Ground-State Properties of the Frustrated 2D Quarter-Filled Hubbard Model

The Hubbard model extended by next-nearest-neighbor (NNN) hopping t′ at quarter filling has been investigated numerically using exact diagonalization technique on a small cluster. We report the role of frustration (NNN hopping) on the ground-state properties of the system. The effects of both positive and negative t′ are considered. Results show that a short range antiferromagnetic order is present in the system, which is favored (suppressed) by negative (positive) t′. It appears that electron-electron correlation decreases with NNN hopping interaction. Inclusion of t′ facilitates electron hopping. Though pairing in the extended s-wave channel is dominant for quarter-filled unfrustrated system, d-wave pairing becomes dominant with the inclusion of negative t′.     

Superconductivity in a Two-Component Model with Local Electron Pairs

Superconductivity in the two-component model of coexisting local electron pairs (hard-core charged bosons) and itinerant fermions coupled via charge exchange mechanism is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s-wave and symmetries are analyzed for a 2D square lattice within the BCS-mean-field approximation (MFA) and the Kosterlitz–Thouless theory. The phase diagrams and superconducting characteristics of this induced pairing model as a function of the position of the local pair (LP) level and the total carrier concentration are reviewed. The model exhibits crossovers between the BCS-like behavior and that of LPs. In addition, the Uemura plots are obtained for extended s and pairing symmetries. Finally, we analyze the pairing fluctuation effects (in 3D) within a generalized T-matrix approach. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for high-temperature superconductors.     

D-Wave Superconductivity and Coulomb Correlations in the Two-Dimensional Hubbard Lattice

We considered anisotropic superconductivity within the two-dimensional Hubbard model extended by pairing correlations originating from the electron–phonon interaction. To discuss the onset of superconductivity close to the insulator–metal transition, we used the Hubbard I approximation to account for the formation of the insulating gap and see the role of Coulomb correlations for superconducting pairing. It has been shown that the Hubbard I approximation reflects effective pairing interactions genuine for correlated electron systems and leads to the stabilization of the superconductivity in the d-wave channel. One may expect the cooperation of phonon-free and phonon-induced mechanism in the formation of thed-wave superconducting state.     

Electron-electron and electron-hole pairing in graphene structures

The superconducting pairing of electrons in doped graphene owing to in-plane and out-of-plane phonons is considered. It is shown that the structure of the order parameter in the valley space substantially affects conditions of the pairing. Electron-hole pairing in a graphene bilayer in the strong coupling regime is also considered. Taking into account retardation of the screened Coulomb pairing potential shows a significant competition between the electron-hole direct attraction and their repulsion owing to virtual plasmons and single-particle excitations.     

协作分集OFDM系统的混合排序子载波配对算法

考虑到信源子载波和中继子载波的配对能在频率选择信道环境下提高协作分集系统的性能,对协作分集正交频分复用(OFDM)系统的子载波配对进行了研究,推导了2种特殊情况下的最佳子载波配对方法,即顺序配对和逆序配对,在此基础上,提出了一种基于混合排序的子载波配对算法.该算法根据信源-中继链路、信源-目标链路和中继-目标链路上的每个子载波的信噪比状态,自适应地选择顺序配对或逆序配对.仿真结果表明,提出的混合排序子载波配对算法能显著提高系统的平均可取得速率,性能明显优于顺序配对和逆序配对.     

Phase Diagram for Mixed-Parity Superconductors

We analyze the response of a two-dimensional superconductor to a magnetic field. We assume that the system can be described by an extended Hubbard model with an attractive nearest-neighbor pairing potential effective both in the singlet and the triplet channels. Treating the model Hamiltonian within the mean-field approximation, we study the competition at zero temperature between superconducting states characterized by different spin and spatial symmetries, as induced by electron density variations and by the explicit time-reversal symmetry breaking due to the presence of a magnetic field. We focus in particular on the competition between unitary and nonunitary states.     

An X‐FEM approach for large sliding contact along discontinuities

The extended finite element method (X‐FEM) has been developed to minimize requirements on the mesh design in a problem with a displacement discontinuity. This advantage, however, still remains limited to the small deformation hypothesis when considering sliding discontinuities. The approach presented in this paper proposes to couple X‐FEM with a Lagrangian large sliding frictionless contact algorithm. A new hybrid X‐FEM contact element was developed with a contact search algorithm allowing for an update of contacting surfaces pairing. The stability of the contact formulation is ensured by an algorithm for fulfilling Ladyzhenskaya‐Babuska‐Brezzi (LBB) condition. Several 2D simple examples are presented in this paper in order to prove its efficiency and stability. Copyright © 2008 John Wiley & Sons, Ltd.     

