System Of Magnetic Impurities Research Articles

Identification of topological superconductivity in magnetic impurity systems using bulk spin polarization

Magnetic impurities on the surface of spin-orbit coupled but otherwise conventional superconductors provide a promising way to engineer topological superconductors with Majorana bound states as for boundary modes. In this work, we show that the spin-polarization in the interior of both one-dimensional impurity chains and two-dimensional islands can be used to determine the topological phase, as it changes sign exactly at the topological phase transition. This offers an independent probe of the topological phase, beyond the zero-energy Majorana bound states appearing at the boundaries of the topological region.

Single-atom gating and magnetic interactions in quantum corrals

Single-atom gating, achieved by manipulation of adatoms on a surface, has been shown in experiments to allow precise control over superposition of electronic states in quantum corrals. Using a Green's function approach, we demonstrate theoretically that such atom gating can also be used to control the coupling between magnetic degrees of freedom in these systems. Atomic gating enables control not only on the direct interaction between magnetic adatoms, but also over superpositions of many-body states which can then control long distance interactions. We illustrate this effect by considering the competition between direct exchange between magnetic impurities and the Kondo screening mediated by the host electrons, and how this is affected by gating. These results suggest that both magnetic and nonmagnetic single-atom gating may be used to investigate magnetic impurity systems with tailored interactions, and may allow the control of entanglement of different spin states.

High-temperature spin polarization of high-mobility charge carriers in hybrid metal-semiconductor structures

We consider magnetic properties of the planar structure consisting of a ferromagnetic metal, diluted magnetic semiconductor, and the quantum well [by the example of the hybrid heterostructure Fe–Ga(Mn)As–InGaAs]. In the framework of the mean-field theory, there is the significant amplification of the ferromagnetism induced by the ferromagnetic metal (Fe) in the system of magnetic impurities (Mn) due to their indirect interaction via the conductivity channel in the quantum well. In the discussed structure, high-temperature ferromagnetism leads to a high spin polarization of the charge carriers (holes, localized in the quantum well), while preserving their high mobility.

The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling

We investigate the effect that Rashba spin-orbit coupling has on the low energy behaviour of a two dimensional magnetic impurity system. It is shown that the Kondo effect, the screening of the magnetic impurity at temperatures T<T K , is robust against such spin-orbit coupling, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature T K in such systems and the prospects of measuring this change are discussed. We conclude that many of the assumptions made in our analysis invalidate our results as applied to recent experiments in semi-conductor quantum dots but may apply to measurements made with magnetic atoms placed on metallic surfaces.

Self-Consistent Perturbation Theory for Thermodynamics of Magnetic Impurity Systems

Integral equations for thermodynamic quantities are derived in the framework of the non-crossing approximation (NCA). Entropy and specific heat of 4f contribution are calculated without numerical differentiations of thermodynamic potential. The formulation is applied to systems such as PrFe4P12 with singlet-triplet crystalline electric field (CEF) levels.

Electron transport and energy relaxation in dilute magnetic alloys

We consider the effect of the RKKY interaction between magnetic impurities on the electron relaxation rates in a normal metal. The interplay between the RKKY interaction and the Kondo effect may result in a non-monotonic temperature dependence of the electron momentum relaxation rate, which determines the Drude conductivity. The electron phase relaxation rate, which determines the magnitude of the weak localization correction to the resistivity, is also a non-monotonic function of temperature. For this function, we find the dependence of the position of its maximum on the concentration of magnetic impurities. We also relate the electron energy relaxation rate to the excitation spectrum of the system of magnetic impurities. The energy relaxation determines the distribution function for the out-of-equilibrium electrons. Measurement of the electron distribution function thus may provide information about the excitations in the spin glass phase.

Spin glass versus superconductivity

A superconductor with interacting paramagnetic impurities is considered. The impurities are coupled via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. At a temperature ${T}_{g},$ the system of magnetic impurities forms a spin-glass state. We study the effect of the spin-spin interactions on the superconducting transition point at $T&lt;{T}_{g}.$ We show that superconducting properties depend on the state of the spin system via spin-spin autocorrelation functions. With the help of the Keldysh technique, a general nonequilibrium Gor'kov equation is derived. Possible aging effects in the superconducting transition point are discussed. The equilibrium superconducting transition point is found explicitly and shown to be shifted towards higher temperatures and impurity concentrations compared to the classical Abrikosov-Gor'kov curve. The corresponding shift of the superconducting quantum critical point is quite small (about 10%). A method of calculating spin-spin correlation function is suggested. The method combines the ideas of random mean-field method and virial expansion. We calculate analytically the first virial term for the spin-spin correlator for the quantum Heisenberg spin glass with the RKKY interactions in the quasiequilibrium regime.

