States of local moment induced by nonmagnetic impurities in cuprate superconductors

By using a model Hamiltonian with d-wave superconductivity and competing antiferromagnetic (AF) orders, the local staggered magnetization distribution due to nonmagnetic impurities in cuprate superconductors is investigated. We show that the net moment induced by a single impurity corresponds to a local spin with S(z)=0 or 1/2 depending on the strength of the AF interaction U and the impurity scattering strength epsilon. Phase diagram of epsilon versus U for the moment formation is presented. We discuss the connection of this result with the Kondo problem. When two impurities are placed at the nearest neighboring sites, the net moment is always zero, unusually robust to parameter changes. For two neighboring strong impurities, separated by a Cu-ion site, the induced net moment has S(z)=0, 1/2, or 1.     

Magnetic response of nonmagnetic impurities in cuprates

A theory of the local magnetic response of a nonmagnetic impurity in a doped antiferromagnet, as relevant to the normal-state in cuprates, is presented. It is based on the assumption of the overdamped collective mode in the bulk system and on the evidence that equal-time spin correlations are only weakly renormalized in the vicinity of the impurity. The theory relates the Kondo-like behavior of the local susceptibility to the anomalous temperature dependence of the bulk magnetic susceptibility.     

Spin resonance in the d-wave superconductor CeCoIn5

Neutron scattering is used to probe antiferromagnetic spin fluctuations in the d-wave heavy fermion superconductor CeCoIn5 (T_(c)=2.3 K). Superconductivity develops from a state with slow (variant Planck's over 2piGamma=0.3+/-0.15 meV) commensurate [Q_(0)=(1/2,1/2,1/2)] antiferromagnetic spin fluctuations and nearly isotropic spin correlations. The characteristic wave vector in CeCoIn5 is the same as CeIn3 but differs from the incommensurate wave vector measured in antiferromagnetically ordered CeRhIn5. A sharp spin resonance (variant Planck's over 2piGamma<0.07 meV) at variant Planck's over 2piomega=0.60+/-0.03 meV develops in the superconducting state removing spectral weight from low-energy transfers. The presence of a resonance peak is indicative of strong coupling between f-electron magnetism and superconductivity and consistent with a d-wave gap order parameter satisfying Delta(q+Q0)=-Delta(q).     

Order and excitation in partially Gutzwiller projected t-t'-t"-J-U models

Extended t-t'-t"-J-U models in which the second-nearest-neighbor hopping (t') and third-nearest-neighbor hopping (t") are included are studied using renormalized mean field theory. The models are meant to be low-energy effective models for the Hubbard models, and hence the Heisenberg exchange integral J and Hubbard repulsion U are related by J = 4t(2)/U. The trial wavefunctions for the ground states are partially Gutzwiller projected Hartree-Fock states. The Gutzwiller projection is implemented by means of a Gutzwiller approximation, and the site double occupancy d is taken as a variational parameter. It is found that a large |t'/t| narrows the band filling range that sustains antiferromagnetism (AFM) in the ground state, enhances the d-wave singlet superconductivity (dSC) in hole overdoped systems, but suppresses the dSC in electron overdoped systems. For a system that has large |t'/t| and |t"/t'|, the superconductivity (SC) at the onset of AFM in hole doped band filling is strongly suppressed. On the excitation occurring, when an electron doped system simultaneously contains SC and AFM, the system is found to have a nodeless gap at the Fermi level. Finally, the result of this study is related to experiments on the superconducting cuprates.     

Orbital ordering and spin-ladder formation in La2RuO5

The semiconductor-semiconductor transition of La2RuO5 is studied by means of augmented spherical wave electronic structure calculations as based on density-functional theory and the local density approximation. This transition has lately been reported to lead to orbital ordering and a quenching of the local spin magnetic moment. Our results hint towards an orbital ordering scenario which, markedly different from the previously proposed scheme, preserves the local S=1 moment at the Ru sites in the low-temperature phase. The unusual magnetic behavior is interpreted by the formation of spin ladders, which result from the structural changes occurring at the transition and are characterized by antiferromagnetic coupling along the rungs.     

非对称Anderson模型的重整微扰展开

利用Yamada微扰论结合重整微扰方法来计算非对称Anderson模型,得到了局域电子占据数、重整化因子、重整化的局域能级以及重整化参数关于裸参数的展开式.计算了局域电子态密度和低温杂质电导,还计算了磁场对它们的影响,这些结果适用于从弱耦合到强耦合的整个耦合强度区域.由于在哈密顿量中已经将局域能级进行了初步重整,采用的重整微扰方法比Hewson的重整微扰方法更适合于研究非对称Anderson模型. 关键词： 非对称Anderson模型 重整化 磁场     

Heavy-mass fermi liquid near a ferromagnetic instability in layered ruthenates

Low temperature magnetic, thermal, and transport measurements in Ca2-xSrxRuO4 clarify the appearance of a cluster glass phase, after the evolution of a nearly ferromagnetic heavy-mass Fermi liquid from the spin-triplet superconductor Sr2RuO4. As the Mott transition is approached across a 2nd-order structural transition, both the magnetization and specific heat decrease considerably while the transport scattering rate diverges. A metamagnetic transition to a highly spin polarized state, with a local moment S=1/2, is observed. We argue that an orbital rearrangement with Ca substitution changes itinerant ferromagnetism to antiferromagnetism of localized moments.     

