2,969 research outputs found
Double exchange and orbital correlations in electron-doped manganites
A double exchange model for degenerate orbitals with intra- and
inter-orbital interactions has been studied for the electron doped manganites
ABMnO (). We show that such a model reproduces the
observed phase diagram and orbital ordering in the intermediate bandwidth
regime and the Jahn-Teller effect, considered to be crucial for the region
, does not play a major role in this region. Brink and Khomskii have
already pointed this out and stressed the relevance of the anistropic hopping
across the degenerate orbitals in the infinite Hund's coupling limit.
From a more realistic calculation with finite Hund's coupling, we show that
inclusion of interactions stabilizes the C-phase, the antiferromagnetic
metallic A-phase moves closer to while the ferromagnetic phase shrinks.
This is in agreement with the recent observations of Kajimoto et. al. and
Akimoto et. al.Comment: text 9 pages, 5 figure
Antiferromagnetism and Superconductivity in a Model with Extended Pairing Interactions
The competition between antiferromagnetism and the superconducting
state is studied in a model with near and next near neighbour interactions in
the absence of any on-site repulsion. A mean field study shows that it is
possible to have simultaneous occurrence of an antiferromagnetic and a singlet
superconducting state in this model. In addition, such a coexistence
generates a triplet superconducting order parameter with centre of mass
momentum {\it dynamically} having the same orbital symmetry as
the singlet superconductor. Inclusion of next nearest neighbour hopping in the
band stabilises the superconducting state away from half filling, the
topology of the phase diagram, though, remains similar to the near neighbour
model. In view of the very recent observation of a broad region of coexistence
of antiferrmagnetic and unconventional superconducting states in organic
superconductors, the possibility of observation of the triplet state has been
outlined.Comment: 12 pages(tex file), 7 figures (ps files
Magnetic properties of doped GdI2
Motivated by the recent experimental studies on layered ferromagnetic
metallic system GdI2 and its doped variant GdI2Hx we develop a model to
understand their ground state magnetic phase diagram. Based on first principle
electronic structure calculations we write down a phenomenological model and
solve it under certain approximations to obtain the ground state energy. In the
process we work out the phase diagram of the correlated double exchange model
on a triangular lattice for the specific band structure at hand.Comment: 13 pages, 5 figures, corrected typo
An Experimental Analysis ofGroup Size and Risk Sharing
We study the relationship between group size and the extent of risk sharing in an insurance game played over a number of periods with random idiosyncratic and aggregate shocks to income in each period. Risk sharing is attained via agents that receive a high endowment in one period making unilateral transfers to agents that receive a low endowment in that period. The complete risk sharing allocation is for all agents to place their endowments in a common pool, which is then shared equally among members of the group in every period. Theoretically, the larger the group size, the smaller the per capita dispersion in consumption and greater is the potential value of insurance. Field evidence however suggests that smaller groups do better than larger groups as far as risk sharing is concerned. Results from our experiments show that the extent of mutual insurance is significantly higher in smaller groups, though contributions to the pool are never close to what complete risk sharing requires.Reciprocity, Risk Sharing, Group Size, Experiments
Effect of many modes on self-polarization and photochemical suppression in cavities
The standard description of cavity-modified molecular reactions typically involves a single (resonant) mode, while in reality, the quantum cavity supports a range of photon modes. Here, we demonstrate that as more photon modes are accounted for, physicochemical phenomena can dramatically change, as illustrated by the cavity-induced suppression of the important and ubiquitous process of proton-coupled electron-transfer. Using a multi-trajectory Ehrenfest treatment for the photon-modes, we find that self-polarization effects become essential, and we introduce the concept of self-polarization-modified BornâOppenheimer surfaces as a new construct to analyze dynamics. As the number of cavity photon modes increases, the increasing deviation of these surfaces from the cavity-free BornâOppenheimer surfaces, together with the interplay between photon emission and absorption inside the widening bands of these surfaces, leads to enhanced suppression. The present findings are general and will have implications for the description and control of cavity-driven physical processes of molecules, nanostructures, and solids embedded in cavities
Magnetic and orbital order in overdoped bilayer manganites
The magnetic and orbital orders for the bilayer manganites in the doping
region have been investigated from a model that incorporates the
two orbitals at each Mn site, the inter-orbital Coulomb interaction and
lattice distortions. The usual double exchange operates via the orbitals.
It is shown that such a model reproduces much of the phase diagram recently
obtained for the bilayer systems in this range of doping. The C-type phase with
() spin order seen by Ling et al. appears as a natural consequence
of the layered geometry and is stabilised by the static distortions of the
system. The orbital order is shown to drive the magnetic order while the
anisotropic hopping across the orbitals, layered nature of the underlying
structure and associated static distortions largely determine the orbital
arrangements.Comment: 8 pages, 5 figure
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