On a weak-coupling theory of superconducting magnetic alloys

The interaction of electrons with magnetic impurities in superconducting magnetic alloys is calculated to second order in the spin exchange coupling. Its effects on the transition temperature, the order parameter, the density of states, and the conductance are studied. The results are as follows. (1) The variation of the transition temperature with impurity concentration is the same as in the Abrikosov-Gor'kov theory; that of the order parameter is similar. (2) The density of states is finite at all nonzero impurity concentrations. Superconductivity with magnetic impurities is always gapless. (3) There are far more states about the Fermi level than in the Abrikosov-Gor'kov theory. At an impurity concentration equal to two-thirds its critical value, the density of states at the Fermi level surpasses one-half the normal value. The conductance curve loses all semblance of an energy gap.     

Electrodynamics of superconducting magnetic alloys in the weak coupling theory

The linear response of a superconducting magnetic alloy to a weak, transverse electromagnetic field is studied in the weak coupling theory of the alloy developed in a previous publication. By weak coupling is here meant that the scattering of the conduction electrons by the magnetic impurities is weak compared to the attractive, superconducting pairing interaction. An expression is derived for the wave-vector-and frequency-dependent transverse conductivity, in the approximation in which only single-particle excitations are included. The result is applied to calculate the dependence of various electromagnetic properties on the temperature and the impurity concentration, such as the magnetic field penetration depths and electromagnetic absorption. Calculations are also presented for the electronic spin susceptibility. The outstanding feature of the results is the finite density of electronic states at the Fermi level.     

Carrier Anisotropy and Impurity Scattering in Ge at mK Temperatures: Modeling and Comparison to Experiment

Improving upon the present background rejection capabilities of the cryogenic germanium detectors for direct dark matter search involves an in-depth comprehension of the charge collection process in these devices. Experimental data point to the combined effects of lattice and impurity scattering on the anisotropy of electron transport in Ge at mK temperatures. A Monte Carlo simulation code has been implemented to incorporate these features in a consistent model for charge collection. In a novel approach to carrier scattering by charged impurities in Ge at cryogenic temperatures, the scattering field of the impurities is treated statistically as a random contribution to the collection field, described by the Holtsmark distribution function with a single adjustable parameter, the mean density of the charged centers. Simulation of charge collection along these lines in devices different by their impurity content shows excellent agreement to experiment. Especially noteworthy is the fact that the strength of impurity scattering is reversed from the known concentration of dopant impurities in the crystals, as the crystal with the higher dopant concentration shows lower scattering at low field than the one with the lower concentration. This raises as an issue for further improvement of these devices, the question of the nature of the scattering centers in high-purity Ge crystals at cryogenic temperatures, associated presumably with deep level impurities or crystal defects.     

Impurity Effects in a Vortex Core in a Chiral p-Wave Superconductor Within the t-Matrix Approximation

We study the effects of non-magnetic impurity scattering on the Andreev bound states (ABS) in an isolated vortex in two-dimensional chiral p-wave superconductors numerically. We incorporate the impurity scattering effects into the quasiclassical Eilenberger formulation through the self-consistent t-matrix approximation. Within this scheme, we calculate the local density of states (LDOS) around two types of vortices: “parallel” (“anti-parallel”) vortex where the phase winding of the pair-potential coming from the vorticity and that coming from the chirality have the same (opposite) sign. When the scattering phase-shift δ ₀ of each impurity is small, we find that the impurities affect differently the spectra of quasiparticles localized around the two types of vortices in a way similar to that in the Born limit (δ ₀→0). For a larger δ ₀(?π/2), ABS in the vortex is strongly suppressed by the impurities for both types of vortices. We find that there are some correlations between the suppression of ABS near vortex cores and the low energy density of states due to the impurity bands in the bulk.     

Effect of localized spin fluctuations on transport properties in the one-band Wolff model

Fischer's calculations of the electrical resistivity, thermal resistivity, and thermopower of dilute alloys with nearly magnetic impurities are extended, using the one-band Wolff model, to the case where magnetic impurity scattering is large. A close similarity is seen to resistivities calculated in the two-band Wolff model, and a comparison is given with experimental data onRhFeand IrFe alloys.     

Resonant spikes of the nuclear spin qubits relaxation rate in quantum-Hall systems with magnetic impurities

The magnetic field dependence of the local phononassisted nuclear spin qubit relaxation rate in 2DES with magnetic impurities is studied theoretically. For weak and strong scattering limits under a strong magnetic field, we show that the magnetic field dependence of T/sub 1//sup -1/ exhibits giant spikes, due to the energy matching of the electron Zeeman splitting with the energy spacing between the vibrational mode energies. The localized mode is created by the lattice distortion around the impurity. This resonance phenomenon could be used as a local probe for studying the localized vibrational modes and their coupling to electrons and for the resonant manipulation of the nuclear spin qubits.     

