140 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
Possible isotope effect on the resonance peak formation in high-T cuprates
Starting from the three-band Hubbard Hamiltonian we derive an effective
model including electron-phonon interaction of quasiparticles with
optical phonons. Within the effective Hamiltonian we analyze the influence of
electronic correlations and electron-phonon interaction on the dynamical spin
susceptibility in layered cuprates. We find a huge isotope effect on the
resonance peak in the magnetic spin susceptibility, ,
seen by inelastic neutron scattering. It results from both the electron-phonon
coupling and the electronic correlation effects taken into account beyond
random phase approximation(RPA) scheme. We find at optimal doping the isotope
coeffiecient which can be further tested
experimentally.Comment: revised version, new figure is added. Phys. Rev. B 69, 0945XX (2004);
in pres
Double-layer Heisenberg antiferromagnet at finite temperature: Brueckner Theory and Quantum Monte Carlo simulations
The double-layer Heisenberg antiferromagnet with intra- and inter-layer
couplings and exhibits a zero temperature quantum phase
transition between a quantum disordered dimer phase for and a Neel
phase with long range antiferromagnetic order for , where
and . We consider the behavior of the system at finite
temperature for using two different and complementary approaches;
an analytical Brueckner approximation and numerically exact quantum Monte Carlo
simulations. We calculate the temperature dependent spin excitation spectrum
(including the triplet gap), dynamic and static structure factors, the specific
heat, and the uniform magnetic susceptibility. The agreement between the
analytical and numerical approaches is excellent. For and , our analytical results for the specific heat and the magnetic
susceptibility coincide with those previously obtained within the nonlinear
model approach for . Our quantum Monte Carlo simulations
extend to significantly lower temperatures than previously, allowing us to
obtain accurate results for the asymptotic quantum critical behavior. We also
obtain an improved estimate for the critical coupling: .Comment: 23 pages, 12 figure
Effect of magnetic frustration on single-hole spectral function in the t-t'-t''-J model
We examine the effect of the magnetic frustration J' on the single-hole
spectral function in the t-t'-t''-J model. At zero temperature, the exact
diagonalization (ED) and the self-consistent Born approximation (SCBA) methods
are used. We find that the frustration suppresses the quasiparticle (QP) weight
at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We
also show the temperature dependence of the single-hole spectral function by
using the ED method. It is found that the lineshapes at (pi/2,0) and
(pi/2,pi/2) show different temperature dependence. These findings are
consistent with the angle-resolved photoemission data on Sr2CuO2Cl2, and
indicate the importance of the magnetic frustration on the electronic states of
the insulating cuprates.Comment: 5 pages, 3 EPS figures, REVTeX, To be published in Phys. Rev. B, Vol.
59, Num. 3 (15 Jan. 1999
Zeeman Perturbed Cu Nuclear Quadrupole Resonance Study of the Vortex State of YBaCuO
We report a Cu nuclear quadrupole resonance (NQR) study of the vortex
state for an aligned polycrystalline sample of a slightly overdoped high-
superconductor YBaCuO (92 K) at a low magnetic
field of 96 mT along the c axis, near a lower critical field . We
observed the frequency distribution of the nuclear spin-lattice relaxation time
in the Zeeman-perturbed Cu NQR spectrum below . The
characteristic behavior of 1/, taking the minimum values with respect
to temperature and frequency, indicates the significant role of
antiferromagnetic spin fluctuations in the Doppler-shifted quasiparticle energy
spectrum inside and outside vortex cores.Comment: 4 pages, 4 figure
Dispersion of the neutron resonance in cuprate superconductors
We argue that recently measured downward dispersion of the neutron resonance
peak in cuprate superconductors is naturally explained if the resonance is
viewed as a spin-1 collective mode in a d-wave superconductor. The reduction of
the resonant frequency away from the antiferromagnetic wave vector is a direct
consequence of the momentum dependence of the d-wave superconducting gap. When
the magnetic correlation length becomes large, the dispersion should become
magnon-like, i.e., curve upwards from (pi,pi).Comment: 4 pages, 3 inline PostScript figures. Added reference
Quasiparticle structure in antiferromagnetism around the vortex and nuclear magnetic relaxation time
On the basis of the Bogoliubov-de Gennes theory for the two-dimensional
extended Hubbard model, the vortex structure in d-wave superconductors is
investigated including the contribution of the induced incommensurate
antiferromagnetism around the vortex core. As the on-site repulsive interaction
increases, the spatial structure of charge and spin changes from the
antiferromagnetic state with checkerboard modulation to that with the stripe
modulation. By the effect of the induced antiferromagnetic moment, the
zero-energy density of states is suppressed, and the vortex core radius
