Pairing Tendencies Research Articles

Pairing tendencies of nuclei

Supplementing the experimental data of excited states of nuclei with those of the canonical assemble, we determine the temperature-dependent specific heat formula, with finite system corrections, in the mass range of 22<A<206. Phase transition structures in shapes of peaks in the specific heat diagram have been identified as distinct pairing processes and are used in comparison to the pairing energy correction term (Gilbert and Cameron, 1965). Through mass dependent pairing gap equations the addition of the proton–neutron pairing procedure and finally the distinction of the pairing tendency, of each nuclei, was made possible.

Electronic structure, magnetic properties, and pairing tendencies of the copper-based honeycomb lattice Na2Cu2TeO6

Spin-$1/2$ chains with alternating antiferromagnetic and ferromagnetic couplings have attracted considerable interest due to the topological character of their spin excitations. Here, using density functional theory and density matrix renormalization group methods, we have systematically studied the dimerized chain system Na$_2$Cu$_2$TeO$_6$. Near the Fermi level, the dominant states are mainly contributed by the Cu $3d_{x^2-y^2}$ orbitals highly hybridized with the O $2p$ orbitals in the nonmagnetic phase, leading to an "effective" single-orbital low-energy model. Furthermore, the bandwidth of the Cu $3d_{x^2-y^2}$ states is small ($\sim 0.8$ eV), suggesting that electronic correlations will strongly affect this system. By introducing such electronic correlations, we found this system is a Mott insulator. Moreover, by calculating the magnetic exchange interactions ($J_1$, $J_2$ and $J_3$), we explained the size and sign of the exchange interactions in Na$_2$Cu$_2$TeO$_6$, in agreement with neutron experiments. In addition, we constructed a single-orbital Hubbard model for this dimerized chain system, where the quantum fluctuations are taken into account. Both AFM and FM coupling ($\uparrow$-$\downarrow$-$\downarrow$-$\uparrow$) along the chain were found in our DMRG and Lanczos calculations, in agreement with DFT and neutron results. We also calculated the hole pairing binding energy $\Delta E$ which becomes negative at Hubbard $U \sim 11$ eV, indicating incipient pairing tendencies. Finally, we also looked at various cases of hole doping that always exhibit tight pairs. Thus, we believe our results for Na$_2$Cu$_2$TeO$_6$ could provide guidance to experimentalists and theorists working on this dimerized chain system, such as short-range magnetic coupling, doping effects, and possible pairing tendencies.

Pair density wave and reentrant superconducting tendencies originating from valley polarization

We investigate superconducting pairing tendencies of a two-dimensional electron fluid with both valley and spin degrees of freedom, both without and in the presence of an in-plane magnetic field. We present suggestive theoretical arguments that spontaneous valley polarization can lead to exotic singlet superconducting tendencies, including pair-density-wave order at zero field and re-entrant superconductivity at high field. We also find a reduced magnetic response in the polarized valley, which allows a finite violation of the Pauli limit. These results are obtained by a mean-field approach to a generalized $t$-$J$ model on a triangular lattice and in the dilute limit. Phenomenological similarities to results of recent magic-angle twisted trilayer graphene experiments are noted.

Topological superconductivity on the honeycomb lattice: Effect of normal state topology

The search for topological superconductors is one of the most pressing and challenging questions in condensed-matter and material research. Despite some early suggestions that doping a topological insulator might be a successful recipe to find topological superconductors, to date there is no general understanding of the relationship of the topology of the superconductor and the topology of its underlying normal state system. One of the major obstacles is the strong effect of the Fermi surface and its subsequent pairing tendencies within the Hubbard model framework, which usually prevents a detailed comparison between different topological superconducting systems. Here we present an analysis of doped insulators---topological and trivial---where the dominant Fermi surface effects have been equalized. Our approach allows us to study and compare superconducting instabilities of different normal state systems and present rigorous results about the influence of the normal state system's topology.