Effect of Non-polar Longitudinal Optical Phonons on Domain Wall Resistance in Diluted Magnetic Semiconductor Nanowires

In the present paper, the effect of non-polar longitudinal optical phonons on domain wall resistance has been studied using the semiclassical approach. The analysis has been based on the Boltzmann transport equation, within the relaxation time approximation. We have determined the magnetoresistance of domain wall in nanowires based on diluted magnetic semiconductors. The modulation of the exchange and the impurity interactions by lattice vibrations, and the hole?Cphonon interaction are considered. The results indicate that the scattering role of phonons becomes significant by increasing temperature, which leads to the resistivity enhancement. It is also found that the magnetic impurities play an impressive role in enhancing the contribution of the domain wall in the resistance. In providing room temperature spintronics devices, understanding the effects of phonons and magnetic impurities on the domain wall resistance is crucial.     

Magnetoresistance of the Two-Dimensional Electron Gas in GaN Quantum Wells in a Parallel Magnetic Field

We present results for the resistivity of the two-dimensional spin-polarized electron gas as realized in GaN quantum wells at zero temperatures. A parallel magnetic field is used to create a spin-polarized electron gas. We discuss the density dependence of the magnetoresistance for impurity scattering and interface-roughness scattering. Finite width effects of the electron gas on the magnetoresistance are described.     

On the problem of superconductors containing polarized magnetic impurities

We investigate theoretically the influence of impurity polarization on the superconducting transition temperature of superconductors containing magnetic impurities. From previous theoretical work it is known that the pair-breaking effect of magnetic impurities is reduced when the impurity spins are fixed by an internal field. This spin-dynamic effect is studied in detail on two models of the magnetic state of the system based on a mean field theory. In some cases the critical concentration of magnetic impurities that destroys superconductivity atT=0 is increased, but we find that the polarization energy of the magnetic impurities must be very large compared to the gap energy of the pure superconductor in order to increase the critical impurity concentration by the factor (S+1)/S reported in the literature. We conclude that the anomalous dependence of the superconducting transition temperature on the concentration of magnetic impurities observed on some dilute alloys with rare-earth impurities can only be attributed to spin-dynamic effects if extraordinary strong polarizing forces between the magnetic impurities exist.Supported in part by Deutsche Forschungsgemeinschaft.     

Impurity states in the excitonic phase

The influence of impurities on the excitonic phase is reinvestigated. Treating the scattering of a particle or a hole by an impurity exactly, we find that a single impurity gives rise to a bound state in the energy gap of the excitonic state. For finite concentrations of impurities the superposition of such bound states leads to the formation of an impurity band. The occurrence of such structures in the gap is due to the pair-breaking effect that normal impurities have on the excitonic state. The reduction in the transition temperature caused by impurities is also considered. It is found that the magnitude of the critical concentration at which the excitonic state is completely destroyed depends on the charge of the impurities.     

Influence of impurity scattering on the critical temperature of superconductors with a partial gap in the electron spectrum

The influence of the scattering potential of nonmagnetic and magnetic impurities on the critical superconducting transition temperatureT _c is considered for superconductors with either a completely or partially dielectric electron spectrum. It is shown thatT _c rises with increasing concentration of nonmagnetic impurities, i.e., the Anderson theorem is violated in such systems. Magnetic impurity scattering lowersT _c. However, this reduction is weakened compared to usual superconductors, due to the presence of the dielectric gap on the Fermi surface.     

Hall effect and magnetoresistance in extrinsic piezoelectric semiconductors

The Hall effect and transverse magnetoresistance in extrinsic piezoelectric semiconductors such as InSb are investigated according to the scattering processes of carriers in semiconductors. These scattering processes contain the acoustic phonon scattering, the piezoelectric scattering, and the ionized-impurity scattering. The energy band structure of carriers in semiconductors is assumed to be nonparabolic. Results show that Hall angle, Hall coefficient, and transverse magnetoresistance depend strongly on the dc magnetic field and carrier density due to the energy-dependent relaxation time. Comparison with experimental data is made. It is also found that the magnetoresistance in degenerate InSb oscillates with the dc magnetic field due to the scattering of carriers with impurities in semiconductors.     

Microwave conductivity from magnetic impurities in an s-wave superconductor

We calculate the optical conductivity in an s-wave superconductor interacting with a low density of magnetic impurities. Interactions are treated beyond the Born approximation so that an impurity band develops inside the superconducting gap. We find that the contribution of these states to the low frequency conductivity is large, and that they have a clear spectroscopic signature which would would make them observable in conventional superconductors.     

The Electronic and Magnetic Properties and the Topological Phase Graphene Sheet with Fe,Co, Si,and Ge Impurities

The structural, electronic, and magnetic properties of pure graphene sheet and graphene sheet with Fe, Co, Si, and Ge impurities are investigated. The calculated results are done within density functional theory in the presence of spin-orbit coupling using the generalized gradient approximation. Electron density of states, band order, electron charge distribution, magnetic moment of these sheets, and the effect of pressure on the band order of graphene sheet with Fe impurity are investigated.     

