On the ubiquity of spin glass concepts and methods

We give a brief overview of some recent developments using concepts and techniques from spin glass theory in the study of other disordered systems. We discuss in particular the equilibrium correlations of a vortex lattice pinned by impurities. We mention how the replica approach, and the spontaneous breaking of replica symmetry, can be brought to bear on these problems.     

Theory of magnetic impurities in superconductors. II

The depression of the superconducting transition temperature due to the pair breaking effect of magnetic impurities is calculated exactly within the Nagaoka-Suhl approach to the Kondo problem in superconductors.     

Indirect exchange interaction in Rashba-spin-orbit-coupled graphene nanoflakes

We study the indirect exchange interaction, named Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, between localized magnetic impurities in graphene nanoflakes with zig-zag edges in the presence of the Rashba spin-orbit interaction (RSOI). We calculate the isotropic and anisotropic RKKY amplitudes by utilizing the tight-binding (TB) model. The RSOI, as a gate tunable variable, is responsible for changes of the RKKY amplitude. We conclude that there is not any switching of the magnetic order (from ferro- to antiferro-magnetic and vice versa) in such a system through the RSOI. The dependence of the RKKY amplitude on the positions of the magnetic impurities and the size of the system is studied. The symmetry breaking, which can occur due to the Rashba interaction, leads to spatial anisotropy in the RKKY amplitude and manifests as collinear and noncollinear terms. Our results show the possibility of control and manipulation of spin correlations in carbon spin-based nanodevices.     

Local symmetry breaking by impurities and mode splitting in doped SmS

We introduce the idea of local symmetry breaking by impurities to explain the recently observed splitting of the J=0-->1 propagating excitation in doped Sm1-xYxS. While preserving the global cubic symmetry of the crystal, Y impurities change the local crystal-field environment of each Sm ion from cubic to tetragonal, thereby splitting the J=1 triplet into a m(i).J=0 level with energy Delta(1) and a m(i).J=+/-1 doublet with energy Delta(2)>Delta(1). A model with a randomly oriented quantization axis m(i) fits not only the observed mode frequencies but also their intensities, which strongly depends on the wave vector.     

Incipient nodal pairing in planar d-wave superconductors

The possibility of a second pairing transition d --> d + is ( d + id') in planar d-wave superconductors which occurs in the absence of external magnetic field, magnetic impurities, or boundaries is established in the framework of the nonperturbative phenomenon of dynamical chiral symmetry breaking in the system of (2+1)-dimensional Dirac-like nodal quasiparticles. We determine the critical exponents and quasiparticle spectral functions that characterize the corresponding quantum-critical behavior and discuss some of its potentially observable spectral and transport features.     

Computer simulations in the study of gold nanowires: the effect of impurities

Suspended gold nanowires have recently been made in an ultra-high vacuum ambient and were imaged by electron microscopy. Two puzzles were presented: one atom thick wires are produced and some of the atomic distances between these atoms before their breaking were too large. Simulations using realistic molecular dynamics method were able to unveil some processes to explain the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. The calculations showed how defects induce the formation of constrictions that eventually will form the one-atom chains. Atomically thin chains, five atoms long were obtained, before breaking. The results were in excellent agreement with experimental results except for the large Au-Au distances. In fact no theoretical calculation of pure gold nanowires have been able to produce such large distances. Light impurities that cannot be imaged in these experiments may be responsible for these large Au-Au distances. Using ab initio total energy calculations based on the density functional theory, we have studied the effect of H, C, O, N, B, S, CH, CH₂, and H₂ impurities on the nanowire’s electronic and structural properties, in particular how they affect the rupture of the nanowire. We find that the impurities tend to locally increase the nanowire’s strength, in such a way that its rupture always occurs at an Au-Au bond and never at an Au-X bond (X being an impurity). In particular, oxygen seems to form very stable bonds that may be used to pull longer Au chains. Regarding the observed large Au-Au bond lengths, it was found, based on quasi-static calculations, that the best candidate to explain the large distances is H. However, some particular experimental conditions may lead to different results. PACS 68.65.-k; 68.37.Lp; 71.15.Pd     

Self-localization of holes in a lightly doped Mott insulator

We show that lightly doped holes will be self-trapped in an antiferromagnetic spin background at low-temperature, resulting in spontaneous translational symmetry breaking. The underlying Mott physics is responsible for such novel self-localization of charge carriers. Interesting transport and dielectric properties are found as the consequences, including large doping-dependent thermopower and dielectric constant, low-temperature variable-range-hopping resistivity, as well as high-temperature strange-metal-like resistivity, which are consistent with experimental measurements in the high-T_c cuprates. Disorder and impurities only play a minor and assistant role here.     

