Theory of the optical properties of phonon systems with disordered force constants,with application to NH4Cl

A theory of systems with substitutional disorder is presented. The theory treats off-diagonal disorder and is thus suitable for application to systems with disordered force constants. An example of such a system, to which particular consideration is given, is NH₄Cl, which has a disordered phase in which the force constants between NH₄⁺ ions are disordered because there are two equally probable orientations of an NH₄⁺ ion within a unit cell. The effect of disorder, in particular the breakdown of k-conservation, on the optical properties of such systems is studied via a calculation of the single-particle Green's function. Both the optical absorption and Raman cross section are shown to be related to the self-energy of this Green's function, and expressions for these quantities are obtained which are exact to second order in the ratio of order-dependent to non order-dependent force constants. The major qualitative features of the expression for the Raman cross section are found to be in good agreement with existing Raman scattering data on NH₄Cl.     

Tri-partite complex for axonal transport drug delivery achieves pharmacological effect

Background

Targeted delivery of pharmaceutical agents into selected populations of CNS (Central Nervous System) neurons is an extremely compelling goal. Currently, systemic methods are generally used for delivery of pain medications, anti-virals for treatment of dermatomal infections, anti-spasmodics, and neuroprotectants. Systemic side effects or undesirable effects on parts of the CNS that are not involved in the pathology limit efficacy and limit clinical utility for many classes of pharmaceuticals. Axonal transport from the periphery offers a possible selective route, but there has been little progress towards design of agents that can accomplish targeted delivery via this intraneural route. To achieve this goal, we developed a tripartite molecular construction concept involving an axonal transport facilitator molecule, a polymer linker, and a large number of drug molecules conjugated to the linker, then sought to evaluate its neurobiology and pharmacological behavior.

Results

We developed chemical synthesis methodologies for assembling these tripartite complexes using a variety of axonal transport facilitators including nerve growth factor, wheat germ agglutinin, and synthetic facilitators derived from phage display work. Loading of up to 100 drug molecules per complex was achieved. Conjugation methods were used that allowed the drugs to be released in active form inside the cell body after transport. Intramuscular and intradermal injection proved effective for introducing pharmacologically effective doses into selected populations of CNS neurons. Pharmacological efficacy with gabapentin in a paw withdrawal latency model revealed a ten fold increase in half life and a 300 fold decrease in necessary dose relative to systemic administration for gabapentin when the drug was delivered by axonal transport using the tripartite vehicle.

Conclusion

Specific targeting of selected subpopulations of CNS neurons for drug delivery by axonal transport holds great promise. The data shown here provide a basic framework for the intraneural pharmacology of this tripartite complex. The pharmacologically efficacious drug delivery demonstrated here verify the fundamental feasibility of using axonal transport for targeted drug delivery.     

Magnetic Field in a Screw Flow with Fluctuations

We consider the influence of fluctuations in a screw flow of a conducting liquid on the effect of magnetic field self-excitation; the solution of this problem is important for experimental realization of a turbulent dynamo. We propose a theoretical approach based on the solution of averaged equations obtained in the limit of a short correlation time. The applicability of this approach is confirmed by direct numerical simulation of the initial equations. We demonstrate the influence of the correlation of fluctuations on the dynamo effect threshold. It is shown that the solution of the mean-field equations differs from the solution based on direct numerical simulation for a finite correlation time. The advantages and disadvantages of the two approaches are estimates, as well as the importance of the discovered difference in the context of problems of magnetic field self-excitation. The influence of helicity and intermittency on the type of the solution is considered.     

The intermittency of vector fields and random-number generators

We examine how well natural random-number generators can reproduce the intermittency phenomena that arise in the transfer of vector fields in random media. A generator based on the analysis of financial indices is suggested as the most promising random-number generator. Is it shown that even this generator, however, fails to reproduce the phenomenon long enough to confidently detect intermittency, while the C++ generator successfully solves this problem. We discuss the prospects of using shell models of turbulence as the desired generator.     

Unusual band structure and exotic electrical conduction for electrons in incommensurate lattice potentials

Electrons in crystals with incommensurate periodic potentials are shown to exhibit a band structure with many very narrow bands separated by narrow band gaps. This makes such systems ideal candidates for observing such exotic electrical conduction phenomena in d.c. fields as Zener tunneling and Stark oscillations. Estimates are made of the conditions under which such phenomena should be observable experimentally.     

Unusual band structure,wave functions and electrical conductance in crystals with incommensurate periodic potentials

The subject of electronic and phonon states in crystals with incommensurate periodic potentials is reviewed. The emphasis is on the physics behind the various theoretical methods of treating this problem and on showing the relationships between them. This review is meant to complement the reviews of mathematically rigorous results on the problem by Simon and by Bellissard. Although the review is primarily theoretical, there is a section which discusses possible experimental application of the theory, as well as experimental observations that have already been made. Some new theoretical results are presented and some old results are reinterpreted.     

Commensurability in one-dimensional lattices at finite temperature

The theory of commensurability transitions in one-dimensional atomic chains has been applied to charge density waves, mercury chain compounds, superionic conductors, etc. Previous numerical and analytical results on a model with chains of atoms with nearest neighbor interactions and periodic external potentials have dealt mainly with equilibrium and dynamical properties atT = 0 K. These studies are extended to nonzero temperatures. It is found that the reversible work per particle to slide the chain vanishes in the thermodynamic limit for any nonzero temperature. The mathematical pathologies associated with the commensurability transition atT= 0 (i.e., the devil's stair) are absent at finite temperature; only thermodynamic evidence of low-order Commensurability transitions remains.Supported by the Department of Energy, Contract number EG-77-S-02-4432.