On the Nagaoka polaron in the t-J model

It is widely believed that a single hole in the two (or three) dimensional t-J model, for sufficiently small exchange coupling J, creates a ferromagnetic bubble around itself, a finite J remnant of the ferromagnetic groundstate at J=0 (the infinite U Hubbard model), first established by Nagaoka. We investigate this phenomenon in two dimensions using the density matrix renormalization group, for system sizes up to 9x9. We find that the polaron forms for J/t<0.02-0.03 (a somewhat larger value than estimated previously). Although finite-size effects appear large, our data seems consistent with the expected 1.1(J/t)^{-1/4} variation of polarion radius. We also test the Brinkman-Rice model of non-retracing paths in a Neel background, showing that it is quite accurate, at larger J. Results are also presented in the case where the Heisenberg interaction is dropped (the t-J^z model). Finally we discuss a "dressed polaron" picture in which the hole propagates freely inside a finite region but makes only self-retracing excursions outside this region.Comment: 7 pages, 9 encapsulated figure

Towards a novel carbon device for the treatment of sepsis

Sepsis is a systemic inflammatory response to infection in which the balance of pro- andanti-inflammatory mediators, which normally isolate and eliminate infection, is disrupted[1]. Gram negative sepsis is initiated by bacterial endotoxin release which activatesmacrophages and circulating monocytes to release TNF and IL-1β followed by IL-6 andother inflammatory cytokines [2]. As the disease progresses, an unregulatedinflammatory response results in, tissue injury, haematological dysfunction and organdysfunction. Severe sepsis, involving organ hypoperfusion may be further complicatedby hypotension that is unresponsive to adequate fluid replacement, resulting in septicshock and finally death [3].Despite improvements in anti-microbial and supportive therapies, sepsis remains asignificant cause of morbidity and mortality in ICUs worldwide [4]. The complexity ofprocesses mediating the progression of sepsis suggests that an extracorporeal devicecombining blood filtration with adsorption of a wide range of toxins, and inflammatorymediators offers the most comprehensive treatment strategy. However, no such deviceexists at present. A novel, uncoated, polymer pyrolysed synthetic carbon device isproposed which combines the superior adsorption properties of uncoated activatedcarbons with the capacity to manipulate porous structure for controlled adsorption oftarget plasma proteins and polypeptides [5]. Preliminary haemocompatibility andadsorptive capacity was assessed using a carbon matrix prototype

Evidence of a massive planet candidate orbiting the young active K5V star BD+20 1790

Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO). DOI: 10.1051/0004-6361/200811000Context. BD+20 1790 is a young active, metal-rich, late-type K5Ve star. We have undertaken a study of stellar activity and kinematics for this star over the past few years. Previous results show a high level of stellar activity, with the presence of prominence-like structures, spots on the surface, and strong flare events, despite the moderate rotational velocity of the star. In addition, radial velocity variations with a semi-amplitude of up to 1 km s-1 were detected. Aims. We investigate the nature of these radial velocity variations, in order to determine whether they are due to stellar activity or the reflex motion of the star induced by a companion. Methods. We have analysed high-resolution echelle spectra by measuring stellar activity indicators and computing radial velocity (RV) and bisector velocity spans. Two-band photometry was also obtained to produce the light curve and determine the photometric period. Results. Based upon the analysis of the bisector velocity span, as well as spectroscopic indices of chromospheric indicators, Ca ii H & K, Hα, and taking the photometric analysis into account, we report that the best explanation for the RV variation is the presence of a substellar companion. The Keplerian fit of the RV data yields a solution for a close-in massive planet with an orbital period of 7.78 days. The presence of the close-in massive planet could also be an interpretation for the high level of stellar activity detected. Since the RV data are not part of a planet search programme, we can consider our results as a serendipitous evidence of a planetary companion. To date, this is the youngest main sequence star for which a planetary candidate has been reported.Peer reviewe

