Luttinger Liquid Instability in the One Dimensional t-J Model

We study the t-J model in one dimension by numerically projecting the true ground state from a Luttinger liquid trial wave function. We find the model exhibits Luttinger liquid behavior for most of the phase diagram in which interaction strength and density are varied. However at small densities and high interaction strengths a new phase with a gap to spin excitations and enhanced superconducting correlations is found. We show this phase is a Luther-Emery liquid and study its correlation functions.Comment: REVTEX, 11 pages. 4 Figures available on request from [email protected]

Phase separation and stripe formation in the 2D t-J model: a comparison of numerical results

We make a critical analysis of numerical results for and against phase separation and stripe formation in the t-J model. We argue that the frustrated phase separation mechanism for stripe formation requires phase separation at too high a doping for it to be consistent with existing numerical studies of the t-J model. We compare variational energies for various methods, and conclude that the most accurate calculations for large systems appear to be from the density matrix renormalization group. These calculations imply that the ground state of the doped t-J model is striped, not phase separated.Comment: This version includes a revised, more careful comparison of numerical results between DMRG and Green's function Monte Carlo. In particular, for the original posted version we were accidentally sent obsolete data by Hellberg and Manousakis; their new results, which are what were used in their Physical Review Letter, are more accurate because a better trial wavefunction was use

Variational state based on the Bethe ansatz solution and a correlated singlet liquid state in the one-dimensional t-J model

The one-dimensional t-J model is investigated by the variational Monte Carlo method. A variational wave function based on the Bethe ansatz solution is newly proposed, where the spin-charge separation is realized, and a long-range correlation factor of Jastrow-type is included. In most regions of the phase diagram, this wave function provides an excellent description of the ground-state properties characterized as a Tomonaga-Luttinger liquid; Both of the amplitude and exponent of correlation functions are correctly reproduced. For the spin-gap phase, another trial state of correlated singlet pairs with a Jastrow factor is introduced. This wave function shows generalized Luther-Emery liquid behavior, exhibiting enhanced superconducting correlations and exponential decay of the spin correlation function. Using these two variational wave functions, the whole phase diagram is determined. In addition, relations between the correlation exponent and variational parameters in the trial functions are derived.Comment: REVTeX 3.0, 27 pages. 7 figures available upon request ([email protected]). To be published in Phys. Rev. B 5

Hybrid speciation driven by multilocus introgression of ecological traits

Hybridization allows adaptations to be shared among lineages and may trigger the evolution of new species1,2. However, convincing examples of homoploid hybrid speciation remain rare because it is challenging to demonstrate that hybridization was crucial in generating reproductive isolation3. Here we combine population genomic analysis with quantitative trait locus mapping of species-specific traits to examine a case of hybrid speciation in Heliconius butterflies. We show that Heliconius elevatus is a hybrid species that is sympatric with both parents and has persisted as an independently evolving lineage for at least 180,000 years. This is despite pervasive and ongoing gene flow with one parent, Heliconius pardalinus, which homogenizes 99% of their genomes. The remaining 1% introgressed from the other parent, Heliconius melpomene, and is scattered widely across the H. elevatus genome in islands of divergence from H. pardalinus. These islands contain multiple traits that are under disruptive selection, including colour pattern, wing shape, host plant preference, sex pheromones and mate choice. Collectively, these traits place H. elevatus on its own adaptive peak and permit coexistence with both parents. Our results show that speciation was driven by introgression of ecological traits, and that speciation with gene flow is possible with a multilocus genetic architecture

From old organisms to new molecules: integrative biology and therapeutic targets in accelerated human ageing

Understanding the basic biology of human ageing is a key milestone in attempting to ameliorate the deleterious consequences of old age. This is an urgent research priority given the global demographic shift towards an ageing population. Although some molecular pathways that have been proposed to contribute to ageing have been discovered using classical biochemistry and genetics, the complex, polygenic and stochastic nature of ageing is such that the process as a whole is not immediately amenable to biochemical analysis. Thus, attempts have been made to elucidate the causes of monogenic progeroid disorders that recapitulate some, if not all, features of normal ageing in the hope that this may contribute to our understanding of normal human ageing. Two canonical progeroid disorders are Werner’s syndrome and Hutchinson-Gilford progeroid syndrome (also known as progeria). Because such disorders are essentially phenocopies of ageing, rather than ageing itself, advances made in understanding their pathogenesis must always be contextualised within theories proposed to help explain how the normal process operates. One such possible ageing mechanism is described by the cell senescence hypothesis of ageing. Here, we discuss this hypothesis and demonstrate that it provides a plausible explanation for many of the ageing phenotypes seen in Werner’s syndrome and Hutchinson-Gilford progeriod syndrome. The recent exciting advances made in potential therapies for these two syndromes are also reviewed

Tiefenverteilung der Anregung charakteristischer Röntgenstrahlung in Kupfer durch 12 bis 30 keV-Elektronen.

The depth distribution of characteristic X-ray production in copper by 12 to 30 keV electrons has been measured with a zink tracer. The results are examined in terms of characteristic distribution parameters which show a potential law energy dependence. Reduction of the depth data and the electron energies by means of electron range and excitation energy, respectively, allows comparison with experimental and theoretical results of other authors on various target-tracer-combinations. Although a general energy dependence is found for the majority of the reduced characteristic parameters the accuracy of the data is not satisfying. Some of the measured distributions obviously contain large systematical errors

Clarification of the AAPM Task Group 21 protocol

NRC publication: Ye