S. D. Mahanti

Structure factor for the periodic walls of mesoporous MCM-41 molecular sieves

A periodic wall model has been developed to calculate the structure factor of hexagonal MCM-41 molecular sieves. This is a discrete lattice version of the continuum periodic cylindrical shell model of Oster and Riley. Using the periodic wall model we can explain the dramatic change in Bragg X-ray scattering that occurs upon removal of the structure-directing surfactant from the pores of the as-synthesized mesostructure. The physical origin of this intensity change can be traced to the sensitive phase relationship between the scattering from the pores and the walls constituting the open inorganic (silica) framework. The scattering intensity is affected only weakly by short-range disorder within the framework walls, provided that the long-range order of the pores channels is maintained. A similar periodic wall model may also be expected to explain the large Bragg intensity changes that occur upon surfactant removal from cubic MCM-48 mesostructures.

Self-assembly of neutral and ionic surfactants: An off-lattice Monte Carlo approach

We study self-assembly of surfactants in two dimensions using off-lattice Monte Carlo moves. Here the Monte Carlo moves consist of slithering snake reptation motion of the surfactant chains and kink-jump of the individual monomers. Unlike many previous studies an important feature of our model is that the solution degrees of freedom are kept implicit in the model by appropriate choice of the phenomenological interaction parameters for the surfactants. This enables us to investigate rather large systems with less number of parameters. The method is powerful enough to study multimicellar systems with regular and inverted micelles for both neutral and ionic surfactants. As a function of several parameters of the model, we study self-assembly of neutral surfactants into micelles of various forms and sizes and compute appropriate cluster-size distributions. Ionic surfactants exhibit, apart from micellization, additional intermicellar ordering. We further study the role of host particles to mimic recent experim...

Observed properties and electronic structure of RNiSb compounds (R = Ho, Er, Tm, Yb and Y). Potential thermoelectric materials

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr 1-x Er x )Ni(Sn 1-x Sb x ) and ErNiSb 1-x Pn x (Pn=As, Sb, Bi) have been studied. They all crystallize in the MgAgAs structure type, which can be considered as a NaCI structure type in which half of the interstitial tetrahedral sites are occupied by Ni atoms. The measured values of the Seebeck coefficients, at room temperature, are positive for RNiSb (R=Ho, Er, Yb and Y) compounds and ErNiSb 1-x Pn x (Pn=As, Sb, Bi) solid solutions, but for (Zr 1-x Er x )Ni(Sn 1-x Sb x ) members vary from negative to positive values when 0

Correlation function studies on the Domany–Kinzel cellular automaton

The Domany–Kinzel cellular automaton is a simple and yet very rich model to study phase transitions in nonequilibrium systems. This model exhibits three characteristic phases: frozen, active and chaotic. In this paper we discuss the behavior of the equal-time two-point correlation functions and that of the associated correlation lengths as one crosses the phase boundary both for the frozen–active and active–chaotic transitions. We have investigated in detail how the correlation lengths diverge as one approaches the phase boundary from both sides. The divergence of the correlation length coupled with the previous studies on the divergence of the susceptibility, suggests that the fluctuation–dissipation theorem holds true in the Domany–Kinzel cellular automaton model. Time dependence of the correlation functions is also discussed.

Layer perovskites : A critical arena for testing concepts of layer rigidity

Using high resolution x-ray diffraction data, we show that the composition dependence of the normalized basal-spacing, d N (x), of the bi-layer alkali perovskites, Cs x Rb 1-x [LaNb 2 O 7 ], measured over the range 0≤x≤1, can be quantitatively accounted for with the anharmonic Lennard-Jones (aLJ) model which incorporates deformations of both the host layers and the guest ions. With the same independently determined value of the Rb/Cs stiffness constant ratio, 1.73, used in previous studies of Cs x Rb 1- x [Ca 2 Nb 3 O 10 ], the tri-layer perovskites, the aLJ model yields a very good one-parameter fit to d N (x) for the bi-layer perovskites. The single fitting parameter, β=[d(0)-t]/[d(1)-t] where t is the host layer thickness, yields a t value of 7.73A in very good (3%) agreement with the value 7.97A deduced from structural studies.

Spin Waves in Doped Manganites

In our study1 3 of the DE model in one dimension, we found that while the SWD differs drastically from Heisenberg shape (HS) for some carrier concentrations, x, there is a range where the shapes are very close. The example La1 xCax MnO3 defines x. Since this range included the value x = 0.3, we offered our result as an explanation for the surprising experimental finding2 that the SWD shape for La. 7 Pb. 3MnO3 at low temperature is HS throughout the Brillouin zone. (We define HS as that of the nearest-neighbor Heisenberg model.) But quite recently there appeared an experimental study6 of Pr. 6 3Sr. 3 7MnO3 that found SWD of drastically different shape from Heisenberg, in contrast to the situation for the La-Pb compound. And since our calculations within the DE model also showed essentially Heisenberg shape for x=.37, we are forced to ask what is the source of the observed shapes. The change in shape of the SWD accompanies changes in dopant and rare earth ions. There is evidence that magnetic properties are correlated with the size of these ions, resulting from a size-dependence of the bandwidth or hopping integral t.1 6 There are three presently pursued corrections to the usual DE model, all of which might possibly be involved in the effects of interest here. Namely, there is orbital degeneracy1 7 2 1 , there are We focus here on the spin waves in the low-temperature ferromagnetic-metal phase of doped manganites. While the experimental discovery of essential remarkable characteristics of these materials occurred as long ago as 19501 , experiments on the fundamental low-lying excitations called magnons or spin waves only appeared within the last 2 years.2 8 The first theoretical consideration of these excitations was in the work of Kubo and Ohata (1972)9 , who presented a semi-classical approximation within the double exchange (DE) model 11 1 0 . This approximation was emphasized recently by Furukawa (1996). The first exact calculations, Zang et al 2 and the present authors1 3, didn’t appear until 1997. An important result of these exact calculations is that the shape of the spin wave dispersion (SWD) curve, energy vs. wave vector, differs in general from that of the famous Heisenberg spin model (with nearest neighbor interactions). In fact such a difference was not at all unexpected in view of fundamental differences found earlier1 4 , 1 5 between the magnetic behavior of the two models. We note though that the semiclassical DE model happens to give precisely Heisenberg shape for the SWD. 9, 11

Physics of the gap formation in half-Heusler compounds and bismuth chalcogenide systems

Using first principles electronic structure calculations based on the density functional theory, we discuss the reasons behind the formation of energy gaps in different classes of narrow-gap semiconductors which are either good or promising thermoelectrics. We find that in half-Heusler compounds such as ZrNiSn and YNiSb, the Ni atoms take active role in the gap formation, both through local symmetry breaking and hybridization. In Bi/sub 2/Te/sub 3/, the best known room temperature thermoelectric, the subtle gap structure is determined by both spin-orbit interaction and hybridization of Bi p and Te p bands. In other Bi chalcogenides and complex ternary systems containing Bi and Te, it appears that spin-orbit interaction does not play as important a role. We discuss possible reasons for this difference.