increases. We also study the effect of the local density of states (LDOS)
change on the site-dependent nuclear relaxation rate . These
results are compared with a variety of experiments performed on high
cuprates.Comment: 10pages, 8 figure
Quantum impurity dynamics in two-dimensional antiferromagnets and superconductors
We present the universal theory of arbitrary, localized impurities in a
confining paramagnetic state of two-dimensional antiferromagnets with global
SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1
excitations, \Delta, is assumed to be significantly smaller than a typical
nearest neighbor exchange. In the absence of impurities, it was argued in
earlier work (Chubukov et al. cond-mat/9304046) that the low-temperature
quantum dynamics is universally and completely determined by the values of
\Delta and a spin-wave velocity c. Here we establish the remarkable fact that
no additional parameters are necessary for an antiferromagnet with a dilute
concentration of impurities, n_{imp} - each impurity is completely
characterized by a integer/half-odd-integer valued spin, S, which measures the
net uncompensated Berry phase due to spin precession in its vicinity. We
compute the impurity-induced damping of the spin-1 collective mode of the
antiferromagnet: the damping occurs on an energy scale \Gamma= n_{imp} (\hbar
c)^2/\Delta, and we predict a universal, asymmetric lineshape for the
collective mode peak. We argue that, under suitable conditions, our results
apply unchanged (or in some cases, with minor modifications) to d-wave
superconductors, and compare them to recent neutron scattering experiments on
YBCO by Fong et al. (cond-mat/9812047). We also describe the universal
evolution of numerous measurable correlations as the host antiferromagnet
undergoes a quantum phase transition to a Neel ordered state.Comment: 36 pages, 12 figures; added reference
Resonant Raman Scattering in Antiferromagnets
Two-magnon Raman scattering provides important information about electronic
correlations in the insulating parent compounds of high- materials. Recent
experiments have shown a strong dependence of the Raman signal in
geometry on the frequency of the incoming photon. We present an analytical and
numerical study of the Raman intensity in the resonant regime. It has been
previously argued by one of us (A.Ch) and D. Frenkel that the most relevant
contribution to the Raman vertex at resonance is given by the triple resonance
diagram. We derive an expression for the Raman intensity in which we
simultaneously include the enhancement due to the triple resonance and a final
state interaction. We compute the two-magnon peak height (TMPH) as a function
of incident frequency and find two maxima at and . We argue that the
high-frequency maximum is cut only by a quasiparticle damping, while the
low-frequency maximum has a finite amplitude even in the absence of damping. We
also obtain an evolution of the Raman profile from an asymmetric form around
to a symmetric form around . We
further show that the TMPH depends on the fermionic quasiparticle damping, the
next-nearest neighbor hopping term and the corrections to the
interaction vertex between light and the fermionic current. We discuss our
results in the context of recent experiments by Blumberg et al. on
and and R\"{u}bhausen et al. on
and show that the triple resonance theory yields a qualitative
and to some extent also quantitative understanding of the experimental data.Comment: 19 pages, RevTeX, 16 figures embedded in the text, ps-file is also
available at http://lifshitz.physics.wisc.edu/www/morr/morr_homepage.htm
Normal-State Spin Dynamics and Temperature-Dependent Spin Resonance Energy in an Optimally Doped Iron Arsenide Superconductor
The proximity of superconductivity and antiferromagnetism in the phase
diagram of iron arsenides, the apparently weak electron-phonon coupling and the
"resonance peak" in the superconducting spin excitation spectrum have fostered
the hypothesis of magnetically mediated Cooper pairing. However, since most
theories of superconductivity are based on a pairing boson of sufficient
spectral weight in the normal state, detailed knowledge of the spin excitation
spectrum above the superconducting transition temperature Tc is required to
assess the viability of this hypothesis. Using inelastic neutron scattering we
have studied the spin excitations in optimally doped BaFe1.85Co0.15As2 (Tc = 25
K) over a wide range of temperatures and energies. We present the results in
absolute units and find that the normal state spectrum carries a weight
comparable to underdoped cuprates. In contrast to cuprates, however, the
spectrum agrees well with predictions of the theory of nearly antiferromagnetic
metals, without complications arising from a pseudogap or competing
incommensurate spin-modulated phases. We also show that the temperature
evolution of the resonance energy follows the superconducting energy gap, as
expected from conventional Fermi-liquid approaches. Our observations point to a
surprisingly simple theoretical description of the spin dynamics in the iron
arsenides and provide a solid foundation for models of magnetically mediated
superconductivity.Comment: 8 pages, 4 figures, and an animatio
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