Demonstration of a robust pseudogap in a three-dimensional correlated electronic system

We outline a partial-fractions decomposition method for determining the one-particle spectral function and single-particle density of states of a correlated electronic system on a finite lattice in the non self-consistent T-matrix approximation to arbitrary numerical accuracy, and demonstrate the application of these ideas to the attractive Hubbard model. We then demonstrate the effectiveness of a finite-size scaling ansatz which allows for the extraction of quantities of interest in the thermodynamic limit from this method. In this approximation, in one or two dimensions, for any finite lattice or in the thermodynamic limit, a pseudogap is present and its energy diverges as Tc is approached from above; this is an unphysical manifestation of using an approximation that predicts a spurious phase transition in one or two dimensions. However, in three dimensions one expects the transition predicted by this approximation to represent a true continuous phase transition, and in the thermodynamic limit any pseudogap predicted by this formulation will remain finite. We have applied our method to the attractive Hubbard model on a three-dimensional simple cubic lattice, and find that for intermediate coupling a prominent pseudogap is found in the single-particle density of states, and this gap persists over a large temperature range. In addition, we also show that for weak coupling a pseudogap is also present. The pseudogap energy at the transition temperature is almost a factor of three larger than the T=0 BCS gap for intermediate coupling, whereas for weak coupling the pseudogap and BCS gap energies are essentially equal.Comment: 28 pages, 9 figure

Hardiness in the apple as correlated with structure and composition

The breeding of hardy apples is naturally such a long time proposition that it would be a distinct advantage if by microscopic examination of the wood, by mechanical tests, or by other observations the hardiness of a tree could be determined while it was still in the nursery, The following is a summary of the facts brought out in the investigation of Hardiness in the Apple as Correlated with Structure and Composition. 1. The cutting, compression and penetration tests seemed to indicate some correlation between hardiness and hardness, but exceptions were found. 2, Twigs from northern sections, except the forms of Metlus rivularis, Red June and Patten, were no harder than twigs from Iowa and further south. 3, Specific gravity tests of dry wood showing density corresponded very closely with the mechanical tests showing hardness, indicating that the two tests are fairly accurate means to the same end. 4. Variat ions in the specific gravity of twigs of the same variety from different sources were noted, as was also the case in the mechanical tests. Twigs of the same variety from the same source gave pretty uniform results. 5. The specific gravity of twigs varies, to some extent depending upon what part of the twig is used. The tenderer varieties, which mature a little later in the summer, gave the greatest specific gravity a short distance back of the tip; earlier maturing varieties increased in specific gravity in proportion to the distance from the tip. As the tests were made in July this difference is in all probability due to the difference in the maturing of the twigs used. 6. The maturity of the wood at the time cold weather sets in undoubtedly has a most important bearing upon the ability of the tree to withstand cold. 7. By noting the formation of terminal buds and the time of leaf fall a pretty accurate idea of maturity can be secured, except in case of a few varieties. 8. The hardier varieties on the average had a slightly lower moisture content than the more tender varieties. 9. The difference in water content can be explained partly at least by the fact that the more tender sorts evaporate water more readily than do the hardy varieties. Freezing tends to dry the twig out, and after a period of very cold weather the twigs of the hardy varieties are generally found to contain the most moisture. 10. The rate of evaporation is due in part to small differences in thickness and structure of bark, including differences in the number of cutinized layers. 11. Most of the hardy varieties contained a large amount of starch stored in the pith and medullary rays. The Malus ioensis, however, proved to be an exception in this regard. 12. Large, thick petals are correlated with hardiness, although the converse of this is not always true. 13. The results of the freezing experiments show that within the limits observed a sudden drop in temperature is more injurious than the actual degree of cold. 14. While all twigs not previously dried were injured to some extent when held in a temperature of -10 degrees F. for 20 minutes, the injury was less in the hardier varieties, thus showing their ability better to withstand such a sudden drop in temperature. 15. While in the various lines of comparison which were made there were found many indications of morphological differences between hardy and tender varieties, yet from the practical view point as yet it is impossible to name anyone test by which the degree of constitutional hardiness of a seedling apple may be foretold. Among the various tests for hardiness, that of the length of season required by the tree to mature the season\u27s growth is of first importance. Perhaps by taking careful notes on a number of trees of any particular variety for two or three years or more, noting their time of starting and cessation of growth, their ability to produce a good root system from the cion, their water content, the resistance of their twigs to sudden zero temperatures, and their rate of evaporation, a pretty accurate idea of the ability of the tree to withstand, ,cold might be obtained. If in addition the variety has hard wood, a good amount of stored starch and large petals these would be further indications of hardiness, although from this study it appears that these points are of less importance than those first named

Dynamical screening in strongly correlated metal SrVO3

The consequences of dynamical screening of Coulomb interaction among correlated electrons in realistic materials have not been widely considered before. In this letter we try to incorporate a frequency dependent Coulomb interaction into the state-of-the-art ab initio electronic structure computing framework of local density approximation plus dynamical mean-field theory, and then choose SrVO3 as a prototype material to demonstrate the importance of dynamical screening effect. It is shown to renormalise the spectral weight near the Fermi level, to increase the effective mass, and to suppress the t2g quasiparticle band width apparently. The calculated results are in accordance with very recent angle-resolved photoemission spectroscopy experiments and Bose factor ansatz calculations.Comment: 6 pages, 4 figures. arXiv admin note: text overlap with arXiv:1107.312

Peierls to superfluid crossover in the one-dimensional, quarter-filled Holstein model

We use continuous-time quantum Monte Carlo simulations to study retardation effects in the metallic, quarter-filled Holstein model in one dimension. Based on results which include the one- and two-particle spectral functions as well as the optical conductivity, we conclude that with increasing phonon frequency the ground state evolves from one with dominant diagonal order---2k_F charge correlations---to one with dominant off-diagonal fluctuations, namely s-wave pairing correlations. In the parameter range of this crossover, our numerical results support the existence of a spin gap for all phonon frequencies. The crossover can hence be interpreted in terms of preformed pairs corresponding to bipolarons, which are essentially localised in the Peierls phase, and "condense" with increasing phonon frequency to generate dominant pairing correlations.Comment: 11 pages, 5 figure

A reliable Pade analytical continuation method based on a high accuracy symbolic computation algorithm

We critique a Pade analytic continuation method whereby a rational polynomial function is fit to a set of input points by means of a single matrix inversion. This procedure is accomplished to an extremely high accuracy using a novel symbolic computation algorithm. As an example of this method in action we apply it to the problem of determining the spectral function of a one-particle thermal Green's function known only at a finite number of Matsubara frequencies with two example self energies drawn from the T-matrix theory of the Hubbard model. We present a systematic analysis of the effects of error in the input points on the analytic continuation, and this leads us to propose a procedure to test quantitatively the reliability of the resulting continuation, thus eliminating the black magic label frequently attached to this procedure.Comment: 11 pages, 8 eps figs, revtex format; revised version includes reference to anonymous ftp site containing example codes (MapleVr5.1 worksheets) displaying the implementation of the algorithm, including the padematinv.m library packag

Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires

Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quantify the DMI for a given material system. Here we extract the DMI at the Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary measurement schemes namely asymmetric domain wall motion and the magnetic stripe annihilation. By using the two different measurement schemes, we find for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and 0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes skyrmions at room temperature and that there is a strong dependence of the DMI on the relative composition of the CoFeB alloy. Finally we optimize the layers and the interfaces using different growth conditions and demonstrate that a higher deposition rate leads to a more uniform film with reduced pinning and skyrmions that can be manipulated by Spin-Orbit Torques

Advanced secondary power system for transport aircraft

A concept for an advanced aircraft power system was identified that uses 20-kHz, 440-V, sin-wave power distribution. This system was integrated with an electrically powered flight control system and with other aircraft systems requiring secondary power. The resulting all-electric secondary power configuration reduced the empty weight of a modern 200-passenger, twin-engine transport by 10 percent and the mission fuel by 9 percent

Analysis of the role of Ser1/Ser2/Thr9 phosphorylation on myosin II assembly and function in live cells

<p>Abstract</p> <p>Background</p> <p>Phosphorylation of non-muscle myosin II regulatory light chain (RLC) at Thr18/Ser19 is well established as a key regulatory event that controls myosin II assembly and activation, both in vitro and in living cells. RLC can also be phosphorylated at Ser1/Ser2/Thr9 by protein kinase C (PKC). Biophysical studies show that phosphorylation at these sites leads to an increase in the Km of myosin light chain kinase (MLCK) for RLC, thereby indirectly inhibiting myosin II activity. Despite unequivocal evidence that PKC phosphorylation at Ser1/Ser2/Thr9 can regulate myosin II function in vitro, there is little evidence that this mechanism regulates myosin II function in live cells.</p> <p>Results</p> <p>The purpose of these studies was to investigate the role of Ser1/Ser2/Thr9 phosphorylation in live cells. To do this we utilized phospho-specific antibodies and created GFP-tagged RLC reporters with phosphomimetic aspartic acid substitutions or unphosphorylatable alanine substitutions at the putative inhibitory sites or the previously characterized activation sites. Cell lines stably expressing the RLC-GFP constructs were assayed for myosin recruitment during cell division, the ability to complete cell division, and myosin assembly levels under resting or spreading conditions. Our data shows that manipulation of the activation sites (Thr18/Ser19) significantly alters myosin II function in a number of these assays while manipulation of the putative inhibitory sites (Ser1/Ser2/Thr9) does not.</p> <p>Conclusions</p> <p>These studies suggest that inhibitory phosphorylation of RLC is not a substantial regulatory mechanism, although we cannot rule out its role in other cellular processes or perhaps other types of cells or tissues in vivo.</p

Development and Validation of a Measure for Seeking Health Information in the Diabetes Online Community: Mixed Methods Study

Background Individuals with chronic diseases often search for health information online. The Diabetes Online Community (DOC) is an active community with members who exchange health information; however, few studies have examined health information brokering in the DOC. Objective The aim of this study was to develop and validate the Attitudes Toward Seeking Health Information Online (ATSHIO) scale in a sample of adults with type 1 diabetes (T1D). Methods People with T1D were recruited through the DOC, specifically Facebook and Twitter. They were provided with a Qualtrics link to complete the survey. This was a mixed methods study that used thematic analysis along with existing theory and formative research to design the quantitative ATSHIO scale. Results A total of 166 people with T1D participated in this study. Confirmatory factor analyses determined a 2-factor scale (Trusting and Evaluating Online Health Information in the DOC and Engaging With Online Health Information in the DOC) with good convergent validity and discriminant validity. Correlations were found between social support, online health information–seeking, diabetes distress, and disease management. Conclusions The ATSHIO scale can be used to investigate how people with diabetes are using the internet for obtaining health information, which is especially relevant in the age of telehealth and Health 2.0