
R~
IN
of the sphere. Errors are smaller than symbol size.0.08
DTRS, sphere
~
0.07
~~
0.06
~
~((
~0.05
"~
(~
~*~0.04
~
°.°3
.j~~J
«
,
o o~
_.jj,
'
'».--q~j",,
~
#a..,_"
__
'~~
.-
-«~'
"".)
i
0
0.7
0.80.9
1-I1.21.3 1.41.5 1.6
1.71.8
1.9 2 2.II
Fig-g-
Susceptibiity XR» of the sphere(Lines
to guide theeye).
5 4.5
4 _,:.." ~:
3.5
~:.."'
3 torus -- _:.""
2 5
sphere
-- __:.""~..."'
A 2
~
_.-..'"
ii'
_,:."~~ 1.5 _:.."
V ,.~..'"
l __:.""
,:a."'
0.5
50 100 150
200
300400500
N
Fig,10.
Scaling of the radius of gyration squared(R~(N))
at the position of the maximum of the specific heat c~~~(o)
denots the data of the torus,(D)
data of the sphere.0.06
0.05 _,..4""
torus -- __:."
~
0.04 sphere
--_:.""'
0.03 __:.-."~
_,:...'~
0.02
~,:.""
%,....""
o.oi
50 100 150 200 300
400
N
Fig,ll.
Scaring of the susceptibility X(Eq. (21), Fig.
9) in the case of a torus(o)
and a sphere(°).
We also measured another
possible
order paramterI'
=
(R'~) IN,
IN
(R'~)
=£ (X; X)~
~
i
which exhibits a small increase near the
phase
transition and a slowerdecay
ofI'
for 1-0.The difference is caused
by
achange
of the internal geometry near thephase
transition[32].
With the data for the
sphere
infigure
8 and thecorresponding
data of the torus one can estimate the critical exponentfl/vd
of the order parameterassuming
a second order transition.Here we use as the effective critical temperature the
position
of thepeak
of thespecific
heat.C~~~
(cf. Fig. 10).
An estimate of
fl/vd using
results in
Torus:
fl/vd
= 0.28 + 0.02
,
Sphere: fl/vd
= 0.35 + 0.04(23)
and with the values of vd in
equation (17)
and(18)
Torus:
fl
= 0.7 + 0.2
,
Sphere: fl
= I.1 + 0.2(24)
Figure
11 shows thescaling
of the maxima of thesusceptiblity
X(Eq. (21), Fig. 9).
The results are
Torus:
7/vd
= 0.62 + 0.06,
Sphere: 7/vd
= 0.66 + 0.06
(25)
and, using
the values of vd inequations (17)
and(18),
Torus: 7 = 1.6 + 0.5
,
Sphere:
7 = 2.1 + 0.5(26)
With these
large
error bars and estimated values a t3 -1.5 the results are almostcompatible
with the
scaling
relationa+2fl+7=2 (27)
Acknowiedgements
Partial support
by
the SFB 123 and the BMFTproject
0326657D and 031240284 isgratefully acknowledged.
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