On-site Impurity Research Articles

Effects of Disordered Ru Substitution inBaFe2As2: Possible Realization of Superdiffusion in Real Materials

An unexpected insensitivity of the Fermi surface to impurity scattering is found in Ru substituted BaFe(2)As(2) from first-principles theory, offering a natural explanation of the unusual resilience of transport and superconductivity to a high level of disordered substitution in this material. This robustness is shown to originate from a coherent interference of correlated on-site and intersite impurity scattering, similar in spirit to the microscopic mechanism of superdiffusion in one dimension. Our result also demonstrates a strong substitution dependence of the Fermi surface and carrier concentration and provides a resolution to current discrepancies in recent photoelectron spectroscopy. These effects offer a natural explanation of the diminishing long-range magnetic, orbital, and superconducting orders with high substitution.

Local suppression of the superfluid density of PuCoGa5by strong onsite disorder

We present superfluid density calculations for the unconventional superconductor PuCoGa$_5$ by solving the real-space Bogoliubov-de Gennes equations on a square lattice within the Swiss Cheese model in the presence of strong on-site disorder. We find that despite strong electronic inhomogeneity, one can establish a one-to-one correspondence between the local maps of the density of states, superconducting order parameter, and superfluid density. In this model, strong on-site impurity scattering punches localized holes into the fabric of d-wave superconductivity similar to a Swiss cheese. Already a two-dimensional impurity concentration of $n_{\rm imp} = 4%$ gives rise to a pronounced short-range suppression of the order parameter and a suppression of the superconducting transition temperature $T_c$ by roughly 20% compared to its pure limit value $T_{c0}$, whereas the superfluid density $\rho_s$ is reduced drastically by about 70%. This result is consistent with available experimental data for aged (400 day-old) and fresh (25 day-old) PuCoGa$_5$ superconducting samples. In addition, we show that the $T^2-$dependence of the low-$T$ superfluid density, a signature of dirty d-wave superconductivity, originates from a combined effect in the density of states of `gap filling' and `gap closing'. Finally, we demonstate that the Uemuera plot of $T_c$ vs.\ $\rho_s$ deviates sharply from the conventional Abrikosov-Gor'kov theory for radiation-induced defects in PuCoGa$_5$, but follows the same trend of short-coherence-length high-$T_c$ cuprate superconductors.

Diffusion and Criticality in Undoped Graphene with Resonant Scatterers

A general theory is developed to describe graphene with an arbitrary number of isolated impurities. The theory provides a basis for an efficient numerical analysis of the charge transport and is applied to calculate the Dirac-point conductivity σ of graphene with resonant scatterers. In the case of smooth resonant impurities the symmetry class is identified as DIII and σ grows logarithmically with increasing impurity concentration. For vacancies (or strong on-site potential impurities, class BDI) σ saturates at a constant value that depends on the vacancy distribution among two sublattices.

Large system asymptotics of persistent currents in mesoscopic quantum rings

We consider a one-dimensional mesoscopic quantum ring filled with spinless electrons and threaded by a magnetic flux, which carries a persistent current at zero temperature. The interplay of Coulomb interactions and a single on-site impurity yields a nontrivial dependence of the persistent current on the size of the ring. We determine numerically the asymptotic power law for systems up to $32\text{ }000$ sites for various impurity strengths and compare with predictions from Bethe ansatz solutions combined with bosonization. The numerical results are obtained using an improved functional renormalization-group (fRG) method. We apply the density-matrix renormalization-group (DMRG) and exact diagonalization methods to benchmark the fRG calculations. We use DMRG to study the persistent current at low electron concentrations in order to extend the validity of our results to quasicontinuous systems. We briefly comment on the quality of calculated fRG ground-state energies by comparison with exact DMRG data.

