195 research outputs found
Impurity-induced spin polarization and NMR line broadening in underdoped cuprates
We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in
underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments
on Cu sites in the proximity of the impurity, a process which is intimately
related to the spin gap phenomenon in cuprates. Below a characteristic Kondo
temperature, the Ni spin is partially screened by the Cu moments, resulting in
an effective impurity spin S=1/2. We further analyze the
Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a
polarization of the effective impurity moments and derive expressions for the
corresponding ^{17}O NMR line broadening. The peculiar aspects of recent
experimental NMR data can be traced back to different spatial characteristics
of Ni and Zn moments as well as to an inherent temperature dependence of local
antiferromagnetic correlations.Comment: PRB B1 01June9
Theory of the density fluctuation spectrum of strongly correlated electrons
The density response function of the two-dimensional
model is studied starting from a mixed gauge formulation of the slave boson
approach. Our results for are in remarkable agreement with exact
diagonalization studies, and provide a natural explanation of the anomalous
features in the density response in terms of the spin polaron nature of the
charge carriers. In particular we have identified unexplained low energy
structures in the diagonalization data as arising from the coherent polaron
motion of holes in a spin liquid.Comment: 4 pages with 4 figures, to be published in Physical Review B (RC
Phase diagram of a Bose gas near a wide Feshbach resonance
In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length exceeds a critical value depending on the atom density
, , the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, , the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure
Theory of anomalous magnon softening in ferromagnetic manganites
In metallic manganites with low Curie temperatures, a peculiar softening of
the magnon spectrum close to the magnetic zone boundary has experimentally been
observed. Here we present a theory of the renormalization of the magnetic
excitation spectrum in colossal magnetoresistance compounds. The theory is
based on the modulation of magnetic exchange bonds by the orbital degree of
freedom of double-degenerate e_g electrons. The model considered is an
orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons which we treat in the limit of strong onsite repulsions. Charge and
coupled orbital-lattice fluctuations are identified as the main origin of the
unusual softening of the magnetic spectrum
Phonons and Magnetic Excitations in Mott-Insulator LaTiO
The polarized Raman spectra of stoichiometric LaTiO (T K) were
measured between 6 and 300 K. In contrast to earlier report on half-metallic
LaTiO, neither strong background scattering, nor Fano shape of the
Raman lines was observed. The high frequency phonon line at 655 cm
exhibits anomalous softening below T: a signature for structural
rearrangement. The assignment of the Raman lines was done by comparison to the
calculations of lattice dynamics and the nature of structural changes upon
magnetic ordering are discussed. The broad Raman band, which appears in the
antiferromagnetic phase, is assigned to two-magnon scattering. The estimated
superexchange constant meV is in excellent agreement with the
result of neutron scattering studies.Comment: 4 pages, 5 figure
Impurity-induced moments in underdoped cuprates
We examine the effect of a nonmagnetic impurity in a two-dimensional spin
liquid in the spin-gap phase, employing a drone-fermion representation of
spin-1/2 operators. The properties of the local moment induced in the vicinity
of the impurity are investigated and an expression for the nuclear-magnetic-
resonance Knight shift is derived, which we compare with experimental results.
Introducing a second impurity into the spin liquid an antiferromagnetic
interaction between the moments is found when the two impurities are located on
different sublattices. The presence of many impurities leads to a screening of
this interaction as is shown by means of a coherent-potential approximation.
Further, the Kondo screening of an impurity-induced local spin by charge
carriers is discussed.Comment: 8 pages, 1 figure. To be published in Phys. Rev. B, Vol. 56, No. 1
Perturbative calculation of the spin-wave dispersion in a disordered double-exchange model
We study the spin-wave dispersion of localized spins in a disordered
double-exchange model using the perturbation theory with respect to the
strength of the disorder potential. We calculate the dispersion upto the
next-leading order, and extensively examine the case of one-dimension. We show
that in that case, disorder yields anomalous gapped-like behavior at the Fermi
wavenumber of the conduction electrons.Comment: 9 pages, 5 figure
Normal Fermi Liquid Behavior of Quasiholes in the Spin-Polaron Model for Copper Oxides
Based on the t-J model and the self-consistent Born approximation, the
damping of quasiparticle hole states near the Fermi surface is calculated in a
low doping regime. Renormalization of spin-wave excitations due to hole doping
is taken into account. The damping is shown to be described by a familiar form
characteristic of the 2-dimensional
Fermi liquid, in contrast with the earlier statement reported by Li and Gong
[Phys. Rev. B {\bf 51}, 6343 (1995)] on the marginal Fermi liquid behavior of
quasiholes
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
Order from Disorder: Non Magnetic Impurities in the Spin-gap Phase of the Cuprates
We solve the problem of non magnetic impurities in the staggered flux
phase of the Heisenberg model which we assume to be a good mean-field
approximation for the spin-gap phase of the cuprates. The density of states is
evaluated exactly in the unitary limit and is porportional to 1/\left (\omega
\ln^2(|\omega|/D)), in analogy with the 1D case of doped spin-Peierls and
two-leg ladders compounds. We argue that the system exhibits a quasi long-range
order at T=0 with instantaneous spin-spin correlations decreasing as n_i/
\ln^2\left (n_i R_{ij}) for large distances and we predict enhanced
low energy fluctuations in Neutron Scattering.Comment: 4 pages, corrected typos, references adde
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