Transition temperature and specific heat discontinuity for superconducting alloys containing Kondo impurities

The dependence of the transition temperature and the specific heat discontinuity of a superconductor on the magnetic impurity concentration is calculated selfconsistently using the spin-boson approach for the Kondo problem. The results agree well with experimental dat for the (La, $\text{Ce}$) Al₂ alloy.     

Kondo effect in the presence of magnetic impurities

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to a RKKY interaction between the spin of the dot and the static spins of the impurities. A magnetic field restores the single Kondo peak in the case of an antiferromagnetic RKKY interaction. This system provides a new platform to study Kondo and RKKY interactions in metals at the level of a single spin.     

Kondo effect in dilute alloy films of CuFe deposited at room and helium temperature

The resistivity of Cu, Cu + 100 ppm Fe and Cu + 200 ppm Fe films was measured as a function of temperature from 1 to 300 K. The results on films evaporated at room temperature are compared with contradicting data of other authors. Disordered CuFe alloy films, produced by evaporation onto a substrate at helium temperature, failed to show the theoretically predicted temperature shift of the resistivity minimum.     

Influence of interstitials and non magnetic impurities on the Kondo effect

The influence of interstitials and non magnetic impurities on the anomalous resistivity, thermopower and Kondo temperature of dilute magnetic alloys was investigated generalizing a model proposed by Bohnen and Fischer. Numerical results are given as a function of the distance between the interstitial (or non magnetic impurity) and the magnetic impurity using their scattering phase shifts as parameters. The Kondo anomalies are altered considerably, if the magnetic impurity is very close to the non magnetic scattering potential, e.g. if it is part of an interstitial dumbbell.This work is part of a doctoral thesis of G.Wehr at the Technische Universität München     

Green's function theory of the Kondo effect in dilute magnetic alloys

The exact solution of thet-matrix integral equation derived from the self-consistent Nagaoka equations in the theory of dilute magnetic alloys is established. It is shown that the unique solution for thet-matrix involving all Kondo type anomalies can be found under quite general assumptions. Using the exact solution we have calculated thermodynamic properties of dilute magnetic alloys. It is found that the excessive specific heat of the system due to the anomalous scattering of conduction electrons from the magnetic impurities is of the order ofBoltzmann's constant per local moment at low temperatures. In the limit of vanishing temperature the specific heat goes to zero asymptotically as (lnT)^?4. Finally the entropy difference of the interacting system as compared to the free system is calculated.     

Kondo temperature of magnetic impurities at surfaces

Based on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.     

Threshold exponents of pseudo-particle spectra of Kondo impurities

Pseudo-particle propagators or resolvents are known to be important ingredients in the theory of a degenerate Anderson impurity. At low temperatures they show conspicuous threshold behavior which is, however, described only approximately in perturbative theories like the non-crossing approximation. In this paper we derive the correct values of the threshold exponents from an argument due to Schotte and Schotte. For degeneracyN=1 we provide a complete perturbative proof of the validity of our results.     

Dynamical parameters for dilute Mössbauer impurities

A simple theoretical analysis is presented for Mössbauer parameters and frequency moments of dilute⁵⁷Fe impurities in different metallic hosts. These parameters have been evaluated for a general harmonic solid using Green's function method. Our results suggest that the contribution due to force constant change term is significant for the mean square displacement whereas that due to mass disordering term is predominating in the evaluations of mean square velocity. The variation of frequency moments for the impurity with host Debye-temperatures modified by the host to impurity mass ratios are shown for a number of solids.     

