Biomedical subjects

K Ø Rasmussen

Publications and source records attributed to K Ø Rasmussen.

7 recordsLinked to original sources

PubMed2006

Probing the mechanical unzipping of DNA.

A study of the micromechanical unzipping of DNA in the framework of the Peyrard-Bishop-Dauxois model is presented. We introduce a Monte Carlo technique that allows accurate determination of the dependence of the unzipping forces on unzipping speed and temperature. Our findings agree quantitatively with experimental results for homogeneous DNA, and for lamda-phage DNA we reproduce the recently obtained experimental force-temperature phase diagram. Finally, we argue that there may be fundamental differences between in vivo and in vitro DNA unzipping.

Bacteriophage lambda↗

PubMed2004

Multipeaked polarons in soft potentials.

We consider a minimal coupled charge / excitation-lattice model capturing a competition between linear polaronic self-trapping and the self-focusing effects of a soft nonlinear on-site potential. The standard single-humped polaron ceases to exist above a critical value of the coupling strength, closely related to the inflection point in the nonlinear potential. For couplings beyond this critical value, we find that successive multihumped polaronic solutions correspond to the lowest-energy stationary states of the system, which may admit interesting quantum resonance behavior.

Journal Article↗

PubMed2004

Elastic moduli of multiblock copolymers in the lamellar phase.

We study the linear elastic response of multiblock copolymer melts in the lamellar phase, where the molecules are composed of tethered symmetric AB diblock copolymers. We use a self-consistent field theory method, and introduce a real space approach to calculate the tensile and shear moduli as a function of block number. The former is found to be in qualitative agreement with experiment. We find that the increase in bridging fraction with block number, that follows the increase in modulus, is not responsible for the increase in modulus. It is demonstrated that the change in modulus is due to an increase in mixing of repulsive A and B monomers. Under extension, this increase originates from a widening of the interface, and more molecules pulled free of the interface. Under compression, only the second of these two processes acts to increase the modulus.

Journal Article↗

PubMed2004

Improved convergence in block copolymer self-consistent field theory by Anderson mixing.

A modification to real space polymeric self-consistent field theory algorithms that greatly improves the convergence properties is presented. The method is based on Anderson mixing [D. G. Anderson, J. Assoc. Comput. Mach. 12, 547 (1965)], and each iteration computed takes negligibly longer to perform than with other methods, but the number of iterations required to reach a high accuracy solution is greatly reduced. No a priori knowledge of possible phases is required to apply this method. We apply our approach to a standard diblock copolymer melt, and demonstrate iteration reductions of more than a factor of 5 in some cases.

Journal Article↗

PubMed2001

Hysteresis and metastability in the quenched turbulent dynamics of the complex Ginzburg-Landau equation.

We consider the quenched dynamics of the two-dimensional complex Ginzburg-Landau equation in its turbulent regime. We initialize the system in a frustrated state and observe how frustration affects the evolution towards the turbulent state. This process is performed for parameter values where, for random initial conditions, the system evolves into the turbulent state. We observe that the glassiness of the initial condition can inhibit the occurrence of the absolute instability close to the critical point for that instability in parameter space. Sufficiently far from the critical point, the turbulent state will develop, but only after spending considerable time in a transient metastable state of fixed vortex density. The parameter distance from the critical point is found to scale as an exponential of a power of the lifetime of the metastable state, and with a power exponent depending on the "depth" of the original quench. The limiting regimes of shallow and deep quench are identified by their respective values of the exponent, and the distinct mechanisms leading to the relaxation to turbulence in each case are highlighted.

Journal Article↗

PubMed2001

Three-dimensional elastic compatibility and varieties of twins in martensites.

We model a cubic-to-tetragonal martensitic transition by a Ginzburg-Landau free energy in the symmetric strain tensor. We show in three dimensions (3D) that solving the St. Venant compatibility relations for strain, treated as independent field equations, generates three anisotropic long-range potentials between the two order parameter components. These potentials encode 3D discrete symmetries, express the energetics of lattice integrity, and determine 3D textures. Simulation predictions include twins with temperature-varying orientation, helical twins, competing metastable states, and compatibility-induced elastic frustration. Our approach also applies to improper ferroelastics.

Journal Article↗

PubMed2001

Twisted localized modes.

In a number of recent papers the so-called twisted localized mode of the discrete nonlinear Schrödinger equation has been proposed. Herein, we study the existence and stability properties of such modes. We analyze the persistence of quasiperiodic modes and study the domains of existence and numerical stability of the exact form of such solutions. We identify the bifurcations through which they lose their stability and follow the behavior of the intrinsic localized modes and their eigenmodes even in the unstable regime.

Journal Article↗