Effects of the Next Nearest Neighbor Hopping on Superconductivity and Antiferromagnetism of Gossamer Superconductivity

The two dimensions hole-doped t-t '-J-U model was studied based on the Gutzwiller approach and the renormalized mean-field theory.The phase diagrams of gossamer superconductors and the effects of the next-nearestneighbor hopping(t ') on superconductivity and antiferromagnetism based on the t-t '-J-U model were investigated.The results show that the qualitative feature of the phase diagrams in the t-t '-J-U model is the same as in the case of the t-J-U model.The antiferromagnetic order coexists with the d-wave superconductivity(dSC) in the underdoped region below the doping δ≈ 0.1 and is enhanced by the t '.The dSC order is slightly suppressed by t ' in the underdoped region and greatly enhanced in the overdoped region.The dSC order is pushed to a larger doping region and the coexistence region of the AF and dSC extends to higher doping.     

Antiferromagnetism and superconductivity in layered organic conductors: Variational cluster approach

The kappa-(ET)2X layered conductors (where ET stands for BEDT-TTF) are studied within the dimer model as a function of the diagonal hopping t' and Hubbard repulsion U. Antiferromagnetism and d-wave superconductivity are investigated at zero temperature using variational cluster perturbation theory (VCPT). For large U, Néel antiferromagnetism exists for t' < t(c2)', with t(c2)' approximately 0.9. For fixed t', as U is decreased (or pressure increased), a d(x2-y2) superconducting phase appears. When U is decreased further, then a d(xy) order takes over. There is a critical value of t(c1)' approximately 0.8 of t' beyond which the AF and dSC phases are separated by the Mott disordered phase.     

空穴掺杂超导体的超导电性

在二维空穴掺杂t-t′-J-U模型和重整化平均场理论的框架下,用Gutzwiller方法研究了电子次近邻跃迁对空穴掺杂超导体的超导电性的影响。在掺杂浓度小于0.1的欠掺杂区,随着电子次近邻跃迁的增大,超导序参数在欠掺杂区域受到抑制,而在过掺杂区得到加强。     

Enhancement of pairing correlation by t' in the two-dimensional extended t-J model

We investigate the effects of the next-nearest-neighbor (t') and the third-nearest-neighbor (t") hopping terms on superconductivity correlation in the 2D hole-doped extended t-J model based on the variational Monte Carlo, mean-field calculation and exact diagonalization method. Despite the diversity of the methods employed, the results all point to a consistent conclusion: While the d-wave superconductivity correlation is slightly suppressed by t' and t" in underdoped regions, it is greatly enhanced in the optimal and overdoped regions. The optimal Tc is a result of the balance of these two opposite trends.     

Drude Weight,Optical Conductivity of Two-Dimensional Hubbard Model at Half Filling

We study the Drude weight D and optical conductivity of the two-dimensional (2D) Hubbard model at half filling with staggered magnetic flux (SMF). When SMF being introduced, the hopping integrals are modulated by the magnetic flux. The optical sum rule, which is related to the mean kinetic energy of band electrons, is evaluated for this 2D Hubbard Hamiltonian. Our present result gives the dependence of the kinetic energy, D and the optical conductivity on SMF and U. At half filling D vanishes exponentially with system size. We also find in the frequency dependence of the optical conductivity, there is δ-function peak at ω≈2|m|U and the incoherent excitations begin to present themselves extended to a higher energy region.     

Kinetic ferromagnetism on a kagome lattice

We study strongly correlated electrons on a kagome lattice at 1/6 (and 5/6) filling. They are described by an extended Hubbard Hamiltonian. We are concerned with the limit |t|相似文献   

Competition between antiferromagnetism and superconductivity in high-Tc cuprates

Using variational cluster perturbation theory we study the competition between d-wave superconductivity (dSC) and antiferromagnetism (AF) in the t-t(')-t(')-U Hubbard model. Large scale computer calculations reproduce the overall ground-state phase diagram of the high-temperature superconductors as well as the one-particle excitation spectra for both hole and electron doping. We identify clear signatures of the Mott gap as well as of AF and of dSC that should be observable in photoemission experiments.     

Single-particle spectral function of the Hubbard chain: frustration induced

The one-electron spectral function of a frustrated Hubbard chain is computed by making use of the cluster perturbation theory. The spectral weight we found turns out to be strongly dependent on the frustrating next-nearest-neighbor hopping t'. A frustration induced pseudogap arises when the system evolves from a gapful Mott insulator to a gapless conductor for an intermediate value of the frustration parameter |t'|. Furthermore, the opening of a pseudogap in the density of states already in the metallic side leads to a continuous opening of the true gap in the insulator. For the hole-doped case, the pseudogap is pinned at the Fermi energy, while the Mott gap is shifted in energy with increasing Hubbard interaction U. The separation of the pseudogap and Mott gap in the hole-doped system demonstrates the validity of the existence of a pseudogap.     

On Superconductivity State in Pure Graphene

We study theoretically the possibility of superconductivity state in pure graphene within the extended attractive Hubbard model. In the absence of disorder, when we use the local attractive interaction potential, U≌5t, where t is hopping term, pure graphene can be in superconductivity state.     

