Pair Hopping Interaction Research Articles

Analysis of collective excitation for multi band superconductor: Frustrated spin model approach

We study properties of multi-band superconductors, focusing on their ground states and collective excitations. Multi-band tight-binding Hamiltonian with repulsive pair-hopping interaction is mapped into a frustrated spin-Hamiltonian. This frustration leads to exotic features in a multi-band superconductor.Applying the mean-field theory to the three- and four-band systems, we systematically examine the superconducting-phase structure between multiple order parameters, and the associated collective modes.

Theory of Charge Kondo Effect on Pair Hopping Mechanism

A new microscopic model of the charge Kondo effect is proposed and is studied on the basis of numerical renormalization group calculations. It is shown that the charge Kondo effect is caused by the pair hopping interaction between the conduction band and the localized band. It is found that the formation of a charge Kondo–Yosida singlet due to its mechanism corresponds to the appearance of a valence skipping phenomenon, and that the charge Kondo–Yosida singlet and the spin Kondo–Yosida singlet state coexist for a tuned parameter set, because the charge and spin degrees of freedom are separated in this model. It is also found that the Sommerfeld coefficient is enhanced by the competition between these two singlet states.

Effect of singlet pairing hopping on the one-dimensional extended Hubbard model with spin exchange interaction

The ground-state properties of the one-dimensional (1D) t-U-J model with singlet pair hopping interaction are studied in the weak-coupling regime. The analysis of the model with U(1) ⊗ SU(2) symmetry is performed using the bosonization and renormalization-group approaches. At half filling the phase diagram is shown to consist of three different insulating states corresponding to the Peierls dimerized bond-charge-density-wave (BCDW) phase, the spin-density-wave (SDW) phase and the bond-spin-density-wave (BSDW) phase, and two different superconducting states with the singlet-superconducting (SS) phase and the triplet-superconducting (TS) phase. Moreover, the SDW, BSDW and TS phases are all degenerated. The result indicates that the pair hopping interaction P has a strong effect on the ground-state properties of 1D correlated electron systems.

Some Properties of the Model of a Superconductor with Pair Hopping and Magnetic Interactions at Half-Filling

This is an author-created, un-copyedited version of an article accepted for publication in Acta Physica Polonica A. The Version of Record is available online at http://przyrbwn.icm.edu.pl/APP/PDF/121/a121z4p104.pdf

The Fulde–Ferrell–Larkin–Ovchinnikov phase in the presence of pair hoppinginteraction

The recent experimental support for the presence of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO)phase in CeCoIn5 directed attention towards the mechanisms responsible for this type of superconductivity.We investigate the FFLO state in a model where on-site/inter-site pairingcoexists with the repulsive pair hopping interaction. The latter interactionis interesting in that it leads to pairing with non-zero momentum of theCooper pairs even in the absence of the external magnetic field (the so-calledη pairing). It turns out that, depending on the strength of the pair hopping interaction, themagnetic field can induce one of two types of the FFLO phase with different spatialmodulations of the order parameter. It is argued that the properties of the FFLO phase maygive information about the magnitude of the pair hopping interaction. We also show thatη pairing and d-wave superconductivity may coexist in the FFLO state. It holds true also forsuperconductors which, in the absence of magnetic field, are of pure d-wave type.

Orbital coupling and superconductivity in the iron pnictides

We demonstrate that strong inter-orbital interaction is very efficient to achieve superconductivity due to magnetic fluctuations in the iron pnictides. Fermi surface states that are coupled by the antiferromagnetic wave vector are often of different orbital nature, causing pair-hopping interactions between distinct Fe-3d orbitals to become important. Performing a self-consistent FLEX calculation below $T_{c}$ we determine the superconducting order parameter as function of intra- and inter-orbital couplings. We find an $s^{\pm}$-pairing state with $T_{c}\simeq80\mathrm{K}$ for realistic parameters.

