On-site Repulsive Interaction Research Articles

The local electronic interaction and strain effects on transverse dynamical spin susceptibility of [formula omitted]-graphyne

We present the behaviors of dynamical transverse spin susceptibilities of undoped β-graphyne monolayer using the Green’s function approach in the context of Hubbard model Hamiltonian. Such dynamical spin susceptibility is proportional to inelastic cross section of neutron beam from the layer. Specially, the effects of magnetic field, on-site coulomb repulsion strength and strain parameter on the spin excitation modes of β-graphyne monolayer are investigated via calculating correlation function of transverse components of spin density operators. Our results show the increase of absolute value of compressive strain parameter leads to move the magnetic excitation modes to higher energies. Also the intensity of peaks in inelastic cross section reduces with absolute value of compressive strain parameter. We also show that applying biaxial tensile strain causes to decrease the intensity of peaks in dynamical transverse spin susceptibility. Finally the effects of magnetic field and on-site coulomb repulsion interaction strength on frequency dependence of dynamical spin structure factor of β-graphyne layer are studied. Moreover the frequency positions of spin excitation mode of β-graphyne monolayer have been investigated due to the effects of biaxial strains, applied magnetic field and Hubbard parameter strength in details.

Half-metallic ferromagnetism in double perovskites Pb2XX'O6 (X = V and Cr and X’ = Zr and Hf)

The structural, magnetic, and electronic properties of Pb-based double perovskite, Pb2XX'O6 (X = V and Cr and X’ = Zr and Hf), have been studied in this paper. This study uses the first-principal projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as considering the on-site Coulomb re-pulsive interaction (GGA+U). The structure of these Pb-based double perovskite is calculated with four magnetic states, antiferromagnetic (AFM), ferromagnetic (FM), ferrimagnetic (FiM), nonmagnetic (NM), and then we compare the energy of these states to find the most stable state. We quickly discovered that all these materials have a lower energy level at the ferromagnetic state, especially Pb2VZrO6 which has an energy difference between AFM and FM state of 120 meV. By analyzing the density of states (DOS) and its magnetic moment, we further discovered that all these materials are half-metallic ferromagnetic (HM-FM). They have an integer magnetic moment because all these materials have a band gap in the spin-down channel. Under GGA + U, the band gap still exists and enlarges, and the V series materials became more stable because of the larger differences between the state of AFM and FM, while the Cr series have the opposite. Lastly, we calculate the electron configuration of V, Cr, Zr, and Hf under GGA and GGA + U calculations in this paper. The HM characteristics of the band gap in one of its spin channels and its ferromagnetic nature mean that there are huge potential applications of these new compounds in spintronics.

Persistent current in a mesoscopic Holstein-Hubbard ring with Dresselhaus interaction

The effect of electron-phonon coupling, onsite repulsive Coulomb interaction and temperature on the persistent current in a quantum ring is studied in the presence of Dresselhaus spin-orbit interaction. The quantum ring threaded by the Aharonov-Bohm flux is modelled by the one-dimensional Holstein-Hubbard-Dresselhaus Hamiltonian. The electron-phonon interaction and Dresselhaus spin-orbit interaction are decoupled by employing the Lang-Firsov coherent transformation and a unitary transformation respectively. Thereafter, a self-consistent diagonalization technique is performed numerically at the Hartree-Fock level to obtain the effective electronic energy and current. It is shown that the intrinsic Dresselhaus spin-orbit interaction enhances the persistent charge and spin currents significantly. On the other hand, the persistent current is reduced by the onsite and nearest-neighbour electron-phonon interaction and Coulomb interaction. Also, the behaviour of the currents is modified by temperature. The spin-splitting of persistent spin current is enhanced considerably by Dresselhaus spin-orbit interaction and this splitting is tuneable in different regimes of magnetic flux, temperature, chemical potential and the interactions present in the system.

