190 research outputs found
Antiferromagnetism in two-dimensional t-J model: pseudospin representation
We discuss a pseudospin representation of the two-dimensional t-J model. We
introduce pseudospins associated with empty sites, deriving a new
representation of the t-J model that consists of local spins and spinless
fermions. We show, within a mean-field approximation, that our representation
of t-J model corresponds to the {\it isotropic} antiferromagnetic Heisenberg
model in an effective magnetic field. The strength and the direction of the
effective field are determined by the hole doping and the
orientation of pseudospins associated with empty sites, respectively. We find
that the staggered magnetization in the standard representation corresponds to
the component of magnetization perpendicular to the effective field in our
pseudospin representation. Using a many-body Green's function method, we show
that the staggered magnetization decreases with increasing hole doping
and disappears at for . Our
results are in good agreement with experiments and numerical calculations in
contradistinction to usual mean-field methods.Comment: 6 pages, 3 figure
Nonzero macroscopic magnetization in half-metallic antiferromagnets at finite temperatures
Combining density-functional theory calculations with many-body
Green's-function technique, we reveal that the macroscopic magnetization in
half-metallic antiferromagnets does not vanish at finite temperature as for the
T=0 limit. This anomalous behavior stems from the inequivalent magnetic
sublattices which lead to different intrasublattice exchange interactions. As a
consequence, the spin fluctuations suppress the magnetic order of the
sublattices in a different way leading to a ferrimagnetic state at finite
temperatures. Computational results are presented for the half-metallic
antiferromagnetic CrMnZ (Z=P,As,Sb) semi-Heusler compounds.Comment: 4 pages, 2 figure
Magnetization Relaxation and Collective Spin Excitations in Correlated Double--Exchange Ferromagnets
We study spin relaxation and dynamics of collective spin excitations in
correlated double--exchange ferromagnets. For this, we introduce an expansion
of the Green's functions equations of motion that treats non--perturbativerly
all correlations between a given number of spin and charge excitations and
becomes exact within a sub--space of states. Our method treats relaxation
beyond Fermi's Golden Rule while recovering previous variational results for
the spin--wave dispersion. We find that the momentum dependence of the
spin--wave dephasing rate changes qualitatively due to the on--site Coulomb
interaction, in a way that resembles experiment, and depends on its interplay
with the magnetic exchange interaction and itinerant spin lifetime. We show
that the collective spin relaxation and its dependence on the carrier
concentration depends sensitively on three--body correlations between a spin
excitation and a Fermi sea electron and hole. The above spin dynamics can be
controlled via the itinerant carrier population.Comment: 13 pages, 10 figures, published in Phys. Rev.
Green's function theory of quasi-two-dimensional spin-half Heisenberg ferromagnets: stacked square versus stacked kagom\'e lattice
We consider the thermodynamic properties of the quasi-two-dimensional
spin-half Heisenberg ferromagnet on the stacked square and the stacked kagom\'e
lattices by using the spin-rotation-invariant Green's function method. We
calculate the critical temperature , the uniform static susceptibility
, the correlation lengths and the magnetization and
investigate the short-range order above . We find that and at
are smaller for the stacked kagom\'e lattice which we attribute to
frustration effects becoming relevant at finite temperatures.Comment: shortened version as published in PR
Thermodynamic properties of Holstein polarons and the effects of disorder
The ground state and finite temperature properties of polarons are studied
considering a two-site and a four-site Holstein model by exact diagonalization
of the Hamiltonian. The kinetic energy, Drude weight, correlation functions
involving charge and lattice deformations, and the specific heat have been
evaluated as a function of electron-phonon (e-ph) coupling strength and
temperature. The effects of site diagonal disorder on the above properties have
been investigated. The disorder is found to suppress the kinetic energy and the
Drude weight, reduces the spatial extension of the polaron, and makes the
large-to-small polaron crossover smoother. Increasing temperature also plays
similar role. For strong coupling the kinetic energy arises mainly from the
incoherent hopping processes owing to the motion of electrons within the
polaron and is almost independent of the disorder strength. From the coherent
and incoherent contributions to the kinetic energy, the temperature above which
the incoherent part dominates is determined as a function of e-ph coupling
strength.Comment: 17 pages. 17 figure
The t-J model on a semi-infinite lattice
The hole spectral function of the t-J model on a two-dimensional
semi-infinite lattice is calculated using the spin-wave and noncrossing
approximations. In the case of small hole concentration and strong
correlations, , several near-boundary site rows appear to be depleted
of holes. The reason for this depletion is a deformation of the magnon cloud,
which surrounds the hole, near the boundary. The hole depletion in the boundary
region leads to a more complicated spectral function in the boundary row in
comparison with its bulk shape.Comment: 8 pages, 5 figure
Superlight small bipolarons
Recent angle-resolved photoemission spectroscopy (ARPES) has identified that
a finite-range Fr\"ohlich electron-phonon interaction (EPI) with c-axis
polarized optical phonons is important in cuprate superconductors, in agreement
with an earlier proposal by Alexandrov and Kornilovitch. The estimated
unscreened EPI is so strong that it could easily transform doped holes into
mobile lattice bipolarons in narrow-band Mott insulators such as cuprates.
Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the
total energy, effective mass, pair radius, number of phonons and isotope
exponent of lattice bipolarons in the region of parameters where any
approximation might fail taking into account the Coulomb repulsion and the
finite-range EPI. The effects of modifying the interaction range and different
lattice geometries are discussed with regards to analytical
strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be
simultaneously small and light, provided suitable conditions on the
electron-phonon and electron-electron interaction are satisfied. Such light
small bipolarons are a necessary precursor to high-temperature Bose-Einstein
condensation in solids. The light bipolaron mass is shown to be universal in
systems made of triangular plaquettes, due to a novel crab-like motion. Another
surprising result is that the triplet-singlet exchange energy is of the first
order in the hopping integral and triplet bipolarons are heavier than singlets
in certain lattice structures at variance with intuitive expectations. Finally,
we identify a range of lattices where superlight small bipolarons may be
formed, and give estimates for their masses in the anti-adiabatic
approximation.Comment: 31 pages. To appear in J. Phys.: Condens. Matter, Special Issue
'Mott's Physics
Fingerprints of Spin-Orbital Physics in Crystalline O
The alkali hyperoxide KO is a molecular analog of strongly-correlated
systems, comprising of orbitally degenerate magnetic O ions. Using
first-principles electronic structure calculations, we set up an effective
spin-orbital model for the low-energy \textit{molecular} orbitals and argue
that many anomalous properties of KO replicate the status of its orbital
system in various temperature regimes.Comment: 4 pages, 2 figures, 1 tabl
Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice
We study the ground state of a spin-half Heisenberg antiferromagnet on the
stacked kagome lattice by using a spin-rotation-invariant Green's-function
method. Since the pure two-dimensional kagome antiferromagnet is most likely a
magnetically disordered quantum spin liquid, we investigate the question
whether the coupling of kagome layers in a stacked three-dimensional system may
lead to a magnetically ordered ground state. We present spin-spin correlation
functions and correlation lengths. For comparison we apply also linear spin
wave theory. Our results provide strong evidence that the system remains
short-range ordered independent of the sign and the strength of the interlayer
coupling
Thermodynamics of layered Heisenberg magnets with arbitrary spin
We present a spin-rotation-invariant Green-function theory of long- and
short-range order in the ferro- and antiferromagnetic Heisenberg model with
arbitrary spin quantum number S on a stacked square lattice. The thermodynamic
quantities (Curie temperature T_C, N\'eel temperature T_N, specific heat C_V,
intralayer and interlayer correlation lengths) are calculated, where the
effects of the interlayer coupling and the S dependence are explored. In
addition, exact diagonalizations on finite two-dimensional (2D) lattices with
S>=1 are performed, and a very good agreement between the results of both
approaches is found. For the quasi-2D and isotropic 3D magnets, our theory
agrees well with available quantum Monte Carlo and high-temperature
series-expansion data. Comparing the quasi-2D S=1/2 magnets, we obtain the
inequalities T_N>T_C and, for small enough interlayer couplings, T_N<T_C. The
results for C_V and the intralayer correlation length are compared to
experiments on the quasi-2D antiferromagnets Zn_2VO(PO_4)_2 with S=1/2 and
La_2NiO_4 with S=1, respectively.Comment: 10 pages, 8 figures, 3 tables, submitted to Physical Review
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