A Majorana perspective on understanding and identifying axion insulators

An axion insulator is theoretically introduced to harbor unique surface states with half-integer Chern number $${{{{{{{\mathcal{C}}}}}}}}$$ C . Recently, experimental progress has been made in different candidate systems, while a unique Hall response to directly reflect the half-integer Chern number is still lacking to distinguish an axion state from other possible insulators. Here we show that the $${{{{{{{\mathcal{C}}}}}}}}=\frac{1}{2}$$ C = 1 2 axion state corresponds to a topological state with Chern number $${{{{{{{\mathcal{N}}}}}}}}=1$$ N = 1 in the Majorana basis. In proximity to an s − wave superconductor, a topological phase transition to an $${{{{{{{\mathcal{N}}}}}}}}=0$$ N = 0 phase takes place at critical superconducting pairing strength. Our theoretical analysis shows that a chiral Majorana hinge mode emerges at the boundary of $${{{{{{{\mathcal{N}}}}}}}}=1$$ N = 1 and $${{{{{{{\mathcal{N}}}}}}}}=0$$ N = 0 regions on the surface of an axion insulator. Furthermore, we propose a half-integer quantized thermal Hall conductance via a thermal transport measurement, which is a signature of the gapless chiral Majorana mode and thus confirms the $${{{{{{{\mathcal{C}}}}}}}}=\frac{1}{2}$$ C = 1 2 ($${{{{{{{\mathcal{N}}}}}}}}=1$$ N = 1 ) topological nature of an axion state. Our proposals help to theoretically comprehend and experimentally identify the axion insulator and may benefit the research of topological quantum computation.

Superconductivity in undoped BaFe2As2by tetrahedral geometry design

Fe-based superconductors exhibit a diverse interplay between charge, orbital, and magnetic ordering. Variations in atomic geometry affect electron hopping between Fe atoms and the Fermi surface topology, influencing magnetic frustration and the pairing strength through changes of orbital overlap and occupancies. Here, we experimentally demonstrate a systematic approach to realize superconductivity without chemical doping in BaFe2As2, employing geometric design within an epitaxial heterostructure. We control both tetragonality and orthorhombicity in BaFe2As2through superlattice engineering, which we experimentally find to induce superconductivity when the As−Fe−As bond angle approaches that in a regular tetrahedron. This approach to superlattice design could lead to insights into low-dimensional superconductivity in Fe-based superconductors.

A Search for Structural Alternatives of RNA

This account describes a search for potentially primordial informational oligomers; the work is the direct outcome of the research program that was initiated by the Eschenmoser group —at ETH Zürich about 20 years ago and was continued at The Scripps Research Institute since 1996— in order to understand the chemical basis on which nucleic acids were chosen by nature as the molecular foundation of its genetic apparatus. The investigation began with the study of the base-pairing properties of structural alternatives of nucleic acids —constructed from different sugars (hexo- & pentopyranoses and tetrofuranose) retaining the canonical nucleobases and phosphates. The outcome from these studies led to the conclusion that Watson-Crick type base-pairing is not unique to RNA/DNA, and that it can be compatible with a wide variety of backbone edifice. This provided the motivation to map the landscape of potentially primordial informational oligomer systems that may contain backbones, recognition elements and linker groups structurally quite different from those known so far. The oligomer systems chosen for study are, conceptually, deemed to be (a) potentially primordial (based on the nature of the starting materials and reaction conditions considered to be prebiotically realistic) and (b) informational (based on their ability to adopt a repetitive conformation such that the information encoded by the recognition elements can be transmitted intermolecularly). Though such studies suggest the possibility of finding informational systems that could lay claim as functional ancestors of RNA —they are more likely to generate results that provide the opportunity to assess the structural and functional uniqueness of nature’s choice. The experimental investigation described here deals with the base-pairing properties of oligomer systems derived from 2,4-disubsituted -triazines, -5-aminopyrimdines and -6-carboxy pyrimidines as recognition elements that are tagged to oligo-dipeptide backbones via different linker groups. The results from the inter- and intra-system cross-pairing studies reveal that there is, on first approximation, a direct correlation between the magnitude of the difference in DpKa of the recognition elements and their base-pairing strength —smaller the Dpka between the base-pairing partners, weaker is the base-pairing strength (in aqueous medium at near neutral pH). These results exemplify the inherent singularity of the canonical nucleobases— their ability to remain un-ionized under physiological conditions based on their constitution – emphasizing the relationship between their physicochemical properties and their functional competence in connection with their role in informational base-pairing.