Role of inter-site Coulomb interaction on the thermodynamic and ground state properties within the [formula omitted] model

In this work, we report the effect of nearest neighbor (NN) inter-site Coulomb interaction (V) with a certain finite value of on-site Coulomb repulsion (U) and NN antiferromagnetic exchange interaction (J) in the context of high-Tc cuprates. An exact diagonalization method has been applied in a 2D t−J−U model extended by NN inter-site Coulomb interaction in the tilted square cluster near optimal doping. The antiferromagnetic tendency is suppressed by the inter-site e−e repulsion. The inter-site Coulomb repulsion produces an effective on-site attraction between electrons, resulting in the creation of doublons against the on-site Coulomb repulsion. The computation of binding energy demonstrates the binding of holes for small U/t, resulting from the interplay between U and V. Superconductivity arises in a limited window of V depending on U. However, the effective dominance of either U or V results in the annihilation of these hole pairs. The presence of two peaks in specific heat is caused by two Coulomb interactions. The development of doublons as V increases leads to a more ordered state, lowering entropy. The thermodynamic properties also show that the t−J−U−V model acts as an attractive Hubbard model for U<V. However, a pure attractive t−J−U model cannot explain these phenomenologies.

Confinement-induced enhancement of superconductivity in a spin- 12 fermion chain coupled to a Z2 lattice gauge field

We investigate a spin-$\frac{1}{2}$ fermion chain minimally coupled to a $\mathbb{Z}_2$ gauge field. In the sector of the gauge generator $\hat G_j =-1$, the model reduces to the Hubbard model with repulsive onsite interaction coupled to a $\mathbb{Z}_2$ gauge field. We uncover how electric fields affect low-energy excitations by both analytical and numerical methods. In the half-filling case, despite electric fields, the system is still a Mott insulator, just like the Hubbard model. For hole-doped systems, holes are confined under nonzero electric fields, resulting in a hole-pair bound state. Furthermore, this bound state also significantly affects the superconductivity, which manifests itself in the emergence of attractive interactions between bond singlet Cooper pairs. Specifically, numerical results reveal that the dimension of the dominant superconducting order parameter becomes smaller when increasing the electric field, signaling an enhancement of the superconducting instability induced by lattice fermion confinement. The superconducting order can even be the dominant order of the system for suitable doping and large applied electric field. The confinement also induces a $\pi$ momentum for the dominant superconducting order parameter leading to a quasi-long-range pair density wave order. Our results provide insights for understanding unconventional superconductivity in $\mathbb{Z}_2$ LGTs and might be experimentally addressed in quantum simulators.

Exploring interacting topological insulator in the extended Su-Schrieffer-Heeger model

Exploring topological phases in interacting systems is a challenging task. We investigate many-body topological physics of interacting fermions in an extended Su-Schrieffer-Heeger (SSH) model, which extends the two sublattices of SSH model into four sublattices and thus is dubbed SSH4 model, based on the density-matrix renormalization-group numerical method. The interaction-driven phase transition from topological insulator to charge density wave (CDW) phase can be identified by analyzing the variations of entanglement spectrum, entanglement entropies, energy gaps, CDW order parameter, and fidelity. We map the global phase diagram of the many-body ground state, which contains nontrivial topological insulator, trivial insulator and CDW phases, respectively. In contrast to interacting SSH model, in which the phase transitions to the CDW phase are argued to be first-order phase transitions, the phase transitions between the CDW phase and topologically trivial/nontrivial phases are shown to be continuous phase transitions. Finally, we {also} show the phase diagram of interacting spinful SSH4 model, where the attractive (repulsive) on-site spin interaction amplifies (suppresses) the CDW phase. The models analyzed here can be implemented with ultracold atoms on optical superlattices.

Metal-insulator transition in the disordered Hubbard model of the Lieb lattice

Using the determinant quantum Monte Carlo method, we investigate the metal-insulator transition in the interacting disordered Hubbard model of a Lieb lattice in which the system characterizes the flat band centered at the Fermi level. By choosing suitable electron densities, we ensure the weak interaction sign problem to improve the reliability of our results. It is found that disorder and on-site Coulomb repulsive interaction produce interesting effects that induce the metal-insulator transition which is impossible in the half-filled case. The density of states at the Fermi energy is still finite in the thermodynamic limit, suggesting that the system is an Anderson insulator rather than a Mott insulator. Moreover, this doping system is paramagnetic, unlike the half-filled system which is ferrimagnetic.

