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.     

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.     

Green's function calculations of the hyperfine interaction for impurities in metals andfor metallic interfaces

We present local density functional calculations for magnetic impurities andmagnetic monolayers in non-magnetic metals. The calculations employ a multiple scattering (KKR) Green's function method for impurities in the bulk andfor ideal surfaces and interfaces. In particular we discuss the moment formation of 3d and4d impurities in alkali andnoble metals. Special emphasis is put on an accurate calculation of the host polarization around 3d impurities in Cu andPd. While the calculated impurity hyperfine fields in Cu contain rather large errors due to the local density approximation, the induced fields of the Cu atoms agree very well with experiments. We also present similar calculations for magnetic monolayers andthe corre-sponding induced host polarization in Cu andPd.     

Local electronic structures in electron-doped cuprates with coexisting orders

Motivated by the observation of a so-called non-monotonic gap in recent angle-resolved photoemission spectroscopy measurement, we study the local electronic structure near impurities in electron-doped cuprates by considering the influence of antiferromagnetic (AF) spin-density-wave (SDW) order. We find that the evolution of density of states (DOS) with AF SDW order clearly indicates the non-monotonic d-wave gap behavior. More interestingly, the local DOS for spin-up is much different from that for spin-down with increasing AF SDW order. As a result, the impurity induced resonance state near the Fermi energy exhibits a spin-polarized feature. These features can be detected by spin-polarized scanning tunneling microscopy experiments.     

Fractional impurity moments in two-dimensional noncollinear magnets

We study dilute magnetic impurities and vacancies in two-dimensional frustrated magnets with noncollinear order. Taking the triangular-lattice Heisenberg model as an example, we use quasiclassical methods to determine the impurity contributions to the magnetization and susceptibility. Most importantly, each impurity moment is not quantized but receives nonuniversal screening corrections due to local relief of frustration. At finite temperatures, where bulk long-range order is absent, this implies an impurity-induced magnetic response of Curie form, with a prefactor corresponding to a fractional moment per impurity. We also discuss the behavior in an applied magnetic field, where we find a singular linear-response limit for overcompensated impurities.     

Local moment formation at interstitial impurity sites in metals

We have calculated the electronic structure and related properties around an interstitial impurity in several metallic hosts. This was done using the real-space linear muffin tin orbital scheme (RS-LMTO-ASA), a first-principles, self-consistent approach implemented directly in real space. We show that interstitial Fe does not develop a local moment in trivalent and tetravalent Sc, Y, Ti and Zr hosts. In divalent Ba, Ca, Sr and Yb we find that the appearance of local moment is extremely dependent on nearest neighbor relaxation, while in alkali metals such as K, we expect the interstitial Fe impurity to be magnetic. We show that these trends can be qualitatively understood using simple ideas based on the Wolff model and the Stoner criterium. We also consider Fe impurities in Gd, a trivalent magnetic host. We find an unusually large induced local moment at the interstitial Fe site and discuss the origin of this effect. Finally, we compare our results with TDPAD and in beam Mössbauer experiments in these systems.     

Persistence of Li induced Kondo moments in the superconducting state of cuprates

Using 7Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T(c) in YBa 2Cu 3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.     

Infrared absorption of various isolated impurities in GaAs

We have calculated the infrared absorption at the local mode frequency for various isolated impurities such as B, Al, Mg, Si, C, and P in GaAs. To calculate the local mode frequency we take into account the effects from impurity mass defect and local force constant shifts and use the symmetry properties of the lattice. To calculate the absorption we consider contributions to the dipole moment from impurity and nearest neighbour charges and from charge migration effects. We suggest a physical model which accounts for impurity and nearest neighbour charges and polarizabilities.     

Convergence of energy scales on the approach to a local quantum critical point

We find the emergence of strong correlations and universality on the approach to the quantum critical points of a two-impurity Anderson model. The two impurities are coupled by an interimpurity exchange interaction J and direct interaction U{12} and are hybridized with separate conduction channels. The low energy behavior is described in terms of renormalized parameters. We show that on the approach to the transitions to a local singlet and a local charged ordered state, the quasiparticle weight factor z→0, and the renormalized parameters can be expressed in terms of a single energy scale T{*}. The values of the renormalized interaction parameters in terms of T{*} can be predicted from the condition of continuity of the spin and charge susceptibilities, and correspond to strong correlation. These predictions are confirmed by the numerical renormalization group calculations, including the case when the on site interaction U=0.     

Direct observation of orbital magnetism in cubic solids

We present x-ray magnetic circular dichroism determinations of the orbital/spin magnetic moment ratios of dilute 3d-series impurities in Au (and Cu) host matrices. This is the first direct measurement of considerable orbital moments in cubic symmetry for a localized impurity in a bulk metal host. It is shown that the unquenching of orbital magnetism depends on a delicate balance of hybridization effects between the local impurity with the host and the filling of the 3d states of the impurity. The results are accompanied by ab initio calculations that support our experimental findings.     

