318 research outputs found
Phenomenological theory of spin excitations in La- and Y-based cuprates
Motivated by recent inelastic neutron scattering (INS) experiments on
La-based cuprates and based on the fermiology theories, we study the spin
susceptibility for La-based (e.g., LaSrCuO) and Y-based (e.g.,
YBaCuO) cuprates, respectively. The spin excitation in
YBaCuO is dominated by a sharp resonance peak at the frequency 40
meV in the superconducting state. Below and above the resonance frequency, the
incommensurate (IC) peaks develop and the intensity of the peaks decreases
dramatically. In the normal state, the resonant excitation does not occur and
the IC peaks are merged into commensurate ones. The spin excitation of
LaSrCuO is significantly different from that of Y-based ones,
namely, the resonance peak does not exist due to the decreasing of the
superconducting gap and the presence of the possible spin-stripe order. The
spectra are only enhanced at the expected resonance frequency (about 18 meV)
while it is still incommensurate. On the other hand, another frequency scale at
the frequency 55 meV is also revealed, namely the spectra are commensurate and
local maximum at this frequency. We elaborate all the results based on the
Fermi surface topology and the d-wave superconductivity, and suggest that the
spin-stripe order be also important in determining the spin excitation of
La-based cuprates. A coherent picture for the spin excitations is presented for
Y-based and La-based cuprates.Comment: 8 pages, 8 figure
Spin susceptibility in bilayered cuprates: resonant magnetic excitations
We study the momentum and frequency dependence of the dynamical spin
susceptibility in the superconducting state of bilayer cuprate superconductors.
We show that there exists a resonance mode in the odd as well as the even
channel of the spin susceptibility, with the even mode being located at higher
energies than the odd mode. We demonstrate that this energy splitting between
the two modes arises not only from a difference in the interaction, but also
from a difference in the free-fermion susceptibilities of the even and odd
channels. Moreover, we show that the even resonance mode disperses downwards at
deviations from . In addition, we demonstrate that there
exists a second branch of the even resonance, similar to the recently observed
second branch (the -mode) of the odd resonance. Finally, we identify the
origin of the qualitatively different doping dependence of the even and odd
resonance. Our results suggest further experimental test that may finally
resolve the long-standing question regarding the origin of the resonance peak.Comment: 8 pages, 5 figure
Doping dependance of the spin resonance peak in bilayer high- superconductors
Motivated by a recent experiment on the bilayer
YCaBaCuO superconductor and based on a bilayer
model, we calculate the spin susceptibility at different doping densities in
the even and odd channels in a bilayer system. It is found that the intensity
of the resonance peak in the even channel is much weaker than that in the odd
one, with the resonance position being at a higher frequency. While this
difference decreases as the doping increases, and both the position and
amplitude of the resonance peaks in the two channels are very similar in the
deeply overdoped sample. Moreover, the resonance frequency in the odd channel
is found to be linear with the critical temperature , while the resonance
frequency increases as doping decreases in the even channel and tends to
saturate at the underdoped sample. We elaborate the results based on the Fermi
surface topology and the d-wave superconductivity.Comment: 6 pages, 5 figure
Drude weight and total optical weight in a t-t'-J model
We study the Drude weight D and the total optical weight K for a t-t'-J model
on a square lattice that exhibits a metallic phase-modulated antiferromagnetic
ground state close to half-filling. Within a suitable 1/N expansion that
includes leading quantum-fluctuation effects, D and K are found to increase
linearly with small hole doping away from the Mott metal-insulator transition
point at half-filling. The slow zero-sound velocity near the latter transition
identifies with the velocity of the lower-energy branch of the twofold
excitation spectrum. At higher doping values, D and K eventually saturate and
then start to decrease. These features are in qualitative agreement with
optical conductivity measurements in doped antiferromagnets.Comment: 7 pages, REVTEX file (3 Postscript figures). To appear in J. Phys.:
Condens. Mattte
Multipole decomposition of LDA+ energy and its application to actinides compounds
A general reformulation of the exchange energy of -shell is applied in
the analysis of the magnetic structure of various actinides compounds in the
framework of LDA+U method. The calculations are performed in an efficient
scheme with essentially only one free parameter, the screening length. The
results are analysed in terms of different polarisation channels, due to
different multipoles. Generally it is found that the spin-orbital polarisation
is dominating. This can be viewed as a strong enhancement of the spin-orbit
coupling in these systems. This leads to a drastic decrease in spin
polarisation, in accordance with experiments. The calculations are able to
correctly differentiate magnetic and non-magnetic Pu system. Finally, in all
magnetic systems a new multipolar order is observed, whose polarisation energy
is often larger in magnitude than that of spin polarisation.Comment: Fixed some references and picture
Resonant magnetic mode in superconducting 2-leg ladders
The spin dynamics of a doped 2-leg spin ladder is investigated by numerical
techniques. We show that a hole pair-magnon boundstate evolves at finite hole
doping into a sharp magnetic excitation below the two-particle continuum. This
is supported by a field theory argument based on a SO(6)-symmetric ladder.