Superconductivity from repulsive interactions on the kagome lattice

The discovery of superconductivity in layered vanadium-based kagome metals $A$V$_3$Sb$_5$ ($A$: K, Rb, Cs) has added a new family of materials to the growing class of possible unconventional superconductors. However, the nature of the superconducting pairing in these materials remains elusive. We present a microscopic theoretical study of the leading superconducting instabilities on the kagome lattice based on spin- and charge-fluctuation mediated Cooper pairing. The applied methodology includes effects of both on-site and nearest-neighbor repulsive Coulomb interactions. Near the upper van Hove filling -- relevant for the $A$V$_3$Sb$_5$ materials -- we find a rich phase diagram with several pairing symmetries being nearly degenerate. In particular, while a substantial fraction of the phase diagram is occupied by a spin-singlet order parameter transforming as a two-dimensional irreducible representation of the point group, several nodal spin-triplet pairing states remain competitive. We compute the band and interaction parameter-dependence of the hierarchy of the leading superconducting instabilities, and determine the detailed momentum dependence of the resulting preferred gap structures. Crucially, for moderate values of the interaction parameters, the individual pairing states depend strongly on momentum and exhibit multiple nodes on the Fermi surface. We discuss the properties of these superconducting gap structures in light of recent experimental developments of the $A$V$_3$Sb$_5$ materials.

Competing insulating phases in a dimerized extended Bose-Hubbard model

We study the ground-state properties of the extended Bose-Hubbard model in a one-dimensional dimerized optical lattice. In the limit of strong on-site repulsion, i.e., hardcore bosons, and strong nearest-neighbor interaction, a stable density-wave (DW) phase is obtained at half-filling as a function of lattice dimerization. Interestingly, at quarter-filling we obtain the signatures of an insulating phase which has the character of both the bond order (BO) and the DW insulators, which we call a bond-order density-wave (BODW) phase. Moreover, we show that for a fixed hopping dimerization there occurs a BO-DW phase crossover as a function of the nearest-neighbor interaction and the BODW phase is more robust when the hopping dimerization is stronger. We further examine the stability of the BODW phase in the limit of finite on-site interactions.

Chiral Ising Gross-Neveu Criticality of a Single Dirac Cone: A Quantum MonteCarlo Study.

We perform large-scale quantum MonteCarlo simulations of SLAC fermions on a two-dimensional square lattice at half filling with a single Dirac cone with N=2 spinor components and repulsive on-site interactions. Despite the presence of a sign problem, we accurately identify the critical interaction strength U_{c}=7.28±0.02 in units of the hopping amplitude, for a continuous quantum phase transition between a paramagnetic Dirac semimetal and a ferromagnetic insulator. Using finite-size scaling, we extract the critical exponents for the corresponding N=2 chiral Ising Gross-Neveu universality class: the inverse correlation length exponent ν^{-1}=1.19±0.03, the order parameter anomalous dimension η_{ϕ}=0.31±0.01, and the fermion anomalous dimension η_{ψ}=0.136±0.005.

Fragment-orbital-dependent spin fluctuations in the single-component molecular conductor [Ni(dmdt)2]

Motivated by recent nuclear magnetic resonance experiments, we calculated the spin susceptibility, Knight shift, and spin-lattice relaxation rate ($1/T_{1}T$) of the single-component molecular conductor [Ni(dmdt)$_2$] using the random phase approximation in a multi-orbital Hubbard model describing the Dirac nodal line electronic system in this compound. This Hubbard model is composed of three fragment orbitals and on-site repulsive interactions obtained using ab initio many-body perturbation theory calculations. We found fragment-orbital-dependent spin fluctuations with the momentum $\textbf{q}$=$\textbf{0}$ and an incommensurate value of the wavenumber $\textbf{q}$=$\textbf{Q}$ at which a diagonal element of the spin susceptibility is maximum. The $\textbf{q}$=$\textbf{0}$ and $\textbf{Q}$ responses become dominant at low and high temperatures, respectively, with the Fermi-pocket energy scale as the boundary. We show that $1/T_{1}T$ decreases with decreasing temperature but starts to increase at low temperature owing to the $\textbf{q}$=$\textbf{0}$ spin fluctuations, while the Knight shift keeps monotonically decreasing. These properties are due to the intra-molecular antiferromagnetic fluctuations caused by the characteristic wave functions of this Dirac nodal line system, which is described by an $n$-band ($n\geq 3$) model. We show that the fragment orbitals play important roles in the magnetic properties of [Ni(dmdt)$_2$].