Multiband effects on fermionic atoms in optical lattices

The superfluid-insulator transition of two-band fermionic atom systems in optical lattices is investigated by the two-site dynamical mean-field theory. Because of the spin-flip and pair-hopping interactions, orbital fluctuations are enhanced, leading to the suppression of the Mott insulating state. The numerical results are discussed in comparison with the effective boson model.

Numerical algorithm for the double-orbital Hubbard model: Hund-coupled pairing symmetry in the doped case

In order to numerically study electron correlation effects in multiorbital systems, we propose a new type of discrete transformation for the exchange (Hund's coupling) and pair-hopping interactions to be used in the dynamical mean field $\text{theory}+\text{quantum}$ Monte Carlo method. The transformation, which is real and exact, turns out to suppress the sign problem in a wide parameter region including non-half-filled bands. This enables us to obtain the dominant pairing symmetry in the double-orbital Hubbard model, which shows that the spin-triplet, orbital-antisymmetric pairing that exploits Hund's coupling is stable in a wide region of the band filling.

A model of strongly correlated electrons with condensed resonating-valence-bond ground states

We propose a new exactly solvable model of strongly correlated electrons. The model is based on a $d$-$p$ model of the CuO$_2$ plane with infinitely large repulsive interactions on Cu-sites, and it contains additional correlated-hopping, pair-hopping and charge-charge interactions of electrons. For even numbers of electrons less than or equal to 2/3-filling, we construct the exact ground states of the model, all of which have the same energy and each of which is the unique ground state for a fixed electron number. It is shown that these ground states are the resonating-valence-bond states which are also regarded as condensed states in which all electrons are in a single two-electron state. We also show that the ground states exhibit off-diagonal long-range order.

Superconductor to spin-density-wave transition in quasi-one-dimensional metals with Ising anisotropy

We study a mechanism for superconductivity in quasi-one-dimensional materials with Ising anisotropy. In an isolated chain Ising anisotropy opens a spin gap; if inter-chain coupling is sufficiently weak, single particle hopping is suppressed and the physics of coupled chains is controlled by a competition between pair hopping and exchange interaction. Spin density wave and triplet superconductivity phases are found separated by a first order phase transition. For particular parameter values a second order transition described by SO(4) symmetry is found.

Phase diagram of the extended hubbard model with pair hopping interaction

A one-dimensional model of interacting electrons with on-site $U$, nearest-neighbor $V$, and pair-hopping interaction $W$ is studied at half-filling using the continuum limit field theory approach. The ground state phase diagram is obtained for a wide range of coupling constants. In addition to the insulating spin- and charge-density wave phases for large $U$ and $V$, respectively, we identify bond-located ordered phases corresponding to an enhanced Peierls instability in the system for $W<0$, $|U-2V|<|2W|$, and to a staggered magnetization located on bonds between sites for $W>0$, $|U-2V|<W$. The general ground state phase diagram including insulating, metallic, and superconducting phases is discussed. A transition to the $\eta_{\pi}$-superconducting phase at $|U-2V| \ll 2t \leq W$ is briefly discussed.

Plaquette boson-fermion model of cuprates

The strongly interacting Hubbard model on the square lattice is reduced to the low energy Plaquette Boson Fermion Model (PBFM). The four bosons (an antiferromagnon triplet and a d-wave hole pair), and the fermions are defined by the lowest plaquette eigenstates. We apply the Contractor Renormalization method of Morningstar and Weinstein to compute the boson effective interactions. The range-3 truncation error is found to be very small, signaling short hole-pair and magnon coherence lengths. The pair-hopping and magnon interactions are comparable, which explains the rapid destruction of antiferromagnetic order with emergence of superconductivity, and validates a key assumption of the projected SO(5) theory. A vacuum crossing at larger doping marks a transition into the overdoped regime. With hole fermions occupying small Fermi pockets and Andreev coupled to hole pair bosons, the PBFM yields several testable predictions for photoemmission, tunneling asymmetry and entropy measurements.