Modulations of the Local Pairing Interaction Near Magnetic Impurities in d-Wave Superconductors

The spin-fluctuation based pairing mechanism has proven successful in explaining the pairing symmetries due to Fermi surface nesting of both cuprates and iron-based materials. In this work, we study signatures of a spin-fluctuation mediated pairing at the local scale. Specifically, we focus on magnetic impurities and calculate both the local antiferromagnetism and the resulting modulated pairing interaction. The latter gives rise to distinct local enhancements of the superconducting gap in the immediate vicinity of the impurities. Our results show that Coulomb-driven pairing naturally leads to unusual superconducting gap modulations near disorder potentials.     

Fermi-surface and charge-spin excitations in strongly correlated electron systems

Fermi-surface and low-energy excitations in a variety of strongly correlated electron systems are studied by using the numerically exact diagonalization method in two-dimensional square lattice. In the one-band Hubbard and extended Hubbard models, the electronic structure in both insulating and metallic states are examined. The structure in the periodic Anderson model is also studied and compared with that in the above models. In the tJ model as well as the negative U Hubbard model, the excitation spectra in the superconducting states are calculated. In particular, in the tJ model the superconductivity with d(x² – y²)-wave pairing is found to be mapped onto the BCS state. It is shown that the systematic study in a variety of models provides an opportunity to construct a unified picture of strongly correlated systems.     

A three-step finite element method for unsteady incompressible flows

This paper describes a three-step finite element method and its applications to unsteady incompressible fluid flows. The stability analysis of the one-dimensional purely convection equation shows that this method has third-order accuracy and an extended numerical stability domain in comparison with the Lax-Wendroff finite element method. The method is cost effective for incompressible flows, because it permits less frequent updates of the pressure field with good accuracy. In contrast with the Taylor-Galerkin method, the present three-step finite element method does not contain any new higher-order derivatives, and is suitable for solving non-linear multi-dimensional problems and flows with complicated outlet boundary conditions. The three-step finite element method has been used to simulate unsteady incompressible flows, such as the vortex pairing in mixing layer. The properties of the flow fields are displayed by the marker and cell technique. The obtained numerical results are in good agreement with the literature.     

Solitonic lattice and Yukawa forces in the rare-earth orthoferrite TbFeO3

The random fluctuations of spins give rise to many interesting physical phenomena, such as the 'order-from-disorder' arising in frustrated magnets and unconventional Cooper pairing in magnetic superconductors. Here we show that the exchange of spin waves between extended topological defects, such as domain walls, can result in novel magnetic states. We report the discovery of an unusual incommensurate phase in the orthoferrite TbFeO(3) using neutron diffraction under an applied magnetic field. The magnetic modulation has a very long period of 340?? at 3?K and exhibits an anomalously large number of higher-order harmonics. These domain walls are formed by Ising-like Tb spins. They interact by exchanging magnons propagating through the Fe magnetic sublattice. The resulting force between the domain walls has a rather long range that determines the period of the incommensurate state and is analogous to the pion-mediated Yukawa interaction between protons and neutrons in nuclei.     

The indirect attraction between holes in the extended hubbard model

The extended Hubbard model [V. J. Emery,Phys. Rev. Lett. 58, 2794 (1987)] is used to study the copper-oxide plane of the high-T _c superconducting material. Due to the Coulomb repulsion between the holes on oxygen and copper atoms, there exists an indirect attraction between oxygen holes. Correlation among holes will enhance rather than screen the attraction. Calculation based on the Hartree-Fock and random-phase approximations show that it is quite large. A comparison with direct Coulomb repulsion shows that it is greater in a certain range of momentum transfer. It is sufficient to provide the pairing mechanism to give high critical temperature.     