Electronic instabilities in Penrose quasicrystals: Competition, coexistence, and collaboration of order

Quasicrystals lack translational symmetry, but can still exhibit long-range order, promoting them to candidates for unconventional physics beyond the paradigm of crystals. Here, we apply a real-space functional renormalization group approach to the prototypical quasicrystalline Penrose tiling Hubbard model treating competing electronic instabilities in an unbiased, beyond-mean-field fashion. Our work reveals a delicate interplay between charge and spin degrees of freedom in quasicrystals. Depending on the range of interactions and hopping amplitudes, we unveil a rich phase diagram including antiferromagnetic orderings, charge density waves, and subleading, superconducting pairing tendencies. For certain parameter regimes, we find a competition of phases, which is also common in crystals, but additionally encounter phases coexisting in a spatially separated fashion and ordering tendencies which mutually collaborate to enhance their strength. We therefore establish that quasicrystalline structures open up a route towards this rich ordering behavior uncommon to crystals and that an unbiased, beyond-mean-field approach is essential to describe this physics of quasicrystals correctly.6 MoreReceived 31 August 2020Revised 17 December 2020Accepted 12 May 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.023180Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAntiferroelectricityCharge density wavesEdge statesElectrical propertiesMagnetic orderPhase transitionsQuasicrystalline structurePhysical Systems2-dimensional systemsQuasicrystalsTechniquesHubbard modelMean field theoryRenormalization groupSecond quantizationSymmetries in condensed matterTight-binding modelCondensed Matter, Materials & Applied Physics

Microscopic origin of Cooper pairing in the iron-based superconductor Ba1\u2212xKxFe2As2

Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue towards this aim by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor Ba1−xKxFe2As2 for 0.22 ≤ x ≤ 0.70 in all symmetry channels, Raman spectroscopy allows us to distill the leading s-wave state. In addition, the spectra collected in the B1g symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet sub-dominant, pairing tendencies of d_{x^2 - y^2} symmetry type. A comprehensive functional Renormalization Group and random-phase approximation study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The consistency between the experimental observations and the theoretical modeling suggests that spin fluctuations play a significant role in superconducting pairing.

Crystallography, Morphology, and Martensite Transformation of Prior Austenite in Intercritically Annealed High-Aluminum Steel

The crystallography and morphology of the intercritical austenite phase in two high-aluminum steels annealed at 850 °C were examined on the basis of electron backscattered diffraction analysis, in concert with a novel orientation relationship determination and prior austenite reconstruction algorithm. The formed intercritical austenite predominantly shared a Kurdjumov–Sachs-type semicoherent boundary with at least one of the neighboring intercritical ferrite grains. If the austenite had nucleated at high-energy sites (such as a grain corner or edge), no orientation relationship was usually observed. The growth rate of the austenite grains was observed to be slow, causing phase inequilibrium even after extended annealing times. The small austenite grain size and phase fraction were consequently shown to affect martensite start temperature. Both steels had distinct variant pairing tendencies under the intercritically annealed condition.

Diffusivity of Mono- and Divalent Salts and Water in Polyelectrolyte Desalination Membranes.

The dynamics of ions and solvent molecules in polyelectrolyte desalination membranes is key to water purification technologies in which selective transport of the different components is desired. Recent experimental and our computational results have shown that nontrivial mechanisms underlie the transport properties of salt ions and water in charged polymer membranes. Explicitly, in polymer electrolytes, we found a reversal in the salt concentration dependence of the mobilities of Na+, Cl- salt ions and water molecules when compared with aqueous solutions. Motivated by such results, in this study, we have used atomistic molecular dynamics simulations to probe whether the mechanisms deduced in our earlier work apply to other salt systems and to mixtures of salts. Specifically, we report results for the ion diffusivities in aqueous KCl, MgCl2, and a 1:1 mixture of NaCl and MgCl2 salt solutions at different concentrations (ranging from 0.06 to 1 M) and investigate, at the molecular level, the mechanisms underlying the behaviors of salt and water transport properties. Our results show that diffusion of salt ions and water in charged polymer membranes are in general influenced by their association with polymer charge groups and ion pairing effects. Divalent ions are more strongly coupled with the polymeric ionic groups than monovalent salt ions and exhibit diffusivity trends that are distinct relative to monovalent salts. Further, we demonstrate that the mobilities of water molecules are influenced by coordination of water with polymer charge groups and their ion pairing tendencies and also exhibit distinct trends in monovalent and divalent salt solutions.

Signatures of pairing in the magnetic excitation spectrum of strongly correlated two-leg ladders

Magnetic interactions are widely believed to play a crucial role in the microscopic mechanism leading to high critical temperature superconductivity. It is therefore important to study the signatures of pairing in the magnetic excitation spectrum of simple models known to show unconventional superconducting tendencies. Using the Density Matrix Renormalization Group technique, we calculate the dynamical spin structure factor $S({\bf k},\omega)$ of a generalized $t-U-J$ Hubbard model away from half-filling in a two-leg ladder geometry. The addition of $J$ enhances pairing tendencies. We analyze quantitatively the signatures of pairing in the magnetic excitation spectra. We found that the superconducting pair-correlation strength, that can be estimated independently from ground state properties, is closely correlated with the integrated low-energy magnetic spectral weight in the vicinity of $(\pi,\pi)$. In this wave-vector region, robust spin incommensurate features develop with increasing doping. The branch of the spectrum with rung direction wave-vector $k_{rung}=0$ does not change substantially with doping where pairing dominates, and thus plays a minor role. We discuss the implications of our results for neutron scattering experiments, where the spin excitation dynamics of hole-doped quasi-one dimensional magnetic materials can be measured, and also address implications for recent resonant inelastic X-ray scattering experiments.

