7,833 research outputs found
Spin dynamics in the generalized ferromagnetic Kondo model for manganites
Dynamical spin susceptibility is calculated for the generalized ferromagnetic
Kondo model which describes itinerant electrons interacting with
localized electrons with antiferromagnetic coupling. The calculations
done in the mean field approximation show that the spin-wave spectrum of the
system in ferromagnetic state has two branches, acoustic and optic ones.
Self-energy corrections to the spectrum are considered and the acoustic
spin-wave damping is evaluated
Classical spin liquid instability driven by off-diagonal exchange in strong spin-orbit magnets
We show that the off-diagonal exchange anisotropy drives Mott insulators with
strong spin-orbit coupling to a classical spin liquid regime, characterized by
an infinite number of ground states and Ising variables living on closed or
open strings. Depending on the sign of the anisotropy, quantum fluctuations
either fail to lift the degeneracy down to very low temperatures, or select
non-collinear magnetic states with unconventional spin correlations. The
results apply to all 2D and 3D tri-coordinated materials with bond-directional
anisotropy, and provide a consistent interpretation of the suppression of the
x-ray magnetic circular dichroism signal reported recently for
-LiIrO under pressure
Magnetic and orbital ordering in cuprates and manganites
The mechanisms of magnetic and orbital interactions due to double exchange
(DE) and superexchange (SE) in transition metal oxides with degenerate e_g
orbitals are presented. Specifically, we study the effective spin-orbital
models derived for the d^9 ions as in KCuF_3, and for the d^4 ions as in
LaMnO_3, for spins S=1/2 and S=2, respectively. Such models are characterized
by three types of elementary excitations: spin waves, orbital waves, and
spin-and-orbital waves. The SE interactions between Cu^{2+} (d^9) ions are
inherently frustrated, which leads to a new mechanism of spin liquid which
operates in three dimensions. The SE between Mn^{3+} (d^4) ions explains the
A-type antiferromagnetic order in LaMnO_3 which coexists with the orbital
order. In contrast, the ferromagnetic metallic phase and isotropic spin waves
observed in doped manganites are explained by DE for degenerate e_g orbitals.
It is shown that although a hole does not couple to spin excitations in
ferromagnetic planes of LaMnO_3, the orbital excitations change the energy
scale for the coherent hole propagation and cause a large redistribution of
spectral weight. Finally, we point out some open problems in the present
understanding of doped manganites.Comment: 155 pages, 66 figure
Spin-1 effective Hamiltonian with three degenerate orbitals: An application to the case of V_2O_3
Motivated by recent neutron and x-ray observations in V_2O_3, we derive the
effective Hamiltonian in the strong coupling limit of an Hubbard model with
three degenerate t_{2g} states containing two electrons coupled to spin S = 1,
and use it to re-examine the low-temperature ground-state properties of this
compound. An axial trigonal distortion of the cubic states is also taken into
account. Since there are no assumptions about the symmetry properties of the
hopping integrals involved, the resulting spin-orbital Hamiltonian can be
generally applied to any crystallographic configuration of the transition metal
ion giving rise to degenerate t_{2g} orbitals. Specializing to the case of
V_2O_3 we consider the antiferromagnetic insulating phase. We find two
variational regimes, depending on the relative size of the correlation energy
of the vertical pairs and the in-plane interaction energy. The former favors
the formation of stable molecules throughout the crystal, while the latter
tends to break this correlated state. We determine in both cases the minimizing
orbital solutions for various spin configurations, and draw the corresponding
phase diagrams. We find that none of the symmetry-breaking stable phases with
the real spin structure presents an orbital ordering compatible with the
magnetic space group indicated by very recent observations of non-reciprocal
x-ray gyrotropy in V_2O_3. We do however find a compatible solution with very
small excitation energy in two distinct regions of the phase space, which might
turn into the true ground state of V_2O_3 due to the favorable coupling with
the lattice. We illustrate merits and drawbacks of the various solutions and
discuss them in relation to the present experimental evidence.Comment: 36 pages, 19 figure
Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality
Quantum spin liquids are long-range entangled states of matter with emergent
gauge fields and fractionalized excitations. While candidate materials, such as
the Kitaev honeycomb ruthenate -RuCl, show magnetic order at low
temperatures , here we demonstrate numerically a dynamical crossover from
magnon-like behavior at low and frequencies to long-lived
fractionalized fermionic quasiparticles at higher and . This
crossover is akin to the presence of spinon continua in quasi-1D spin chains.
It is further shown to go hand in hand with persistent typicality down to very
low . This aspect, which has also been observed in the spin-1/2 kagome
Heisenberg antiferromagnet, is a signature of proximate spin liquidity and
emergent gauge degrees of freedom more generally, and can be the basis for the
numerical study of many finite- properties of putative spin liquids.Comment: 13 pages, 11 figures, accepted versio
An implicit finite-difference solution to the viscous shock layer, including the effects of radiation and strong blowing
An implicit finite-difference scheme is developed for the fully coupled solution of the viscous, radiating stagnation-streamline equations, including strong blowing. Solutions are presented for both air injection and injection of carbon-phenolic ablation products into air at conditions near the peak radiative heating point in an earth entry trajectory from interplanetary return missions. A detailed radiative-transport code that accounts for the important radiative exchange processes for gaseous mixtures in local thermodynamic and chemical equilibrium is utilized in the study. With minimum number of assumptions for the initially unknown parameters and profile distributions, convergent solutions to the full stagnation-line equations are rapidly obtained by a method of successive approximations. Damping of selected profiles is required to aid convergence of the solutions for massive blowing. It is shown that certain finite-difference approximations to the governing differential equations stabilize and improve the solutions. Detailed comparisons are made with the numerical results of previous investigations. Results of the present study indicate lower radiative heat fluxes at the wall for carbonphenolic ablation than previously predicted
A modified method of integral relations approach to the blunt-body equilibrium air flow field, including comparisons with inverse solutions
Numerical calculation of inviscid adiabatic flow field around blunt bodies at hypersonic speed
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