1,323 research outputs found
Orbital fluctuations in the VO perovskites
The properties of Mott insulators with orbital degrees of freedom are
described by spin-orbital superexchange models, which provide a theoretical
framework for understanding their magnetic and optical properties. We introduce
such a model derived for configuration of V ions in
the VO perovskites, =Lu,Yb,,La, and demonstrate that
orbital fluctuations along the axis are responsible for the
huge magnetic and optical anisotropies observed in the almost perfectly cubic
compound LaVO. We argue that the GdFeO distortion and the large
difference in entropy of -AF and -AF phases is responsible for the second
magnetic transition observed at in YVO. Next we address the
variation of orbital and magnetic transition temperature, and
, in the VO perovskites, after extending the spin-orbital model
by the crystal-field and the orbital interactions which arise from the
GdFeO and Jahn-Teller distortions of the VO octahedra. We further find
that the orthorhombic distortion which increases from LaVO to LuVO
plays a crucial role by controlling the orbital fluctuations, and via the
modified orbital correlations influences the onset of both magnetic and orbital
order.Comment: 25 pages, 10 figure
Defects, disorder and strong electron correlations in orbital degenerate, doped Mott insulators
We elucidate the effects of defect disorder and - interaction on the
spectral density of the defect states emerging in the Mott-Hubbard gap of doped
transition-metal oxides, such as YCaVO. A soft gap of
kinetic origin develops in the defect band and survives defect disorder for
- interaction strengths comparable to the defect potential and hopping
integral values above a doping dependent threshold, otherwise only a pseudogap
persists. These two regimes naturally emerge in the statistical distribution of
gaps among different defect realizations, which turns out to be of Weibull
type. Its shape parameter determines the exponent of the power-law
dependence of the density of states at the chemical potential () and hence
distinguishes between the soft gap () and the pseudogap ()
regimes. Both and the effective gap scale with the hopping integral and the
- interaction in a wide doping range. The motion of doped holes is
confined by the closest defect potential and the overall spin-orbital
structure. Such a generic behavior leads to complex non-hydrogen-like defect
states that tend to preserve the underlying -type spin and -type orbital
order and can be detected and analyzed via scanning tunneling microscopy.Comment: 5 pages, 4 figure
Magnetism of one-dimensional Wigner lattices and its impact on charge order
The magnetic phase diagram of the quarter-filled generalized Wigner lattice
with nearest- and next-nearest-neighbor hopping t_1 and t_2 is explored. We
find a region at negative t_2 with fully saturated ferromagnetic ground states
that we attribute to kinetic exchange. Such interaction disfavors
antiferromagnetism at t_2 <0 and stems from virtual excitations across the
charge gap of the Wigner lattice, which is much smaller than the Mott-Hubbard
gap proportional to U. Remarkably, we find a strong dependence of the charge
structure factor on magnetism even in the limit U to infinity, in contrast to
the expectation that charge ordering in the Wigner lattice regime should be
well described by spinless fermions. Our results, obtained using the
density-matrix renormalization group and exact diagonalization, can be
transparently explained by means of an effective low-energy Hamiltonian
Magnetic properties of spin-orbital polarons in lightly doped cobaltates
We present a numerical treatment of a spin-orbital polaron model for
Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the
polarons account for the peculiar magnetic properties of this layered compound:
They explain the large susceptibility; their internal degrees of freedom lead
both to a negative Curie-Weiss temperature and yet to a ferromagnetic
intra-layer interaction, thereby resolving a puzzling contradiction between
these observations. We make specific predictions on the momentum and energy
location of excitations resulting from the internal degrees of freedom of the
polaron, and discuss their impact on spin-wave damping.Comment: 4+ pages, 6 figures, accepted for publication in Phys. Rev. Let
Quantum phase transitions in exactly solvable one-dimensional compass models
We present an exact solution for a class of one-dimensional compass models
which stand for interacting orbital degrees of freedom in a Mott insulator. By
employing the Jordan-Wigner transformation we map these models on
noninteracting fermions and discuss how spin correlations, high degeneracy of
the ground state, and symmetry in the quantum compass model are visible
in the fermionic language. Considering a zigzag chain of ions with singly
occupied orbitals ( orbital model) we demonstrate that the orbital
excitations change qualitatively with increasing transverse field, and that the
excitation gap closes at the quantum phase transition to a polarized state.
This phase transition disappears in the quantum compass model with maximally
frustrated orbital interactions which resembles the Kitaev model. Here we find
that finite transverse field destabilizes the orbital-liquid ground state with
macroscopic degeneracy, and leads to peculiar behavior of the specific heat and
orbital susceptibility at finite temperature. We show that the entropy and the
cooling rate at finite temperature exhibit quite different behavior near the
critical point for these two models.Comment: 15 pages, 14 figure
Contact angle of sessile drops in Lennard-Jones systems
Molecular dynamics simulation is used for studying the contact angle of
nanoscale sessile drops on a planar solid wall in a system interacting via the
truncated and shifted Lennard-Jones potential. The entire range between total
wetting and dewetting is investigated by varying the solid--fluid dispersive
interaction energy. The temperature is varied between the triple point and the
critical temperature. A correlation is obtained for the contact angle in
dependence of the temperature and the dispersive interaction energy. Size
effects are studied by varying the number of fluid particles at otherwise
constant conditions, using up to 150 000 particles. For particle numbers below
10 000, a decrease of the contact angle is found. This is attributed to a
dependence of the solid-liquid surface tension on the droplet size. A
convergence to a constant contact angle is observed for larger system sizes.
The influence of the wall model is studied by varying the density of the wall.
The effective solid-fluid dispersive interaction energy at a contact angle of
90 degrees is found to be independent of temperature and to decrease linearly
with the solid density. A correlation is developed which describes the contact
angle as a function of the dispersive interaction, the temperature and the
solid density. The density profile of the sessile drop and the surrounding
vapor phase is described by a correlation combining a sigmoidal function and an
oscillation term
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