The possibility of measuring intrinsic electronic correlations in graphene using a d-wave contact Josephson junction

While not widely recognized, electronic correlations might play an important role in graphene. Indeed, Pauling's resonance valence bond (RVB) theory for the pp-bonded planar organic molecules, of which graphene is the infinite extension, already established the importance of the nearest neighbor spin-singlet bond (SB) state in these materials. However, despite the recent growth of interest in graphene, there is still no quantitative estimate of the effects of Coulomb repulsion in either undoped or doped graphene. Here we use a tight-binding Bogoliubov-de Gennes (TB BdG) formalism to show that in unconventional d-wave contact graphene Josephson junctions the intrinsic SB correlations are strongly enhanced. We show on a striking effect of the SB correlations in both proximity effect and Josephson current as well as establishing a 1/(T-T_c) functional dependence for the superconducting decay length. Here T_c is the superconducting transition temperature for the intrinsic SB correlations, which depends on both the effects of Coulomb repulsion and the doping level. We therefore propose that d-wave contact graphene Josephson junctions will provide a promising experimental system for the measurement of the effective strength of intrinsic SB correlations in graphene.Comment: 4 pages, 4 figure

The effect of nearest neighbor spin-singlet correlations in conventional graphene SNS Josephson junctions

Using the self-consistent tight-binding Bogoliubov-de Gennes formalism we have studied the effect of nearest neighbor spin-singlet bond (SB) correlations on Josephson coupling and proximity effect in graphene SNS Josephson junctions with conventional s-wave superconducting contacts. Despite the s-wave superconducting state in the contacts, the SB pairing state inside the junction has d-wave symmetry and clean, sharp interface junctions resemble a 'bulk-meets-bulk' situation with very little interaction between the two different superconducting states. In fact, due to a finite-size suppression of the superconducting state, a stronger SB coupling constant than in the bulk is needed in order to achieve SB pairing in a junction. For both short clean zigzag and armchair junctions a d-wave state that has a zero Josephson coupling to the s-wave state is chosen and therefore the Josephson current decreases when a SB pairing state develops in these junctions. In more realistic junctions, with smoother doping profiles and atomic scale disorder at the interfaces, it is possible to achieve some coupling between the contact s-wave state and the SB d-wave states. In addition, by breaking the appropriate lattice symmetry at the interface in order to induce another d-wave state, a non-zero Josephson coupling can be achieved which leads to a substantial increase in the Josephson current. We also report on the LDOS of the junctions and on a lack of zero energy states at interfaces despite the unconventional order parameters, which we attribute to the near degeneracy of the two d-wave solutions and their mixing at a general interface.Comment: 13 pages, 9 figures. Typos correcte

Josephson current in graphene: the role of unconventional pairing symmetries

We investigate the Josephson current in a graphene superconductor/normal/superconductor junction, where superconductivity is induced by means of the proximity effect from external contacts. We take into account the possibility of anisotropic pairing by also including singlet nearest-neighbor interactions, and investigate how the transport properties are affected by the symmetry of the superconducting order parameter. This corresponds to an extension of the usual on-site interaction assumption, which yields an isotropic s-wave order parameter near the Dirac points. Here, we employ a full numerical solution as well as an analytical treatment, and show how the proximity effect may induce exotic types of superconducting states near the Dirac points, e.g. $p_x$- and $p_y$-wave pairing or a combination of s-wave and p+\i p-wave pairing. We find that the Josephson current exhibits a weakly-damped, oscillatory dependence on the length of the junction when the graphene sheet is strongly doped. The analytical and numerical treatments are found to agree well with each other in the s-wave case when calculating the critical current and current-phase relationship. For the scenarios with anisotropic superconducting pairing, there is a deviation between the two treatments, especially for the effective $p_x$-wave order parameter near the Dirac cones which features zero-energy states at the interfaces. This indicates that a numerical, self-consistent approach becomes necessary when treating anisotropic superconducting pairing in graphene.Comment: 15 pages, 12 figure

Pyrolysis of cotton stalks and utilization of pyrolysis char for sustainable soil enhancement and carbon storage

Worldwide, a large amount of biomass accumulates in the form of ligno-cellulosic agricultural by-products that can neither be efficiently used as animal feed nor as feedstock for anaerobic digestion. A clean and energy-efficient utilization in combustion plants is counter-indicated by high contents of elements like Cl, K and Na that decrease the ash-melting-point and lead to fouling and corrosion. Although this biomass is, therefore, not suitable for conventional combustion plants, a pyrolysis process can be applied that works at lower temperatures of 500-600°C. Please click on the file below for full content of the abstract

Self-consistent solution for proximity effect and Josephson current in ballistic graphene SNS Josephson junctions

We use a tight-binding Bogoliubov-de Gennes (BdG) formalism to self-consistently calculate the proximity effect, Josephson current, and local density of states in ballistic graphene SNS Josephson junctions. Both short and long junctions, with respect to the superconducting coherence length, are considered, as well as different doping levels of the graphene. We show that self-consistency does not notably change the current-phase relationship derived earlier for short junctions using the non-selfconsistent Dirac-BdG formalism but predict a significantly increased critical current with a stronger junction length dependence. In addition, we show that in junctions with no Fermi level mismatch between the N and S regions superconductivity persists even in the longest junctions we can investigate, indicating a diverging Ginzburg-Landau superconducting coherence length in the normal region.Comment: 8 pages, 6 figure

Geistig fit durch mediterrane Kost? : Wie Menschen gesünder alt werden können

In südlichen Gefilden wächst so manches, was in Maßen genossen dem Wohlbefinden dient. Dies gilt nicht nur für Heilkräuter und Rotwein, sondern vermutlich auch für andere für den Mittelmeerraum typische Getränke und Speisen. Auf der Suche nach diesen "natürlichen Apotheken" erfassen Wissenschaftler aus Deutschland und sechs weiteren europäischen Ländern derzeit seltene Unterarten bewährter Nutzpflanzen wie Thymian, Olive, Wein und Orange. Sie erforschen, ob die seit Jahrhunderten überlieferten Schutzund Heilungskräfte der Gewächse einer wissenschaftlichen Prüfung standhalten und worauf sie beruhen. Die Frankfurter Gruppe um Prof. Dr. Walter Müller hat dabei insbesondere Stoffe im Blick, die das Nervensystem beeinflussen. Macht mediterrane Kost wirklich geistig fit

GOing Bayesian: model-based gene set analysis of genome-scale data

The interpretation of data-driven experiments in genomics often involves a search for biological categories that are enriched for the responder genes identified by the experiments. However, knowledge bases such as the Gene Ontology (GO) contain hundreds or thousands of categories with very high overlap between categories. Thus, enrichment analysis performed on one category at a time frequently returns large numbers of correlated categories, leaving the choice of the most relevant ones to the user's; interpretation