Observation of the Kondo effect in a spin-3/2 hole quantum dot

We report the observation of Kondo physics in a spin-3/2 hole quantum dot. The dot is formed close to pinch-off in a hole quantum wire defined in an undoped AlGaAs/GaAs heterostructure. We clearly observe two distinctive hallmarks of quantum dot Kondo physics. First, the Zeeman spin splitting of the zero-bias peak in the differential conductance is independent of the gate voltage. Second, this splitting is twice as large as the splitting for the lowest one-dimensional subband. We show that the Zeeman splitting of the zero-bias peak is highly anisotropic and attribute this to the strong spin-orbit interaction for holes in GaAs.     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

Numerical calculation of the fidelity for the Kondo and the Friedel-Anderson impurities

The fidelities of the Kondo and the Friedel-Anderson (FA) impurities are calculated numerically. The ground states of both systems are calculated with the FAIR (Friedel artificially inserted resonance) theory. The ground state in the interacting systems is compared with a nullstate in which the interaction is zero. The different multi-electron states are expressed in terms of Wilson states. The use of N Wilson states simulates the use of a large effective number N _eff of states. A plot of ln(F) versus N ∝ ln(N _eff ) reveals whether one has an Anderson orthogonality catastrophe at zero energy. The results are at first glance surprising. The ln(F) – ln(N _eff ) plot for the Kondo impurity diverges for large N _eff . On the other hand, the corresponding plot for the symmetric FA impurity saturates for large N _eff when the level spacing at the Fermi level is of the order of the singlet-triplet excitation energy. The behavior of the fidelity allows one to determine the phase shift of the electron states in this regime.     

The longitudinal magnetophonon effect in semiconductors containing magnetic impurities

The dependence of longitudinal magnetoresistance on magnetic field in semiconductors containing magnetic impurities is investigated theoretically. The calculation takes into account the scattering of electrons on magnetic impurities and on optical phonons. The inelastic optical phonon scattering itself is responsible for magnetophonon oscillations of the magnetoresistance, the extremes of these oscillations occuring when energy distance between Landau levels is equal to the energy of optical phonon, $\text{h}\text{?}$ω₀. The scattering on magnetic impurities may lead to spin flip electronic transitions. The spin flip electronic transitions manifest themselves as additional minima on the oscillatory picture of magnetoresistance. These new minima occur when the energy separation between spin-split Landau levels is equal to $\text{h}\text{?}$ω₀.     

Calculation of host nuclear relaxation in dilute Kondo alloys for T ⪡ Tk

We present a microscopic calculation of the host nuclear spin-lattice relaxation rate as a function of impurity-host distance r for dilute Kondo alloy at T ? T_k. The net relaxation rate involves two main parts. One, dependent up-on the local conduction electron density of states, exhibits kinematic oscillations which are due to resonant scattering. The other part, due to the impurity spin fluctuations, is shown to dominate the first mechanism allowing the Korringa relation to hold in systems with low T_k (such as Cu:Fe). In A?:Mn both mechanisms are of equal importance, and the net host relaxation rate and the excess Knight shift do not satisfy a Korringa relation.     

Kondo effect in a quantum antidot

We report Kondo-like behavior in a quantum antidot (a submicron depleted region in a two-dimensional electron gas) in the quantum-Hall regime. When both spins of the lowest Landau level are present all around the antidot, the resonances between extended edge states via antidot bound states show an abnormal feature in alternate Coulomb-blockaded regions. The feature becomes suppressed when the temperature or source-drain bias is raised as for Kondo resonances in quantum dots. Although the exact mechanism is unknown, Kondo-like correlated tunneling may arise from a Skyrmion-type edge reconstruction. This observation demonstrates the generality of the Kondo phenomenon.     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Ce impurities

The dependence of the superconducting transition temperature T_c on Ce impurity concentration and the specific heat jump at T_c as a function of T_c are reported for the system ($\text{La}$Ce)Sn₃. The experimental results are analyzed using a theory due to Kaiser concerning the effect on superconductivity of nonmagnetic localized resonant impurity states This analysis yields values for the intraatomic Coulomb repulsion parameter U_eff and the Ce local density of states at the Fermi level N_? (E_F). The results of low temperature normal state heat capacity and magnetic susceptibility measurements which give independent estimates of N_? (E_F) are also reported. A large pressure dependence of the T_c of ($\text{La}$Ce)Sn₃ alloys was observed for pressures up to 20 kbar. This behavior is similar to that previously observed in several other superconducting matrix-Ce impurity systems in which the Ce solute 4f electron shell undergoes a continuous-pressure induced demagnetization.