217 research outputs found
Indication of antiferromagnetic interaction between paramagnetic Co ions in the diluted magnetic semiconductor ZnCoO
The magnetic properties of ZnCoO ( and 0.10) thin films,
which were homo-epitaxially grown on a ZnO(0001) substrates with varying
relatively high oxygen pressure, have been investigated using x-ray magnetic
circular dichroism (XMCD) at Co core-level absorption edge. The line
shapes of the absorption spectra are the same in all the films and indicate
that the Co ions substitute for the Zn sites. The magnetic-field and
temperature dependences of the XMCD intensity are consistent with the
magnetization measurements, indicating that except for Co there are no
additional sources for the magnetic moment, and demonstrate the coexistence of
paramagnetic and ferromagnetic components in the homo-epitaxial
ZnCoO thin films, in contrast to the ferromagnetism in the
hetero-epitaxial ZnCoO films studied previously. The analysis of
the XMCD intensities using the Curie-Weiss law reveals the presence of
antiferromagnetic interaction between the paramagnetic Co ions. Missing XMCD
intensities and magnetization signals indicate that most of Co ions are
non-magnetic probably because they are strongly coupled antiferromagnetically
with each other. Annealing in a high vacuum reduces both the paramagnetic and
ferromagnetic signals. We attribute the reductions to thermal diffusion and
aggregation of Co ions with antiferromagnetic nanoclusters in
ZnCoO.Comment: 21 pages, 7 figures, accepted for Physical Review
Spin Gap and Superconductivity in Weakly Coupled Ladders: Interladder One-particle vs. Two-particle Crossover
Effects of the interladder one-particle hopping, , on the
low-energy asymptotics of a weakly coupled Hubbard ladder system have been
studied, based on the perturbative renormalization-group approach. We found
that for finite intraladder Hubbard repulsion, , there exists a crossover
value of the interladder one-particle hopping, . For
, the spin gap metal (SGM) phase of the isolated
ladder transits at a finite transition temperature, , to the d-wave
superconducting (SCd) phase via a two-particle crossover. In the temperature
region, , interladder coherent Josephson tunneling of the Cooper pairs
occurs, while the interladder coherent one-particle process is strongly
suppressed. For , around a crossover temperature,
, the system crosses over to the two-dimensional (2D) phase via a
one-particle crossover. In the temperature region, , the
interladdercoherent band motion occurs.Comment: 4 pages, 5 eps figures, uses jpsj.st
Perturbation Analysis of Superconductivity in the Trellis-Lattice Hubbard Model
We investigate pairing symmetry and transition temperature in the
trellis-lattice Hubbard model. We solve the \'Eliashberg equation using the
third-order perturbation theory with respect to the on-site repulsion . We
find that a spin-singlet state is very stable in a wide range of parameters. On
the other hand, when the electron number density is shifted from the
half-filled state and the band gap between two bands is small, a spin-triplet
superconductivity is expected. Finally, we discuss a possibility of
unconventional superconductivity and pairing symmetry in
SrCaCuO.Comment: 7pages, 10 figures. To be published in J. Phys. Soc. Jp
Quantum Spin Systems: From Spin Gaps to Pseudo Gaps
Many low dimensional spin systems with a dimerized or ladder-like
antiferromagnetic exchange coupling have a gapped excitation spectrum with
magnetic bound states within the spin gap. For spin ladders with an even number
of legs the existence of spin gaps and within the t-J model a tendency toward
superconductivity with d-wave symmetry is predicted. In the following we will
characterize the spin excitation spectra of different low dimensional spin
systems taking into account strong spin phonon interaction (),
charge ordering () and doping on chains and ladders (\ladder).
