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.

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

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.

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

Phase Diagram of Coupled Ladders

The 2-leg t-J ladder forms a spin liquid at half-filling which evolves to a Luther-Emery liquid upon doping. Our aim is to obtain a complete phase diagram for isotropic coupling (i.e. rungs and legs equal) as a function of electron density n and the ratio J/t (>0). Two known limiting cases are: n<1/2 which is a single band Luttinger liquid and small hole doping for J/t close to 0 which is a Nagaoka ferromagnet. Using Lanczos techniques we examine the region between the Nagaoka and Luther-Emery phases for 1>n>1/2. We find evidences for gapless behavior in both spin and charge channels for J/t<0.3 consistent with Luttinger liquids in both bonding and anti-bonding bands (i.e., C2S2). This proposal is based on the behavior of spin and charge correlation functions. For example the hole-hole correlation function which displays hole pairing at larger J/t, shows hole-hole repulsion in this region. As a further test, we examined the dependence of the energy on a relative phase shift between bonding and antibonding bands. For J/t < 0.3 this is very weak, indicating a lack of pairing between these channels.Comment: 21 pages, 18 figure

Quantum-enhanced protocols with mixed states using cold atoms in dipole traps

We discuss the use of cold atoms in dipole traps to demonstrate experimentally a particular class of protocols for computation and metrology based on mixed states. Modelling of the system shows that, for a specific class of problems (tracing, phase estimation), a quantum advantage can be achieved over classical algorithms for very realistic conditions and strong decoherence. We discuss the results of the models and the experimental implementation

New Young Star Candidates in CG4 and Sa101

The CG4 and Sa101 regions together cover a region of ~0.5 square degree in the vicinity of a "cometary globule" that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6 to 70 microns) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared excesses in all 6 of the previously identified young stars in our maps, and we identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the "fingers" of the dust being disturbed by the ionization front from the heart of the Gum Nebula.Comment: Accepted for publication in A