244 research outputs found
Depth profile photoemission study of thermally diffused Mn/GaAs (001) interfaces
We have performed a depth profile study of thermally diffused Mn/GaAs (001)
interfaces using photoemission spectroscopy combined with Ar-ion
sputtering. We found that Mn ion was thermally diffused into the deep region of
the GaAs substrate and completely reacted with GaAs. In the deep region, the Mn
2 core-level and Mn 3 valence-band spectra of the Mn/GaAs (001) sample
heated to 600 C were similar to those of GaMnAs,
zinc-blende-type MnAs dots, and/or interstitial Mn in tetrahedrally coordinated
by As atoms, suggesting that the Mn 3 states were essentially localized but
were hybridized with the electronic states of the host GaAs. Ferromagnetism was
observed in the dilute Mn phase.Comment: 5 pages, 4 figure
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
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
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
Nonlinear optical response and spin-charge separation in one-dimensional Mott insulators
We theoretically study the nonlinear optical response and photoexcited states
of the Mott insulators. The nonlinear optical susceptibility \chi^(3) is
calculated by using the exact diagonalization technique on small clusters. From
the systematic study of the dependence of \chi^(3) on dimensionality, we find
that the spin-charge separation plays a crucial role in enhancing \chi^(3) in
the one-dimensional (1D) Mott insulators. Based on this result, we propose a
holon-doublon model, which describes the nonlinear response in the 1D Mott
insulators. These findings show that the spin-charge separation will become a
key concept of optoelectronic devices.Comment: 5 pages with 3 figures, to appear in PRB RC, 15 August 200
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
Superconductivuty versus Tunneling in a Doped Antiferromagnetic Ladder
The low-energy charge excitations of a doped antiferromagnetic ladder are
modeled by a system of interacting spinless fermions that live on the same
ladder. A relatively large spin gap is assumed to ``freeze out'' all spin
fluctuations. We find that the formation of rung hole pairs coincides with the
opening of a single-particle gap for charge excitations along chains and with
the absence of coherent tunneling in between chains. We also find that such
hole pairs condense into either a crystalline or superconducting state as a
function of the binding energy.Comment: 15 pgs. in PLAIN TeX, 2 figs. in postscript, to appear in Phys. Rev.
Cu-O network-dependent core-hole screening in low-dimensional cuprate systems: a high-resolution X-ray photoemission study
We present an experimental study of the dynamics of holes in the valence bands of zero-, one-, and two-dimensional undoped model cuprates, as expressed via the screening of a Cu 2p core hole. The response depends strongly upon the dimensionality and the details of the Cu-O-Cu network geometry and clearly goes beyond the present theoretical state-of-the-art description within the three-band d-p model
Electronic Structure of Ladder Cuprates
We study the electronic structure of the ladder compounds (SrCa)CuO 14-24-41
and SrCuO 123. LDA calculations for both give similar Cu 3d-bands near the
Fermi energy. The hopping parameters estimated by fitting LDA energy bands show
a strong anisotropy between the t_perp t_par intra-ladder hopping and small
inter-ladder hopping. A downfolding method shows that this anisotropy arises
from the ladder structure.The conductivity perpendicular to the ladders is
computed assuming incoherent tunneling giving a value close to experiment.Comment: 5 pages, 3 figure
Magnetic excitations and structural change in the S=1/2 quasi-one-dimensional magnet Sr_{14-x}Y_{x}Cu_{24}O_{41} (0<x<1)
Neutron scattering measurements have been performed on the S=1/2
quasi-one-dimensional system Sr_{14-x}Y_{x}Cu_{24}O_{41}, which has both simple
chains and two-leg ladders of copper ions. We observed that when a small amount
of yttrium is substituted for strontium, which is expected to reduce the number
of holes, the dimerized state and the structure in the chain are changed
drastically. The inelastic peaks originating from the dimerized state of the
chain becomes broader in energy but not in momentum space. This implies that
the dimerized state becomes unstable but the spin correlations are unchanged
with yttrium substitution. Furthermore, it was observed that nuclear Bragg peak
intensities originating from the chain show strong temperature and x
dependence, which suggests that the chains slide along the c axis as
temperature and x are varied.Comment: 5 pages, 6 figures, to appear in Phys. Rev.
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