Electron-phonon interaction and antiferromagnetic correlations

We study effects of the Coulomb repulsion on the electron-phonon interaction (EPI) in the Holstein-Hubbard model, using the antiferromagnetic (AF) dynamical mean-field approximation. AF correlations strongly enhance EPI effects on the electron Green's function with respect to the paramagnetic correlated system, but the net effect of the Coulomb interaction is a moderate suppression of the EPI. Doping leads to additional suppression. In contrast, the Coulomb interaction strongly suppresses EPI effects on phonons, but the suppression weakens with doping.     

铁基氟化物超导体SrFe_(1-x)Co_xAsF(x=0,0.125)声子特性的第一性原理计算研究

采用基于第一性原理的平面波赝势方法,计算了铁基氟化物及其钴掺杂超导体SrFe1-xCoxAsF(x=0,0.125)在四方非磁态与正交条纹反铁磁态下的声子谱(声子色散曲线、声子态密度)及电-声子耦合常数.计算发现:条纹反铁磁相互作用下的自旋-声子耦合效应强于电-声子耦合作用使声子谱的宽度减小;自旋效应使声子的有效质量增加导致条纹反铁磁态下Fe原子与As原子的耦合振动频率减小.另外,掺杂和自旋效应是提高电-声子耦合常数的两个有效方法,但计算所得超导转变温度远小于实验测量值,表明铁基超导电性非简单的电-声子耦合配对机理.     

Hot spots and pseudogaps for hole- and electron-doped high-temperature superconductors

Using cluster perturbation theory, it is shown that the spectral weight and pseudogap observed at the Fermi energy in recent angle resolved photoemission spectroscopy of both electron- and hole-doped high-temperature superconductors find their natural explanation within the t-t(')-t(")-U Hubbard model in two dimensions. The value of the interaction U needed to explain the experiments for electron-doped systems at optimal doping is in the weak to intermediate coupling regime where the t-J model is inappropriate. At strong coupling, short-range correlations suffice to create a pseudogap, but at weak-coupling long correlation lengths associated with the antiferromagnetic wave vector are necessary.     

Self-localization of composite spin-lattice polarons

Self-localization of holes in the Holstein t-J model is studied in the adiabatic limit using exact diagonalization and the retraceable path approximation. It is shown that the critical electron-phonon coupling lambda c decreases with increasing J and that this behavior is determined mainly by the incoherent rather than by the coherent motion of the hole. The obtained spin correlation functions in the localized region can be understood within a percolation picture where antiferromagnetic order can persist up to a substantial hole doping. These results restrict the possibility of self-localization of holes in lightly doped cuprates.     

Antiferromagnetism and lattice-distortion in the Peierls-Hubbard model

The ground state of the Peierls-Hubbard model is investigated. It is found to be dimerized or antiferromagnetic, depending on the strength of the effective electron-phonon coupling and the electron-electron interaction.Lattice distortion and magnetization do not coexist in the ground state.     

Effect of Lattice Distortion on Charge Order in Manganites at Doping x = 0.5

In the present paper, we continue our investigation on the antiferromagnetic origin of the charge order observed in the half-doped manganese. By introducing a Su-Schrieffer-Heeger (SSH) type of perturbation interaction to the double-exchange Hamiltonian, we calculate again its ground-state phase diagram at filling x=0.5 by the unrestricted real-space Hartree-Fock approximation method. We find that, as the SSH electron-phonon interaction increases, the charge order parameter decreases to zero rapidly but the CE-type antiferromagnetic order becomes more stable. In other words, the charge order is much more fragile than the CE-type or the Neel-type antiferromagnetic orders under the electron-phonon perturbation. These results support the proposed theory in the recent publications that the charge order in these systems is induced by the antiferromagnetic correlations.     

Band structure of a one-dimensional Peierls-Hubbard-model

A mean field theory for a model Hamiltonian including the Fröhlich electron-phonon coupling term as well as the Hubbard electron-electron interaction term is presented.For the non antiferromagnetic case the band structure is derived; the Peierls-gap is found to be reduced but not to be quenched.     

