Mikko P. Haataja (PhD, McGill University)
Assistant Professor
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Welcome to the homepages of the COMBO
(COmputational Materials &
BiOphysics)
group! I am the leader of a young group, whose research focuses on theoretical
and computational materials science,
physics of materials, and biophysics. The underlying theme of our research can be stated
as "evolving microstructures from materials to biology".
Our research is currently supported by the National Science Foundation (NSF).
Current work includes
studies of microstructure formation during solid-solid phase transformations and
solidification, dislocation dynamics, mechanical properties of metallic glasses, and
compositional domain formation on surfaces for catalysis applications. More recently, I have also developed an
interest in various problems in biophysics/biomechanics, such as signaling pathways
in cells and thermodynamics and kinetics of spatial heterogeneities ("lipid rafts")
in the plasma membrane of mammalian cells. To give
you a flavor of my research interests, a brief description
of some of the problems I've been working on recently can be found below.
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PAST,
PRESENT & FUTURE RESEARCH ACTIVITIES
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I am interested in understanding how elastic strains affect thin film
growth in heteroepitaxy. Initially, the growing film is under strain
and the relaxation of these strains can be facilitated either through the
development of an undulated morphology or nucleation of misfit dislocations,
or both. Such structures are very interesting as they can be employed as quantum dots
for semiconductor optoelectronics applications. To this end, I have
constructed and studied a phase-field model which
included both the morphology of the film as well as dislocation density field.
The figure on the left shows a typical thin film morphology during strain relaxation while
the panel on the right shows the x-component of the corresponding dislocation density field. Dislocations
accumulate at the film-substrate interface and facilitate strain relaxation. More details
can be found in the following references: M. Haataja et al., PRB 65, 035401 (2001);
PRB 65, 165414 (2002).
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Another problem of great interest to me has to do
with the role of dislocations in phase separating
binary alloys. If the lattice spacings of the two new phases are
unequal, coherency strains build up which eventually are relaxed through
nucleation of misfit dislocations. We have examined this process through
analytical calculations and simulations of the dislocation density field coupled to the evolving
composition. The figure on the left shows the alloy morphology during the phase separation process
the panel on the right shows the x-component of the corresponding dislocation density field. Discrete dislocations
accumulate at the compositional domain walls and facilitate strain relaxation. On a
quantitative level, we have
shown that the effect of mobile dislocations on domain coarsening kinetics can be captured in a
single scaling form, amenable to experimental verification. More details
can be found in the following references: M. Haataja and F. Leonard, PRB 69, 081201 (2004);
F. Leonard and M. Haataja, APL 86, 181909 (2005); M. Haataja et al., APL 87, 251901 (2005).
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Yet another problem of great interest to me has to do
with extending the time scales of atomic scale simulations. To this end,
we introduced a continuum "phase-field crystal model" capable of simulating crystalline phases with atomic
scale spatial resolution across diffusive time scales [for details, see K. R. Elder
et al., PRL 88, 245701 (2002)]. More recently, together with Peter Stephanovic and
Nick Provatas, we have extended this method to include elastic
relaxation through the propagation of "quasi-phonons" [Stefanovic et al., PRL 95, 225504 (2006)]. The figures below
show the propagation of a crack from a circular void.
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Related to the PFC method, we have recently attempted to extend it ferroelectric materials
by coupling the density field to a continuum polarization field. The figures below
illustrate the possibility of manipulating the grain structure of a nanocrystalline ferroelectric material
with externally applied electric field. Left panel: grain structure in the absence of polarization. Middle panel:
grain structure in the presence of a rotating E-field. Right panel: local polarization vector. [Haataja et al.,
manuscript in preparation.]
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Yet another problem of great interest to me has to do
with the role of dislocations in polycrystalline materials which undergo
a recrystallization process. The figure below (from a phase-field model)
shows the microstructure of a recrystallized material. We have developed a coupled coarse-grained dislocation density and phase-field method
for studying the effect of heterogeneous dislocation networks on the
growth morphology of isolated recrystallized grains [S. Sreekala and
M. Haataja, Phys. Rev. B 76, 094109 (2007)].
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We have also recently started investigating mechanical properties of metallic glasses and metallic glass-nanocrystalline material
composites through simulations of a novel continuum model. The figures below
illustrate the deformation of such a composite. From left to right and top to bottom: (1) local slip threshold in the composite; (2)-(6) local shear stresses at increasing
external strains. Note the development of localized shear bands in (4) which ultimately propagate into the nanoscale crystals (5,6). [Abdeljawad and Haataja,
manuscript in preparation.]
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I am also interested in the dynamics of driven systems. We have recently studied
the dynamics of interfaces in an Ising model coupled to diffusing impurities as well as driven
dislocation lines in the presence of immobile misfitting solute. In the former case, we have
shown that the behavior of the system can be described (in a certain limit) in terms
of an effective free-energy which decribes the kinetics of impurities attaching to and
detaching from the interface. The figure on the left below shows typical pinned configuration
where the local impurity density along the interface is very large. The figure on the right displays
a typical configuration of a dislocation line driven through a field of misfitting solute atoms. For the Ising model, more details
can be found in M. Haataja et al., PRL 92, 160603 (2004).
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We have also recently worked on developing physically-based models which explain morphologies of wurtzite nanoribbons.
In particular, such nanoribbons have been experimentally observed to spontaneously bend into nanorings or nanoarcs. Our theory suggests that bending
is controlled by dopant density, spontaneous polarization, and nanoribbon thickness. The figures below display the crystal
structure of ZnO and predicted nanoribbon curvature as a function of thickness H and dopant density [C. Majidi, Z. Chen, D. J. Srolovitz, and
M. Haataja, submitted for publication (2009)].
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We have recently launched a research effort in soft condensed matter systems.
More specifically, we are currently investigating (1) self-organization and
non-equilibrium dynamics in lipid bilayers, (2) molecular structure and properties of
lipid bilayers, and (3) self-assembly of detergents in bulk and on surfaces. The
first figure from the left shows a typical microstructure in a three component
lipid bilayer undergoing phase separation in the presence of non-equilibrium flux
of lipid to and from the monolayer in view. This "lipid recycling" leads to a
steady-state where the domains attain a characteristic size dependent on the
recycling rate. We argue that this recycling-induced steady-state
can explain the presence of the elusive
"lipid rafts" thought to exist in the mammalian plasma membrane. The second
figure from the left displays a single Sodium Dodecyl Sulphate molecule on top
of a graphite surface. We are interested in the micellization dynamics
and structures on such surfaces; an example from a simulation is shown in the
third figure from the left. Finally, the rightmost figure displays a snapshot from
a simulation where micelle formation takes place. In addition to lipids and detergents,
we are interested in the behavior of synthetic molecular switches in biology. To this end,
we have constructed physically-based models for the tether length dependence of signal integration proteins, shown
schematically in the bottom figure [Van Valen, Haataja, and Phillips, Biophys. J. 96, 1275 (2009)].
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RECENT PAPERS I: Condensed matter physics
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[1] M. Haataja J. Muller, A. D. Rutenberg, and Martin Grant,
"Dynamics of dislocations and surface instabilities in misfitting
heteroepitaxial films", Phys. Rev. B 65, 035401 (2001).
