Local moment formation in the superconducting state of a doped Mott insulator

A microscopic theory is presented for the local moment formation near a nonmagnetic impurity or a copper defect in high-Tc superconductors. We use a renormalized mean-field theory of the t-J model for a doped Mott insulator and study the fully self-consistent, spatially unrestricted solutions of the d-wave superconducting (SC) state in both the spin S=0 and S=1/2 sectors. We find a transition from the singlet d-wave SC state to a spin doublet SC state when the renormalized exchange coupling exceeds a doping dependent critical value. The induced S=1/2 moment is staggered and localized around the impurity. It arises from the binding of an S=1/2 nodal quasiparticle to the impurity. The local density of states is calculated and connections to NMR and STM experiments are discussed.     

Influence of local spin polarization to the Kondo effect

We use the spin non-degenerate single impurity Anderson model to investigate the influence of the local spin polarization to the Kondo effect. By using the Schrieffer-Wolff transformation, we obtain a generalized s-d exchange Hamiltonian, which describes the interaction between a polarized local spin and conduction electrons. In this case, the singlet is no longer an eigenstate as shown by variational calculations where the splitting of the local energy Δ = ɛ _d↑ − ɛ _d↓ can be arbitrarily small. The local spin polarization generates the instability of the singlet ground state of the S = 1/2 s-d exchange model.      

Mesoscopic spin clusters, phase separation, and induced order in spin-gap magnets: A review

In many low-dimensional systems with antiferromagnetic interactions, a magnetic order of the Néel type is absent. These systems remain in a quantum-disordered (spin-liquid) state down to zero temperature. The disordered state appears to be stable to weak perturbations when magnetic excitations are separated from the ground state by an energy gap. The stability of the spin-liquid ground state is destroyed upon introduction of impurities or in a sufficiently strong magnetic field. This paper presents a review of the main results of the experimental investigations performed in order to reveal and identify mesoscopic spin clusters formed in the vicinity of impurity ions, to determine the spatial structure of an impurity-induced magnetic order in spin-gap systems, and to examine the low-frequency excitation spectra of field-induced antiferromagnetic phases.     

The Kondo necklace model with planar anisotropy

We study the one-dimensional anisotropic Kondo necklace model at zero temperature through White's density matrix renormalization group technique. The ground state energy and the spin gap were calculated as a function of the exchange parameter for two anisotropy values. We found a finite critical point separating a Kondo singlet from an antiferromagnetic phase. The transition is highly congruent with a Kosterlitz–Thouless form. We observed that the critical point increases with the anisotropy.     

The spectrum of interacting metallic carbon nanotubes: exchange effects and universality

The low energy spectrum of finite size metallic single-walled carbon nanotubes (SWNTs) is determined. Starting from a tight binding model for the p_z electrons, we derive the low energy Hamiltonian containing all relevant scattering processes resulting from the Coulomb interaction, including the short ranged contributions becoming relevant for small diameter tubes. In combination with the substructure of the underlying honeycomb lattice the short ranged processes lead to various exchange effects. Using bosonization the spectrum is determined. We find that the ground state is formed by a spin 1 triplet, if 4n+2 electrons occupy the SWNT and the branch mismatch is smaller than the exchange splitting. Additionally, we calculate the excitation spectra for the different charge states and find the lifting of spin-charge separation as well as the formation of a quasi-continuum at higher excitation energies.     

Collective state of electrons and impurities with a spin

The problem of states of an electron system interacting with impurities that have a spin of 1/2 is considered. It is shown that in the calculation of the energy of the system, the electron spin-flip processes and the formation of electron–hole–impurity flip spin (hole against the background of electrons with another spin projection) play the major role. Such complexes are accumulated in the system (a sort of Bose condensate of complexes is formed); this reduces the energy of the system, which is a linear function of the initial interaction of an electron with the impurity spin (in contrast, for example, to the result obtained in perturbation theory). The hole-type excitation and the spin excitation have a gap in the spectrum. Small parameters of the problem are the interaction of electrons with impurity spins and the number of impurities. The electron–electron interaction is not taken into account. Impurities are assumed to be distributed at random, and calculations are performed using the known averaging over the positions of impurities.     