Magnetic properties and pairing tendencies of the iron-based superconducting ladderBaFe2S3: Combinedab initioand density matrix renormalization group study

The recent discovery of superconductivity under high pressure in the two-leg ladder compound BaFe$_2$S$_3$ [H. Takahashi et al., Nature Materials 14, 1008 (2015)] opens a broad avenue of research, because it represents the first report of pairing tendencies in a quasi one-dimensional iron-based high critical temperature superconductor. Similarly as in the case of the cuprates, ladders and chains can be far more accurately studied using many-body techniques and model Hamiltonians than their layered counterparts, particularly if several orbitals are active. In this publication, we derive a two-orbital Hubbard model from first principles that describes individual ladders of BaFe$_2$S$_3$. The model is studied with the density matrix renormalization group. These first reported results are exciting for two reasons: (i) at half-filling, ferromagnetic order emerges as the dominant magnetic pattern along the rungs of the ladder, and antiferromagnetic order along the legs, in excellent agreement with neutron experiments; (ii) pairs form in the strong coupling regime, as found by studying the binding energy of two holes doped on the half-filled system. In addition, Orbital Selective Mott Phase characteristics develop with doping, with only one Wannier orbital receiving the hole carriers while the other remains half-filled. These results suggest that the analysis of models for iron-based two-leg ladders could clarify the origin of pairing tendencies and other exotic properties of iron-based high critical temperature superconductors.

Kinetic Energy Driven Stripe Formation and Pairing in Repulsive Electronic Systems

We study repulsive Hubbard and t − J type systems on a square lattice (long believed to capture certain quintessential aspects of the high temperature superconductors). These models (alongside the parent compounds of the high temperature superconductors) are antiferromagnetic in the absence of hole doping. As we illustrate, a unifying underlying principle for the dynamics of holes introduced by doping rationalizes the emergence of nonuniform electronic structures– “stripes” and possible pairing tendencies therein. Specifically, our analysis invokes the following (numerically *Corresponding author: E-mail: zohar@wuphys.wustl.edu; Bosch and Nussinov; PSIJ, 10(3), 1-33, 2016; Article no.PSIJ.24117 verified) sublattice parity principle: A strong antiferromagnetic background forces injected holes to hop in steps of two such that they always remain on the same sublattice. When applied to a domain wall in an antiferromagnet, this simple principle naturally gives rise to (bond centered) stripes. We demonstrate that the holes are self-consistently localized on stripes. Extending this picture, we then show that the holes on a stripe favor the formation of pairs on neighboring rungs or sites. Throughout this work much emphasis is placed on the problem of a two leg ladder immersed in a staggered magnetic field. Although we will focus on the square lattice, our considerations may be extended to similar electronic structures appearing in other models on bipartite lattices when these exhibit antiferromagnetic correlations with an underlying sublattice structure.

New Tailored Sodium Salts for Battery Applications

This article describes synthesis and basic electrochemical and structural properties of newly designed sodium salts for application in liquid nonaqueous sodium electrolytes. There has been two imidazole fluorine derivative sodium salts synthesized: sodium 4,5-dicyano-2-(trifluoromethyl)imidazolate (NaTDI) and sodium 4,5-dicyano-2-(pentafluoroethyl)imidazolate (NaPDI). The structure of the salts has been confirmed by means of Raman spectroscopy, nuclear magnetic resonance (13C NMR and 19F NMR), X-ray diffraction, thermogravimetry (TGA), and differential scanning calorimetry (DSC). Electrochemical characterization included ionic conductivity measurements, dynamic viscosity, and electrochemical stability of solutions of the salts in propylene carbonate (PC) at different temperatures. Raman spectra of the electrolytes have been performed to carefully monitor the degree of ionic associations specially ion pairing tendencies.

Strong contributions from vertical triads to helix-partner preferences in parallel coiled coils.

