Hot Nuclei - Landau Theory, Thermal Fluctuations and Dissipation

Isotropic-genesis nematic elastomers (IGNEs) are liquid crystalline polymers (LCPs) that have been randomly, permanently cross-linked in the high-temperature state so as to form an equilibrium random solid. Thus, instead of being free to diffuse throughout the entire volume, as they would be in the liquid state, the constituent LCPs in an IGNE are mobile only over a finite, segment specific, length-scale controlled by the density of cross-links. We address the effects that such network-induced localization have on the liquid–crystalline characteristics of an IGNE, as probed via measurements made at high temperatures. In contrast with the case of uncross-linked LCPs, for IGNEs these characteristics are determined not only by thermal fluctuations but also by the quenched disorder associated with the cross-link constraints. To study IGNEs, we consider a microscopic model of dimer nematogens in which the dimers interact via orientation-dependent excluded volume forces. The dimers are, furthermore, randomly, permanently cross-linked via short Hookean springs, the statistics of which we model by means of a Deam–Edwards type of distribution. We show that at length-scales larger than the size of the nematogens this approach leads to a recently proposed, phenomenological Landau theory of IGNEs [Lu et al., Phys. Rev. Lett.108, 257803 (2012)], and hence predicts a regime of short-ranged oscillatory spatial correlations in the nematic alignment, of both thermal and glassy types. In addition, we consider two alternative microscopic models of IGNEs: (i) a wormlike chain model of IGNEs that are formed via the cross-linking of side-chain LCPs; and (ii) a jointed chain model of IGNEs that are formed via the cross-linking of main-chain LCPs. At large length-scales, both of these models give rise to liquid–crystalline characteristics that are qualitatively in line with those predicted on the basis of the dimer-and-springs model, reflecting the fact that the three models inhabit a common universality class.

Download Full-text

SURFACE-DIRECTED SPINODAL DECOMPOSITION: LATTICE MODEL VERSUS GINZBURG–LANDAU THEORY

Modern Physics Letters B ◽

10.1142/s0217984909018886 ◽

2009 ◽

Vol 23 (04) ◽

pp. 549-565 ◽

Cited By ~ 7

Author(s):

KURT BINDER ◽

SUBIR K. DAS ◽

JÜRGEN HORBACH

Keyword(s):

Phase Separation ◽

Boundary Conditions ◽

Spinodal Decomposition ◽

Large Scale ◽

Binary Systems ◽

Landau Theory ◽

Lattice Models ◽

Thermal Fluctuations ◽

Full Account ◽

Ginzburg Landau

When a binary mixture is quenched into the unstable region of the phase diagram, phase separation starts by spontaneous growth of long-wavelength concentration fluctuations ("spinodal decomposition"). In the presence of surfaces, the latter provide nontrivial boundary conditions for this growth. These boundary conditions can be derived from lattice models by suitable continuum approximations. But the lattice models can also be simulated directly, and thus used to clarify the conditions under which the Ginzburg–Landau type theory is valid. This comparison shows that the latter is accurate only in the immediate vicinity of the bulk critical point, if thermal fluctuations can also be neglected (true for the late stages of phase separation). In contrast, a local kinetic molecular field theory can take full account of nonlinearities and of rapid concentration variations, and thus has a much wider validity. This enables the detailed study of phase separation processes in thin films of solid binary alloys. However, the extension to spinodal decomposition in fluid binary systems (which can be simulated by brute force large scale molecular dynamics methods, of course) remains an unsolved challenge.

Download Full-text

ZERO—POINT SPIN FLUCTUATION EFFECTS IN ANHARMONIC WEAK ITINERANT ELECTRON MAGNETS

International Journal of Modern Physics B ◽

10.1142/s0217979293001220 ◽

1993 ◽

Vol 07 (01n03) ◽

pp. 585-588 ◽

Cited By ~ 6

Author(s):

ALEXANDER SOLONTSOV

Keyword(s):

Inelastic Neutron Scattering ◽

Spin Fluctuation ◽

Landau Theory ◽

Zero Point ◽

Thermal Fluctuations ◽

Inelastic Neutron ◽

Spin Fluctuations ◽

Itinerant Electron ◽

Fluctuation Effects

The spin fluctuation theory of weak itinerant magnets is critically reviewed and is shown to be well established in the weak spin anharmonicity limit. A new self—consistent approach is formulated in terms of the Landau theory of phase transitions to account for zero—point and thermal fluctuations in anharmonic itinerant magnets. The role of spin fluctuations and Fermi excitations in MnSi, Ni3Al and ZrZn2 is discussed using the inelastic neutron scattering experiments.

Download Full-text

Recent Development of the Magnetic Shape Memory Materials Research in Finland

MRS Proceedings ◽

10.1557/proc-1200-g03-03 ◽

2009 ◽

Vol 1200 ◽

Author(s):

Outi Söderberg ◽

Ilkka Aaltio ◽

Yanling Ge ◽

Alexandr Soroka ◽

Raisa Niemi ◽

...

Keyword(s):

Fatigue Life ◽

Los Angeles ◽

Shape Memory ◽

Landau Theory ◽

Thermal Fluctuations ◽

Magnetic Shape Memory ◽

Twin Boundaries ◽

Mechanical Cycling ◽

Materials Research

AbstractNi-Mn-Ga based magnetic shape memory (MSM) materials have been studied since 1998 in Finland at the Helsinki University of Technology (TKK, previously HUT). The large HUT-MSM-project resulted in MSM-alloys with high service temperature, 10 % field-induced-strain, as well as circumstances when and how a Ni-Mn-Ga alloy exhibits this phenomenon. The understanding of the structure and behavior of twin boundaries, and their role, for example, in the vibration damping and long-term actuation has been enhanced in the recent projects. Twin boundaries have been studied by XRD, by high-resolution transmission electron microscopy (HRTEM), and by in-situ straining in TEM, the last one in co-operation with the Institute of Physics in Prague (ASCR-IP), Czech Republic. The results obtained by neutron diffraction in co-operation with Hahn-Meitner-Institut Berlin, Institute for Metal Physics (IMP), Kiev, and Institut Laue-Langevin (ILL), Grenoble, have given new crystallographic information. Damping of Ni-Mn-Ga polymer composites has been proved to be excellent at high stiffness levels with the loss factor = 0.6 at E ≈ 1 GPa. This research was carried out in co-operation with the University of California Los Angeles (UCLA), USA. In the long-term actuation, a fatigue life of 2×109 has been recorded for a five-layered modulated Ni-Mn-Ga structure in mechanical cycling. The evolution of the MSM parameters during the long-term use is recorded and used as an input data for the models developed in the European MAFESMA co-operation. The search for alloys with wide stable thermal property range showing MSM effect has continued and alloys that are stable down to 4 K have been established. Modeling based on Ginsburg-Landau theory has been applied to evaluate aging and thermal fluctuations in the modulated Ni-Mn-Ga structures. As a commercial target, AdaptaMat Ltd. develops technology to produce Ni-Mn-Ga magnetic shape memory material with improved quality, lower twinning stress, longer fatigue life as well as lower cost and better availability for use in research and development.

Download Full-text