27 research outputs found
Metal-Insulator Transition Accompanied with a Charge Ordering in the One-dimensional t-J' Model
We study the metal-insulator transition accompanied with a charge ordering in
the one-dimensional (1D) t-J' model at quarter filling by the density matrix
renormalization group method. In this model the nearest-neighbor hopping energy
t competes with the next-nearest-neighbor exchange energy J'. We have found
that a metal-insulator transition occurs at a finite value of t/J'; (t/J')_C =
0.18 and the transition is of first order. In the insulating phase for small
t/J', there is an alternating charge ordering and the system behaves as a 1D
quantum Heisenberg antiferromagnet. The metallic side belongs to the
universality class of the Tomonaga-Luttinger liquids. The quantum phase
transition is an example of melting of the 1D quantum Heisenberg
antiferromagnet.Comment: 4 pages, 6 Postscript figures, REVTeX, submitted to Phys. Rev.
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.
Quasi-two-dimensional hole ordering and dimerized state in the CuO2-chain layers in Sr14Cu24O41
Neutron scattering experiments have been performed on
SrCuO which consists of both chains and ladders of copper
ions. We observed that the magnetic excitations from the CuO chain have two
branches and that both branches are weakly dispersive along the and
axes. The - dispersion relation as well as the intensities can be
reasonably described by a random phase approximation with intradimer coupling
between next-nearest-neighbor copper spins =11 meV, interdimer coupling
along the c axis =0.75 meV, and interdimer coupling along the a axis
=0.75 meV. The dimer configuration indicates a quasi-two-dimensional hole
ordering, resulting in an ordering of magnetic Cu with spin-1/2 and
nonmagnetic Cu, which forms the Zhang-Rice singlet. We have also studied the
effect of Ca substitution for Sr on the dimer and the hole ordering.Comment: 7 pages, Revtex, 10 figures, Submitted to Phys. Rev.
Reversible melting and equilibrium phase formation of (Bi,Pb)2Sr2Ca2Cu3O10+d
The decomposition and the reformation of the (Bi,Pb)2Sr2Ca2Cu3O10+d
(?Bi,Pb(2223)?) phase have been investigated in-situ by means of
High-Temperature Neutron Diffraction, both in sintered bulk samples and in
Ag-sheathed monofilamentary tapes. Several decomposition experiments were
performed at various temperatures and under various annealing atmospheres,
under flowing gas as well as in sealed tubes, in order to study the appropriate
conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was
found to melt incongruently into (Ca,Sr)2CuO3, (Sr,Ca)14Cu24O41 and a
Pb,Bi-rich liquid phase. Phase reformation after melting was successfully
obtained both in bulk samples and Ag-sheathed tapes. The possibility of
crystallising the Bi,Pb(2223) phase from the melt was found to be extremely
sensitive to the temperature and strongly dependent on the Pb losses. The study
of the mass losses due to Pb evaporation was complemented by thermogravimetric
analysis which proved that Pb losses are responsible for moving away from
equilibrium and therefore hinder the reformation of the Bi,Pb(2223) phase from
the melt. Thanks to the full pattern profile refinement, a quantitative phase
analysis was carried out as a function of time and temperature and the role of
the secondary phases was investigated. Lattice distortions and/or transitions
were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212),
(Ca,Sr)2CuO3 and (Sr,Ca)14Cu24O41, due to cation diffusion and stoichiometry
changes. The results indicate that it is possible to form the Bi,Pb(2223) phase
from a liquid close to equilibrium conditions, like Bi(2212) and Bi(2201), and
open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape
processing.Comment: 45 pages (including references,figures and captions), 13 figures
Submitted to Supercond. Sci. Techno
Electronic and Magnetic Structures of Chain Structured Iron Selenide Compounds
Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2
(2212\ast) and BaFe2Se3(123\ast) are studied by the first-principles
calculations. We find that while all these compounds are composed of
one-dimensional (1D) Fe chain (or ladder) structures, their electronic
structures are not close to be quasi-1D. The magnetic exchange couplings
between two nearest-neighbor (NN) chains in 2212\ast and between two NN
two-leg-ladders in 123\ast are both antiferromagnetic (AFM), which is
consistent with the presence of significant third NN AFM coupling, a common
feature shared in other iron-chalcogenides, FeTe (11\ast) and KyFe2-xSe2
(122\ast). In magnetic ground states, each Fe chain of 2212\ast is
ferromagnetic and each two-leg ladder of 123\ast form a block-AFM structure. We
suggest that all magnetic structures in iron-selenide compounds can be unified
into an extended J1-J2-J3 model. Spin-wave excitations of the model are
calculated and can be tested by future experiments on these two systems.Comment: 6 pages, 6 figures, 2 table