The structural, electronic, magnetic and optical properties of 4d transition metal adatoms (Tc, Ru, Rh and Cd) adsorbed on monolayer blue phosphorene

The structural, electronic, magnetic and optical properties of 4d transition metal adatoms (Tc, Ru, Rh and Cd) adsorbed on blue phosphorene (blue P) monolayer have been systematically investigated by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive interactions (GGA + U). The pristine blue P monolayer is a nonmagnetic semiconductor, but adatom-doping with these 4d transition metals is an effective way to modify its electronic, magnetic and optical properties and thus expands its applications in spintronics, magnetoelectronics and optoelectronics. Except for the close-filled Cd adatom is unadsorbed, the Tc and Ru adatom-adsorbed blue P monolayers change to bipolar magnetic semiconductor, while the Rh adatom-adsorbed blue P monolayer turns into magnetic spin-gapless semiconductor. The binding energy and optical absorption results confirm that, the Tc, Ru and Rh adatom-adsorbed blue P monolayers are not only fabricable experimentally but also have enhanced absorption for visible and infrared lights.

Enhancement of pairing correlations due to nearly flat bands in the plaquette-Lieb Hubbard model

Motivated by recent research works on the flat-band correlated electron systems, we construct a plaquette-Lieb lattice, which exhibits nearly flat bands close to the Fermi level around half-filling, when the inter-plaquette hopping integral is much smaller than the intra-plaquette one. A detailed comparison study with the well-known checkerboard lattice indicates that there exist significant differences for the pairing property between the two lattices, and the nearly flat bands on the plaquette-Lieb lattice strongly enhance the [Formula: see text]-wave pairing correlation for repulsive on-site Hubbard interaction and [Formula: see text]-wave pairing correlation for attractive on-site Hubbard interaction. Our study not only provides a new lattice with nearly flat bands, but also deepens the understanding of electronic structure on superconductivity.

Impurity effect on order modulations in superconducting systems with mixed singlet-triplet pairing

Based on a model Hamiltonian including the on-site repulsive interaction among the electrons, we investigate the effect induced by nonmagnetic impurities in bulk superconducting systems with mixed singlet-triplet pairing by numerically solving the Bogoliubov-de Gennes equations. For the emergent spin-triplet p-wave as well as coexisted spin-singlet s-wave and d-wave pairing symmetries, the order-parameter patterns for the quantized vortex states with point-like cores can be effectively tuned by the located sites of nonmagnetic impurities. The next-nearest-neighbor hopping effect on the evolution of vortex configurations is also discussed in the presence of vortex pinning by the impurity. Besides the unconventional vortex lattice, several types of topological defects, including the unclosed domain walls and enclosed skyrmionic modes, can be obtained when the impurity position is varied. All these configurations can be resolved by the zero-energy peaks in the local density of states.

Collective spin and charge excitations in the t-J-U model of high-[formula omitted] cuprates

The t-J-U model of high-Tc copper-oxide superconductors incorporates both the on-site Coulomb repulsion and kinetic exchange interaction and yields a semi-quantitative description of the static properties of those materials. We extend this analysis to dynamic quantities and address collective spin- and charge excitations in the correlated metallic state of the t-J-U model. We employ VWF+1/Nf approach that combines the variational wave function (VWF) approach with the expansion in the inverse number of fermionic flavors (1/Nf). It is shown that the resonant (paramagnon) contribution to the dynamic magnetic susceptibility remains robust as one interpolates between the Hubbard- and t-J-model limits, whereas the incoherent continuum undergoes substantial renormalization. Energy of the collective charge mode diminishes as the strong-coupling limit is approached. We also introduce the concept of effective kinetic exchange interaction that allows for a unified interpretation of magnetic dynamics in the Hubbard, t-J, and t-J-U models. The results are discussed in the context of recent resonant inelastic x-ray scattering experiments for the high-Tc cuprates.

Scattering of two particles in a one-dimensional lattice

This study concerns the two-body scattering of particles in a one-dimensional periodic potential. A convenient ansatz allows for the separation of center-of-mass and relative motion, leading to a discrete Schr\"odinger equation in the relative motion that resembles a tight-binding model. A lattice Green's function is used to develop the Lippmann-Schwinger equation, and ultimately derive a multiband scattering $K$ matrix which is described in detail in the two-band approximation. Two distinct scattering lengths are defined according to the limits of zero relative quasimomentum at the top and bottom edges of the two-body collision band. Scattering resonances occur in the collision band when the energy is coincident with a bound state attached to another higher or lower band. Notably, repulsive on-site interactions in an energetically closed lower band lead to collision resonances in an excited band.

