Qualitative understanding of the sign of t' asymmetry in the extended t-J Model and relevance for pairing properties

Numerical calculations illustrate the effect of the sign of the next nearest-neighbor hopping term t' on the 2-hole properties of the t-t'-J model. Working mainly on 2-leg ladders, in the -1.0 < t'/t < 1.0 regime, it is shown that introducing t' in the t-J model is equivalent to effectively renormalizing J, namely t' negative (positive) is equivalent to an effective t-J model with smaller (bigger) J. This effect is present even at the level of a 2x2 plaquette toy model, and was observed also in calculations on small square clusters. Analyzing the transition probabilities of a hole-pair in the plaquette toy model, it is argued that the coherent propagation of such hole-pair is enhanced by a constructive interference between both t and t' for t'>0. This interference is destructive for t'<0.Comment: 5 pages, 4 figures, to appear in PRB as a Rapid Communicatio

Spectral density for a hole in an antiferromagnetic stripe phase

Using variational trial wave function based on the string picture we study the motion of a single mobile hole in the stripe phase of the doped antiferromagnet. The holes within the stripes are taken to be static, the undoped antiferromagnetic domains in between the hole stripes are assumed to have alternating staggered magnetization, as is suggested by neutron scattering experiments. The system is described by the t-t'-t''-J model with realistic parameters and we compute the single particle spectral density.Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in PRB. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: [email protected]

Enhancement of long-range magnetic order by magnetic field in superconducting La2CuO(4+y)

We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC order. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed.Comment: Version accepted for publication in Phys. Rev. B. Improved discussion in connection with the muSR result

Quantum dots in magnetic fields: thermal response of broken symmetry phases

We investigate the thermal properties of circular semiconductor quantum dots in high magnetic fields using finite temperature Hartree-Fock techniques. We demonstrate that for a given magnetic field strength quantum dots undergo various shape phase transitions as a function of temperature, and we outline possible observable consequences.Comment: In Press, Phys. Rev. B (2001

The influence of gene expression time delays on Gierer-Meinhardt pattern formation systems

There are numerous examples of morphogen gradients controlling long range signalling in developmental and cellular systems. The prospect of two such interacting morphogens instigating long range self-organisation in biological systems via a Turing bifurcation has been explored, postulated, or implicated in the context of numerous developmental processes. However, modelling investigations of cellular systems typically neglect the influence of gene expression on such dynamics, even though transcription and translation are observed to be important in morphogenetic systems. In particular, the influence of gene expression on a large class of Turing bifurcation models, namely those with pure kinetics such as the Gierer–Meinhardt system, is unexplored. Our investigations demonstrate that the behaviour of the Gierer–Meinhardt model profoundly changes on the inclusion of gene expression dynamics and is sensitive to the sub-cellular details of gene expression. Features such as concentration blow up, morphogen oscillations and radical sensitivities to the duration of gene expression are observed and, at best, severely restrict the possible parameter spaces for feasible biological behaviour. These results also indicate that the behaviour of Turing pattern formation systems on the inclusion of gene expression time delays may provide a means of distinguishing between possible forms of interaction kinetics. Finally, this study also emphasises that sub-cellular and gene expression dynamics should not be simply neglected in models of long range biological pattern formation via morphogens

The high-pressure phase of boron, {\gamma}-B28: disputes and conclusions of 5 years after discovery

{\gamma}-B28 is a recently established high-pressure phase of boron. Its structure consists of icosahedral B12 clusters and B2 dumbbells in a NaCl-type arrangement (B2){\delta}+(B12){\delta}- and displays a significant charge transfer {\delta}~0.5- 0.6. The discovery of this phase proved essential for the understanding and construction of the phase diagram of boron. {\gamma}-B28 was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and in particular deals with the contentious issues related to the equation of state, hardness, putative isostructural phase transformation at ~40 GPa, and debates on the nature of chemical bonding in this phase. Our analysis confirms that (a) calculations based on density functional theory give an accurate description of its equation of state, (b) the reported isostructural phase transformation in {\gamma}-B28 is an artifact rather than a fact, (c) the best estimate of hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a significant degree of ionicity. Apart from presenting an overview of previous results within a consistent view grounded in experiment, thermodynamics and quantum mechanics, we present new results on Bader charges in {\gamma}-B28 using different levels of quantum-mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about significant degree of partial ionicity in this phase is very robust

The STAR Time Projection Chamber: A Unique Tool for Studying High Multiplicity Events at RHIC

The STAR Time Projection Chamber (TPC) is used to record collisions at the Relativistic Heavy Ion Collider (RHIC). The TPC is the central element in a suite of detectors that surrounds the interaction vertex. The TPC provides complete coverage around the beam-line, and provides complete tracking for charged particles within +- 1.8 units of pseudo-rapidity of the center-of-mass frame. Charged particles with momenta greater than 100 MeV/c are recorded. Multiplicities in excess of 3,000 tracks per event are routinely reconstructed in the software. The TPC measures 4 m in diameter by 4.2 m long, making it the largest TPC in the world.Comment: 28 pages, 11 figure

One particle spectral weight of the three dimensional single band Hubbard model

Dynamic properties of the three-dimensional single-band Hubbard model are studied using Quantum Monte Carlo combined with the maximum entropy technique. At half-filling, there is a clear gap in the density of states and well-defined quasiparticle peaks at the top (bottom) of the lower (upper) Hubbard band. We find an antiferromagnetically induced weight above the naive Fermi momentum. Upon hole doping, the chemical potential moves to the top of the lower band where a robust peak is observed. Results are compared with spin-density-wave (SDW) mean-field and self consistent Born approximation results, and also with the infinite dimensional Hubbard model, and experimental photoemission (PES) for three dimensional transition-metal oxides.Comment: 11 pages, REVTeX, 16 figures included using psfig.sty. Ref.30 correcte

Dispersion of Ordered Stripe Phases in the Cuprates

A phase separation model is presented for the stripe phase of the cuprates, which allows the doping dependence of the photoemission spectra to be calculated. The idealized limit of a well-ordered array of magnetic and charged stripes is analyzed, including effects of long-range Coulomb repulsion. Remarkably, down to the limit of two-cell wide stripes, the dispersion can be interpreted as essentially a superposition of the two end-phase dispersions, with superposed minigaps associated with the lattice periodicity. The largest minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk) Van Hove singularity. The calculated spectra are dominated by two features -- this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a strong correlation between these two features and the experimental photoemission results of a two-peak dispersion in La$_{2-x}$Sr$_x$CuO$_4$, and the peak-dip-hump spectra in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The differences are suggestive of the role of increasing stripe fluctuations. The 1/8 anomaly is associated with a quantum critical point, here expressed as a percolation-like crossover. A model is proposed for the limiting minority magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure