Ferromagnetism in one-dimensional metals: breakdown of the Hartree-Fock approximation and possible first order phase transition

We calculate the Gibbs potential Gamma (M) of a one-dimensional metal at constant magnetization M to second order in the screened electron-electron interaction U. At zero temperature we find that Gamma (M) contains non-analytic corrections proportional to M^2 \ln | M| and | M |^3, implying that a possible paramagnetic-ferromagnetic quantum phase transition in one-dimensional metals must be first order.Comment: 8 pages, 7 figures; we have changed our choice of the ultraviolet cutoff, as suggested by a referee. Phase transition occurs now for Stoner factor smaller than unity. Fig 5 and Fig.7 are have been redraw

Systematic Perturbation Theory for Dynamical Coarse-Graining

We demonstrate how the dynamical coarse-graining approach can be systematically extended to higher orders in the coupling between system and reservoir. Up to second order in the coupling constant we explicitly show that dynamical coarse-graining unconditionally preserves positivity of the density matrix -- even for bath density matrices that are not in equilibrium and also for time-dependent system Hamiltonians. By construction, the approach correctly captures the short-time dynamics, i.e., it is suitable to analyze non-Markovian effects. We compare the dynamics with the exact solution for highly non-Markovian systems and find a remarkable quality of the coarse-graining approach. The extension to higher orders is straightforward but rather tedious. The approach is especially useful for bath correlation functions of simple structure and for small system dimensions.Comment: 17 pages, 5 figures, version accepted for publication in PR

Weak coupling approximations in non-Markovian Transport

We study the transport properties of the Fano-Anderson model with a Lorentzian-shaped density of states in one of the electronic reservoirs. We explicitly show that the energy dependence of the density of states can cause non-Markovian effects and that the non-Markovian master equation may fail if these effects are strong. We evaluate the stationary current, the zero frequency current noise and the occupation dynamics of the resonant level by means of a quantum master equation approach within different approximation schemes and compare the results to the exact solution obtained by scattering theory and Green's functions.Comment: 9 pages, 6 figures; due to suggestions of a referee we have added an appendix where our kernel is derived in detail; a few typos are correcte

Vegetative Ecological Characteristics of Restored Reed (Phragmites australis) Wetlands in the Yellow River Delta, China

In this study, we compared ecological characteristics of wetland vegetation in a series of restoration projects that were carried out in the wetlands of Yellow River Delta. The investigated characteristics include plant composition structure, species diversity and community similarity in three kinds of Phragmites australis wetlands, i.e. restored P. australis wetlands (R1, R2, R3 and R4: restored in 2002, 2005, 2007 and 2009, respectively), natural P. australis wetland (N) and degraded P. australis wetland (D) to assess the process of wetlands restoration. The coverage of the R1 was 99%, which was similar to natural wetland. Among all studied wetlands, the highest and lowest stem density was observed in R1 and R2, respectively, Plant height and stem diameter show the same trend as N > R2 > R1 > R3 > D > R4. Species diversity of restored P. australis wetlands became closed to natural wetland. Both species richness and Shannon–Wiener index had similar tendency: increased first and then decreased with restored time. The highest species richness and species diversity were observed in R2, while the lowest values of those parameters were found in natural P. australis wetland. Similarity indexes between restored wetlands and natural wetland increased with the restoration time, but they were still less than 50%. The results indicate that the vegetation of P. australis wetlands has experienced a great improvement after several years’ restoration, and it is feasible to restored degraded P. australis wetlands by pouring fresh water into those wetlands in the Yellow River Delta. However, it is notable that costal degraded P. australis wetland in this region may take years to decades to reach the status of natural wetland

Mastergleichungen in nicht markoffschem Transport: Erfolg und Misserfolg von nicht markoffschen Korrekturen und Korrekturen höherer Ordnung

Wie lässt sich kohärentes Tunneln von Elektronen gut mit Mastergleichungen beschreiben? Diese Frage stellt sich, weil Mastergleichungen ursprünglich für sequentielles, also klassisches Tunneln konzipiert wurden. Die große Stärke von Mastergleichungen liegt darin, dass sich mit ihnen sehr gut und einfach Mehrteilchen-Wechselwirkungen innerhalb eines mesoskopischen Systems beschreiben lassen. Diese Stärke beruht allerdings darauf, dass die Kopplung zur Außenwelt nur näherungsweise berücksichtigt wird, so dass eine perfekte Beschreibung von kohärenten Tunnelprozessen in das System hinein oder aus dem System heraus im Allgemeinen ausgeschlossen ist. Allerdings wurden Methoden entwickelt, mit denen sich systematisch Korrekturen zur gewöhnlichen Mastergleichung berechnen lassen, etwa die Realzeit-Diagrammatik. Normalerweise sollten kohärente Tunnelprozesse deutlich besser beschrieben werden, wenn höhere Korrekturen berücksichtigt werden. In dieser Arbeit beschäftigen wir uns mit einer bestimmten Methode, um solche Korrekturen selbstkonsistent auszurechnen. Sie wurde kürzlich von Jonas Pedersen und Andreas Wacker entwickelt. Um die Qualität der Korrekturen beurteilen zu können, vergleichen wir alle Ergebnisse mit der exakten Lösung eines einfachen Modells in nichttrivialen Realisierungen. Um Hinweise zu erhalten, ob auch Korrelationen zwischen Elektronen gut beschrieben werden, untersuchen wir Größen aus dem Gebiet der vollen Zählstatistik (Full Counting Statistics). Eine weitere Möglichkeit, Mastergleichungen zu verbessern, besteht darin, Gedächniseffekte (auch: nicht markoffsche Effekte) zu berücksichtigen. Diese werden bei der Herleitung der Mastergleichung erzeugt und häufig hinterher weggelassen. Wir verwenden neue Formalismen, die die Berechnung nicht markoffscher Korrekturen vereinfachen und testen die Ergebnisse anhand der exakten Lösung. Außer den Näherungen stellen wir auch mehrere exakte Lösungen unseres Testmodells vor (ein resonantes Niveau mit Zu- und Abfluss von Elektronen). Mit diesen berechnen wir verschiedene physikalische Größen bis hin zu frequenzabhängigen Kumulanten, bei denen sich besonders gut der Übergang von rein sequentiellem zu rein kohärentem Tunneln veranschaulichen lässt. Außerdem geben wir ein Beispiel, wie sich Wechselwirkungseffekte in die exakten Lösungen einbauen lassen. Die Arbeit schließt mit einer Beurteilung der untersuchten Lösungen und Näherungen, insbesondere mit der Empfehlung, bei kleinen Spannungen alle behandelten Korrekturen zu berücksichtigen, aber bei ungewöhnlichen spektralen Eigenschaften generell mit Mastergleichungen vorsichtig zu sein.How can we gain a good description of coherent tunneling of electrons? This question arises as originally the master equation was made to describe sequential tunneling which is classical. The strength of master equations lies in the fact that they give an easy and good description of many particle effects within a mesoscopic system. However, this stength emerges because the coupling to the outside world is only approximately incorporated. Thus, a perfect description of tunneling processes into and out of the system is in general excluded. But methods have been developed that give systematic corrections to usual master equations, for example real time diagrammatics. Usually, when higher order corrections are incorporated this should lead to a remarkably better description of coherent tunneling processes. In this thesis we are dealing with a certain method to evaluate such corrections in a self-consistent way which was recently developed by Jonas Pedersen and Andreas Wacker. To be able to make statements about its accuracy we compare all results to the exact solution of a simple model in non-trivial realizations. To get a hint if also correlations will be described well, we investigate quantities form the field of Full Counting Statistics. A further possibility to improve master equations consists of including memory effects (also: Non-Markovian effects). They are created in the derivation of the master equation and often neglected afterwards. We will use a new formulation which simplifies the evaluation of Non-Markovian corrections and we test the results using the exact solution. Apart from the approximations we also present several exact solutions of our test model (a resonant level with a source and a drain of electrons). We use them to evaluate various physical quantities up to the frequency-dependent cumulants where it is especially easy to visualize the transition form purely sequential to purely coherent tunneling. We furthermore give an example of how interaction effects can be built into our exact solutions. The work concludes with statements about the solutions and approximations. Particularly, we suggest that one should incorporate all corrections considered here for systems with small bias, but that if the spectral properties are unusual one should be careful with master equations

Photophysics of a Ruthenium Complex with a π-Extended Dipyridophenazine Ligand for DNA Quadruplex Labeling

International audienceThe light-switch mechanism of the complex [Ru(bpy)2(Br-dpqp)](PF6)2 (1, bpy = 2,2′-bipyridine, Br-dpqp = 12-bromo-14-ethoxydipyrido[3,2-a:2′,3′-c]quinolino[3,2-h]phenazine), i.e., a light-up probe for the selective labeling of G-quadruplexes, is investigated by time-resolved transient absorption and emission spectroscopy. We show that, in contrast to the prototypical light-switch complex [Ru(bpy)2(dppz)](PF6)2 (2, dppz = dipyrido[3,2-a:2′,3′-c]phenazine), a 3ππ* state localized on the π-extended ligand is the state determining the excited-state properties in both protic and aprotic environments. In aprotic environments, emission originates from a bright 3MLCTphen state, which is thermally accessible from the 3ππ* state at ambient temperature. In the presence of water, i.e., in environments resembling in cellulo situations, the thermally accessible 3MLCT state is altered and becomes close in energy to the 3ππ* state, which induces a rapid excited-state deactivation of the 3ππ* state and a comparably weak emission

Clinical Relevance of Right Atrial Functional Response to Treatment in Pulmonary Arterial Hypertension

Background: Right atrial (RA) function has emerged as an important determinant of outcome in pulmonary arterial hypertension (PAH). However, studies exploring RA function after initiation of specific pulmonary vascular treatment and its association with outcome in patients with incident PAH are lacking.Methods: RA peak longitudinal strain (PLS), passive strain (PS), and peak active contraction strain (PACS) were retrospectively assessed in 56 treatment-naive patients with PAH at baseline and during follow-up after initiation of specific monotherapy or combination therapy. Patients were grouped according to their individual RA functional response to treatment, based on change from baseline (Delta): worsened (first Delta-tertile), stable (second Delta-tertile), and improved (third Delta-tertile). The Spearman's rho correlation and linear regression analysis were used to determine associations. Time to clinical worsening (defined as deterioration of functional class or 6-min walking distance, disease-related hospital admission, or death) was measured from the follow-up assessment. The association of RA functional treatment response with time to clinical worsening was assessed using the Kaplan-Meier and the Cox regression analyses.Results: Median (interquartile range) time to echocardiographic follow-up was 11 (9-12) months. Of the 56 patients, 37 patients (66%) received specific dual or triple combination therapy. Delta RA PLS during follow-up was significantly associated with changes in key hemodynamic and echocardiographic parameters. The change of pulmonary vascular resistance, right ventricular (RV) end-systolic area, and global longitudinal strain were independently associated with Delta RA PLS. The median time to clinical worsening after echocardiographic follow-up was 6 (2-14) months [17 events (30%)]. In the multivariate Cox regression analysis, worsening of RA PLS was significantly associated with clinical deterioration (hazard ratio: 4.87; 95% CI: 1.26-18.76; p = 0.022). Patients with worsened RA PLS had a significantly poorer prognosis than those with stable or improved RA PLS (log-rank p = 0.012). By contrast, PS and PACS did not yield significant prognostic information.Conclusion: Treatment-naive patients with PAH may show different RA functional response patterns to PAH therapy. These functional patterns are significantly associated with clinically relevant outcome measures. Improvements of RA function are driven by reductions of afterload, RV remodeling, and RV dysfunction

Echocardiographic pressure–strain loop‐derived stroke work of the right ventricle: validation against the gold standard

Abstract Aims Commercially available integrated software for echocardiographic measurement of stroke work (SW) is increasingly used for the right ventricle, despite a lack of validation. We sought to assess the validity of this method [echo‐based myocardial work (MW) module] vs. gold‐standard invasive right ventricular (RV) pressure–volume (PV) loops. Methods and results From the prospectively recruiting EXERTION study (NCT04663217), we included 42 patients [34 patients with pulmonary arterial hypertension (PAH) or chronic thromboembolic pulmonary hypertension (CTEPH) and 8 patients with absence of cardiopulmonary disease] with RV echocardiography and invasive PV catheterization. Echocardiographic SW was assessed as RV global work index (RVGWI) generated via the integrated pressure–strain MW software. Invasive SW was calculated as the area bounded by the PV loop. An additional parameter derived from the MW module, RV global wasted work (RVGWW), was correlated with PV loop measures. RVGWI significantly correlated with invasive PV loop‐derived RV SW in the overall cohort [rho = 0.546 (P < 0.001)] and the PAH/CTEPH subgroup [rho = 0.568 (P < 0.001)]. Overall, RVGWW correlated with invasive measures of arterial elastance (Ea), the ratio of end‐systolic elastance (Ees)/Ea, and end‐diastolic elastance (Eed) significantly. Conclusions Integrated echo measurement of pressure–strain loop‐derived SW correlates with PV loop‐based assessment of RV SW. Wasted work correlates with invasive measures of load‐independent RV function. Given the methodological and anatomical challenges of RV work assessment, evolution of this approach by incorporating more elaborated echo analysis data and an RV reference curve might improve its reliability to mirror invasively assessed RV SW