Low temperature anomaly in mesoscopic kondo wires

Nonperturbative scaling theory of free magnetic moment phases in disordered metals

The crossover between a free magnetic moment phase and a Kondo phase in low-dimensional disordered metals with dilute magnetic impurities is studied. We perform a finite-size scaling analysis of the distribution of the Kondo temperature obtained from a numerical renormalization group calculation of the local magnetic susceptibility for a fixed disorder realization and from the solution of the self-consistent Nagaoka-Suhl equation. We find a sizable fraction of free (unscreened) magnetic moments when the exchange coupling falls below a critical value Jc. Between the free moment phase due to Anderson localization and the Kondo-screened phase we find a phase where free moments occur due to the appearance of random local pseudogaps at the Fermi energy whose width and power scale with the elastic scattering rate 1/tau.

Phase coherence of conduction electrons below the Kondo temperature

We have measured the phase decoherence rate tau_{varphi};{-1} of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, T_{K} approximately 4 K, is in the ideal temperature range to study the progressive screening of the Fe spins as the temperature T falls below T_{K}. The contribution to tau_{varphi};{-1} from the Fe impurities is clearly visible over the temperature range 40 mK-10 K. Below T_{K}, tau_{varphi};{-1} falls rapidly until T/T_{K} approximately 0.1, in agreement with recent theoretical calculations. At lower T tau_{varphi};{-1} deviates from theory with a flatter T-dependence. Understanding this anomalous dephasing for T/T_{K}<0.1 may require theoretical models with larger spin and number of channels.

Universal dephasing rate due to diluted Kondo impurities

We calculate the dephasing rate due to magnetic impurities in a weakly disordered metal as measured in a weak-localization experiment. If the density nS of magnetic impurities is sufficiently low, the dephasing rate 1/tauphi is a universal function, 1/tauphi=(nS/nu)f(T/TK), where TK is the Kondo temperature and nu is the density of states. We show that inelastic vertex corrections with a typical energy transfer DeltaE are suppressed by powers of 1/(tauphiDeltaE) proportional to nS. Therefore, the dephasing rate can be calculated from the inelastic cross section proportional to pinu ImT-/pinuT/2, where T is the T matrix which is evaluated numerically exactly using the numerical renormalization group.

Experimental test of the numerical renormalization-group theory for inelastic scattering from magnetic impurities

We present measurements of the phase coherence time taupsi in quasi-one-dimensional Au/Fe Kondo wires and compare the temperature dependence taupsi of with a recent theory of inelastic scattering from magnetic impurities [Phys. Rev. Lett. 93, 107204 (2004)10.1103/PhysRevLett.93.107204]. A very good agreement is obtained for temperatures down to 0.2T(K). Below the Kondo temperature T(K), the inverse of the phase coherence time varies linearly with temperature over almost one decade in temperature.

Role of surface anisotropy for magnetic impurities in electron dephasing and energy relaxation and their size effect

Recently the electron dephasing and energy relaxation due to different magnetic impurities have been extensively investigated experimentally in thin wires, and in this Letter these quantities are theoretically studied. It was shown earlier that a magnetic impurity in a metallic host with strong spin-orbit interaction experiences a surface anisotropy of the form H=K(d)(nS)(2) which causes size effects for impurities with integer spin. Here we show that the dephasing and the energy relaxation are influenced by the surface anisotropy in very different ways for integer spin having a singlet ground state. That must result also in strong size effects and may resolve the puzzle between the concentrations estimated from the two kinds of experiments.

High-field measurements of electron decoherence time in metallic nanowires: switching off magnetic impurity spins

We report low-temperature measurements of electron decoherence time in a series of pure gold wires, 18 nm thick and 30 nm wide. At fields up to 15 T, large enough to polarize any concentration of magnetic impurity spins, conductance fluctuation measurements show almost no temperature dependence of the decoherence time below 300 mK, both in the correlation field for interference and the root-mean-square value of the fluctuations. Combined with previous low-field weak localization measurements on samples from similar material, our experiment suggests that the ubiquitous saturation of decoherence time in these samples is not due to any mechanism based on magnetic impurity spins.

Localization length in anderson insulator with Kondo impurities

The localization length, xi, in a two-dimensional Anderson insulator depends on the electron spin scattering rate by magnetic impurities, tau(-1)(s). For antiferromagnetic sign of the exchange, the time tau(s) is itself a function of xi, due to the Kondo correlations. We demonstrate that the unitary regime of localization is impossible when the concentration of magnetic impurities, n(M), is smaller than a critical value, n(c). For n(M)>n(c), the dependence of xi on the dimensionless conductance, g, is reentrant, crossing over to unitary, and back to orthogonal behavior upon increasing g. Sensitivity of Kondo correlations to a weak parallel magnetic field results in a giant parallel magnetoresistance.