η-pairing superconductivity in the Hubbard chain with pair hopping

The ground-state phase diagram of the one-dimensional Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper-pair center-of-mass momentum $Q=0 (\mathrm{BCS}\ensuremath{-}{\ensuremath{\eta}}_{0}$ phase) and $Q=\ensuremath{\pi} ({\ensuremath{\eta}}_{\ensuremath{\pi}}$ phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator, and an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting $\mathrm{BCS}\ensuremath{-}{\ensuremath{\eta}}_{0}$ and ${\ensuremath{\eta}}_{\ensuremath{\pi}}$ phases and the metallic Luttinger-liquid phase. The $\mathrm{BCS}\ensuremath{-}{\ensuremath{\eta}}_{0}$ phase exhibits a smooth crossover from a weak-coupling BCS type to a strong-coupling local-pair regime. The ${\ensuremath{\eta}}_{\ensuremath{\pi}}$ phase shows the properties of the doublon (zero-size Cooper-pair) superconductor with Cooper-pair center-of-mass momentum $Q=\ensuremath{\pi}.$ The transition into the ${\ensuremath{\eta}}_{\ensuremath{\pi}}$-paired state corresponds to an abrupt change in the ground-state structure. After the transition the conduction band is completely destroyed and a new ${\ensuremath{\eta}}_{\ensuremath{\pi}}$-pair band, corresponding to the strongly correlated doublon motion, is created.

Superconductivity of the Two-Dimensional Penson-Kolb Model

Two-dimensional (d = 2) Penson-Kolb model, i.e. the tight-binding model with the pair-hopping (intersite charge exchange) interaction, is considered and the effects of phase fluctuations on the s-wave superconductivity ofthis system are discussed within the Kosterlitz-Thouless scenario. The London penetration depth A at T = 0, the Kosterlitz-Thouless critical temperature T c , and the Hartree-Fock approximation critical temperature Tp are determined as a function of particle concentration and interaction. The Uemura type plots (T c vs. λ - 2 (0)) are derived. Beyond weak coupling and for low concentrations they show the existence of universal scaling: T c ∼ 1/λ 2 (0), as it was previously found for the attractive Hubbard model and for the models with intersite electron pairing.

Superconducting Characteristics of the Penson-Kolb Model

We study superconducting properties of the Penson—Kolb model, i.e. the tight-binding model with the pair-hopping ( inters ite charge exchange) interaction J. The evolution of the critical fields, the coherence length, the Ginzburg ratio, and the London penetration depth with particle concentration n and pairing strength are determined. The results are compared with those found earlier for the attractive Hubbard model.

Superconductivity in the Hubbard model with pair hopping

The phase diagrams and superconducting properties of the extended Hubbard model with pair hopping interaction, i.e. the Penson-Kolb-Hubbard model are studied. The analysis of the model is performed for d-dimensional hypercubic lattices, including d=1 and d=$\infty$, by means of the (broken symmetry) Hartree-Fock approximations and, for d=$\infty$, by the slave-boson mean-field method. For d=1, at half-filling the phase diagram is shown to consist of nine different phases including two superconducting states with center-of-mass momentum q=0 and q=Q ($\eta$-pairing), site and bond-located antiferromagnetic and charge-density wave states as well as three mixed phases with coexisting site and bond orderings. The stability range of the bond-type orderings is shrank with increasing lattice dimensionality d and for d=$\infty$ the corresponding diagram consists of four phases only, involving exclusively site-located orderings. Comparing the pair hopping model with the attractive Hubbard model we found in the both cases gradual evolution from the BCS-like limit to the tightly bound pairs regime and a monotonic increase of the gap in the excitation spectrum with increasing coupling. However, the dynamics of electron pairs in both models is qualitatively different, which results in different dependences of condensation energies and critical temperatures on interaction parameters as well as in different electrodynamic properties, especially in a strong coupling regime.

