Two-channel Kondo Physics Research Articles

Two-channel Kondo effects inAl/AlOx/Scplanar tunnel junctions

We have measured the differential conductances $G(V,T)$ in several $\text{Al}/{\text{AlO}}_{x}/\text{Sc}$ planar tunnel junctions between 2 and 35 K. As the temperature decreases to $\ensuremath{\sim}16\text{ }\text{K}$, the zero-bias conductance $G(0,T)$ crosses over from a standard $\ensuremath{-}\text{ln}\text{ }T$ dependence to a novel $\ensuremath{-}\sqrt{T}$ dependence. Correspondingly, the finite bias conductance $G(V,T)$ reveals a two-channel Kondo scaling behavior between $\ensuremath{\sim}4$ and 16 K. The observed two-channel Kondo physics is ascribed to originating from a few localized spin-$\frac{1}{2}$ Sc atoms situated slightly inside the ${\text{AlO}}_{x}/\text{Sc}$ interface.

Magnetic field induced two-channel Kondo effect in multiple quantum dots

We study the possibility of observing the two-channel Kondo physics in multiple planar quantum dot heterostructures in the presence of a strong in-plane magnetic field. We show that a fine tuning of the coupling parameters of the system and an external magnetic field may stabilize the two-channel Kondo critical point. We make predictions for the behavior of the scaling of the differential conductance in the vicinity of the quantum critical point, as a function of magnetic field, temperature, and source-drain potential.

Kondo screening in unconventional superconductors: The role of anomalous propagators

The Kondo effect in superconductors is frequently investigated using the local quasiparticle density of states as sole bath characteristics, i.e., the presence of anomalous propagators is ignored. Here we point out that this treatment is exact for a number of situations, including pointlike impurities in $d$-wave superconductors. We comment on recent investigations [M. Matsumoto and M. Koga, J. Phys. Soc. Jpn. 70, 2860 (2001); Phys. Rev. B 65, 024508 (2002)] which reached different conclusions: while their numerical results are likely correct, their interpretation in terms of two-channel Kondo physics and an ``orbital effect of Cooper pairs'' is incorrect.

Prediction of the Capacitance Line Shape in Two-Channel Quantum Dots

We propose a setup to realize two-channel Kondo physics using quantum dots. We discuss how the charge fluctuations on a small dot can be accessed by using a system of two single-electron transistors arranged in parallel. We derive a microscopic Hamiltonian description of the setup that allows us to make the connection with the two-channel Anderson model (of extended use in the context of heavy-fermion systems) and in turn make detailed predictions for the differential capacitance of the dot. Its line shape, which we determined precisely, shows a robust behavior that should be experimentally verifiable.

From quantum critical to two-channel Kondo physics via charge fluctuations in a quantumdot

We consider charge fluctuations in a quantum dot coupled to an interactingone-dimensional electron liquid. By tuning the coupling between the dot and theone-dimensional electron liquid, one can access the various regimes which arise, whichinclude a quantum critical and a two-channel Kondo regime. The differentialcapacitance is computed and is shown to contain detailed information about the system.

Kondo effect in quantum dots at high voltage: universality and scaling.

We examine the properties of a dc-biased quantum dot in the Coulomb blockade regime. For voltages V that are large compared to the Kondo temperature T(K), the physics is governed by the scales V and gamma, where gamma approximately V/ln(2)(V/T(K)) is the nonequilibrium decoherence rate induced by the voltage-driven current. Based on scaling arguments, self-consistent perturbation theory, and perturbative renormalization group, we argue that due to the large gamma the system can be described by renormalized perturbation theory in 1/ln(V/T(K))<<1. However, in certain variants of the Kondo problem, two-channel Kondo physics is induced by a large voltage V.

A two-channel Kondo impurity in the spin-1/2 chain: consequences for Knight shift experiments

A magnetic impurity in the spin- 1 2 chain is a simple realization of the two-channel Kondo problem since the field theoretical descriptions in the spin sector are identical. The correlation functions near the impurity can be calculated. Using a modified version of the numerical transfer matrix DMRG, we are now able to accurately determine local properties close to the impurity in the thermodynamic limit. The local susceptibilities (Knight-shifts) show an interesting behavior in a large range around the impurities. We are able to make quantitative experimental predictions which would allow one to observe two-channel Kondo physics for the first time directly by doping of spin- 1 2 chain compounds.

Universal Crossover Behavior of a Magnetic Impurity and Consequences for Doping in Spin-1/2Chains

We consider a magnetic impurity in the antiferromagnetic spin- $1/2$ chain which is equivalent to the two-channel Kondo problem in terms of the field theoretical description. Using a modification of the transfer-matrix density matrix renormalization group we are able to determine the crossover function for the impurity susceptibility over a large temperature range, which exhibits universal data collapse. We also calculate the local susceptibilities near the impurity, which show an interesting competition of boundary effects. This results in quantitative predictions for experiments on doped spin- $1/2$ chains, which could observe two-channel Kondo physics directly.

Two-channel Kondo physics from tunneling impurities with triangular symmetry

Tunnelling impurities in metals have been known for some time to have the potential for exhibiting Kondo-like physics. However previous models based on an impurity hopping between two equivalent positions have run into trouble due to the existence of relevant operators that drive the system away from the non-Fermi-liquid Kondo fixed point. In the case of an impurity hopping among positions with higher symmetry, such as triangular symmetry, it is shown here that the non-Fermi-liquid behavior at low temperatures can be generic. Using various bosonization techniques, the fixed point is shown to be {\em stable}. However, unlike the conventional two-channel Kondo (2CK) model, it has {\em four} leading irrelevant operators, implying that while the form of the singular temperature dependence of physical quantities is similar to the 2CK model, there will not be simple universal amplitude ratios. The phase diagram of this system is analyzed and a critical manifold is found to separate the non-Fermi-liquid from a conventional Fermi liquid fixed point. Generalization to higher symmetries, such as cubic, and the possibility of physical realizations with dynamic Jahn-Teller impurities is discussed.