Atomic bond fluctuations and crossover to potential-energy-landscape-influenced regime
in supercooled liquid
V. A. Levashov
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
T. Egami
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA;
Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA;
and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
R. S. Aga
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
J. R. Morris
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6115 USA
and Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200, USA
Received 16 April 2008; revised manuscript received 14 August 2008; published 17 October 2008
The ideas related to potential-energy landscape and cooperativity of atomic rearrangements are widely
discussed in the research field of glass transition. The crossover transition from high-temperature regime to
potential-energy-landscape-influenced regime was extensively studied using the concept of inherent structure.
However, the interpretation of this crossover behavior in terms of microscopic changes in real structures is still
lacking. In this paper we present several observations on the crossover behavior on real structures. We compare
fluctuations in the global properties total number of bonds, total potential energy, pressure versus fluctuations
in the local properties coordination number, atomic potential energy, local atomic pressure by means of
molecular dynamics simulations. We then show that the total and local fluctuations in the number of atomic
bonds in the system depend on temperature differently above and below the temperature of crossover to the
landscape-influenced regime. Similarly, the ratio between the global and local fluctuations in the potential
energy and pressure changes in the vicinity of the crossover temperature, whereas the change is less distinct
than in the case of the bond fluctuations. Our results indicate that local fluctuations become more correlated
below the crossover temperature, most likely via the interaction through the dynamic shear elastic field.
DOI: 10.1103/PhysRevE.78.041202 PACS numbers: 61.20.Ne, 61.20.Ja, 61.43.Fs
I. INTRODUCTION
The connection between structure and dynamics is a cen-
tral question in the research field of glass transition 1–3.
After many years of study it is still unclear why such prop-
erties as diffusivity, viscosity, and relaxation time change by
so many orders of magnitude while there are only small ap-
parent changes in the structure. It is also not clear if there
exist universal parameters that can describe these subtle
changes in the structure for all classes of liquids and glasses
4 –7.
In the past decades extensive numerical studies were per-
formed on different model systems exhibiting the glass tran-
sition. These studies indicated the presence of different dy-
namic regimes in supercooled liquids 1,8–10. In particular,
two temperatures are often considered to separate these re-
gimes, in addition to the glass transition temperature T
g
. One
is the mode-coupling temperature T
c
which is above, but is
rather close to T
g
11,12. The other is the so-called cross-
over temperature T
A
which lies significantly above T
g
or T
c
,
but usually below the melting temperature T
m
2. It is often
assumed that “the true supercooled liquid behavior” starts
below T
A
2.
Most often the presence of the crossover temperature T
A
is revealed in molecular dynamics MD simulations by ex-
amining the inherent structure, i.e., the structure obtained by
steepest decent relaxation quenching of the structure at a
given “real” temperature. In particular, it was shown 10
that the mean potential energy of the inherent structures ex-
hibits a nontrivial dependence on the temperature of the
original “real” structure. Thus when the system is quenched
from a high temperature, the energy of the inherent structure
is almost temperature independent. However, as temperature
is reduced below T
A
, the energy of the inherent structure
starts to decrease with the decreasing original temperature.
There are also several observations of crossover behavior
on real structure. In particular it was observed in the tem-
perature dependence of the parameter in the stretched ex-
ponential fit to the intermediate self-scattering function
10,11,13, in the temperature dependence of viscosity i.e.,
breakdown of Stokes-Einstein relationship13,14, and in
deviation from the fluctuation-dissipation relation 13. How-
ever, while these observations are important, in our view
they did not yield enough insight about the relations of these
phenomena to the microscopic structure of the real liquid.
The interpretation of the crossover phenomenon in terms of
microscopic parameters is still absent.
In this paper we present several evidences of the cross-
over behavior on the real, not the inherent, structure. In our
PHYSICAL REVIEW E 78, 041202 2008
1539-3755/2008/784/0412029 ©2008 The American Physical Society 041202-1