Strings in charge-transfer Mott insulators: Effects of lattice vibrations and the Coulomb interaction

Applying the canonical transformation with the 1/λ perturbation expansion in the nonadiabatic and intermediate regime and the discrete generalization of Pekar’s continuous nonlinear equation in the extreme adiabatic regime, we show that there are no strings in narrow-band ionic insulators due to the Fröhlich electron-phonon interaction alone. The multipolaron system is a homogeneous state in a wide range of physically interesting parameters, no matter how strong the correlations are. At the same time, the Fröhlich interaction allows the antiferromagnetic interactions and/or short-range electron-phonon interactions to form short strings in doped antiferromagnetic insulators if the static dielectric constant is large enough.     

Co掺杂(ZnO)12团簇的结构和磁性质

采用第一性原理密度泛函理论系统地研究Co原子单掺杂和双掺杂(ZnO)₁₂团簇的结构和磁性质.考虑三种掺杂方式:替代掺杂,外掺杂和内掺杂.首先比较各种掺杂团簇的稳定性.结果表明,不管是单掺杂还是双掺杂,外掺杂团簇都是最稳定结构.在结构优化的基础上,对掺杂的(ZnO)₁₂团簇进行磁性计算.发现团簇磁矩主要来自Co-3d态的贡献,4s和4p态也贡献了一小部分磁矩.由于轨道杂化,相邻的Zn和O原子也产生少量自旋.Co原子之间的磁性耦合由直接的Co-Co反铁磁耦合和Co和O原子之间通过p-d杂化产生的铁磁耦合这两种相互作用的竞争决定.研究发现外双掺杂团簇存在铁磁耦合,在纳米量子器件有潜在的应用价值.     

Pseudogap and antiferromagnetic correlations in the hubbard model

Using the dynamical cluster approximation and quantum Monte Carlo simulations we calculate the single-particle spectra of the Hubbard model with next-nearest neighbor hopping . In the underdoped region, we find that the pseudogap along the zone diagonal in the electron doped systems is due to long-range antiferromagnetic correlations. The physics in the proximity of (0, pi) is dramatically influenced by t' and determined by the short range correlations. The effect t' of on the low-energy angle-resolved photoemission spectroscopy spectra is weak except close to the zone edge. The short range correlations are sufficient to yield a pseudogap signal in the magnetic susceptibility and produce a concomitant gap in the single-particle spectra near (pi, pi/2), but not necessarily at a location in the proximity of the Fermi surface.     

d-wave superconductivity in the hubbard model

The superconducting instabilities of the doped repulsive 2D Hubbard model are studied in the intermediate to strong coupling regime with the help of the dynamical cluster approximation. To solve the effective cluster problem we employ an extended noncrossing approximation, which allows for a transition to the broken symmetry state. At sufficiently low temperatures we find stable d-wave solutions with off-diagonal long-range order. The maximal T(c) approximately 150 K occurs for a doping delta approximately 20% and the doping dependence of the transition temperatures agrees well with the generic high- T(c) phase diagram.     

Stable Intrinsic Long Range Antiferromagnetic Coupling in Dilutely V Doped Chalcopyrite

A stable and long-range antiferromagnetic(AFM) coupling without charge carrier mediators has been searched for a long time,but the existence of this kind of coupling is still lacking.Based on first principle calculations,we systematically study carrier free long-range AFM coupling in four transition metal chalcopyrite systems:ABTe_2(A=Cu or Ag,B=Ga or In) in the dilute doping case.The AFM coupling is mainly due to the p-d coupling and electron redistribution along the interacting chains.The relatively small energy difference between p and d orbitals,as well as between dopants and atoms in the middle of the chain can enhance the stability of longrange AFM configurations.A multi-band Hubbard model is proposed to provide fundamental understanding of long-range AFM coupling.     