Vortical patterns in bulk superconducting systems with mixed pairing orders

Based on a model Hamiltonian including the on-site repulsive interaction among the electrons, we investigate the spatial configurations of mixed spin-triplet (dominant p-wave) and spin-singlet (coexisted s-wave and d-wave) pairing symmetries in bulk superconducting systems by numerically solving the Bogoliubov–de Gennes equations. The influences of the Zeeman field and the next-nearest-neighbor hopping on the order-parameter modulations are mainly discussed. Besides the quantized vortex states with point-like cores, the unclosed vortex chains and enclosed skyrmionic modes can remain stable in the present bulk sample. Furthermore, nonuniform patterns of the vortex lattice and the elliptical-like vortices can be obtained at appropriate applied flux. The corresponding zero-energy local density of states as well as the relative phase differences useful to display the skyrmionic texture are provided

On the Kinetic Energy Driven Superconductivity in the Two-Dimensional Hubbard Model

We investigate the role of kinetic energy for the stability of superconducting state in the two-dimensional Hubbard model on the basis of an optimization variational Monte Carlo method. The wave function is optimized by multiplying by correlation operators of site off-diagonal type. This wave function is written in an exponential-type form given as ψλ=exp(−λK)ψG for the Gutzwiller wave function ψG and a kinetic operator K. The kinetic correlation operator exp(−λK) plays an important role in the emergence of superconductivity in large-U region of the two-dimensional Hubbard model, where U is the on-site Coulomb repulsive interaction. We show that the superconducting condensation energy mainly originates from the kinetic energy in the strongly correlated region. This may indicate a possibility of high-temperature superconductivity due to the kinetic energy effect in correlated electron systems.

Spin-fluctuation-induced pairing in twisted bilayer graphene

We investigate the interplay of magnetic fluctuations and Cooper pairing in twisted bilayer graphene from a purely microscopic model within a large-scale tight-binding approach resolving the \AA ngstr\"om scale. For local onsite repulsive interactions and using the random-phase approximation for spin fluctuations, we derive a microscopic effective pairing interaction that we use for self-consistent solutions of the Bogoliubov-de-Gennes equations of superconductivity. We study the predominant pairing types as function of interaction strength, temperature and band filling. For large regions of this parameter space, we find chiral $d$-wave pairing regimes, spontaneously breaking time-reversal symmetry, separated by magnetic instabilities at integer band fillings. Interestingly, the $d$-wave pairing is strongly concentrated in the AA regions of the moir\'e unit cell and exhibits phase windings of integer multiples of $2\pi$ around these superconducting islands, i.e. pinned vortices. The spontaneous circulating current creates a distinctive magnetic field pattern. This signature of the chiral pairing should be measurable by state-of-the-art experimental techniques.

Mott transition of fermions in anisotropic ruby lattice

&lt;sec&gt;In this work, the Hubbard model is adopted to describe fermions with on-site repulsive interaction and the nearest-neighbor hopping in anisotropic ruby lattice. The combination of cluster dynamical mean field theory and continuous-time quantum Monte Carlo algorithm is used to solve the theoretical model.&lt;/sec&gt; &lt;sec&gt;It is widely accepted that the density of states and the double occupancy are two important quantities for determining the phase transition of two-dimensional strongly correlated system. Therefore, based on the self-consistent calculation, using the maximum entropy method to calculate the single particle density of states and double occupancy of fermions in anisotropic ruby lattice. Here in this work, there are 6 sites in a cluster.&lt;/sec&gt; &lt;sec&gt;The influences of temperature, interaction and anisotropic parameter on metal-insulator phase transition of fermions in anisotropic ruby lattice are discussed based on the calculations of single particle density of state and double occupancy. Finally, the metal-Mott insulator phase diagram which shows the competition between temperature and on-site repulsive interaction in the phase transition of fermions in anisotropic ruby lattice is presented. The results shows that the system is in metallic state for the regime of weak interaction and low temperature, and the Mott insulator appears in the regime of strong interaction and high temperature. The metallic state and Mott insulating one are separated by the second-order transition line in the phase diagram.&lt;/sec&gt;