Extended thermodynamic approach to ion interaction chromatography

The chromatographic behavior of charged analytes in ion interaction chromatography (IIC) is theoretically investigated. The chemical modifications of the stationary and mobile phases in the presence of ion interaction reagent (IIR) are theoretically shown to change the partition coefficient for charged molecules. The most reliable literature experimental results concerning retention behavior of charged molecules in IIC were used to test the new theory. Retention equations are compared with those that can be obtained from the most important retention models in IIC. The present exhaustive retention model, which is well-founded in physical chemistry, goes further than the previous ones whose retention equations can be viewed as limiting cases of the present theory. The present extended thermodynamic approach reduces to stoichiometric or electrostatic retention models if the surface potential or pairing equilibria are respectively neglected. Moreover, it is able to quantitatively explain experimental evidences that cannot be rationalized by the existing retention models.     

Polynomial generating pairing and its criterion for optimal pairing

We define a polynomial generating pairing (PGP) and propose a method to construct a family of pairing friendly curves from PGP. We show that a bilinear map over the family is directly determined by the coefficients of the PGP and the map is non-degenerate under a minor condition which is satisfied with cryptographic parameters. Finally, we provide a criterion for PGP to obtain an optimal pairing.     

Effects of disorder on the electron pairing

The electron pairing in randomly disordered lattices is studied by using an attractive Hubbard model, and by mapping the many-body problem onto a tight-binding one in a higher dimensional space, where a diagonal disorder is considered within the coherent-potential approximation. The results show an enhancement of the pair-binding energy as the self-energy difference increases in a binary alloy A_xB_1–x. This fact suggests that the pairing process is highly sensitive to the one-particle localization condition. A ground-state phase diagram for on-site interaction disorder shows regions where pairing is avoid for ordered diatomic systems but not for disordered case.     

Superconductivity-type fluctuations in the electronic spectra of charge-transfer organic solids

The interaction between an excited electron and a large-radius exciton (Wannier-Mott exciton) has been studied in the visible and ultra-violet frequency regions for a two-level system of a molecular solid. Using a self-consistent approach, it is shown that at low temperatures and when certain conditions prevail, a bound-state may exist arising from the electron-exciton pairing. The excitation spectrum is found to be of the superconductivity type and the electron- exciton quasiparticle migrates through the crystal with definite energy and wave vector. The gap function due to the electron-exciton pairing is calculated at zero temperature and then the expression for the transition temperature is established. A formula for the ground-state energy is derived corresponding to the electron-exciton pairing and a discussion of the parameters that appear in the theory is presented.     

Magnetic Properties and Strong-Coupling Corrections in an Ultracold Fermi Gas with Population Imbalance

We investigate magnetic properties of an ultracold Fermi gas with population imbalance. In the presence of population imbalance, the strong-coupling theory developed by Nozières and Schmitt-Rink (which is frequently referred to as the NSR theory, or Gaussian fluctuation theory) is known to give unphysical results in the BCS-BEC crossover region. We point out that this problem comes from how to treat pseudogap effects originating from pairing fluctuations and many-body corrections to the spin susceptibility. We also clarify how to overcome this problem by including higher order fluctuations beyond the ordinary T-matrix approximation. Calculated spin susceptibility based on our extended T-matrix approximation agrees well with the recent experiment on a ⁶Li Fermi gas.     

Pairing and Short-Range Correlations in Nuclear Systems

The structure and density dependence of the pairing gap in infinite matter is relevant for astrophysical phenomena and provides a starting point for the discussion of pairing properties in nuclear structure. Short-range correlations can significantly deplete the available single-particle strength around the Fermi surface and thus provide a reduction mechanism of the pairing gap. Here, we study this effect in the singlet and triplet channels of both neutron matter and symmetric nuclear matter. Our calculations use phase-shift equivalent interactions and chiral two-body and three-body interactions as a starting point. We find an unambiguous reduction of the gap in all channels with very small dependence on the NN force in the singlet neutron matter and the triplet nuclear matter channel. In the latter channel, SRC alone provide a 50% reduction of the pairing gap.     

Interband Model Scenario of Cuprate Superconductivity

A multiband approach for hole- and electron-doped cuprate superconductors is developed. The model electron spectrum includes nodal and antinodal defect (polaronic) subbands created by doping. Bare gaps between itinerant and defect subbands close with extended doping. The overlap conditions determine phase diagram special points. There are analogies for both types of doping despite the activation of different sublattices. Illustrative calculations have given self-consistent results reproducing qualitatively doping dependences of T _c , superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, etc. Interband pairing scenario is suggested to be an essential aspect of cuprate superconductivity. This work was supported by the Estonian Science Foundation grant No. 7296.