Dynamics of fluctuations below a stationary bifurcation to electroconvection in the planar nematic liquid crystal N4

We fitted C(k,tau,epsilon) proportional to exp([-sigma(k,epsilon)tau] to time-correlation functions C(k,tau,epsilon) of structure factors S(k,t,epsilon) of shadowgraph images of fluctuations below a supercritical bifurcation at V(0)=V(c) to electroconvection of a planar nematic liquid crystal in the presence of a voltage V=sqrt[2]V(0)cos((2pift) [k=(p,q) is the wave vector and epsilon identical with V(2)(0)/V(2)(c)-1]. There were stationary oblique (normal) rolls at small (large) f. Fits of a modified Swift-Hohenberg form to sigma(k,epsilon) gave f-dependent critical behavior for the minimum decay rates sigma(0)(epsilon) and the correlation lengths xi(p,q)(epsilon).     

Local mode sideband IR absorption and second harmonic intensity calculation for various isolated impurities in GaAs

We have calculated the intensity and shape of the local mode sidebands and the second harmonic intensity for various isolated impurities in GaAs. To calculate the absorption we have considered the second order dipole moment in the harmonic approximation. The absorption depends on the values of the impurity charge and polarizability, on the corresponding quantities for the nearest neighbours of each impurity and on a charge migration factor. The latter accounts for charge variations during the motion. Although these quantities vary from impurity to impurity, the results are presented in a general form valid also for other impurities. The point group symmetry properties are used in order to give a physical meaning to the results. The effects on the dynamical properties from the mass defect and the local changes in the force constant are also taken into account.     

U impurities in Au

The properties of U impurities in Au have been studied by magnetic susceptibility, electrical resistivity, and electron spectroscopic measurements. The results indicate that U forms a well-defined local magnetic moment of 3.6 _B , giving rise to a Kondo resistance anomaly at low temperature. This is in agreement with spectroscopic data, which show localized impurity states well removed from the Fermi level. Comparison to calculated atomic multiplets shows the need for inclusion of crystal or ligand field effects for the multiplets. The intra-5f Coulomb correlation energy is found to be 2.6 eV.     

Photoselection of impurity molecules with respect to the orientation of their stark moments induced by an amorphous matrix

A method of selective (relative to the magnitude of the linear Stark effect in zero-dipole impurity molecules) burning of holes in the spectra of amorphous matrices is studied. It is shown that this method makes it possible not only to select a subensemble of impurity molecules having both preset energy of the 0-0 transition and magnitude of the matrix-induced Stark moment, but also to divide this subensemble into two fractions whose Stark moments are predominantly oriented either along the transition moment or perpendicular to it. The possibility of experimental verification of the most popular hypothesis about the origin of induced Stark moments, which neglects a possible correlation between the direction of the Stark moment and that of the transition dipole moment, is considered. The method described also allows one to study experimentally the correlation of some other photophysical parameters of impurity molecules with the direction of local internal fields relative to the vector of the transition dipole moment.     

Critical local-moment fluctuations,anomalous exponents,and omega/T scaling in the Kondo problem with a pseudogap

Experiments in heavy-fermion metals and related theoretical work suggest that critical local-moment fluctuations can play an important role near a zero-temperature phase transition. We study such fluctuations at the quantum critical point of a Kondo impurity model in which the density of band states vanishes as /epsilon/(r) at the Fermi energy (epsilon=0). The local spin response is described by a set of critical exponents that vary continuously with r. For 0相似文献   

Quasiparticle states and quantum interference induced by magnetic impurities on a two-dimensional topological superconductor

We theoretically study the effect of localized magnetic impurities on two-dimensional topological superconductor (TSC). We show that the local density of states (LDOS) can be tuned by the effective exchange field m, the chemical potential μ of TSC, and the distance Δr as well as the relative spin angle α between two impurities. The changes in Δr between two impurities alter the interference and result in significant modifications to the bonding and antibonding states. Furthermore, the bound-state spin LDOS induced by single and double magnetic impurity scattering, the quantum corrals and the quantum mirages are also discussed. Finally, we briefly compare the impurities in TSC with those in topological insulators.     

Effect of magnetic impurities in chiral <Emphasis Type="Italic">p</Emphasis>-wave superconducting nanoloops

The effect of many magnetic impurities in symmetric chiral p-wave superconducting nanoloops is investigated by numerically solving the BdG equations self-consistently. Two magnetic impurities can lead to the appearance of two impurity bound levels close to the Fermi level. The arising bound states can cross the Fermi level at the same impurity strength for the case of two independent midway impurities, while multiple zero-energy states can be obtained at two separated values of impurity strength when two independent edge impurities are present. Moreover, the multiple zero modes can only show up for appropriate relative positions between two edge impurities due to the quantum interference effect. Particularly, for some appropriate strength of two independent midway impurities, the impurity bound levels cross the Fermi level twice with increasing threaded flux, while the multiple zero modes can not emerge in the flux evolution.     

Chemically active substitutional nitrogen impurity in carbon nanotubes

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 at. %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.     

Self-consistent numerical study of pure and impurity doped three-band Hubbard model

The three-band Hubbard model — both pure and with static non-magnetic impurities — has been studied within a self-consistent numerical Hartree-Fock (HF) scheme. The system shows nesting properties only in the absence of direct O-O hopping. Spin excitations in the system are gapless with the existence of a Goldstone mode in the broken-symmetry state. The variation of spinwave velocity with Cu-site Coulomb repulsion shows a (1/(2U _d)+(1/Δ)) dependence in the strong-coupling limit. Each non-magnetic impurity in the system gives rise to two gap states for a particular spin and the local moment produced is robust even at finite concentration of mobile hole doping. The gapless Goldstone mode is preserved even in case of unequal concentration of impurities on the two sublattices.