Thermal fluctuations and NMR spectra of incommensurate insulators

Journal ArticleThe effects of thermal order-parameter fluctuations on the NMR line shape of incommensurate systems are evaluated within the mean-field Landau theory and the results are compared with the 8 7Rb and 3 9K 1/2 -1/2 NMR spectra of Rb2ZnCl4 and K2Se04 just below the paraelectric-incommensurate transitions. We show that thermal fluctuations do not only reduce the effective incommensurate splitting as compared to the static case but also change the shape of the spectrum. In particular they remove the 8- function-like form of the two edge singularities in analogy to the Debye-Waller factor in x-ray scattering. Two-dimensional NMR separation techniques allow for a separate observation of static and dynamic incommensurate line shapes close to the paraelectric-incommensurate transition temperature Tj. The static inhomogeneously broadened NMR line shape results from the static distribution of quadrupole perturbed Zeeman frequencies, reflecting the frozen-out incommensurate modulation wave. The dynamic line shape reflects the time-dependent part of the electric quadrupole interaction resulting from phason and amplitudon thermal fluctuations of the modulation wave. Close to Tl these fluctuations become so low in frequency that they influence the line shape. A precise determination of 77 can be obtained from the maximum in the width of the dynamic line shape

The Electron-Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering

Atom scattering is becoming recognized as a sensitive probe of the electron-phonon interaction parameter $\lambda$ at metal and metal-overlayer surfaces. Here, the theory is developed linking $\lambda$ to the thermal attenuation of atom scattering spectra (in particular, the Debye-Waller factor), to conducting materials of different dimensions, from quasi-one dimensional systems such as W(110):H(1$\times$1) and Bi(114), to quasi-two dimensional layered chalcogenides and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of $\lambda$ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition the number of layers contributing to the electron-phonon interaction that is measured in an atom surface collision.Comment: 23 pages, 5 figures, 1 tabl

Visual isolation, habitat complexity, and the density of stream salmonids

The effect of visual isolation on the density of territorial fish such as Atlantic salmon ( Salmo salar ) was examined. Kalleberg's (1958) hypothesis that the density of salmon can be increased by adding boulders was tested for the first time in a natural stream. Eight study reaches were established in Catamaran Brook and the Little Southwest Miramichi River (New Brunswick). Each reach was divided into three sites of 3 x 2 m, and received one of three treatments: boulder-added, 36 boulders (median diameter = 0.21 m) were added to increase visual isolation; boulder-removed, where all boulders were removed; and a control, where the reach was left unchanged. A three-fold increase in the density of salmon was observed in the boulder-added treatment compared to the boulder-removed and control sites. The impact of adding boulders was more important for the young-of-the-year than for the 1+ and 2+ salmon. However, for non-salmonid fishes, no effect was found. A Geographic Information System (GIS) viewshed analysis showed that the visible area and the distance to nearest neighbour were significantly smaller in the boulder-added treatment than in the other two treatments. Although mean water velocity was reduced in the boulder-added treatments, the snout velocity of individual salmon did not differ among treatments, suggesting that the boulders were not used as a velocity refuge. The results support Kalleberg's hypothesis that visual isolation is a key factor controlling salmon density, and that it might be used as a low-cost method for increasing the density of territorial fish

Orbital domain state and finite size scaling in ferromagnetic insulating manganites

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied with strong anomalous increase of certain FM neutron Bragg peaks. (55,139)(1/T1) spin-lattice relaxation rates diverge on approaching this temperature from below, signalling a genuine phase transition at T(tr) approx. 70 K. The increased local magnetic anisotropy of the low temperature phase, the cooling-rate dependence of the Bragg peaks, and the observed finite size scaling of T(tr) with Ca (hole) doping, are suggestive of freezing into an orbital domain state, precursor to a phase transition into an inhomogeneous orbitally ordered state embodying hole-rich walls.Comment: 4 pages, 4 figure

The ecology of fish movement in six Lake Ontario tributaries

Despite a rich literature on the ecology of freshwater fishes, the movement behaviour of many stream fish remains poorly understood, yet is expected to be important for key ecological and micro-evolutionary processes. The main purpose of my thesis was to improve our understanding of fish movements by evaluating the movement of multiple species across multiple streams and entire watersheds. I undertook three steps to achieve this: 1) a comparative study addressing whether information in natural history accounts provide reliable information for predicting the behaviour of other populations of stream fishes (Chapter 1); 2) adapted, tested and extended existing theory on sex-biased arrival by conducting a broad test of four hypotheses on stream fishes at spawning sites (Chapter 2); and 3) to extend and contribute to a management concern of broad interest regarding invasive species control by examining the degree to which fishes move between adjacent streams (Chapter 3). Comparisons between my data and those from the literature demonstrated a strong correlation regarding key biological indices, suggesting that natural history information is a reliable source of information and can be used in most management decisions regarding freshwater stream fishes. My detailed data set also revealed novel findings unknown to the natural history literature: juveniles moved into the streams along with spawning adults; and species with known "anadromous-like" life histories spend more time in the streams than previously thought. Secondly, my data suggested that the most common form of sex-biased timing is males arriving before females, consistent with hypotheses that males are better able to cope with the prevailing conditions at the spawning sites, as well as to increase their reproductive success by encountering more females. However, there was considerable variation within and among species of fishes. Finally, my thesis suggested that the fish assemblages in these Lake Ontario catchments are dynamic, exchanging individuals at rates and spatial extents likely to be important for metapopulation dynamics and gene flow. Understanding the movement behaviour of fishes, a critical aspect of their life histories, will be important in developing proper conservation plans. Knowledge gained from this study will serve to improve our understanding of how concepts such as metapopulations, habitat fragmentation, and movement apply to stream fishes and to efforts to control invasive fish species

Spin-polarized oxygen hole states in cation deficient La(1-x)CaxMnO(3+delta)

When holes are doped into a Mott-Hubbard type insulator, like lightly doped manganites of the La(1-x)CaxMnO3 family, the cooperative Jahn-Teller distortions and the appearance of orbital ordering require an arrangement of Mn(3+)/Mn(4+) for the establishment of the insulating canted antiferromagnetic (for x<=0.1), or of the insulating ferromagnetic (for 0.1<x<= 0.2) ground state. In the present work we provide NMR evidence about a novel and at the same time puzzling effect in La(1-x)CaxMnO(3+delta) systems with cation deficience. We show that in the low Ca-doping regime, these systems exhibit a very strong hyperfine field at certain La nuclear sites, which is not present in the stoichiometric compounds. Comparison of our NMR results with recent x-ray absorption data at the Mn K edge, suggests the formation of a spin-polarized hole arrangement on the 2p oxygen orbitals as the origin of this effect.Comment: 10 pages, 4 Figures, submitted to PR