Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)

Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15). Static dielectric constants are determined from the low-T limiting behavior of the permittivity. The estimated polarizability for bound holes ~ 10^{-22} cm^{-3} implies a radius comparable to the interatomic spacing, consistent with the small polaron picture established from prior transport studies near room temperature and above on nearby compositions. Relaxation peaks in the dielectric loss associated with charge-carrier hopping yield activation energies in good agreement with low-T hopping energies determined from variable-range hopping fits of the dc resistivity. The doping dependence of these energies suggests that the orthorhombic, canted antiferromagnetic ground state tends toward an insulator-metal transition that is not realized due to the formation of the ferromagnetic insulating state near Mn(4+) concentration ~ 0.13.Comment: PRB in press, 5 pages, 6 figure

Towards first-principles understanding of the metal-insulator transition in fluid alkali metals

By treating the electron-ion interaction as perturbation in the first-principles Hamiltonian, we have calculated the density response functions of a fluid alkali metal to find an interesting charge instability due to anomalous electronic density fluctuations occurring at some finite wave vector {\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since |{\bi Q}| is smaller than the diameter of the Fermi surface, this instability necessarily impedes the electric conduction, implying its close relevance to the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure

Paleoecology of Organic Deposits of Probable Last Interglacial Age in Northern Ontario

Nonglacial deposits in northern Ontario that may date to the last interglacial interval are well known from the Hudson Bay Lowlands where they have been described in sections along several river valleys. Soil horizons, peat beds and other organic sediment sequences comprise the Missinaibi Formation of the Moose River Basin studied for pollen and macrofossils. Results suggest that the climate was as warm or warmer than present, and spruce woodlands prevailed among broad expanses of bog and fen. The Beaver River peat records conditions similar to the present in the Fort Severn area with open spruce woodlands dispersed in peatlands. South of the Lowlands in the Timmins area, a widespread organic-silt horizon termed the Owl Creek beds is stratigraphically equivalent to the Missinaibi Formation. The waning phase of a warm interval is represented, with early climate possibly similar to the present and the later climate much cooler. Correlation of the Missinaibi Formation with substage 5e of the deep-sea oxygen isotope record is corroborated by amino acid results on marine shells from some associated units. Analysis of shells from beneath the Beaver River peat bed indicate that this interval may be considerably younger, possibly substage 5c or, more likely, 5a. The Owl Creek beds may relate to sub-stage 5e, or to one of the younger intervals, 5c or 5a.Les dépôts non glaciaires du nord de l'Ontario qui datent peut-être du dernier interglaciaire sont bien connus dans les basses terres de la baie d'Hudson où ils ont été décrits dans des coupes le long de plusieurs vallées. Les horizons de sols, les lits de tourbes et autres séquences de sédiments organiques comprennent la Formation de Missinaibi dans le bassin de Moose River dont on a étudié le pollen et les macrofossiles. Les résultats indiquent que le climat était aussi chaud, sinon plus, que maintenant et que les forêts d'épinettes dominaient de vastes étendues de tourbières minérotrophes et ombrothrophes. Les données sur la tourbe de Beaver River révèlent des conditions similaires à celles d'aujourd'hui dans la région de Fort Severn où les forêts ouvertes d'épinettes sont dispersées dans les tourbières. Dans la région de Timmins, au sud des basses terres, se trouve un horizon de silt organique étendu (les lits de Owl Creek), qui est l'équivalent stratigraphique de la Formation de Missinaibi. La fin de l'intervalle chaud y est représentée. Ie climat étant au début probablement aussi chaud qu'aujourd'hui puis, par la suite, beaucoup plus froid. La corrélation de la Formation de Missinaibi avec le stade isotopique 5e est corroborée par les résultats des acides aminés sur des coquillages marins de quelques unités associées. Les analyses faites sur des coquillages recueillis sous le lit de tourbe de Beaver River indiquent que cet intervalle pourrait être plus jeune, probablement du stade 5c ou mieux du stade 5a. Les lits de Owl Creek pourraient être associés au stade 5e ou à un intervalle plus récent, 5c ou 5a.Nichtglaziale Ablagerungen im nôrdlichen Ontario, die môglicherweise aus dem letzten Interglazial stammen, sind aus dem Tiefland der Hudson Bay wohlbekannt. wo sie in Schnitten entlang mehrerer Flusstâler beschrieben worden sind. Boden-Horizonte. Torfberten und andere organische Ablagerungssequenzen umfassen die Missinaibi Formation des Moose River-Beckens, welche auf Pollen und Makrofossile untersucht wurde. Die Ergebnisse deuten darauf, dass das Klima so warm wie gegenwârtig oder warmer war und dass Rottannenwàlder zwischen weiten Flâchen von Sùmpfen und Mooren vorherrschten. Der Torf von Beaver River belegt der Gegenwart âhnliche Bedingungen im Gebiet von Fort Severn, wo offene Rottannenwàlder zwischen die Torfmoore verteilt sind. Sùdlich des Tieflands im Gebiet von Timmins ist ein ausgedehnter organischer Schlamm-Horizont, genannt die Betten von Owl Creek. das stratigraphische Gegenstùck zur Missinaibi Formation. Die Endphase eines warmen Intervalls ist festgehalten, mit einem frùhen Klima, das wahrscheinlich dem heutigen âhnlich war, und einem spâteren viel kâlteren Klima. Die Korrelation der Missinaibi Formation mit dem Unterstadium 5e des Tiefsee Sauerstoff Isotop-Belegs wird durch Aminosauren-Ergebnisse auf marinen Muscheln von einigen verbundenen Einheiten bestatigt. Analysen von Muscheln, die unter dem Torfbett von Beaver River gefunden wurden, zeigen, dass dies Interval sehr viel junger sein kônnte, môglicherweise Unterstadium 5c oder noch eher 5a. Die Owl Creek-Betten kônnten mit dem Unterstadium 5e oder einem der jùngeren Intervalle 5c oder 5a verbunden sein