A Study of the S=1/2 Alternating Chain using Multiprecision Methods
In this paper we present results for the ground state and low-lying
excitations of the alternating Heisenberg antiferromagnetic chain. Our
more conventional techniques include perturbation theory about the dimer limit
and numerical diagonalization of systems of up to 28 spins. A novel application
of multiple precision numerical diagonalization allows us to determine
analytical perturbation series to high order; the results found using this
approach include ninth-order perturbation series for the ground state energy
and one magnon gap, which were previously known only to third order. We also
give the fifth-order dispersion relation and third-order exclusive neutron
scattering structure factor for one-magnon modes and numerical and analytical
binding energies of S=0 and S=1 two-magnon bound states.Comment: 16 pages, 9 figures. for submission to Phys.Rev.B. PICT files of figs
available at http://csep2.phy.ornl.gov/theory_group/people/barnes/barnes.htm
Electronic States and Magnetic Propertis of Edge-sharing Cu-O Chains
The electronic states and magnetic properties for the copper oxides
containing edge-sharing Cu-O chains such as LiCuO,
LaCaCuO and CuGeO are systematically studied. The
optical conductivity and the temperature dependence of the
magnetic susceptibility for single crystalline samples LiCuO
are measured as a reference system and analyzed by using the exact
diagonalization method for small Cu-O clusters. It is shown that the spectral
distribution of is different between edge-sharing and
corner-sharing Cu-O-Cu bonds. The charge transfer gap in edge-sharing chains is
larger than that of high- cuprates. The exchange interaction between
nearest-neighbor copper ions in edge-sharing chains depends sensitively
on the Cu-O-Cu bond angles. In addition to , the exchange interaction
between next-nearest-neighbor copper ions has sufficient contribution to
the magnetic properties. We calculate and for all the copper oxides
containing edge-sharing Cu-O chains and discuss the magnetic properties.Comment: 10 pages,RevTeX,8 postscript figures. Accepted for publication in
Phys. Rev.
Sliding conduction by the quasi one-dimensional charge-ordered state in SrCaCuO
Nonlinear conduction (NLC) of the two-leg spin ladder,
SrCaCuO, was investigated for the =0, 1 and 12
materials . Although insulating materials (=0 and 1) exibited the NLC both
in the ladder- and rung directions, the NLC in the ladder direction of the
=0-material was found to be very special. We considered this to be due to
the sliding motion of the charge ordered state, which was responsible for the
resonance at microwave frequencies. We discussed possible candidates for the
charge ordered state responsible for the NLC, including Wigner crystal in quasi
one dimension (4-CDW).Comment: 5 figure
Potency analysis of cellular therapies: the emerging role of molecular assays
Potency testing is an important part of the evaluation of cellular therapy products. Potency assays are quantitative measures of a product-specific biological activity that is linked to a relevant biological property and, ideally, a product's in vivo mechanism of action. Both in vivo and in vitro assays can be used for potency testing. Since there is often a limited period of time between the completion of production and the release from the laboratory for administration to the patient, in vitro assays such are flow cytometry, ELISA, and cytotoxicity are typically used. Better potency assays are needed to assess the complex and multiple functions of cellular therapy products, some of which are not well understood. Gene expression profiling using microarray technology has been widely and effectively used to assess changes of cells in response to stimuli and to classify cancers. Preliminary studies have shown that the expression of noncoding microRNA which play an important role in cellular development, differentiation, metabolism and signal transduction can distinguish different types of stem cells and leukocytes. Both gene and microRNA expression profiling have the potential to be important tools for testing the potency of cellular therapies. Potency testing, the complexities associated with potency testing of cellular therapies, and the potential role of gene and microRNA expression microarrays in potency testing of cellular therapies is discussed
MAGNETIC STUDIES OF Pr1Ba2Cu3O7-δ AND La1Ba2Cu3O7-δ
We report magnetic susceptibility (xspin(T)), and magnetic scattering (neutron) studies for the superconducting complex La1Ba2Cu3O7 and the insulating complex Pr1Ba2Cu3O7-δ. The magnetic susceptibility and scattering measurements on the compound Pr1Ba2Cu3O7-δ show no sign of magnetic ordering. For the La1Ba2Cu3O7-δ samples we observe only a small change in the Pauli susceptibility and a decrease in the Curie-constant as Tc of the samples is increased