Resonant States of a Momentum-Dependent Impurity Potential in a d-Wave Superconductor

Controlled impurity substitution is an important tool in the identification of a symmetry of the superconducting ground state. Particularly efficient are the scanning tunneling microscopy (STM) measurements of the local density of states (LDOS) which probe directly and with atomic precision excited states in the vicinity of the impurity atom. STM measurements of LDOS around single Zn [1] or Ni [2] impurity atom as well as Cu vacancy [3] in superconducting Bi2Sr2CaCu2O8+δ revealed a distinct fourfold symmetry of LDOS specific for a d-wave superconductor. Although, the main features of complex STM patterns in cuprates are captured by a model on-site impurity scattering [4–6], studies of extended impurity potentials [7–9] are motivated by some still unresolved features of LDOS in these compounds [10]. We continue the issue of anisotropic in the reciprocal space impurity potential [11–15] and discuss its effect on the impurity-bound states in high-temperature superconductors.

Nonlocal effect of local nonmagnetic impurity in high- [formula omitted] superconductors: Induced local moment and huge residual resistivity

We study a Hubbard model with a strong onsite impurity potential based on an improved fluctuation-exchange (FLEX) approximation, which we call the GV I -FLEX method. We find that (i) both local and staggered susceptibilities are strongly enhanced around the impurity. By this reason, (ii) the quasiparticle lifetime as well as the local density of states (DOS) are strongly suppressed in a wide area around the impurity (like a Swiss cheese hole), which causes the “huge residual resistivity” beyond the s-wave unitary scattering value. These results by the GV I method naturally explains the various impurity effects in HTSCs in a unified way, which had been a long-standing theoretical problem.

Effect of a nonmagnetic impurity in a nearly antiferromagnetic Fermi liquid: Magnetic correlations and transport phenomena

In nearly antiferromagnetic (AF) metals such as high-Tc superconductors (HTSC's), a single nonmagnetic impurity frequently causes nontrivial widespread change of the electronic states. To elucidate this long-standing issue, we study a Hubbard model with a strong onsite impurity potential based on an improved fluctuation-exchange (FLEX) approximation, which we call the GV^I-FLEX method. This model corresponds to the HTSC with dilute nonmagnetic impurity concentration. We find that (i) both local and staggered susceptibilities are strongly enhanced around the impurity. By this reason, (ii) the quasiparticle lifetime as well as the local density of states (DOS) are strongly suppressed in a wide area around the impurity (like a Swiss cheese hole), which causes the ``huge residual resistivity'' beyond the s-wave unitary scattering limit. We stress that the excess quasiparticle damping rate caused by impurities has strong momentum-dependence due to non-s-wave scatterings induced by many-body effects, so the structure of the ``hot spot/cold spot'' in the host system persists against impurity doping. This result could be examined by the ARPES measurements. In addition, (iii) only a few percent of impurities can causes a ``Kondo-like'' upturn of resistivity ($d\rho/dT<0$) at low temperatures when the system is very close to the AF quantum critical point (QCP). The results (i)-(iii) obtained in the present study, which cannot be derived by the simple FLEX approximation, naturally explains the main impurity effects in HTSC's. We also discuss the impurity effect in heavy fermion systems and organic superconductors.

Instabilities and bifurcations of nonlinear impurity modes.

We study the structure and stability of nonlinear impurity modes in the discrete nonlinear Schrödinger equation with a single on-site nonlinear impurity emphasizing the effects of interplay between discreteness, nonlinearity, and disorder. We show how the interaction of a nonlinear localized mode (a discrete soliton or discrete breather) with a repulsive impurity generates a family of stationary states near the impurity site, as well as examine both theoretical and numerical criteria for the transition between different localized states via a cascade of bifurcations.

Multimode soliton dynamics in perturbed ladder lattices.

