170 research outputs found
Nonlocal Excitations and 1/8 Singularity in Cuprates
Momentum-dependent excitation spectra of the two-dimensional Hubbard model on
the square lattice have been investigated at zero temperature on the basis of
the full self-consistent projection operator method in order to clarify
nonlocal effects of electron correlations on the spectra. It is found that
intersite antiferromagnetic correlations cause shadow bands and enhance the
Mott-Hubbard splittings near the half-filling. Furthermore nonlocal excitations
are shown to move the critical doping concentration , at
which the singular quasiparticle peak is located just on the Fermi level, from
(the single-site value) to .
The latter suggests the occurance of an instability such as the stripe at
.Comment: 4 pages, 5 figures; to be published in the Journal of Korean Physical
Society (ICM12
Kink Structure in the Quasiparticle Band of Doped Hubbard Systems
By making use of the self-consistent projection operator method with
high-momentum and high-energy resolutions, we find a kink structure in the
quasiparticle excitation spectrum of the two-dimensional Hubbard model in the
underdoped regime. The kink is caused by a mixing between the quasiparticle
state and excitations with short-range antiferromagnetic order. We suggest that
this might be the origin of the strong concentration dependence of the 'kink'
found in La_{2-x}Sr_{x}CuO_{4} (x=0.03-0.07).Comment: 3 pages, 4 figures. to be published in J. Phys. Soc. Jpn., Vol. 74,
No. 9, September 15, 200
First-Principles Dynamical Coherent-Potential Approximation Approach to the Ferromagnetism of Fe, Co, and Ni
Magnetic properties of Fe, Co, and Ni at finite temperatures have been
investigated on the basis of the first-principles dynamical CPA (Coherent
Potential Approximation) combined with the LDA (Local Density Approximation) +
Hamiltonian in the Tight-Binding Linear Muffintin Orbital (TB-LMTO)
representation. The Hamiltonian includes the transverse spin fluctuation terms.
Numerical calculations have been performed within the harmonic approximation
with 4th-order dynamical corrections. Calculated single-particle densities of
states in the ferromagnetic state indicate that the dynamical effects reduce
the exchange splitting, suppress the band width of the quasi-particle state,
and causes incoherent excitations corresponding the 6 eV satellites. Results of
the magnetization vs temperature curves, paramagnetic spin susceptibilities,
and the amplitudes of local moments are presented. Calculated Curie
temperatures () are reported to be 1930K for Fe, 2550K for Co, and
620K for Ni; for Fe and Co are overestimated by a factor of 1.8,
while in Ni agrees with the experimental result. Effective Bohr
magneton numbers calculated from the inverse susceptibilities are 3.0 (Fe), 3.0 (Co), and 1.6 (Ni), being in
agreement with the experimental ones. Overestimate of in Fe and Co
is attributed to the neglects of the higher-order dynamical effects as well as
the magnetic short range order.Comment: 10 pages, 13 figure
Frustrated electron liquids in the Hubbard model
The ground state of the Hubbard model is studied within the constrained
Hilbert space where no order parameter exists. The self-energy of electrons is
decomposed into the single-site and multisite self-energies. The calculation of
the single-site self-energy is mapped to a problem of self-consistently
determining and solving the Anderson model. When an electron reservoir is
explicitly considered, it is proved that the single-site self-energy is that of
a normal Fermi liquid even if the multisite self-energy is anomalous. Thus, the
ground state is a normal Fermi liquid in the supreme single-site approximation
(S^3A). In the strong-coupling regime, the Fermi liquid is stabilized by the
Kondo effect in the S^3A and is further stabilized by the Fock-type term of the
superexchange interaction or the resonating-valence-bond (RVB) mechanism beyond
the S^3A. The stabilized Fermi liquid is frustrated as much as an RVB spin
liquid in the Heisenberg model. It is a relevant unperturbed state that can be
used to study a normal or anomalous Fermi liquid and an ordered state in the
whole Hilbert space by Kondo lattice theory. Even if higher-order multisite
terms than the Fock-type term are considered, the ground state cannot be a Mott
insulator. It can be merely a gapless semiconductor even if the multisite
self-energy is so anomalous that it is divergent at the chemical potential. A
Mott insulator is only possible as a high temperature phase.Comment: 11 pages, no figur
Geography-dependent horizontal gene transfer from vertebrate predators to their prey
Horizontal transfer (HT) of genes between multicellular animals, once thought to be extremely rare, is being more commonly detected, but its global geographic trend and transfer mechanism have not been investigated. We discovered a unique HT pattern of Bovine-B (BovB) LINE retrotransposons in vertebrates, with a bizarre transfer direction from predators (snakes) to their prey (frogs). At least 54 instances of BovB HT were detected, which we estimate to have occurred across time between 85 and 1.3â
