2,998 research outputs found
Theoretical prediction of Jahn-Teller distortions and orbital ordering in Cs2CuCl2Br2
With the use of the density function calculations we show that the actual
crystal structure of CsCuClBr should contain elongated in the
plane CuClBr octahedra, in contrast to the experimentally observed
compression in direction. We also predict that the spins on Cu ions
should be ferromagnetically ordered in plane, while the exchange
interaction along direction is small and its sign is uncertain.Comment: 4 pages, 3 figure
Jahn-Teller distortions and phase separation in doped manganites
A "minimal model" of the Kondo-lattice type is used to describe a competition
between the localization and metallicity in doped manganites and related
magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant
charge carriers can be significantly lower than that implied by the doping
level x. A strong tendency to the phase separation is demonstrated for a wide
range of intermediate doping concentrations vanishing at low and high doping.
The phase diagram of the model in the x-T plane is constructed. At low
temperatures, the system is in a state with a long-range magnetic order:
antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS)
state. At high temperatures, there can exist two types of the paramagnetic (PM)
state with zero and nonzero density of the itinerant electrons. In the
intermediate temperature range, the phase diagram includes different kinds of
the PS states: AF-FM, FM-PM, and PM with different content of itinerant
electrons. The applied magnetic field changes the phase diagram favoring the FM
ordering. It is shown that the variation of temperature or magnetic field can
induce the metal-insulator transition in a certain range of doping levels.Comment: 14 pages, 7 figures, submitted to Phys. Rev. B.; v.2 contains the
changes introduced according to comments of the PRB Referees; in v. 3, some
misprints are correcte
Current-driven orbital order-disorder transition in LaMnO3
We report significant influence of electric current on the orbital
order-disorder transition in LaMnO3. The transition temperature T_OO, thermal
hysteresis in the resistivity (rho) versus temperature (T) plot around T_OO,
and latent heat L associated with the transition decrease with the increase in
current density. Eventually, at a critical current density, L reaches zero. The
transition zone, on the other hand, broadens with the increase in current
density. The states at ordered, disordered, and transition zone are all found
to be stable within the time window from ~10^-3 to ~10^4 seconds.Comment: 7 pages including 5 figures; resolution of Fig.1 is better here than
the published versio
Orbital Ordering and Spin-Ladder Formation in La2RuO5
The semiconductor-semiconductor transition of La2RuO5 is studied by means of
augmented spherical wave (ASW) electronic structure calculations as based on
density functional theory and the local density approximation. This transition
has lately been reported to lead to orbital ordering and a quenching of the
local spin magnetic moment. Our results hint towards an orbital ordering
scenario which, markedly different from the previously proposed scheme,
preserves the local S = 1 moment at the Ru sites in the low-temperature phase.
The unusual magnetic behaviour is interpreted by the formation of spin-ladders,
which result from the structural changes occurring at the transition and are
characterized by antiferromagnetic coupling along the rungs.Comment: 5 pages, 4 figures, for more information see
http://www.physik.uni-augsburg.de/~eyert
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
First order phase transition in the anisotropic quantum orbital compass model
We investigate the anisotropic quantum orbital compass model on an infinite
square lattice by means of the infinite projected entangled-pair state
algorithm. For varying values of the and coupling constants of the
model, we approximate the ground state and evaluate quantities such as its
expected energy and local order parameters. We also compute adiabatic time
evolutions of the ground state, and show that several ground states with
different local properties coexist at . All our calculations are
fully consistent with a first order quantum phase transition at this point,
thus corroborating previous numerical evidence. Our results also suggest that
tensor network algorithms are particularly fitted to characterize first order
quantum phase transitions.Comment: 4 pages, 3 figures, major revision with new result
Clear Experimental Signature of Charge-Orbital density wave in NdCaMnO
Single Crystals of NdCaMnO have been prepared by the
travelling floating-zone method, and possible evidence of a charge -orbital
density wave in this material presented earlier [PRB68,092405 (2003)] using
High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In
the current note we present direct evidence of charge-orbital ordering in this
material using heat capacity measurements. Our heat capacity measurements
indicate a clear transition consistent with prior observation. We find two main
transitions, one at temperature K, and other at
K. In addition, we may also conclude that there is a strong electron-phonon
coupling in this material.Comment: 7 pages, 8 figure
One-dimensional orbital fluctuations and the exotic magnetic properties of YVO
Starting from the Mott insulator picture for cubic vanadates, we derive and
investigate the model of superexchange interactions between V ions, with
nearly degenerate orbitals occupied by two electrons each. The
superexchange interactions are strongly frustrated and demonstrate a strong
interrelation between possible types of magnetic and orbital order. We
elucidate the prominent role played by fluctuations of and orbitals
which generate ferromagnetic superexchange interactions even in the absence of
Hund's exchange. In this limit we find orbital valence bond state which is
replaced either by -type antiferromagnetic order with weak -type orbital
order at increasing Hund's exchange, or instead by -type antiferromagnetic
order when the lattice distortions stabilize -type orbital order. Both
phases are observed in YVO and we argue that a dimerized -type
antiferromagnetic phase with stronger and weaker FM bonds alternating along the
c axis may be stabilized by large spin-orbital entropy at finite temperature.
This suggests a scenario which explains the origin of the exotic -AF order
observed in YVO in the regime of intermediate temperatures and allows one
to specify the necessary ingredients of a more complete future theory.Comment: 23 pages, 15 figure
Role of local geometry in spin and orbital structure of transition metal compounds
We analyze the role of local geometry in the spin and orbital interaction in
transition metal compounds with orbital degeneracy. We stress that the tendency
observed for the most studied case (transition metals in O octahedra with
one common oxygen -- common corner of neighboring octahedra and with metal--oxygen--metal bonds), that ferro-orbital ordering renders
antiferro-spin coupling, and, {\it vice versa}, antiferro-orbitals give
ferro-spin ordering, is not valid in general case, in particular for octahedra
with common edge and with M--O--M bonds. Special attention is
paid to the ``third case'', neighboring octahedra with common face (three
common oxygens) -- the case practically not considered until now, although
there are many real systems with this geometry. Interestingly enough, the
spin--orbital exchange in this case turns out to be to be simpler and more
symmetric than in the first two cases. We also consider, which form the
effective exchange takes for different geometries in case of strong spin--orbit
coupling.Comment: 31 pages, 9 figures, submitted to JET
- …