Enhanced Bound State Formation in Two Dimensions via Stripe-Like Hopping Anisotropies

We have investigated two-electron bound state formation in a square two-dimensional t-J-U model with hopping anisotropies for zero electron density; these anisotropies are introduced to mimic the hopping energies similar to those expected in stripe-like arrangements of holes and spins found in various transition metal oxides. In this report we provide analytical solutions to this problem, and thus demonstrate that bound-state formation occurs at a critical exchange coupling, J_c, that decreases to zero in the limit of extreme hopping anisotropy t_y/t_x -> 0. This result should be contrasted with J_c/t = 2 for either a one-dimensional chain, or a two-dimensional plane with isotropic hopping. Most importantly, this behaviour is found to be qualitatively similar to that of two electrons on the two-leg ladder problem in the limit of t_interchain/t_intrachain -> 0. Using the latter result as guidance, we have evaluated the pair correlation function, thus determining that the bound state corresponds to one electron moving along one chain, with the second electron moving along the opposite chain, similar to two electrons confined to move along parallel, neighbouring, metallic stripes. We emphasize that the above results are not restricted to the zero density limit - we have completed an exact diagonalization study of two holes in a 12 X 2 two-leg ladder described by the t-J model and have found that the above-mentioned lowering of the binding energy with hopping anisotropy persists near half filling.Comment: 6 pages, 3 eps figure

Cardiovascular risk assessments at occupational health services: employee experiences

Background: Across England in the UK, population screening for cardiovascular disease (CVD) primarily takes place within general practice in the form of the National Health Service Health Check. Additional screening sites such as occupational health are advocated to improve the population impact. Aims: To investigate participant experiences with cardiovascular and type 2 diabetes risk assessment (RA) at occupational health and subsequent support-seeking at general practice. Methods: Face-to-face interviews were conducted for this qualitative study. Participants were recruited at three workplaces; a steel works and two hospital sites. Using interpretive phenomenological analyses, themes were drawn from salient narratives and categorically organized. Results: There were 29 participants. Themes (n = 16) were organized into two domains; factors that facilitated (n = 9) or thwarted (n = 7) participant engagement with the RA and general practice. All participants described the RA as worthwhile and strongly valued RA at occupational health. Those with obesity and high CVD risk highlighted their difficulties in making lifestyle changes. Participants reported confusion and anxiety when GP advice about medication appeared to contradict what participants had interpreted during RA at occupational health. Conclusions: This study highlights factors that facilitate or thwart engagement in cardiovascular RA at occupational health services and general practice follow-up. Stakeholders can integrate these factors into standard operating procedures to enhance participant engagement and enable safeguards that minimize potential harm to participants

Phase Diagram of the 1D Anderson Lattice

We map out the phase diagram of the one--dimensional Anderson lattice by studying the ground state magnetization as a function of band--filling using the density matrix renormalization group technique. For strong coupling, we find that the quarter--filled system has an S=0 ground state with strong antiferromagnetic correlations. As additional electrons are put in, we find first a ferromagnetic phase, as reported by M\"{o}ller and W\"{o}lfle, and then a phase in which the ground state has total spin $S=0$. Within this $S=0$ phase, we find RKKY oscillations in the spin--spin correlation functions.Comment: REVTEX manuscript with 5 Postcript figures included in uu file. Submitted to Phys. Rev.

Dynamical Properties of Two Coupled Hubbard Chains at Half-filling

Using grand canonical Quantum Monte Carlo (QMC) simulations combined with Maximum Entropy analytic continuation, as well as analytical methods, we examine the one- and two-particle dynamical properties of the Hubbard model on two coupled chains at half-filling. The one-particle spectral weight function, $A({\bf k},\omega)$, undergoes a qualitative change with interchain hopping $t_\perp$ associated with a transition from a four-band insulator to a two-band insulator. A simple analytical model based on the propagation of exact rung singlet states gives a good description of the features at large $t_\perp$. For smaller $t_\perp$, $A({\bf k}, \omega)$ is similar to that of the one-dimensional model, with a coherent band of width the effective antiferromagnetic exchange $J$ reasonably well-described by renormalized spin-wave theory. The coherent band rides on a broad background of width several times the parallel hopping integral $t$, an incoherent structure similar to that found in calculations on both the one- and two-dimensional models. We also present QMC results for the two-particle spin and charge excitation spectra, and relate their behavior to the rung singlet picture for large $t_\perp$ and to the results of spin-wave theory for small $t_\perp$.Comment: 9 pages + 10 postscript figures, submitted to Phys.Rev.B, revised version with isotropic t_perp=t data include