Enhanced superconductivity by correlations between two Kondo impurities

We study the interplay between magnetic correlations of two Kondo impurities and superconducting singlet pairing. Performing a Schrieffer-Wolff transformation in the zero-bandwidth limit of the two-impurity Anderson model we obtain the Hamiltonian of two magnetic impurities and we add a superconducting term to the conduction electrons. The model allows us to study the effect of the magnetic correlation between the impurities on the superconducting ground state. At zero temperature, different superconducting ground states can be obtained depending on the magnitude of magnetic coupling between S₁ and S₂. For increasing coupling, the superconducting region is enlarged showing an interesting result: in the strong coupling limit, where the impurities are in a very strong ferromagnetic correlation state, half of the conduction electrons are decoupled from the local moments of the impurities and take advantage of the superconducting pairing lowering the ground state energy. On the contrary, when the coupling between S₁and S₂ decreases, the scenario of the two independent Kondo impurities in presence of superconductivity emerges and all the conduction electrons are involved in the pair breaking physics. At finite temperature, we obtain the phase diagram and we observe a region of parameters where the re-entrance phenomenon occurs.     

Anomalous temperature dependence of the dephasing time in mesoscopic kondo wires

We present measurements of the magnetoconductance of long and narrow quasi-one-dimensional gold wires containing magnetic iron impurities in a temperature range extending from 15 mK to 4.2 K. The dephasing rate extracted from the weak antilocalization shows a pronounced plateau in a temperature region of 300-800 mK, associated with the phase breaking due to the Kondo effect. Below the Kondo temperature, the dephasing rate decreases linearly with temperature, in contradiction with standard Fermi-liquid theory. Our data suggest that the formation of a spin glass due to the interactions between the magnetic moments is responsible for the observed anomalous temperature dependence.     

Topological response theory of doped topological insulators

We generalize the topological response theory of three-dimensional topological insulators (TI) to metallic systems-specifically, doped TI with finite bulk carrier density and a time-reversal symmetry breaking field near the surface. We show that there is an inhomogeneity-induced Berry phase contribution to the surface Hall conductivity that is completely determined by the occupied states and is independent of other details such as band dispersion and impurities. In the limit of zero bulk carrier density, this intrinsic surface Hall conductivity reduces to the half-integer quantized surface Hall conductivity of TI. Based on our theory we predict the behavior of the surface Hall conductivity for a doped topological insulator with a top gate, which can be directly compared with experiments.     

Crossover between weak antilocalization and weak localization in a magnetically doped topological insulator

We report transport studies on magnetically doped Bi(2)Se(3) topological insulator ultrathin films grown by molecular beam epitaxy. The magnetotransport behavior exhibits a systematic crossover between weak antilocalization and weak localization with the change of magnetic impurity concentration, temperature, and magnetic field. We show that the localization property is closely related to the magnetization of the sample, and the complex crossover is due to the transformation of Bi(2)Se(3) from a topological insulator to a topologically trivial dilute magnetic semiconductor driven by magnetic impurities. This work demonstrates an effective way to manipulate the quantum transport properties of the topological insulators by breaking time-reversal symmetry.     

Impurity scattering,Friedel oscillations and RKKY interaction in graphene

We investigate Friedel Oscillations (FO) surrounding a point scatterer in graphene. We find that the long-distance decay of FO depends on the symmetry of the scatterer. In particular, the FO of the charge density around a Coulomb impurity show a faster, δρ∼1/ r³, decay than in conventional 2D electron systems. In contrast, the FO of the exchange field which surrounds atomically sharp defects breaking the hexagonal symmetry of the honeycomb lattice decay according to the 1/r² law. We discuss the consequences of these findings for the temperature dependence of the resistivity of the material and the RKKY interaction between magnetic impurities.     

Pair-breaking in kondo superconductors

An earlier theory for superconductors with magnetic impurities is extended by taking into account explicitly the energy dependence of pair breaking. Results on the transition temperature and the specific heat jumps are in very good agreement with experimental data on (La_1-cCe_c)Al₂.     

Quantum theory of metallic nanocohesion

The conducting and mechanical properties of ultrasmall metallic structures are calculated using the electronic scattering matrix, evaluated in the free electron approximation. Force oscillations of the order are predicted when a metallic quantum wire is stretched to the breaking point, which are synchronized with quantized jumps in the conductance. Coherent backscattering from impurities is shown to lead to fine structure (a “quantum fingerprint”) in the force oscillations. 1997 Elsevier Science B.V. All rights reserved.     