Spin filtering in graphene nanoribbons with Mn-doped boron nitride inclusions

We investigate the spin filtering effects in graphene nanoribbons, where inclusions of hexagonal boron nitride were introduced together with substitutional magnetic impurities. The embedded Mn-doped boron nitride regions serve as quasi-0D islands of diluted magnetic semiconductor in the otherwise metallic graphene nanoribbon. Our first principle approach based on non-equilibrium Green's functions gives the polarization of the spin current for structures with one or two Mn impurities as a function of the applied bias. For the two impurity case, ferromagnetic and antiferromagnetic spin configurations of the magnetic impurities are considered. The spin resolved current indicates that the analyzed structures are suitable for spin filter applications or for spin current switching devices.     

Pair breaking and charge relaxation in superconductors

We present a general formalism based on the quasiclassical Green's function for calculating charge imbalance in nonequilibrium superconductors. Our discussion is sufficiently general that it applies at arbitrary temperatures, and under conditions when the width of quasiparticle states are appreciable due to pair breaking processes, and when strong coupling effects are significant. As a first application we demonstrate in detail how in the limit of small pair breaking and for a weak coupling superconductor the collision term in the formalism reduces to the one in the quasiparticle Boltzmann equation. We next treat the case of charge imbalance generated by tunnel injection, with pair breaking by phonons and magnetic impurities. Over the range of temperatures investigated experimentally to date, the calculated charge imbalance is rather close to that evaluated using the Boltzmann equation, even if pair breaking is so strong as almost to destroy superconductivity. Finally we consider charge imbalance generated by the combined influence of a supercurrent and a temperature gradient. We give calculations for a dirty superconductor with scattering by phonons as the pair breaking mechanism, and the results give a reasonable account of the experimental data of Clarke, Fjordbøge, and Lindelof. We carry out calculations for the case of impurity scattering alone which are valid not only in the clean and dirty limits, but also for intermediate situations. These enable us to see how the large contribution to the charge imbalance found for energies close to the gap edge in the clean case is reduced with increasing impurity scattering.The research was supported in part by the National Science Foundation grant NSF DMR-78-21068.     

Magnetotransport in field-induced spin density waves in Bechgaard salts

We study theoretically the electrical conductivity in a field-induced spin density wave (FISDW) in Bechgaard salts in a high magnetic field. We find a new contribution to the Hall conductivity, which appears only in a FISDW withN0. Furthermore, this Hall conductance has an activated form with an energy gap ₀ at low temperatures, unlike the quantum Hall effect. Therefore, the present theory predicts that the Hall resistance increases exponentially at low temperatures, which can be tested experimentally.On leave of abence from Nankai University, China.     

Energetic pinning of magnetic impurity levels in quantum-confined semiconductors

Donor- and acceptor-type (D/A) impurities play central roles in controlling the physical properties of semiconductors. With continued miniaturization of information processing devices, the relationship between quantum confinement and D/A ionization energies becomes increasingly important. Here, we provide direct spectroscopic evidence that impurity D/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional theory calculations. The resulting experimental demonstration of pinned impurity levels in quantum dots has far-reaching implications for physical phenomena involving impurity-carrier interactions in doped semiconductor nanostructures, including in the emerging field of semiconductor spintronics where magnetic-dopant-carrier exchange interactions define the functionally relevant properties of diluted magnetic semiconductors.     

Nickel-related defect in diamond: A tight-binding molecular-dynamics study

Structural, electronic, and magnetic properties of isolated Ni impurities at point defects and a dislocation core in diamond are investigated using tight-binding molecular-dynamics simulations. The results show the structural stabilization associated by lowering local symmetry in the cases of point defects. The segregation energies of Ni impurity for a substitutional site and for an interstitial site in the dislocation core are estimated to be in the same order within 0.2 eV. The local electronic density-of-states reveals that the gap states appeared by the insertion of Ni impurity are strongly localized around the Ni sites. Magnetic moments on neighboring C atoms are induced so as to screen the moment on the Ni atom except for the case of interstitial Ni impurity in which the total magnetic moment remains non-zero. Analyses indicate that localized atomic d states on the Ni atom and the p–d coupling between Ni and neighboring C atoms are responsible for the residual magnetic moment in the system with an interstitial Ni defect. In the other systems investigated, on the other hand, the bonding states between Ni impurity and its neighboring C atoms are dominated by the s–p coupling.     

Effects of impurities on the phase transition of an itinerant electron antiferromagnet

Theory of the phase transition of an itinerant electron antiferromagnet is similar to superconductivity theory, including singular contributions to the specific heat and other properties at the transition temperature. The effects of a small concentration of normal impurities, which are pair breaking for the antiferromagnet, are calculated for temperatures nearT _c . The principal effect of normal homovalent impurities is temperature smearing of the phase transition due to inhomogeneous electron-impurity scattering. This temperature smearing removes the singularities, leaving peaks with finite amplitudes which depend on the impurity concentration in a simple manner. Recent experimental results for chromium are discussed.