We investigate the interplay between the longitudinal and lateral solitonic modes in perturbed ladder lattices in regard to the transmission of soliton wave packet. (1) In a longitudinal uniform field the lateral and longitudinal solitonic modes are shown to be independent. However, unlike in the unperturbed case the dynamics of the soliton center of mass becomes confined within a finite spatial domain via the Bloch-Zener mechanism in the longitudinal direction and due to the transverse finiteness of the ladder in the lateral one. (2) The segment of on-site impurities causes the soliton mode-mode mixing. As a result the soliton exhibits rather complex two- or three-dimensional dynamics accompanied by wave radiation which may give rise to soliton trapping. Nevertheless, under some specific conditions the soliton is able to bypass even the strong impurities slaloming between them. In particular, the slalom soliton dynamics is possible on a ladder lattice with a segment of zigzig-distributed on-site impurities. We formulate the conditions favorable to the case and show that their violation gives rise to either soliton trapping on or soliton reflection from the impure segment. (3) Finally, we study the effect of the modified transverse bond on the longitudinal soliton dynamics and reveal that it might act on the soliton as either an attractive or a repulsive potential, depending on the sign of the transverse energy of the ingoing soliton. The effect is essentially a solitonic one and becomes strictly pronounced for heavy solitons, when imperfection-induced radiation effects are exponentially suppressed. We expect that transverse-bond imperfection could serve as a filter selecting the solitons with prescribed properties. A similar function is feasible for zigzag-distributed on-site impurities too.

Slalom soliton dynamics on a ladder lattice with zig-zag distributed impurities

The center of mass of a soliton distribution on a regular ladder lattice is shown to exhibit a sinusoidal motion. Provided that the initial parameters of soliton are properly adjusted, this sort of dynamics persists on an irregular ladder lattice with a segment of zig-zag distributed on-site impurities of rather strong intensity. As a result, the soliton is able to slalom between such obstacles without any visible changes on its main characteristics. We formulate the conditions favorable to the case and show that their violation gives rise to either the soliton reflection from or soliton trapping on the impure segment, besides to some radiation effects of the spatially extended waves.

Crossover between the ballistic and diffusive regime of the spin-conductance and giant magnetoresistance in magnetic multilayered nanostructures

We analyze the interplay between disorder and band structure in current perpendicular to the planes giant magnetoresistance (GMR). We consider finite magnetic multilayers attached to pure crystalline leads, described by a tight-binding simple cubic two-band model $(s\ensuremath{-}d).$ Several models of disorder are considered, including random on-site potentials, lattice distortions, impurities, vacancies, and cross-section fluctuations. Magnetotransport properties are calculated in the zero-temperature zero-bias limit, within the Landauer-B\"uttiker formalism. Using a very efficient numerical scattering technique, we are able to perform simulations, over large length scales, and to investigate spin transport in the ballistic, diffusive, and localized regimes, as well as the crossover between them. The competition between disorder-induced mean-free-path reduction and disorder-induced spin asymmetry enhancement of the conductance highlights several different regimes of GMR.

Tunneling Ionization of Autolocalized DX- Centers in Terahertz Fields.

Tunneling ionization of ${\mathrm{DX}}^{\ensuremath{-}}$ centers in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ has been observed in terahertz radiation fields. Tunneling times have been measured for autolocalized and on-site deep impurities. It is shown that in one case the tunneling time is smaller and in the other larger than the reciprocal temperature multiplied by a universal constant due to the different tunneling trajectories. This allows one to distinguish in a direct way between the two types of configuration potentials of impurities.

Lyapunov exponent and transfer-matrix spectrum of the random binary alloy.

The one-dimensional Anderson model with binary distributed onsite disorder is investigated using the transfer-matrix approach. The random matrix product is transformed into an iterated conformal map and the convergence of the mapping is studied for various values of the concentration of onsite impurities and the energy. For dilute impurity concentrations the convergence of the mapping is strongly energy dependent and exhibits interesting behavior in the complex mapping plane. As the impurity concentration increases the mapping converges to the unit circle which results from the underlying conformal structure. Explicit expressions for the Lyapunov exponent and the eigenvalues of the transfer matrix are obtained in terms of the conformal map. From the Lyapunov exponent, the localization length and the integrated density of states are calculated. In the dilute limit there exist states in which the localizaiton length exceeds the system size. These states correspond to extended electronic states and it is shown that the number of extended states scales with system size as ${\mathit{N}}_{\mathrm{extended}}$\ensuremath{\propto}${\mathit{N}}^{\mathrm{\ensuremath{-}}2}$. Increasing the impurity concentration beyond a critical value destroys these states and the localization length over the entire energy range becomes smaller than the system size.