Localized electronic excitations in NiO studied with resonant inelastic X-Ray scattering at the Ni M threshold: evidence of spin flip

We studied the neutral electronic excitations of NiO localized at the Ni sites by measuring the resonant inelastic x-ray scattering (RIXS) spectra at the Ni M2,3 edges. The good energy resolution allows an unambiguous identification of several spectral features due to excitations. The dependence of the RIXS spectra on the excitation energy gives evidence of local spin flip and yields a value of 125 +/- 15 meV for the antiferromagnetic exchange interaction. Accurate crystal field parameters are also obtained.     

Elementary excitations for the Anderson model at finite temperatures

The elementary excitation spectrums for the Anderson model at finite temperatures are calculated by using the Bethe-ansatz solution. The formulation is based on the method of Yang and Yang, which was developed for the one-dimensional boson systems with the -function type interaction. We obtain the temperature dependence of the spin and the charge excitation spectrums. When the impurity level lies deeply from the Fermi level and the Coulomb interaction is suitably large, the resonant peak structure develops in the low energy region of the spin excitation spectrum and the hump structure grows around the impurity level of the charge excitation spectrum with decreasing temperature. Received: 21 January 1998 / Accepted: 17 March 1998     

Ultrafast solvent-induced spin-flip and nonadiabatic coupling: ClF in argon solids

Femtosecond pump-probe spectra show direct evidence for ultrafast solvent-induced spin flip in photodissociation-recombination events of ClF, a light diatomic molecule, for which the spin-orbit coupling is weak. The bound triplet states ((3)Pi) of ClF are probed and the dynamics for excitation to the singlet state ((1)Pi(1)) is compared with excitation to the triplet state B((3)Pi(0)). The population initially excited to the singlet state (1)Pi(1) is transferred to the bound triplet states (3)Pi within tau(f)=0.5 ps. Oscillations in the spectra indicate wave packet dynamics with the triplet state period of 300 to 400 fs in both cases. According to simulations of F(2)/Ar, most of the initially excited singlet state population is converted to repulsive and weakly bound triplet states within approximately 60 fs. In the first ps, 40% of the triplet population accumulates in the weakly bound (3)Pi states, in good accord with the experiment.     

Interchain impurity substitution effects on the magnetoelastic coupling in CuGeO3

We report the polarized far-infrared transmittance of Si-doped CuGeO₃ single crystals as a function of impurity concentration and applied magnetic field at low temperature. We use the behavior of the 44 cm⁻¹ spin gap excitation and the 98 cm⁻¹ zone-folding mode structure to investigate the interaction between the magnetic system and the lattice distortion. We find that interchain impurity substitution collapses the spin gap before it suppresses the lattice dimerization, a result that is understood in terms of the relative length scales for lattice distortion and spin singlet formation as well as criteria for magnetoelastic coupling in chains.     

Theory of an induced-moment spin glass

We review the derivation of generalised TAP equations for general quantum spin systems and apply the theory to a simple induced-moment spin glass model. We consider two-level systems with a singlet ground state and a triplet excited state, which interact via long range random exchange couplings. For not too large energy splitting the spin glass state can be stabilised.     

Interaction between localized moments in dilute alloys: Correlation effects

The effect of including dynamical correlations between electrons of opposite spins in determining the ground state energy of a pair of magnetically interacting impurity atoms in an otherwise normal metal is discussed. It is found that in the ground state of such a system the spins of the magnetic impurity atoms are aligned antiparallel. In other words, the interaction between the localized states is of antiferromagnetic exchange type. This result differs sharply from that predicted by the Hartree-Fock (H-F) theory, in which the ground state of the system can be either ferro- or antiferromagnetic, depending on the energies of the spin up and spin down electrons relative to the Fermi energy. The calculations are performed using many-body Green's function techniques in thet-matrix approximation.     