Pairing preferences in heterodimeric coiled coils are determined by complementarities among side chains that pack against one another at the helix-helix interface. However, relationships between dimer stability and interfacial residue identity are not fully understood. In the context of the "knobs-into-holes" (KIH) packing pattern, one can identify two classes of interactions between side chains from different helices: "lateral", in which a line connecting the adjacent side chains is perpendicular to the helix axes, and "vertical", in which the connecting line is parallel to the helix axes. We have previously analyzed vertical interactions in antiparallel coiled coils and found that one type of triad constellation (a'-a-a') exerts a strong effect on pairing preferences, while the other type of triad (d'-d-d') has relatively little impact on pairing tendencies. Here, we ask whether vertical interactions (d'-a-d') influence pairing in parallel coiled-coil dimers. Our results indicate that vertical interactions can exert a substantial impact on pairing specificity, and that the influence of the d'-a-d' triad depends on the lateral a' contact within the local KIH motif. Structure-informed bioinformatic analyses of protein sequences reveal trends consistent with the thermodynamic data derived from our experimental model system in suggesting that heterotriads involving Leu and Ile are preferred over homotriads involving Leu and Ile.

Properties of the multiorbital Hubbard models for the iron-based superconductors

A brief review of the main properties of multiorbital Hubbard models for the Fe-based superconductors is presented. The emphasis is on the results obtained by our group at the University of Tennessee and Oak Ridge National Laboratory, Tennessee, USA, but results by several other groups are also discussed. The models studied here have two, three, and five orbitals, and they are analyzed using a variety of computational and mean-field approximations. A “physical region” where the properties of the models are in qualitative agreement with neutron scattering, photoemission, and transport results is revealed. A variety of interesting open questions are briefly discussed such as: what are the dominant pairing tendencies in Hubbard models? Can pairing occur in an interorbital channel? Are nesting effects of fundamental relevance in the pnictides or approaches based on local moments are more important? What kind of magnetic states are found in the presence of iron vacancies? Can charge stripes exist in iron-based superconductors? Why is transport in the pnictides anisotropic? The discussion of results includes the description of these and other open problems in this fascinating area of research.

Neutron and ARPES constraints on the couplings of the multiorbital Hubbard model for the iron pnictides

The results of neutron scattering and angle-resolved photoemission experiments for the Fe-pnictide parent compounds, and their metallic nature, are shown to impose severe constraints on the range of values that can be considered "realistic" for the intraorbital Hubbard repulsion U and Hund coupling J in multiorbital Hubbard models treated in the mean-field approximation. Phase diagrams for three- and five-orbital models are here provided, and the physically realistic regime of couplings is highlighted, to guide future theoretical work into the proper region of parameters of Hubbard models. In addition, using the random phase approximation, the pairing tendencies in these realistic coupling regions are investigated. It is shown that the dominant spin-singlet pairing channels in these coupling regimes correspond to nodal superconductivity, with strong competition between several states that belong to different irreducible representations. This is compatible with experimental bulk measurements that have reported the existence of nodes in several Fe-pnictide compounds.

Constraints imposed by symmetry on pairing operators for the iron pnictides

Considering model Hamiltonians that respect the symmetry properties of the pnictides, it is argued that pairing interactions that couple electrons at different orbitals with an orbital-dependent pairing strength inevitably lead to interband pairing matrix elements, at least in some regions of the Brillouin zone. Such interband pairing has not been considered of relevance in multiorbital systems in previous investigations. It is also observed that if, instead, a purely intraband pairing interaction is postulated, this requires that the pairing operator has the form $\Delta^+(k)=f(k) \sum_{\alpha} d^+_{k,\alpha,\uparrow}d^+_{-k, \alpha,\downarrow}$ where $\alpha$ labels the orbitals considered in the model and f(k) arises from the spatial location of the coupled electrons or holes. This means that the gaps at two different Fermi surfaces involving momenta $k_F$ and $k'_F$ can only differ by the ratio $f(k_F)/ f(k'_F)$ and that electrons in different orbitals must be subject to the same pairing attraction, thus requiring fine tuning. These results suggest that previously neglected interband pairing tendencies could actually be of relevance in a microscopic description of the pairing mechanism in the pnictides.

Coexistence of pairing tendencies and ferromagnetism in a doped two-orbital Hubbard model on two-leg ladders

Using the Density Matrix Renormalization Group and two-leg ladders, we investigate an electronic two-orbital Hubbard model including plaquette diagonal hopping amplitudes. Our goal is to search for regimes where charges added to the undoped state form pairs, presumably a precursor of a superconducting state.For the electronic density $\rho=2$, i.e. the undoped limit, our investigations show a robust $(\pi,0)$ antiferromagnetic ground state, as in previous investigations. Doping away from $\rho=2$ and for large values of the Hund coupling $J$, a ferromagnetic region is found to be stable. Moreover, when the interorbital on-site Hubbard repulsion is smaller than the Hund coupling, i.e. for $U'<J$ in the standard notation of multiorbital Hubbard models, our results indicate the coexistence of pairing tendencies and ferromagnetism close to $\rho=2$. These results are compatible with previous investigations using one dimensional systems. Although further research is needed to clarify if the range of couplings used here is of relevance for real materials, such as superconducting heavy fermions or pnictides, our theoretical results address a possible mechanism for pairing that may be active in the presence of short-range ferromagnetic fluctuations.