Effect of optical irradiation on the indirect exchange constant of a magnetic impurity system and an antiferromagnetic insulator

Based on a multisite s-d mixing model, in which electron-photon coupling was taken into account, a scheme to calculate the indirect exchange constant of a magnetic impurity system and an antiferromagnetic insulator in thepresence of radiation field was advanced. The effect of optical irradiation on the exchange constant was calculated within a single valence model for imperfect NiO. A notable change of antiferromagnetic exchange interaction occurs while ‖S d , i ‖ 2 N p q is larger than 1.0×10 - 4 (eV) 2 (S d , i is the transition matrix element between a d state and an impurity state, and N p q is photon population). The decrease of antiferromagnetic exchange interaction reaches about 10% in the radiation field provided by a typical pulsed laser source while there exist appropriate impurity states in the imperfect NiO. The effect can be used to explain the photoinduced decrease of pinning strength in NiO/FeNi bilayer.

Phase coherence phenomena in superconducting films

Superconducting films subject to an in-plane magnetic field exhibit a gapless superconducting phase. We explore the quasi-particle spectral properties of the gapless phase and comment on the transport properties. Of particular interest is the sensitivity of the quantum interference phenomena in this phase to the nature of the impurity scattering. We find that films subject to columnar defects exhibit a `Berry-Robnik' symmetry which changes the fundamental properties of the system. Furthermore, we explore the integrity of the gapped phase. As in the magnetic impurity system, we show that optimal fluctuations of the random impurity potential conspire with the in-plane magnetic field to induce a band of localized sub-gap states. Finally, we investigate the interplay of the proximity effect and gapless superconductivity in thin normal metal-superconductor bi-layers.

Phase transition in a system of magnetic impurities

A new effect is described. Jumps of resistivity were observed with decrease of temperature in a system of magnetic impurities exemplified by Mn-doped InSb. Effect is caused by phase transition in a system.

Fermi liquid theory and magnetic impurity systems. I. Quasi-particle Hamiltonians and mean field theory

The author gives explicit quasi-particle Hamiltonians, including interactions, for several magnetic impurity models and shows that the application of mean field theory to these gives asymptotically exact results as T to 0 and H to 0 corresponding to Fermi liquid theory. This approach provides a conceptual link between the intuitive phenomenological theory of Landau and Nozieres, and the microscopic theory of Luttinger and Yamada.

Reentrant spin glass properties of Cr1−x−yMnxFeyGe

DC and ac magnetic measurements of the exchange enhanced magnetic impurity system Cr1−xMnxGe with x = 0.21 and 0.19 suggest reentrant spin glass behaviours with mixed ferromagnetic and spin glass phase characterized by anisotropy fields. Substitution of Fe for Cr breaks the ferromagnetic ordering due to negative coupling between moments on Fe and Mn.

Spin-resolved scattering in the Cu(Mn) magnetic-impurity system

We report spin-resolved scattering measurements on the Cu(Mn) magnetic-impurity system using the third-harmonic de Haas---van Alphen wave-shape---analysis technique (dHvA WA). Magnetic field and temperature dependences of spin-up and spin-down scattering and the magnetic-moment---conduction-electron interaction ${H}_{\mathrm{ex}}$ are presented. The exchange field ${H}_{\mathrm{ex}}$ is found not to scale with $\frac{H}{T}$. The $H$ and $T$ dependences of the spin-resolved scattering agree qualitatively with existing theories and spin-averaged experimental data such as the negative magnetoresistivity. The spin-resolved $d$-impurity scattering phase shifts derived from our data agree with results obtained by other, complementary methods. Fundamental dHvA amplitude measurements are presented on all symmetry orbits. For the low-field data, a partial-wave shift analysis indicates predominately $d$-type impurity scattering. The high-field data deviate from a linear extrapolation of the low-field data on all orbits. The magnetic field where the deviation occurs scales with the amount of $d$ wave present in that orbit.

Magnetic impurity ordering in dilute alloys

The interaction of conduction electrons with the ordered system of magnetic impurities randomly distributed in a normal metal is investigated in terms of the s-d exchange model. In the calculation of the electronic self-energy part the most divergent terms are summed. The internal exchange field arising due to the impurity ordering is determined in a self-consistent manner. It is shown that in the system under investigation the electron spectrum is greatly renormalized; in particular, the effective mass on the Fermi surface and also the low-temperature electronic heat capacity become several times larger. The addition to the electronic heat capacity has a broad temperature maximum and depends on the impurity concentration. The concentration and temperature dependences are qualitatively in good agreement with the experimental results. The mechanism responsible for the appearance of the long-range ferromagnetic interaction is also discussed.

Magnetic ordering of localized spins in dilute alloys

The interaction of electrons with the ordered magnetic impurity system was shown to lead to great electron spectrum renormalization. Thus the effective mass on the Fermi surface becomes several times as large. For the addition to the effective mass the concentration and temperature dependences are obtained which are in good agreement with the experimental ones.

Anomalous low-temperature thermal properties of glasses and spin glasses

Abstract We show that a linear specific heat at low temperatures for glass follows naturally from general considerations on the glassy state. From the same considerations we obtain the experimentally observed anomalous low-temperature thermal conductivity, and we predict an ultrasonic attenuation which increases at low temperatures. Possible relationships with the linear specific heat in magnetic impurity systems are pointed out. We suggest experimental study of the relaxation of thermal and other properties.