Ma. Using comprehensive transcontinental sampling, our study demonstrates that BovB HT is highly prevalent in one geographical region, Madagascar, suggesting important regional differences in the occurrence of HTs. We discovered parasite vectors that may plausibly transmit BovB and found that the proportion of BovB-positive parasites is also high in Madagascar where BovB thus might be physically transported by parasites to diverse vertebrates, potentially including humans. Remarkably, in two frog lineages, BovB HT occurred after migration from a non-HT area (Africa) to the HT hotspot (Madagascar). These results provide a novel perspective on how the prevalence of parasites influences the occurrence of HT in a region, similar to pathogens and their vectors in some endemic diseases
Magnetic properties of X-Pt (X=Fe,Co,Ni) alloy systems
We have studied the electronic and magnetic properties of Fe-Pt, Co-Pt and
Ni-Pt alloy systems in ordered and disordered phases. The influence of various
exchange-correlation functionals on values of equilibrium lattice parameters
and magnetic moments in ordered Fe-Pt, Co-Pt and Ni-Pt alloys have been studied
using linearized muffin-tin orbital method. The electronic structure
calculations for the disordered alloys have been carried out using augmented
space recursion technique in the framework of tight binding linearized
muffin-tin orbital method. The effect of short range order has also been
studied in the disordered phase of these systems. The results show good
agreements with available experimental values.Comment: 21 pages, 4 eps figures, accepted for publication in Journal of
Physics Condensed Matte
Orbital Physics in the Perovskite Ti Oxides
In the perovskite Ti oxide RTiO3 (R=rare-earth ions), the Ti t2g orbitals and
spins in the 3d^1 state couple each other through the strong electron
correlations, resulting in a rich variety of orbital-spin phases. The origin
and nature of orbital-spin states of these Mott insulators have been
intensively studied. In this article, we review the studies on orbital physics
in the perovskite titanates. We focus on the following three topics: (1) the
origin and nature of the ferromagnetism as well as the orbital ordering in the
compounds with relatively small R ions such as GdTiO3 and YTiO3, (2) the origin
of the G-type antiferromagnetism and the orbital state in LaTiO3, and (3) the
orbital-spin structures in other AFM(G) compounds with relatively large R ions
(R=Ce, Pr, Nd and Sm). On the basis of these discussions, we discuss the whole
phase diagram together with mechanisms of the magnetic phase transition. We
also show that the Ti t2g degeneracy is inherently lifted in the titanates,
which allows the single-band descriptions of the ground-state and low-energy
electronic structures as a good starting point. Our analyses indicate that
these compounds offer touchstone materials described by the single-band Hubbard
model on the cubic lattice. From this insight, we also reanalyze the hole-doped
titanates. Experimentally revealed filling-dependent and bandwidth-dependent
properties and the critical behavior of the metal-insulator transitions are
discussed in the light of theories based on the single-band Hubbard models.Comment: Review article, 26 pages, to appear in New Journal of Physic
G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO_3-type distortion in RTiO_3 with R=La, Pr, Nd and Sm
The origin of the antiferromagnetic order and puzzling properties of LaTiO_3
as well as the magnetic phase diagram of the perovskite titanates are studied
theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually
lifted by the La cations in the GdFeO_3-type structure, which generates a
crystal field with nearly trigonal symmetry. This allows the description of the
low-energy structure of LaTiO_3 by a single-band Hubbard model as a good
starting point. The lowest-orbital occupation in this crystal field stabilizes
the AFM(G) state, and well explains the spin-wave spectrum of LaTiO_3 obtained
by the neutron scattering experiment. The orbital-spin structures for RTiO_3
with R=Pr, Nd and Sm are also accounted for by the same mechanism. We point out
that through generating the R crystal field, the GdFeO_3-type distortion has a
universal relevance in determining the orbital-spin structure of the perovskite
compounds in competition with the Jahn-Teller mechanism, which has been
overlooked in the literature. Since the GdFeO_3-type distortion is a universal
phenomenon as is seen in a large number of perovskite compounds, this mechanism
may also play important roles in other compounds of this type.Comment: 20 pages, 15 figure
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