Magnetically driven pair potential in CuO 2 sheets

Within a Bethe lattice approach we obtain the first-neighbor, temperature dependent, effective pair potential for the hole-induced frustration Ising model, in the short-range potential approximation. Frustration- and fluctuation-driven pairing tendencies occur at distinct ranges of the temperature scale for low hole concentrations. However, as concentration increases these two mechanisms overlap.

Quadrupolar-fluctuation model for high-Tc superconductivity: A quantum Monte Carlo study.

We present results for quantum Monte Carlo simulations of the quadrupolar fluctuation model of high-temperature superconductivity. The simulations focused on measurement of the pairing susceptibility in various interesting pairing channels (on-site, nearest-neighbor, and second-nearest-neighbor {ital s}- and {ital d}-wave pairing). The lattice was 4{times}4 in the oxygen sites, with eight copper sites bearing quadrupole moments. Exchange of these quadrupolar fluctuations may lead to attraction between oxygen holes. As in weak-coupling theory, we find (i) favorable tendencies towards second-nearest-neighbor {ital s}- and {ital d}-wave filling, with larger doping favoring {ital d}-wave symmetry; (ii) a nonmonotonic dependence of the pairing tendencies upon the on-site quadrupole level splitting; and (iii) a strong suppression of all nearest-neighbor pairing.

A study of Paridae community in Japan

This paper discusses the problems of ecological distribution in avian mixed population of closely related sympatric species as revealed in the Paridae community in Japan. Structure and distribution of home ranges of five species of tits (Aegithalidae and Paridae) are compared by analysing intra-and inter-specific relations in the ecological separation of social structure between species.1. The vegetation of artificial mixed forests of study area in the mountain region about 1000m of altitude in central Japan, consisted of deciduous woods of Quercus serrata, Castanea crenata, Betula platyphylla and Alnus Japonica and coniferous plantations of Pinus densiflora and Larix leptolepis.2. In these mixed forests, five tit species Aegithalos caudatus, Parus major, P. ater P. montanus and P. varius are sympatric and Ae. caudatus, P. major and P. ater maintain annually stable high population densities, P. montanus has low density and P. varius is represented by only one pair.3. Social systems of Aegithalos and Parus are essentially different. That of Aegithalos is characterized by flock unit which is maintained by flock territoriality, consisting of 'flock range' and 'flock members'. During winter, the territoriality of flock unit is strengthened by 'group displays' and in the breeding season the means of maintenance of territoriality are shifted from 'flock unit'to 'pair unit'. In Parus, on the other hand, the base of social system is the 'pair unit', which is emphasized by singing activities and fighting behaviour and in winter the territoriality is weakened, with the result that the home range of pair unit is extended and the tendency of flocking together with adjacent pair unit is raised.4. The pair unit in Parus is maintained through the winter, especially in milder weather. The tendency of pair unit is strongest in P. montanus, while P. major and P. ater show the tendency for flocking. However, although there are gradations of types in the species of Parus between flocking and pairing tendencies which are also subject to population densities, there esists no intermediate type between flock unit system of Aegithalos and that of pair unit in Parus. This is the essential difference of social systems of these two genera.5. The patterns of ecological separation (or segregation) in the interspecific relations are different by winter and breeding seasons. In winter, the separation occurs in vertical layers of the forest and in breeding season it is horizontally mosaic, with distinct interspecific overlaps of home ranges, but main feeding areas are segregated. The pair unit supported by territoriality has important function in dividing same resources intraspecifically, but interspecifically the pair units are overlapped.6. No case of interspecific territoriality is known among Parus species, although interspecific territorial overlaps are found in P. montanus-varius group and P. major-ater group along the marginal zones of habitat of each species. Wide overlaps of distribution among tit species are, in my opinion, the effect of mutual avoidance within a mixed flock, not being the process of the development of interspecific territoriality.7. Parus species make mixed groups (or flocks) attracted by and attached to Aegithalos flock which is 'flock-unit' society, but Parus species society is basically 'pair-unit', although it makes flock by grouping of pair-units.