Tunneling Conductance in a Normal Metal/Ferromagnetic Superconductor Nano-Junction at a Finite Temperature

In this study, we investigate the tunneling conductance at a finite temperature in a normal metal/ferromagnetic superconductor nano-junction where the ferromagnetic superconductor (FS) is in three different cooper pairing states: spin singlet s-wave pairing (SWP), spin triplet opposite spin pairing (OSP), and spin triplet equal spin pairing (ESP) while including Fermiwave mismatch (FWM) and effective mass mismatch (EMM) in two sides of the nano-junction. We find that the conductance shows clearly different behaviors all depending on the symmetries of cooper pairing in a mannerthat the conductance spectra shows a gap-like structure, two interior dipsstructure and zero bias peak for SWP, OSP, and ESP, respectively. Also, theeffective FS gap (δ_eff) is a linear and decreasing function of exchange field. The slope of (δ_eff) versus exchange field for OSP is twice the SWP. Thus, we can determine the spin polarization of N/FS nano-junction based on the dependence of (δ_eff) to exchange field.     

Role of surface anisotropy for magnetic impurities in electron dephasing and energy relaxation and their size effect

Recently the electron dephasing and energy relaxation due to different magnetic impurities have been extensively investigated experimentally in thin wires, and in this Letter these quantities are theoretically studied. It was shown earlier that a magnetic impurity in a metallic host with strong spin-orbit interaction experiences a surface anisotropy of the form H=K(d)(nS)(2) which causes size effects for impurities with integer spin. Here we show that the dephasing and the energy relaxation are influenced by the surface anisotropy in very different ways for integer spin having a singlet ground state. That must result also in strong size effects and may resolve the puzzle between the concentrations estimated from the two kinds of experiments.     

Effects of Coulomb interactions on spin states in vertical semiconductor quantum dots

The effects of direct Coulomb and exchange interactions on spin states are studied for quantum dots contained in circular and rectangular mesas. For a circular mesa a spin-triplet favored by these interactions is observed at zero and nonzero magnetic fields. We tune and measure the relative strengths of these interactions as a function of the number of confined electrons. We find that electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle state degeneracy, and find that the spin-configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states. For a rectangular mesa we observe no signatures of the spin-triplet at zero magnetic field. Due to the anisotropy in the lateral confinement single-particle state degeneracy present in the circular mesa is lifted, and Coulomb interactions become weak. We evaluate the degree of the anisotropy by measuring the magnetic field dependence of the energy spectrum for the ground and excited states, and find that at zero magnetic field the spin-singlet is more significantly favored by the lifting of level degeneracy than by the reduction in the Coulomb interaction. We also find that the spin-triplet is recovered by adjusting the level degeneracy with magnetic field. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

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.     

Magnetic states of an isotropic magnet with the “large” ion spin <Emphasis Type="Italic">S</Emphasis> = 3/2

The magnetic state of a system of particles with a “large” spin of 3/2 in the presence of isotropic exchange interaction in the system has been studied on the basis of a derived spin Hamiltonian. It has been shown that, at a positive contribution of the exchange interaction, an unstable nematic state appears and transforms to a stable ferromagnetic state (with an average spin of 3/2). The excitation spectrum in the ferromagnetic state is a triply degenerate Goldstone-type gapless magnon mode. At a negative sign of the exchange contribution, an antinematic state is stable with respect to a transition to a ferromagnetic state, which is forbidden. In this case, the antinematic always occurs in the state of a phase transition to an unstable antiferromagnetic state, the excitation spectrum of which is characterized by a single nondegenerate Goldstone mode.     

Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock–Darwin levels. In QDs with very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified.     

Coherent magnetic oscillation in the spin ladder system alpha(')- NaV2O5

We report the first observation of coherent magnetic excitations in a spin ladder system NaV2O5 by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at 127 cm(-1) (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at T(C) = 34 K. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at 303 cm(-1).     

Electronic States and Spatial Charge Distribution of Single Mn Impurity in Diluted Magnetic Semiconductors

The electronic and magnetic properties as well as the spatial charge distribution of single Mn impurity in III--V diluted magnetic semiconductors are obtained when the degeneracy of the p orbits contributed from the four nearest-neighbouring As(N) atoms is taken into account. We show that in the ground state, the Mn spin is strongly antiferromagnetically coupled to the surrounding As(N) atoms when the p-d hybridization V_pd is large and both the hole level E_v and the impurity level E_d are close to the Fermi energy. The spatial charge distribution of the Mn acceptor in the (110) plane is non-spherically symmetric, in good agreement with the recent STM images.