550 research outputs found
Dilution effect in correlated electron system with orbital degeneracy
Theory of dilution effect in orbital ordered system is presented. The
orbital model without spin degree of freedom and the spin-orbital coupled model
in a three-dimensional simple-cubic lattice are analyzed by the Monte-Carlo
simulation and the cluster expansion method. In the orbital model without
spin degree of freedom, reduction of the orbital ordering temperature due to
dilution is steeper than that in the dilute magnet. This is attributed to a
modification of the orbital wave-function around vacant sites. In the
spin-orbital coupled model, it is found that magnetic structure is changed from
the A-type antiferromagnetic order into the ferromagnetic one. Orbital
dependent exchange interaction and a sign change of this interaction around
vacant sites bring about this novel phenomena. Present results explain the
recent experiments in transition-metal compounds with orbital dilution.Comment: 12 pages, 16 figure
Dilution Effects in Two-dimensional Quantum Orbital System
We study dilution effects in a Mott insulating state with quantum orbital
degree of freedom, termed the two-dimensional orbital compass model. This is a
quantum and two-dimensional version of the orbital model where the interactions
along different bond directions cause frustration between different orbital
configurations. A long-range correlation of a kind of orbital at each row or
column, termed the directional order, is studied by means of the quantum
Monte-Carlo method. It is shown that decrease of the ordering temperature due
to dilution is much stronger than that in spin models. Quantum effect enhances
the effective dimensionality in the system and makes the directional order
robust against dilution. We discuss an essential mechanism of the dilute
orbital systems.Comment: 5pages, 4 figure
Numerical study of t2g orbital system with ferromagnetic polarization
Finite temperature orbital state in a ferromagnetic Mott insulator with
triply-degenerate orbital is investigated numerically. We employ the
quantum Monte Carlo simulation with the loop algorithm. Indications for
conventional staggered-type orbital order are not remarkable down to the lowest
temperature to which the present simulation can get access. Physical parameters
monitoring the off-diagonal orbital order, which is characterized by a linear
combination of the orbital-wave functions with equal
weights, are not conspicuous. It is found that a orbital gap-like behavior
appears in the uniform orbital susceptibility. This is supported by a threshold
behavior in the staggered correlation function in a calculation with the
additional Ising-type interaction. Some rigorous remarks for the long-range
orbital order are also presented.Comment: 5 pages, 3 figure
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