Effects of bulk charged impurities on the bulk and surface transport in three-dimensional topological insulators

In the three-dimensional topological insulator (TI), the physics of doped semiconductors exists literally side-by-side with the physics of ultrarelativistic Dirac fermions. This unusual pairing creates a novel playground for studying the interplay between disorder and electronic transport. In this mini-review, we focus on the disorder caused by the three-dimensionally distributed charged impurities that are ubiquitous in TIs, and we outline the effects it has on both the bulk and surface transport in TIs. We present self-consistent theories for Coulomb screening both in the bulk and at the surface, discuss the magnitude of the disorder potential in each case, and present results for the conductivity. In the bulk, where the band gap leads to thermally activated transport, we show how disorder leads to a smaller-than-expected activation energy that gives way to variable-range hopping at low temperatures. We confirm this enhanced conductivity with numerical simulations that also allow us to explore different degrees of impurity compensation. For the surface, where the TI has gapless Dirac modes, we present a theory of disorder and screening of deep impurities, and we calculate the corresponding zero-temperature conductivity. We also comment on the growth of the disorder potential in passing from the surface of the TI into the bulk. Finally, we discuss how the presence of a gap at the Dirac point, introduced by some source of time-reversal symmetry breaking, affects the disorder potential at the surface and the mid-gap density of states.     

Electron spin-lattice relaxation anomaly and the local pseudo freeze-out of impurity dynamics in H-bonded ferroelectrics

The strong decrease in the electron spin-lattice relaxation rate at the ferroelectric transition temperature T_c and the simultaneous increase in the transverse spin-spin relaxation rate can be both understood in terms of the local “spontaneous freeze-out” model of impurity dynamics recently proposed to explain the spontaneous dynamic symmetry breaking observed far above T_c in the EPR spectra of H-bonded ferroelectrics doped with paramagnetic impurities.     

Effect of structure doping profile on the current switching-off process in power semiconductor opening switches

Using a physicomathematical model, the process of current breaking in power semiconductor opening switches was investigated in p ⁺-p-n-n ⁺ structures with different doping profiles. The model takes account of the actual doping profile of a structure, diffusion and drift of current carriers in a strong electric field, recombination via deep impurities and Auger recombination, and impact ionization in a dense plasma. The calculation of the electrical circuit of an opening switch is based on solution of Kirchhoff’s equations. It has been shown that in the nanosecond regime of breaking superhigh current densities with densities of the interrupted currents from a few to tens of kA/cm², the dominant factor in the current breaking process is the width of the p-region in the initial doping profile of a structure. An increase in the p-region width from 100 to 200 μm makes the velocity of the excess plasma front propagating in the p-region in the reverse pumping stage higher by a factor of 5–7. Higher propagation velocity of the plasma front makes the current breaking process more intensive, which is manifested in the shorter current breaking time and higher overvoltage across the opening switch.     

Effects of nonmagnetic impurities on spin-fluctuation induced superconductivity

We study the effects of nonmagnetic impurities on the phase diagram of a system of interacting electrons with a flat Fermi surface. The one-loop Wilsonian renormalization group flow of the angle dependent diffusion function D(theta;(1),theta;(2),theta;(3)) and interaction U(theta;(1),theta;(2),theta;(3)) determines the critical temperature and the nature of the low temperature state. As the imperfect nesting increases, the critical temperature decreases, and the low temperature phase changes from the spin-density wave (SDW) to the d-wave superconductivity (dSC) and finally, for bad nesting, to the charge-localized state. Both SDW and dSC phases are affected by disorder. The pair breaking depends on the imperfect nesting and is the most efficient when the critical temperature for superconductivity is maximal.     

High-T(c) ferroelectricity emerging from magnetic degeneracy in cupric oxide

Cupric oxide is multiferroic at unusually high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric, and the higher-T one is ferroelectric with a size and direction of polarization in good agreement with experiments. By mapping the ab?initio results on to an effective spin model, we show that the system has a manifold of almost degenerate ground states. In the high-T magnetic state noncollinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii-Moriya interaction. This leads to an unconventional mechanism for multiferroicity, with the particular property that nonmagnetic impurities enhance the effect.     

Strain enhanced exchange interaction between impurities in graphene

We study the effect of strain on the exchange interaction between two magnetic impurities in graphene. We calculate the exchange integrals between the impurities using a Green function technique. The calculation results indicate that strain may remarkably enhance the exchange interaction between the impurities within small distance. We explain the micromechanism of generating this phenomenon.