Orbit spaces of free involutions on the product of two projective spaces

Let $X$ be a finitistic space having the mod 2 cohomology algebra of the product of two projective spaces. We study free involutions on $X$ and determine the possible mod 2 cohomology algebra of orbit space of any free involution, using the Leray spectral sequence associated to the Borel fibration $X \hookrightarrow X_{\mathbb{Z}_2} \longrightarrow B_{\mathbb{Z}_2}$. We also give an application of our result to show that if $X$ has the mod 2 cohomology algebra of the product of two real projective spaces (respectively complex projective spaces), then there does not exist any $\mathbb{Z}_2$-equivariant map from $\mathbb{S}^k \to X$ for $k \geq 2$ (respectively $k \geq 3$), where $\mathbb{S}^k$ is equipped with the antipodal involution.Comment: 14 pages, to appear in Results in Mathematic

Insulating and Conducting Phases of RbC60

Optical measurements were performed on thin films of Rb$_{x}$C$_{60}$, identified by X-ray diffraction as mostly $x=1$ material. The samples were subjected to various heat treatments, including quenching and slow cooling from 400K. The dramatic increase in the transmission of the quenched samples, and the relaxation towards the transmission observed in slow cooled samples provides direct evidence for the existence of a metastable insulating phase. Slow cooling results in a phase transition between two electrically conducting phases.Comment: Minor revisions. Submitted to PRB, RevTeX 3.0 file, 2 postscript figures included, ir_dop

The ground state of the two-leg Hubbard ladder: a density--matrix renormalization group study

We present density-matrix renormalization group results for the ground state properties of two-leg Hubbard ladders. The half-filled Hubbard ladder is an insulating spin-gapped system, exhibiting a crossover from a spin-liquid to a band-insulator as a function of the interchain hopping matrix element. When the system is doped, there is a parameter range in which the spin gap remains. In this phase, the doped holes form singlet pairs and the pair-field and the "$4 k_F$" density correlations associated with pair density fluctuations decay as power laws, while the "$2 k_F$" charge density wave correlations decay exponentially. We discuss the behavior of the exponents of the pairing and density correlations within this spin gapped phase. Additional one-band Luttinger liquid phases which occur in the large interband hopping regime are also discussed.Comment: 14 pages, 18 figures, uses Revtex with epsfig to include the figure

Ground State Properties of the Doped 3-Leg t-J Ladder

Results for a doped 3-leg t-J ladder obtained using the density matrix renormalization group are reported. At low hole doping, the holes form a dilute gas with a uniform density. The momentum occupation of the odd band shows a sharp decrease at a large value of k_F similar to the behavior of a lightly doped t-J chain, while the even modes appear gapped. The spin-spin correlations decay as a power law consistent with the absence of a spin gap, but the pair field correlations are negligible. At larger doping we find evidence for a spin gap and as x increases further we find 3-hole diagonal domain walls. In this regime there are pair field correlations and the internal pair orbital has d_x^2-y^2 - like symmetry. However, the pair field correlations appear to fall exponentially at large distances.Comment: 14 pages, 11 postscript figure

The transition between hole-pairs and four-hole clusters in four-leg tJ ladders

Holes weakly doped into a four-leg \tj ladder bind in pairs. At dopings exceeding a critical doping of $\delta_c\simeq {1/8}$ four hole clusters are observed to form in DMRG calculations. The symmetry of the ground state wavefunction does not change and we are able to reproduce this behavior qualitatively with an effective bosonic model in which the four-leg ladder is represented as two coupled two-leg ladders and hole-pairs are mapped on hard core bosons moving along and between these ladders. At lower dopings, $\delta<\delta_c$, a one dimensional bosonic representation for hole-pairs works and allows us to calculate accurately the Luttinger liquid parameter \krho, which takes the universal value \krho=1 as half-filling is approached