379 research outputs found
Spin and orbital excitation spectrum in the Kugel-Khomskii model
We discuss spin and orbital ordering in the twofold orbital degenerate
superexchange model in three dimensions relevant to perovskite transition metal
oxides. We focus on the particular point on the classical phase diagram where
orbital degeneracy is lifted by quantum effects exclusively. Dispersion and
damping of the spin and orbital excitations are calculated at this point taking
into account their mutual interaction. Interaction corrections to the
mean-field order parameters are found to be small. We conclude that
quasi-one-dimensional Neel spin order accompanied by the uniform
d_{3z^2-r^2}-type orbital ordering is stable against quantum fluctuations.Comment: 4 pages with 3 PS figures, 1 table, RevTeX, accepted to Phys. Rev. B.
Rapid Communicatio
Spin-wave softening and Hund's coupling in ferromagnetic manganites
Using one-orbital model of hole-doped manganites, we show with the help of
Holstein-Primakov transformation that finite Hund's coupling is responsible for
the spin-wave softening in the ferromagnetic -phase manganites. We obtain an
analytical result for the spin-wave spectrum for \JH\gg t. In the limit of
infinte Hund's coupling, the spectrum is the conventional nearest-neighbor
Heisenberg ferromagnetic spin-wave. The o(t/\JH)-order correction is negative
and thus accounts for the softening near the zone boundary.Comment: 5 pages, 3 figure
Non-perturbative corrections to mean-field behavior: spherical model on spider-web graph
We consider the spherical model on a spider-web graph. This graph is
effectively infinite-dimensional, similar to the Bethe lattice, but has loops.
We show that these lead to non-trivial corrections to the simple mean-field
behavior. We first determine all normal modes of the coupled springs problem on
this graph, using its large symmetry group. In the thermodynamic limit, the
spectrum is a set of -functions, and all the modes are localized. The
fractional number of modes with frequency less than varies as for tending to zero, where is a constant. For an
unbiased random walk on the vertices of this graph, this implies that the
probability of return to the origin at time varies as ,
for large , where is a constant. For the spherical model, we show that
while the critical exponents take the values expected from the mean-field
theory, the free-energy per site at temperature , near and above the
critical temperature , also has an essential singularity of the type
.Comment: substantially revised, a section adde
Single-electron tunneling in InP nanowires
We report on the fabrication and electrical characterization of field-effect
devices based on wire-shaped InP crystals grown from Au catalyst particles by a
vapor-liquid-solid process. Our InP wires are n-type doped with diameters in
the 40-55 nm range and lengths of several microns. After being deposited on an
oxidized Si substrate, wires are contacted individually via e-beam fabricated
Ti/Al electrodes. We obtain contact resistances as low as ~10 kOhm, with minor
temperature dependence. The distance between the electrodes varies between 0.2
and 2 micron. The electron density in the wires is changed with a back gate.
Low-temperature transport measurements show Coulomb-blockade behavior with
single-electron charging energies of ~1 meV. We also demonstrate energy
quantization resulting from the confinement in the wire.Comment: 4 pages, 3 figure
Multicomponent intervention for patients admitted to an emergency unit for suicide attempt: an exploratory study
Suicide is a major cause of premature deaths worldwide and belongs to the top priority public health issues. While suicide attempt is the most important risk factor for completed suicide, intervention for suicide attempters (SA) have produced mixed results. Since an important proportion of SA request medical care, emergency units (EU) are an opportune setting to implement such interventions. This exploratory study evaluated the feasibility and acceptability of a multicomponent intervention for SA admitted to an EU. The intervention consisted of coordination by a case manager of a joint crisis plan (JCP), an early meeting with relatives and the existing care network, as well as phone contacts during 3 months after suicide attempt. Among 107 SA admitted to the emergency unit during the study period, 51 could not be included for logistical reason, 22 were excluded, and intervention was offered to 34. Of these, 15 refused the intervention, which was thus piloted with 19 SA. First-time attempters most frequently declined the intervention. Feasibility and acceptability of phone contacts and case manager were good, while JCPs and meetings were difficult to implement and perceived as less acceptable. Refusal pattern questions the global acceptability and is discussed: JCPs and meetings will have to be modified in order to improve their feasibility and acceptability, especially among first-time attempters
Orbital dynamics in ferromagnetic transition metal oxides
We consider a model of strongly correlated electrons interacting by
superexchange orbital interactions in the ferromagnetic phase of LaMnO. It
is found that the classical orbital order with alternating occupied
orbitals has a full rotational symmetry at orbital degeneracy, and the
excitation spectrum derived using the linear spin-wave theory is gapless. The
quantum (fluctuation) corrections to the order parameter and to the ground
state energy restore the cubic symmetry of the model. By applying a uniaxial
pressure orbital degeneracy is lifted in a tetragonal field and one finds an
orbital-flop phase with a gap in the excitation spectrum. In two dimensions the
classical order is more robust near the orbital degeneracy point and quantum
effects are suppressed. The orbital excitation spectra obtained using finite
temperature diagonalization of two-dimensional clusters consist of a
quasiparticle accompanied by satellite structures. The orbital waves found
within the linear spin-wave theory provide an excellent description of the
dominant pole of these spectra.Comment: 13 pages, 12 figures, to appear in Phys. Rev.
Theory of Raman Scattering from Orbital Excitations in Manganese Oxides
We present a theory of the Raman scattering from the orbital wave excitations
in manganese oxides. Two excitation processes of the Raman scattering are
proposed. The Raman scattering cross section is formulated by using the
pseudospin operator for orbital degree of freedom in a Mn ion. The Raman
spectra from the orbital wave excitations are calculated and their implications
in the recent experimental results reported in LaMnO are discussed.Comment: 10 pages, 7 figure
Orbital liquid in three dimensional Mott insulator:
We present a theory of spin and orbital states in Mott insulator .
The spin-orbital superexchange interaction between ions in cubic
crystal suffers from a pathological degeneracy of orbital states at classical
level. Quantum effects remove this degeneracy and result in the formation of
the coherent ground state, in which the orbital moment of level is
fully quenched. We find a finite gap for orbital excitations. Such a disordered
state of local degrees of freedom on unfrustrated, simple cubic lattice is
highly unusual. Orbital liquid state naturally explains observed anomalies of
.Comment: 5 pages, 3 figure
Exact Ground States in Spin Systems with Orbital Degeneracy
We present exact ground states in spin models with orbital generacy in one
and higher dimensions. A method to obtain the exact ground states of the models
when the Hamiltonians are composed of the products of two commutable operators
is proposed. For the case of the spin-1/2 model with two-fold degeneracy some
exact ground states are given, such as the Valence-Bond (VB), the magnetically
ordered, and the orbitally ordered states under particular parameter regimes.
We also find the models with the higher spin and degeneracy which have the new
types of VB ground states in the spin and the orbital sectors.Comment: 4 pages(JPSJ.sty), 2 figures(EPS), to appear in J. Phys. Soc. Jpn.
68, No.2 (1999) 32
Orbital and spin physics in LiNiO2 and NaNiO2
We derive a spin-orbital Hamiltonian for a triangular lattice of e_g orbital
degenerate (Ni^{3+}) transition metal ions interacting via 90 degree
superexchange involving (O^{2-}) anions, taking into account the on-site
Coulomb interactions on both the anions and the transition metal ions. The
derived interactions in the spin-orbital model are strongly frustrated, with
the strongest orbital interactions selecting different orbitals for pairs of Ni
ions along the three different lattice directions. In the orbital ordered
phase, favoured in mean field theory, the spin-orbital interaction can play an
important role by breaking the U(1) symmetry generated by the much stronger
orbital interaction and restoring the threefold symmetry of the lattice. As a
result the effective magnetic exchange is non-uniform and includes both
ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic
interactions still dominate, this offers yet insufficient explanation for the
absence of magnetic order and the low-temperature behaviour of the magnetic
susceptibility of stoichiometric LiNiO_2. The scenario proposed to explain the
observed difference in the physical properties of LiNiO_2 and NaNiO_2 includes
small covalency of Ni-O-Li-O-Ni bonds inducing weaker interplane superexchange
in LiNiO_2, insufficient to stabilize orbital long-range order in the presence
of stronger intraplane competition between superexchange and Jahn-Teller
coupling.Comment: 33 pages, 12 postscript figures, uses iopams.sty . This article
features in New Journal of Physics as part of a Focus Issue on Orbital
Physics - all contributions may be freely accessed at
(http://stacks.iop.org/1367-2630/6/i=1/a=E05). The published version of this
article may be found at http://stacks.iop.org/1367-2630/7/12
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