Failure of saturated ferromagnetism for the Hubbard model with two holes

We consider the Hubbard model on a finite set of sites with nonpositive hopping matrix elements and infinitely strong on-site repulsion. Nagaoka's theorem states that in this model the relative ground state in the sector with one unoccupied site is maximally ferromagnetic. We show that this phenomenon is a consequence of a combinatorial coincidence valid in the one-hole regime only. In the case of more than one hole there is no reason to expect maximally ferromagnetic ground states. We prove this claim for the case of two holes for models defined on a class of graphs which contains all tori that are not too small.     

Investigation on the Ground State of the Hubbard Model

The absolute ground states of the Hubbard model are explicitly presented within the framework of pseudospin when U > 2μ and U < 2μ. It is shown that the states do not possess true off-diagonal long-range order in the two cases. It is also discovered that in the ground states the U(1) phase symmetry of the Hubbard model is spontaneously broken away from the half-filling, and the total spin is zero, which is independent of the sign of U and the electron filling in the considered periodic lattice.     

Mott transition, antiferromagnetism, and d-wave superconductivity in two-dimensional organic conductors

We study the Mott transition, antiferromagnetism, and superconductivity in layered organic conductors using the cellular dynamical mean-field theory for the frustrated Hubbard model. A d-wave superconducting phase appears between an antiferromagnetic insulator and a metal for t'/t=0.3-0.7 or between a nonmagnetic Mott insulator (spin liquid) and a metal for t'/t>or=0.8, in agreement with experiments on layered organic conductors including kappa-(ET)2Cu2(CN)3. These phases are separated by a strong first-order transition. The phase diagram gives much insight into the mechanism for -wave superconductivity. Two predictions are made.     

Lateral transport in boron doped SiGe quantum wells

The temperature dependence of lateral conductivity and hole mobility in boron doped Si/SiGe/Si quantum well structures were studied. The conductivity at the temperatures below 20 K is shown to be due to hopping over B centers while at higher temperatures it is due to two-stage excitation consisting of thermal activation of holes from the ground to strain-split B states and the next hole tunneling into the valence band.     

Critical point and the nature of the pseudogap of single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta superconductors

We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by 63Cu nuclear spin-lattice relaxation rate (1/T1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x approximately 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (Tc=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.     

Striped phase in the cuprates as a semiclassical phenomenon

The striped phase, a novel type of electron solid, has been observed recently in a number of doped Mott-Hubbard insulators (including cuprates). This solid consists of a parallel array of charged-domain walls, bound states of carriers and Néel walls in the antiferromagnetic spin system. The existence of these states has been predicted well in advance of their experimental observation on the basis of semiclassical (‘Hartree-Fock’) theory. Nevertheless, it is not at all clear whether semiclassics yields a correct explanation. In this paper we will focus especially on the variety of striped phases realized in the cuprates, characterized by a domain wall filling of half a hole per domain wall unit cell. We will unfold the reasons why semiclassics, as applied to simple Hubbard models, favours strongly a filling of one hole per domain wall unit cell, as is for instance the case in the nickelates. Nevertheless, the occurrence of half-filled walls as semiclassical ground states cannot be excluded on general grounds. It might be that Hubbard models do not incorporate the microscopic situation correctly. Instead, we derive a qualitative criterion: in order to acquire a special stability on the semiclassical level, the half-filled domain walls should be characterized by a quadrupling of the period along the walls, involving a modulation in the longitudinal spin- and/or charge channel. 64.60. - i, 71.27. + a, 74.72. - h, 75.10. - b     

The Gutzwiller approximation for degenerate bands: a formal derivation

We present a detailed derivation of the Gutzwiller approximation for multi-band Hubbard models with density-density Coulomb interactions. For the one-band Hubbard model we introduce a mathematically well-defined formalism which is easily generalized to the multi-band case. In contrast to earlier attempts, our approach allows us to include inter-orbital hopping terms in the Hamiltonian. Received: 9 December 1997 / Revised and accepted: 6 March 1998     

Electronic state in the 2D Hubbard model

Electronic state of the 2D Hubbard model near the half-filling is analyzed by use of the composite operator method. Doping and temperature dependence of density of states show similar behaviors obtained in numerical simulation. The weight of the upper and lower Hubbard bands at the half filling are not evenly distributed in the Brillouin zone, keeping roughly the original band distribution. With hole doping the lower Hubbard band spreads in the whole zone.     

Stability of ferromagnetism in the Hubbard model on the kagome lattice

The Hubbard model on the kagome lattice has highly degenerate ground states (the flat lowest band) in the corresponding single-electron problem and exhibits the so-called flat-band ferromagnetism in the many-electron ground states as was found by Mielke [J. Phys. A 24, L73 (1991)]]. Here we study the model obtained by adding extra hopping terms to the above model. The lowest single-electron band becomes dispersive, and there is no band gap between the lowest band and the other band. We prove that, at half filling of the lowest band, the ground states of this perturbed model remain saturated ferromagnetic if the lowest band is nearly flat.     

Metallic ferromagnetism in the Hubbard model: a rigorous example

We present the first rigorous example of the Hubbard model in any dimension which exhibits metallic ferromagnetism. The model is a genuine Hubbard model with short-range hopping and on-site Coulomb repulsion, and has many single-electron bands. In the limit where the band gap and the Coulomb repulsion become infinite, we prove that the ground states are completely ferromagnetic and at the same time conducting.