Ground State and Thermodynamic Properties of an Induced-Pairing Model

We study the phase diagrams and thermodynamic properties of a system of coexisting wide-band current carriers (c-particles) and narrow-band electrons (d-electrons) which can form local pairs. There are two distinct mechanisms of superconductivity in the model considered: (i) the intersubsystem charge exchange, which leads to the superconducting state involving both types of electrons and (ii) the pair hopping interaction of d-electrons, leading to the superconducting state of d-particles only. In contrast to previous works on the subject, we assume an arbitrary value of on-site density interaction of d-electrons U, which allows us to study the effects of reduced d-pair binding energy. Within the approach in which the U term is exactly treated we determine the evolution of superconducting properties as a function of interactions and relative position of the bands. In a definite range of parameters the system shows features which are intermediate between those of a local pair superconductor and those of a classical BCS model. The increasing on-site Coulomb repulsion U competing with the intersubsystem charge exchange and the pair hopping interaction reduces the superconducting critical temperature. Moreover, it can induce a change of the superconducting transition into the first order one. Above a critical value of U, dependent on concentration of electrons and other interactions, the superconducting state cannot be stable at any temperature.

Bond-located ordering in the one-dimensional Penson - Kolb - Hubbard model

In this paper we study the ground-state phase diagram of the one-dimensional half-filled repulsive (U > 0) Hubbard model supplemented with the pair-hopping interaction (W) (the Penson - Kolb - Hubbard model) using the continuum-limit field theory approach. We compare the low-energy properties of the U > 0 Hubbard model and W > 0 Penson - Kolb model. We show that, despite similar excitation spectra, the character of instabilities in these models is completely different. In contrast to the Hubbard model, in the case of the Penson - Kolb model, the dynamical generation of a charge gap leads to the suppression of spin-density-wave (SDW) fluctuations. The charge-density-wave fluctuations survive and coexist with bond-located SDW (Bd-SDW) instabilities. The Bd-SDW corresponds to a magnetically ordered state with staggered magnetization located on bonds between sites. The possibility of bond-located ordering is connected with the site-off-diagonal nature of the pair-hopping interaction. In the case of PKH the bond-ordered states exist at . For W > U/2 the Bd-SDW is realized while for the dimerized phase is realized.

Metallic ferromagnetism in a single-band model: Effect of band filling and Coulomb interactions.

A single-band tight-binding model with on-site repulsion and nearest-neighbor exchange interaction has been proposed as a simple model to describe metallic ferromagnetism. Here we extend previously obtained exact-diagonalization studies for a one-dimensional \textonehalf{}-filled band system to other band fillings, and consider the effect of including various other Coulomb matrix elements in the Hamiltonian that are expected to be of appreciable magnitude in real materials. Results of exact diagonalization and mean-field theory for the one-dimensional case are compared. As the band filling decreases from \textonehalf{}, the tendency to ferromagnetism is found to decrease in exact diagonalization, while mean-field theory predicts the opposite behavior. A nearest-neighbor Coulomb repulsion term is found to suppress the tendency to ferromagnetism; however, the effect becomes small for large on-site repulsion. A pair hopping interaction enhances the tendency to ferromagnetism. A nearest-neighbor hybrid Coulomb matrix element breaks electron-hole symmetry and causes metallic ferromagnetism to occur preferentially for more than half-filled rather than less-than-half-filled bands in this model. Mean-field theory is found to yield qualitatively incorrect results for the effect of these interactions on the tendency to ferromagnetism. The implications of these results for the understanding of ferromagnetism in real materials is discussed.

Ground-state instabilities in the one-dimensional Penson-Kolb-Hubbard model.

Different kinds of instabilities (CDW, SDW, SS) in the 1D Hubbard model with pair-hopping interaction are investigated using an approximate Bethe-Salpeter equation. The study is performed at any density of electrons and for arbitrary values of the model parameters. In the absence of the on-site interaction, no transition occurs at half filling for any finite negative pair-hopping parameter, in agreement with recent results; our calculation suggests that the Penson-Kolb model (t,W) with $\mid W/t\mid<\pi/\sin k^{}_F$ behaves qualitatively like the Hubbard model (t,U). Phase diagrams of the Penson-Kolb-Hubbard model (t,U,W) at various densities are presented.