Base pairing interactions between substrate RNA and H/ACA guide RNA modulate the kinetics of pseudouridylation, but not the affinity of substrate binding by H/ACA small nucleolar Ribonucleoproteins

H/ACA small nucleolar ribonucleoproteins (snoRNPs) pseudouridylate RNA in eukaryotes and archaea. They target many RNAs site-specifically through base-pairing interactions between H/ACA guide and substrate RNA. Besides ribosomal RNA (rRNA) and small nuclear RNA (snRNA), H/ACA snoRNPs are thought to also modify messenger RNA (mRNA) with potential impacts on gene expression. However, the base-pairing between known target RNAs and H/ACA guide RNAs varies widely in nature, and therefore the rules governing substrate RNA selection are still not fully understood. To provide quantitative insight into substrate RNA recognition, we systematically altered the sequence of a substrate RNA target by the Saccharomyces cerevisiae H/ACA guide RNA snR34. Time courses measuring pseudouridine formation revealed a gradual decrease in the initial velocity of pseudouridylation upon reducing the number of base pairs between substrate and guide RNA. Changing or inserting nucleotides close to the target uridine severely impairs pseudouridine formation. Interestingly, filter binding experiments show that all substrate RNA variants bind to H/ACA snoRNPs with nanomolar affinity. Next, we showed that binding of inactive, near-cognate RNAs to H/ACA snoRNPs does not inhibit their activity for cognate RNAs, presumably because near-cognate RNAs dissociate rapidly. We discuss that the modulation of initial velocities by the base pairing strength might affect the order and efficiency of pseudouridylation in rRNA during ribosome biogenesis. Moreover, the binding of H/ACA snoRNPs to near-cognate RNAs may be a mechanism to search for cognate target sites. Together, our data provide critical information to aid in the prediction of productive H/ACA guide – substrate RNA pairs.

Pairing corrections within the density-dependent delta interaction

The validity of the Strutinsky prescriptions is tested within a state-dependent pairing interaction instead of a constant pairing strength. The amplitude of the pairing field is determined to reproduce the experimental values of the pairing gaps in the ground state of the parent [Formula: see text]Th. The density-dependent delta interaction was used to compute the quantities related to the pairing corrections along a fission trajectory, such as Fermi energies, pairing gaps, and pairing effects. An enhancement of the pairing gaps during the penetration of the second barrier was evidenced in the case of the state-dependent pairing formalism. Moreover, the outer barrier calculated in the framework of the macroscopic–microscopic model is strongly suppressed.

Nuclear structure investigation of even–even and odd Pb isotopes by using the Hartree–Fock–Bogoliubov method

The nuclear structure of even–even and odd lead isotopes ([Formula: see text]Pb) is investigated within the Hartree–Fock–Bogoliubov (HFB) theory. Calculations are performed for a wide range of neutron numbers, starting from the proton-rich side up to the neutron-rich side, by using the SLy4 Skyrme interaction and a new proposed formula for the pairing strength which is more precise for this region of nuclei as we did in previous works in the regions of Neodymium (Nd, [Formula: see text]) [Y. El Bassem and M. Oulne, Int. J. Mod. Phys. E 24 (2015) 1550073] and Molybdenum (Mo, [Formula: see text]) [Y. El Bassem and M. Oulne, Nucl. Phys. A 957 (2017) 22]. Such a new pairing strength formula allows reaching exotic nuclei region where the experimental data are not available. Calculated values of various physical quantities such as binding energy, two-neutron separation energy, quadrupole deformation, and rms-radii for protons and neutrons are discussed and compared with experimental data and some estimates of other nuclear models like Finite Range Droplet Model (FRDM), Relativistic Mean Field (RMF) model with NL3 functional (NL3), Density-Dependent Meson-Exchange Relativistic Energy Functional (DD-ME2) and results of HFB calculations based on the D1S Gogny effective nucleon–nucleon interaction (Gogny D1S).

Determination of the pairing-strength constants in the isovector plus isoscalar pairing case

A method for the determination of the pairing-strength constants, in the neutron–proton (n–p) isovector plus isoscalar pairing case, is proposed in the framework of the BCS theory. It is based on the fitting of these constants to reproduce the experimentally known pairing gap parameters as well as the root-mean-squared (r.m.s) charge radii values. The method is applied to some proton-rich even–even nuclei. The single-particle energies used are those of a deformed Woods–Saxon mean field. It is shown that the obtained value of the ratio [Formula: see text] is of the same order as the ones, arbitrary chosen, of some previous works. The effect of the inclusion of the isoscalar n–p pairing in the r.m.s matter radii is then numerically studied for the same nuclei.

Ground state properties of even–even and odd Nd, Ce and Sm isotopes in Hartree–Fock–Bogoliubov method

In this work, we have studied the ground state properties of both even–even and odd Nd isotopes within Hartree–Fock–Bogoliubov method with SLy5 Skyrme force in which the pairing strength has been generalized with a new proposed formula. We calculated binding energies, two-neutron separation energies, quadrupole deformation, charge, neutron and proton radii. Similar calculations have been carried out for Ce and Sm in order to verify the validity of our pairing strength formula. The results have been compared with available experimental data, the results of Hartree–Fock–Bogoliubov calculations based on the D1S Gogny effective nucleon–nucleon interaction and predictions of some nuclear models such as finite range droplet model (FRDM) and relativistic mean field (RMF) theory.