Band structure and antiferromagnetism of a single CuO2 layer

We have solved a set of self-consistency equations for the three-band model describing electrons coupled strongly by antiferromagnetic correlations in a single CuO₂ layer. Strong but finite Hubbard correlations are taken into account by using a random phase approximation for the electronic propagators which contain the combined effect of both the Hubbard correlation and the hybridization of copper and nearest neighbor oxygen states. From the Green functions the band structure is determined, which depends strongly on the doping fraction and the antiferromagnetic order parameter 〈s_Q〉. The main impact of doping is to decrease the magnitude of antiferromagnetic fluctuations, although the decrease appears to be quite slow when compared with experimental data.     

Why standard estimates of electron-phonon coupling in cuprates do not work

The problem of estimating the coupling strength between oxygen-breathing phonons and electrons in cuprates is discussed. Some of these modes are found in experiments to be strongly coupled, in particular at low doping concentrations. Standard tools, like local density approximation, give a too small coupling. Many-body techniques compare instead much better with experiments. This suggests that electronic correlation effects play a crucial role in the estimate of the electron-phonon coupling from first principles.     

Quantum Monte Carlo study of an interaction-driven band-insulator-to-metal transition

We study the transitions from band insulator to metal to Mott insulator in the ionic Hubbard model on a two-dimensional square lattice using determinant quantum Monte Carlo. Evaluation of the temperature dependence of the conductivity demonstrates that the metallic region extends for a finite range of interaction values. The Mott phase at strong coupling is accompanied by antiferromagnetic order. Inclusion of these intersite correlations changes the phase diagram qualitatively compared to dynamical mean field theory.     

Tuning magnetism of MnO by doping with 2p elements

We show that the antiferromagnetic coupling between Mn atoms in MnO can be tuned to a ferromagnetic one by doping it with a small concentration of 2p elements like boron, carbon and nitrogen. Our ab initio density functional calculations in the Hubbard U framework show that the coupling between Mn atoms turns ferromagnetic for 3 at% B doping. B is found to be the most effective one in stabilizing ferromagnetism. We discuss the role of 2p states of the dopant atoms to explain the trend of ferromagnetism in MnO.     

Double exchange model for nanoscopic clusters

We solve the double exchange model on nanoscopic clusters exactly, and specifically consider a six-site benzene-like nanocluster. This simple model is an ideal testbed for studying magnetism in nanoclusters and for validating approximations such as the dynamical mean field theory (DMFT). Non-local correlations arise between neighboring localized spins due to the Hund’s rule coupling, favoring a short-range magnetic order of ferro or antiferromagnetic type. For a geometry with more neighboring sites or a sufficiently strong hybridization between leads and the nanocluster, these non-local correlations are less relevant, and DMFT can be applied reliably.     

Effect of canted antiferromagnetic order on the electronic structure in the <Emphasis Type="Italic">t–J</Emphasis>* model within the cluster perturbation theory

The electronic structure in the two-dimensional t–J* model with canted antiferromagnetic order in an external magnetic field has been calculated within the cluster perturbation theory. In zero external field, the evolution of the Fermi surface with n-type doping has been obtained in good agreement with experimental data on cuprate superconductors. It has been shown that the inclusion of short-range correlations can result in a nonmonotonic dependence of the spectral weight distribution at the Fermi level on the external magnetic field. In contrast to the case of electron doping, such changes in the case of hole doping can be expected at experimentally achievable fields.     

Electronic structure and phonon spectrum of binary iron-based superconductor FeSe in both nonmagnetic and striped antiferromagnetic phases

Using first-principles calculations, we investigate electronic structure and phonon spectrum of binary iron-based superconductor FeSe in both tetragonal nonmagnetic (NM) phase and orthorhombic striped antiferromagnetic (SAF) phase. It is found that the softening of atomic vibration modes and main electron-phonon coupling contribution from low-frequency Eliashberg spectral function α²F(ω) in SAF phase of FeSe lead to the enhancement of electron-phonon coupling strength λ_ep and logarithmically average frequency ω_ln. However, the obtained superconducting T_c in SAF phase just increases up to 0.34 K, even though Coulomb pseudopotential μ^∗ is limited to zero. As a result, our magnetic phonons calculation still rules out phonon mediated superconductivity, although the electron-phonon coupling through the spin channel play an important role in FeSe.     

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.