Superconductivity of spin glasses

The coexistence of superconducting and spin glass phases is investigated. The dependence of the phase transition temperature in the spin glass state both on the impurity concentration and the superconductivity state is given. The influence of the spin glass phase on the renormalized frequency and the order parameter of the conduction electrons is determined.     

Impurity in a d-wave superconductor: Kondo effect and STM spectra

We present a theory for recent STM studies of Zn impurities in the superconductor Bi2Sr2CaCu2O8+delta, using insights from NMR experiments which show that there is a net S = 1/2 moment on the Cu ions near the Zn. We argue that the Kondo spin dynamics of this moment is the origin of the low bias peak in the differential conductance, rather than a resonance in a purely potential scattering model. The spatial and energy dependence of the STM spectra of our model can also fit the experiments.     

Characterizing a high spin magnetic impurity via Andreev reflection spectroscopy

The ground state properties of a high spin magnetic impurity and its interaction with an electronic spin are probed via Andreev reflection. We see that through the charge and spin conductance one can effectively estimate the interaction strength, the ground state spin and magnetic moment of any high spin magnetic impurity. We show how a high spin magnetic impurity at the junction between a normal metal and superconductor can contribute to superconducting spintronics applications. Particularly, while spin conductance is absent below the gap for Ferromagnet-Insulator-Superconductor junctions we show that in the case of a Normal metal-High spin magnetic impurity-Normal Metal-Insulator-Superconductor (NMNIS) junction it is present. Further, it is seen that pure spin conduction can exist without any accompanying charge conduction in the NMNIS junction.     

Phase Diagram of One-Dimensional Bosons in an Array of Local Nonlinear Potentials at Zero Temperature

The Gaussian wave functional method is applied to a boson system with an array of local nonlinear potentials cos[βφ(nR)] to study the phase diagram of its ground state. The stable ground state is determined by the renormalized mass μ which is a function of the parameter γ=β²/4φ, the strength of potential α and the potential concentration c. In different cases γ<γ₁, γ₁<γ<γ₂ and γ>γ₂, μ can have different multiplicities, the phase diagram in parameter space is thus depicted. The value γ=γ₁ depends on the concentration c, for c→0, it coincides with that of the single impurity model; while γ₂=2 coincides with the conclusion of the continuous model.     

Competition between Kondo and RKKY correlations in the presence of strong randomness

We propose that competition between Kondo and magnetic correlations results in a novel universality class for heavy fermion quantum criticality in the presence of strong randomness. Starting from an Anderson lattice model with disorder, we derive an effective local field theory in the dynamical mean-field theory approximation, where randomness is introduced into both hybridization and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Performing the saddle-point analysis in the U(1) slave-boson representation, we reveal its phase diagram which shows a quantum phase transition from a spin liquid state to a local Fermi liquid phase. In contrast with the clean limit case of the Anderson lattice model, the effective hybridization given by holon condensation turns out to vanish, resulting from the zero mean value of the hybridization coupling constant. However, we show that the holon density becomes finite when the variance of the hybridization is sufficiently larger than that of the RKKY coupling, giving rise to the Kondo effect. On the other hand, when the variance of the hybridization becomes smaller than that of the RKKY coupling, the Kondo effect disappears, resulting in a fully symmetric paramagnetic state, adiabatically connected to the spin liquid state of the disordered Heisenberg model. We investigate the quantum critical point beyond the mean-field approximation. Introducing quantum corrections fully self-consistently in the non-crossing approximation, we prove that the local charge susceptibility has exactly the same critical exponent as the local spin susceptibility, suggesting an enhanced symmetry at the local quantum critical point. This leads us to propose novel duality between the Kondo singlet phase and the critical local moment state beyond the Landau-Ginzburg-Wilson paradigm. The Landau-Ginzburg-Wilson forbidden duality serves the mechanism of electron fractionalization in critical impurity dynamics, where such fractionalized excitations are identified with topological excitations.     

Phase diagram of an impurity in the spin-1/2 chain: two-channel Kondo effect versus Curie law

We consider a magnetic S = 1/2 impurity in the antiferromagnetic spin chain as a function of two coupling parameters: the symmetric coupling of the impurity to two sites in the chain J1 and the coupling between the two sites J2. By using field theory arguments and numerical calculations we can identify all possible fixed points and classify the renormalization flow between them, which leads to a nontrivial phase diagram. Depending on the detailed choice of the two (frustrating) coupling strengths, the stable phases correspond either to a decoupled spin with Curie law behavior or to a non-Fermi-liquid fixed point with a logarithmically diverging impurity susceptibility as in the two-channel Kondo effect. Our results resolve a controversy about the renormalization flow.     

