764 research outputs found
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
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.
Benzo-dipteridine derivatives as organic cathodes for Li- and Na-ion batteries
Organic-based electrodes for Li- and Na-ion batteries present attractive alternatives to commonly applied inorganic counterparts which can often carry with them supply-chain risks, safety concerns with thermal runaway, and adverse environmental impact. The ability to chemically direct the structure of organic electrodes through control over functional groups is of particular importance, as this provides a route to fine-tune electrochemical performance parameters. Here, we report two benzo-dipteridine derivatives, BF-Me2 and BF-H2, as high-capacity electrodes for use in Li- and Na-ion batteries. These moieties permit binding of multiple Li-ions per molecule while simultaneously ensuring low solubility in the supporting electrolyte, often a precluding issue with organic electrodes. Both display excellent electrochemical stability, with discharge capacities of 142 and 182 mAh g–1 after 100 cycles at a C/10 rate and Coulombic efficiencies of 96% and ∼ 100% demonstrated for BF-Me2 and BF-H2, respectively. The application of a Na-ion cell has also been demonstrated, showing discharge capacities of 88.8 and 137 mAh g–1 after 100 cycles at a C/2 rate for BF-Me2 and BF-H2, respectively. This work provides an encouraging precedent for these and related structures to provide versatile, high-energy density, and long cycle-life electrochemical energy storage materials
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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Structural phase transitions and superconductivity in lanthanum copper oxides
Despite the enormous effort expended over the past ten years to determine the mechanism underlying high temperature superconductivity in cuprates there is still no consensus on the physical origin of this fascinating phenomenon. This is a consequence of a number of factors, among which are the intrinsic difficulties in understanding the strong electron correlations in the copper oxides, determining the roles played by antiferromagnetic interactions and low dimensionality, analyzing the complex phonon dispersion relationships, and characterizing the phase diagrams which are functions of the physical parameters of temperature and pressure, as well as the chemical parameters of stoichiometry and hole concentration. In addition to all of these intrinsic difficulties, extrinsic materials issues such as sample quality and homogeneity present additional complications. Within the field of high temperature superconductivity there exists a subfield centered around the material originally reported to exhibit high temperature superconductivity by Bednorz and Mueller, Ba doped La{sub 2}CuO{sub 4}. This is structurally the simplest cuprate superconductor. The authors report on studies of phase differences observed between such base superconductors doped with Ba or Sr. What these studies have revealed is a fascinating interplay of structural, magnetic and superconducting properties which is unique in the field of high temperature superconductivity and is summarized in this paper
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
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