Epitaxial Growth of La1/3_{1/3}Sr2/3_{2/3}FeO3_3 thin films by laser ablation

We report on the synthesis of high quality La$_{1/3}$Sr$_{2/3}$FeO$_3$ (LSFO) thin films using the pulsed laser deposition technique on both SrTiO$_3$ (STO) and LaAlO$_3$ (LAO) substrates (100)-oriented. From X-Ray diffraction (XRD) studies, we find that the films have an out-of-plane lattice parameter around 0.3865nm, almost independent of the substrate (i.e. the nature of the strains). The transport properties reveal that, while LSFO films deposited on STO exhibit an anomaly in the resistivity vs temperature at 180K (corresponding to the charge-ordered transition and associated with a transition from a paramagnetic to an antiferromagnetic state), the films grown on LAO display a very small magnetoresistance behavior and present an hysteresis around 270K under the application of a 4T magnetic field. The changes in transport properties between both substrates are discussed and compared with the corresponding single crystals.Comment: 9 pages, 4 figure

Pollen and Macrofossil Study of an Interglacial Deposit in Nova Scotia

Overburden removal for quarrying operations at the Milford Gypsum Quarry, East Milford, Nova Scotia, exposed 2 m of compacted and distorted peat and organic clays with abundant plant remains, associated with inorganic and laminated clays and sands. The non-glacial sediments were underlain by a grey till-like deposit and overlain by more than 20 m of red till. Wood (Larix sp.) from the nonglacial sediments produced a radiocarbon date of > 50,000 years BP (GSC-1642). Pollen analysis of the organic sediments shows a basal assemblage characterized by a variety of hardwood pollen genera including Fagus, Ulmus, Acer, Quercus and Tilia. Higher in the sequence Betula pollen increases and becomes the dominant pollen type. Towards the upper part of the organic unit, Picea and Abies balsamea replace the hardwood genera. Alnus is the most abundant pollen type at the top of the sequence. Wood, seeds, moss and Coleoptera remains add to the environmental reconstruction. The evidence as a whole leads to the conclusion that the latter part of an interglacial interval, probably the Sangamon, is represented. Hardwood forests dominated by a variety of thermophilous hardwood genera attest to a climate at least as warm as the present in the area. As the climate deteriorated, Betula became dominant. Continued cooling induced a transition to coniferous forests in which Picea and Abies balsamea predominated.L'enlèvement des formations superficielles, lors d'excavations dans la carrière de gypse d'East Milford en Nouvelle-Ecosse, a mis à jour 2 m de tourbe et d'argile organique compactées et déformées, ainsi que d'abondants restes végétaux, associés à des argiles et sables inorganiques et laminés. Les sédiments non glaciaires reposaient sur un dépôt gris ayant l'apparence de till et étaient recouverts par une couche de plus de 20 m de till rouge. Du bois met de la séquence. Des restes de bois, de graines et de mousses de > 50 000 ans BP (GSC-1642). L'analyse pollinique des sédiments organiques montre à la base un assemblage caractérisé par du pollen provenant de divers genres de feuillus, dont Fagus, Ulmus, Acer, Quercus et Tilia. Le pollen de Betula augmente vers le haut dans la séquence et devient le type dominant. Dans la partie supérieure de la séquence organique, Picea et Abies balsamea remplacent les bois durs. Alnus est le type de pollen le plus abondant au sommet de la séquence. Des restes de bois, de graines, de mousses et de Coléoptères aident à reconstituer l'environnement. Dans l'ensemble, les preuves permettent de conclure que la séquence date de la dernière partie d'un interglaciaire, probablement le Sangamonien. Les forêts de feuillus, dominées par une variété de genres thermophiles, témoignent d'un climat au moins aussi chaud que le climat actuel dans la région. À mesure que le climat s'est détérioré, les Betula ont proliféré. Le refroidissement continu a provoqué une transition vers les forêts de conifères dans lesquelles ont prédominé Picea et Abies balsamea

Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange

The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d) (LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions.Comment: 18 pages, 4 color figure

Model solution for volume reflection of relativistic particles in a bent crystal

For volume reflection process in a bent crystal, exact analytic expressions for positively- and negatively-charged particle trajectories are obtained within a model of parabolic continuous potential in each interplanar interval, with the neglect of incoherent multiple scattering. In the limit of the crystal bending radius greatly exceeding the critical value, asymptotic formulas are obtained for the particle mean deflection angle in units of Lindhard's critical angle, and for the final beam profile. Volume reflection of negatively charged particles is shown to contain effects of rainbow scattering and orbiting, whereas with positively charged particles none of these effects arise within the given model. The model predictions are compared with experimental results and numerical simulations. Estimates of the volume reflection mean angle and the final beam profile robustness under multiple scattering are performed.Comment: 21 pages, 11 figure

Mott scattering of polarized electrons in a strong laser field

We present analytical and numerical results of the relativistic calculation of the transition matrix element $S_{fi}$ and differential cross section for Mott scattering of initially polarized Dirac particles (electrons) in the presence of strong laser field with linear polarization. We use exact Dirac-Volkov wave functions to describe the dressed electrons and the collision process is treated in the first Born approximation. The influence of the laser field on the degree of polarization of the scattered electron is reported.Comment: 12 pages, 11 figures, Revte

Mott physics and first-order transition between two metals in the normal state phase diagram of the two-dimensional Hubbard model

For doped two-dimensional Mott insulators in their normal state, the challenge is to understand the evolution from a conventional metal at high doping to a strongly correlated metal near the Mott insulator at zero doping. To this end, we solve the cellular dynamical mean-field equations for the two-dimensional Hubbard model using a plaquette as the reference quantum impurity model and continuous-time quantum Monte Carlo method as impurity solver. The normal-state phase diagram as a function of interaction strength $U$, temperature $T$, and filling $n$ shows that, upon increasing $n$ towards the Mott insulator, there is a surface of first-order transition between two metals at nonzero doping. That surface ends at a finite temperature critical line originating at the half-filled Mott critical point. Associated with this transition, there is a maximum in scattering rate as well as thermodynamic signatures. These findings suggest a new scenario for the normal-state phase diagram of the high temperature superconductors. The criticality surmised in these systems can originate not from a T=0 quantum critical point, nor from the proximity of a long-range ordered phase, but from a low temperature transition between two types of metals at finite doping. The influence of Mott physics therefore extends well beyond half-filling.Comment: 27 pages, 16 figures, LaTeX, published versio

Nonequilibrium brittle fracture propagation: Steady state, oscillations and intermittency

A minimal model is constructed for two-dimensional fracture propagation. The heterogeneous process zone is presumed to suppress stress relaxation rate, leading to non-quasistatic behavior. Using the Yoffe solution, I construct and solve a dynamical equation for the tip stress. I discuss a generic tip velocity response to local stress and find that noise-free propagation is either at steady state or oscillatory, depending only on one material parameter. Noise gives rise to intermittency and quasi-periodicity. The theory explains the velocity oscillations and the complicated behavior seen in polymeric and amorphous brittle materials. I suggest experimental verifications and new connections between velocity measurements and material properties.Comment: To appear in Phys. Rev. Lett., 6 pages, self-contained TeX file, 3 postscript figures upon request from author at [email protected] or [email protected], http://cnls-www.lanl.gov/homepages/rafi/rafindex.htm