Hubbard model on decorated lattices

We introduce a family of lattices for which the Hubbard model and its natural extensions can be quasiexactly solved, i.e., solved for the ground and low energy states. In particular, we show rigorously that the ground state of the Hubbard model with off-site Coulomb repulsions on a decorated Kagomè lattice is an ordered array of local currents. The low energy theory describing this chiral state is an S=1 / 2 XY model, where each spin degree of freedom represents the two possible chiralities of each local current.     

Spin-triplet superconductivity in UNi(2)Al(3) revealed by the (27)Al Knight shift measurement

We report (27)Al Knight shift ( (27)K) measurement on a single-crystal UNi(2)Al(3) that reveals a coexistence of superconductivity and a spin-density-wave (SDW) type of magnetic ordering ( T(SDW) = 4.5 K). The spin part of (27)K, (27)K(s), does not change down to 50 mK across the superconducting (SC) transition temperature T(c) approximately 0.9 K. In contrast with the isostructural compound UPd(2)Al(3) ( T(c) approximately 2 K), which was identified to be a spin-singlet d-wave superconductor, the behavior of (27)K strongly supports that UNi(2)Al(3) , like UPt(3) and Sr(2)RuO(4), belongs to a class of spin-triplet SC pairing state superconductors.     

Mott-hubbard metal-insulator transition in paramagnetic V2O3: an LDA+DMFT(QMC) study

The electronic properties of paramagnetic V2O3 are investigated by the computational scheme LDA+DMFT(QMC). This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic V2O3 and insulating (V0.962Cr0.038)2O3 we find a Mott-Hubbard transition at a Coulomb interaction U approximately 5 eV. The calculated spectrum is in very good agreement with experiment. Furthermore, the orbital occupation and the spin state S = 1 determined by us agree with recent polarization dependent x-ray-absorption experiments.     

Tunneling Conductance and Magnetoresistance in Ferromagnet/Ferromagnet/d-Wave Superconductor Double Tunnel Junctions

The tunneling conductance and tunneling magnetoresistance (TMR) are investigated in ferromagnet/insulator/ferromagnet/insulator/d-wave superconductor (FM/I/FM/I/d-wave SC) structures by applying an extended Blonder-Tinkham-Klapwijk (BTK) approach. We study the effects of the exchange splitting in the FM, the magnetic impurity scattering in the thin insulator interface of FM/I/FM, and noncollinear magnetizations in adjacent magnetic layers on the TMR. It is shown (1) that the tunneling conductance and TMR exhibit amplitude-varying oscillating behavior with exchange splitting, (2) that with the presence of spin-flip scattering in insulator interface of FM/I/FM, the TMR can be dramatically enhanced, and (3) that the TMR depends strongly on the angle between the magnetization of two FMs.     

Local spin density in the Cr(7)Ni antiferromagnetic molecular ring and (53)Cr-NMR

We present (53)Cr-NMR spectra collected at low temperature in a single crystal of the heterometallic antiferromagnetic (AF) ring Cr(7)Ni in the S?=?1/2 ground state with the aim of establishing the distribution of the local electronic moment in the ring. Due to the poor S/N we observed only one signal which is ascribed to three almost equivalent (53)Cr nuclei in the ring. The calculated spin density in Cr(7)Ni in the ground state, with the applied magnetic field both parallel and perpendicular to the plane of the ring, turns out to be AF staggered with the greatest component of the local spin ?s? for the Cr(3+) ions next to the Ni(2+) ion. The (53)Cr-NMR frequency was found to be in good agreement with the local spin density calculated theoretically by assuming a core polarization field of H(cp)?=-?11?T/μ(B) for both orientations, close to the value found previously in Cr(7)Cd. The observed orientation dependence of the local spin moments is well reproduced by the theoretical calculation and evidences the importance of single-ion and dipolar anisotropies.     

Local moment and ferromagnetic state formation in dilute,degenerate d-electron alloys: Ferromagnetic versus antiferromagnetic interactions

In this note the Kim [1] non-degenerate Anderson model (NDAM) of random dilute alloys treatment of local moment and ferromagnetic state formation is generalized to the ten-fold degenerate Anderson model (TDAM) of Siegel and Kemeny [2], Siegel [3,4] and Moriya [12]. We first determine how an impurity state is modified by neighboring impurities. For a simple binary alloy the local electron state at each impurity site depends upon the local distribution of other impurities. Second we derive a TDAM general relationship for the occurrence of a local moment on one impurity and the ferromagnetic ordering of the total impurity spins. Lastly we derive the impurity-impurity TDAM magnetic interaction; for the direct transfer interaction the impurity-impurity magnetic interaction can be ferromagnetic or antiferromagnetic depending upon the fractional occupation of impurity states. At each stage we compare our results with those of Kim's NDAM treatment.     

Ground state of ferromagnet with antiferromagnetic impurity

The Heisenberg ferromagnet with an antiferromagnetic impurity and arbitrary spin is considered. The method is suggested for constructing the ground state of such a system, the method using the Jacobi matrix technique. As an example, there has been investigated the ferromagnet with a simple cubic lattice and the matrix spin $\text{S =}\text{1}\text{2}$, and the impurity spin S′ = 1. The energy and wave function of the ground state are found as dependent on the system parameters.