4 research outputs found
Doping dependence of spin and orbital correlations in layered manganites
We investigate the interplay between spin and orbital correlations in
monolayer and bilayer manganites using an effective spin-orbital t-J model
which treats explicitly the e_g orbital degrees of freedom coupled to classical
t_{2g} spins. Using finite clusters with periodic boundary conditions, the
orbital many-body problem is solved by exact diagonalization, either by
optimizing spin configuration at zero temperature, or by using classical
Monte-Carlo for the spin subsystem at finite temperature. In undoped
two-dimensional clusters, a complementary behavior of orbital and spin
correlations is found - the ferromagnetic spin order coexists with alternating
orbital order, while the antiferromagnetic spin order, triggered by t_{2g} spin
superexchange, coexists with ferro-orbital order. With finite crystal field
term, we introduce a realistic model for La_{1-x}Sr_{1+x}MnO_4, describing a
gradual change from predominantly out-of-plane 3z^2-r^2 to in-plane x^2-y^2
orbital occupation under increasing doping. The present electronic model is
sufficient to explain the stability of the CE phase in monolayer manganites at
doping x=0.5, and also yields the C-type antiferromagnetic phase found in
Nd_{1-x}Sr_{1+x}MnO_4 at high doping. Also in bilayer manganites magnetic
phases and the accompanying orbital order change with increasing doping. Here
the model predicts C-AF and G-AF phases at high doping x>0.75, as found
experimentally in La_{2-2x}Sr_{1+2x}Mn_2O_7.Comment: 23 pages, 21 figure
Data Analysis for Quantum Monte Carlo Simulations
Quantum Monte Carlo simulations for dynamic quantities involve the solution of a severely ill-conditioned inverse problem. Although Maximum-Entropy techniques have been successfully applied, we find that the conventional evidence approximation tends to overfitting. We describe an alternative approximation of the evidence integral and a method based on adaptive basis functions and compare the results for several test cases