The spectroscopic characterization of the model systems mentioned above has
been performed using magnetic inelastic light scattering originating from a
spin conserving exchange scattering mechanism. This is also bound to yield more
insight into the interrelation between these spin gap excitations and the
origin of the pseudo gap in high temperature superconductors.Comment: 10 pages, 5 figure
Conductivity of Doped Two-Leg Ladders
Recently, conductivity measurements were performed on the hole-doped two-leg
ladder material Sr_{14-x}Ca_xCu_{24}O_{41}. In this work, we calculate the
conductivity for doped two-leg ladders using a model of hole-pairs forming a
strongly correlated liquid - a single component Luttinger liquid - in the
presence of disorder. Quantum interference effects are handled using
renormalization group methods. We find that our model can account for the low
energy features of the experimental results. However, at higher energies the
experiments show deviations from the predictions of this model. Using the
results of our calculations as well as results on the ground state of doped
two-leg ladders, we suggest a scenario to account for the higher energy
features of the experimental results.Comment: 5 pages, 3 postscript figure
Optical and transport properties in doped two-leg ladder antiferromagnet
Within the t-J model, the optical and transport properties of the doped
two-leg ladder antiferromagnet are studied based on the fermion-spin theory. It
is shown that the optical and transport properties of the doped two-leg ladder
antiferromagnet are mainly governed by the holon scattering. The low energy
peak in the optical conductivity is located at a finite energy, while the
resistivity exhibits a crossover from the high temperature metallic-like
behavior to the low temperature insulating-like behavior, which are consistent
with the experiments.Comment: 13 pages, 5 figures, accepted for publication in Phys. Rev. B65
(2002) (April 15 issue
Transport Properties of Doped t-J Ladders
Conductivity and Hall coefficient for various types of t-J ladders are
calculated as a function of temperature and frequency by numerical
diagonalization. A crossover from an incoherent to a coherent charge dynamics
is found at a temperature T_{coh}. There exists another crossover at T_{PG}
below which a pseudogap opens in the optical spectra, induced by the opening of
a spin gap. In the absence of the spin gap, T_{coh} and the coherent weight are
suppressed especially with increasing dimensionality. On the contrary, T_{coh}
is strongly enhanced by the pseudogap formation below T_{PG}, where the
coherent Drude weight decreases with increasing dimensionality. The Hall
coefficient shows a strong crossover at T_{PG} below which it has large
amplitude for small doping concentration.Comment: 4 pages, RevTeX, 5 PostScript figure
Charge-density wave formation in Sr_{14}Cu_{24}O_{41}
The electrodynamic response of the spin-ladder compound
SrCaCuO () has been studied from
radiofrequencies up to the infrared. At temperatures below 250 K a pronounced
absorption peak appears around 12 cm in SrCuO for
the radiation polarized along the chains/ladders ().
In addition a strongly temperature dependent dielectric relaxation is observed
in the kHz - MHz range. We explain this behavior by a charge density wave which
develops in the ladders sub-system and produces a mode pinned at 12 cm.
With increasing Ca doping the mode shifts up in frequency and eventually
disappears for because the dimensionality of the system crosses over from
one to two dimensions, giving way to the superconducting ground state under
pressure.Comment: One name added to author list 4 pages, 2 figures, email:
[email protected]
Nuclear Spin Relaxation in Hole Doped Two-Leg Ladders
The nuclear spin-lattice relaxation rate () has been measured in the
single crystals of hole doped two-leg ladder compounds
SrCaCuO and in the undoped parent material
LaCaCuO. Comparison of at the Cu and the two
distinct oxygen sites revealed that the major spectral weight of low frequency
spin fluctuations is located near for most of the
temperature and doping ranges investigated. Remarkable difference in the
temperature dependence of for the two oxygen sites in the heavily doped
=12 sample revealed reduction of singlet correlations between two legs in
place of growing antiferromagnetic correlations along the leg direction with
increasing temperature. Such behavior is most likely caused by the dissociation
of bound hole pairs.Comment: 4 pages. to appear in J. Phys. Soc. Jpn. Vol. 6
Charge-Density-Wave Formation in the Doped Two-Leg Extended Hubbard Ladder
We investigate electronic properties of the doped two-leg Hubbard ladder with
both the onsite and the nearest-neighbor Coulomb repulsions, by using the the
weak-coupling renormalization-group method. It is shown that, for strong
nearest-neighbor repulsions, the charge-density-wave state coexisting with the
p-density-wave state becomes dominant fluctuation where spins form intrachain
singlets. By increasing doping rate, we have also shown that the effects of the
nearest-neighbor repulsions are reduced and the system exhibits a quantum phase
transition into the d-wave-like (or rung-singlet) superconducting state. We
derive the effective fermion theory which describes the critical properties of
the transition point with the gapless excitation of magnon. The phase diagram
of the two-leg ladder compound, Sr_{14-x}Ca_xCu_{24}O_{41}, is discussed.Comment: 4 pages, 2 figure
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