DMRG Study of Critical Behavior of the Spin-1/2 Alternating Heisenberg Chain

We investigate the critical behavior of the S=1/2 alternating Heisenberg chain using the density matrix renormalization group (DMRG). The ground-state energy per spin and singlet-triplet energy gap are determined for a range of alternations. Our results for the approach of the ground-state energy to the uniform chain limit are well described by a power law with exponent p=1.45. The singlet-triplet gap is also well described by a power law, with a critical exponent of p=0.73, half of the ground-state energy exponent. The renormalization group predictions of power laws with logarithmic corrections can also accurately describe our data provided that a surprisingly large scale parameter is present in the logarithm.Comment: 6 pages, 4 eps-figure

String order in spin liquid phases of spin ladders

Two-leg spin ladders have a rich phase diagram if rung, diagonal and plaquette couplings are allowed for. Among the possible phases there are two Haldane-type spin liquid phases without local order parameter, which differ, however, in the topology of the short range valence bonds. We show that these phases can be distinguished numerically by two different string order parameters. We also point out that long range string- and dimer orders can coexist

Plasma Turbulence in the Local Bubble

Turbulence in the Local Bubble could play an important role in the thermodynamics of the gas that is there. The best astronomical technique for measuring turbulence in astrophysical plasmas is radio scintillation. Measurements of the level of scattering to the nearby pulsar B0950+08 by Philips and Clegg in 1992 showed a markedly lower value for the line-of-sight averaged turbulent intensity parameter $$than is observed for other pulsars, consistent with radio wave propagation through a highly rarefied plasma. In this paper, we discuss the observational progress that has been made since that time. At present, there are four pulsars (B0950+08, B1133+16, J0437-4715, and B0809+74) whose lines of sight seem to lie mainly within the local bubble. The mean densities and line of sight components of the interstellar magnetic field along these lines of sight are smaller than nominal values for pulsars, but not by as much expected. Three of the four pulsars also have measurements of interstellar scintillation. The value of the parameter$$ is smaller than normal for two of them, but is completely nominal for the third. This inconclusive status of affairs could be improved by measurements and analysis of ``arcs'' in ``secondary spectra'' of pulsars.Comment: Submitted to Space Science Reviews as contribution to Proceedings of ISSI (International Space Science Institute) workshop "From the Heliosphere to the Local Bubble". Refereed version accepted for publicatio

Optical absorption of spin ladders

We present a theory of phonon-assisted optical two-magnon absorption in two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show that collective excitations of total spin S=0, 1 and 2 exist outside of the two-magnon continuum. It is demonstrated that the singlet collective state has a clear signature in the optical spectrum.Comment: 4 pages, 3 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

Low-cost locally manufacturable unilateral imperial external fixator for low- and middle-income countries

Treating open fractures in long bones can be challenging and if not performed properly can lead to poor outcomes such as mal/non-union, deformity, and amputation. One of the most common methods of treating these fracture types is temporary external fixation followed by definitive fixation. The shortage of high-quality affordable external fixators is a long-recognised need, particularly in Low- and Middle-Income Countries (LMICs). This research aimed to develop a low-cost device that can be manufactured locally to international standards. This can provide surge capacity for conflict zones or in response to unpredictable incidents and situations. The fixator presented here and developed by us, the Imperial external fixator, was tested on femur and tibia specimens under 100 cycles of 100 N compression-tension and the results were compared with those of the Stryker Hoffmann 3 frame. The Imperial device was stiffer than the Stryker Hoffmann 3 with a lower median interfragmentary motion (of 0.94 vs. 1.48 mm). The low-cost, easy to use, relatively lightweight, and easy to manufacture (since minimum skillset and basic workshop equipment and materials are needed) device can address a critical shortage and need in LMICs particularly in conflict-affected regions with unpredictable demand and supply. The device is currently being piloted in three countries for road traffic accidents, gunshot wounds and other conflict trauma—including blast cohorts

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