Similarities and differences with the resonant mode of the high-T cuprates
are discussed.Comment: 5 pages, 5 figure
Multi-k magnetic structures in USb_{0.9}Te_{0.1} and UAs_{0.8}Se_{0.2} observed via resonant x-ray scattering at the U M4 edge
Experiments with resonant photons at the U M4 edge have been performed on a
sample of USb_{0.9}Te_{0.1}, which has an incommensurate magnetic structure
with k = 0.596(2) reciprocal lattice units. The reflections of the form ,
as observed previously in a commensurate k = 1/2 system [N. Bernhoeft et al.,
Phys. Rev. B 69 174415 (2004)] are observed, removing any doubt that these
occur because of multiple scattering or high-order contamination of the
incident photon beam. They are clearly connected with the presence of a 3k
configuration. Measurements of the reflections from the sample
UAs_{0.8}Se_{0.2} in a magnetic field show that the transition at T* ~ 50 K is
between a low-temperature 2k and high-temperature 3k state and that this
transition is sensitive to an applied magnetic field. These experiments stress
the need for quantitative theory to explain the intensities of these
reflections.Comment: submitted to Phys. Rev.
Strongly Correlated Cerium Systems: Non-Kondo Mechanism for Moment Collapse
We present an ab initio based method which gives clear insight into the
interplay between the hybridization, the coulomb exchange, and the
crystal-field interactions, as the degree of 4f localization is varied across a
series of strongly correlated cerium systems. The results for the ordered
magnetic moments, magnetic structure, and ordering temperatures are in
excellent agreement with experiment, including the occurence of a moment
collapse of non-Kondo origin. In contrast, standard ab initio density
functional calculations fail to predict, even qualitatively, the trend of the
unusual magentic properties.Comment: A shorter version of this has been submitted to PR
Dispersion of Magnetic Excitations in Superconducting Optimally Doped YBa_2Cu_3O_6.95
Detailed neutron scattering measurements of YBa_2Cu_3O_6.95 found that the
resonance peak and incommensurate magnetic scattering induced by
superconductivity represent the same physical phenomenon: two dispersive
branches that converge near 41 meV and the in-plane wave-vector q_af=(pi/a,
pi/a) to form the resonance peak. One branch has a circular symmetry around
q_af and quadratic downward dispersion from ~41 meV to the spin gap of
33+-1meV. The other, of lower intensity, disperses from ~41 meV to at least 55
meV. Our results exclude a quartet of vertical incommensurate rods in q-w space
expected from spin waves produced by dynamical charge stripes as an origin of
the observed incommensurate scattering in optimally-doped YBCO.Comment: Version 3: Author change. Changes made throughout the text and minor
changes in figures, Model parameters slightly changed after a small error in
the calculation was discovere
Identifying Collective Modes via Impurities in the Cuprate Superconductors
We show that the pinning of collective charge and spin modes by impurities in
the cuprate superconductors leads to qualitatively different fingerprints in
the local density of states (LDOS). In particular, in a pinned (static) spin
droplet, the creation of a resonant impurity state is suppressed, the
spin-resolved LDOS exhibits a characteristic spatial pattern, and the LDOS
undergoes significant changes with increasing magnetic field. Since all of
these fingerprints are absent in a charge droplet, impurities are a new probe
for identifying the nature and relative strength of collective modes.Comment: 4 pages, 4 figure
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