The 2-Channel Kondo Model

Abstract

Certain zero-bias anomalies (ZBAs) in the voltage, temperature and magnetic field dependence of the conductance G(V, T, H) of quenched Cu point contacts have previously been interpreted to be due to non-magnetic 2-channel Kondo (2CK) scattering from near-degenerate atomic two-level tunneling systems (Ralph and Buhrman, 1992;, and hence to represent an experimental realization of the non-Fermi-liquid physics of the T = 0 fixed point of the 2-channel Kondo model. In this, the first in a series of three papers (I,II,III) devoted to 2-channel Kondo physics, we present a comprehensive review of the quenched Cu ZBA experiments and their 2CK interpretation, including new results on ZBAs in constrictions made from Ti or from metallic glasses. We first review the evidence that the ZBAs are due to electron scattering from stuctural defects that are not static, but possess internal dynamics. In order to distinguish between several mechanisms proposed to explain the experiments, we then analyze the scaling properties of the conductance at low temperature and voltage and extract from the data a universal scaling function Γ(v). The theoretical calculation of the corresponding scaling function within the 2CK model is the subject of papers II and III. The main conclusion of our work is that the properties of the ZBAs, and most notably their scaling behavior, are in good agreement with the 2CK model and clearly different from several other proposed mechanisms.

ThAsSe diamagnet: Evidence for a Kondo effect derived from structural two-level systems

“…10 Recently performed point-contact spectroscopy measurements on Cu nanoconstrictions revealed the anomalous zero-bias response that points to a coupling of the tunneling centers to the conduction electrons. 11,12 To date, however, there exists no convincing example for this electron-TLS interaction on a macroscopic scale. Experimental evidence for such a scattering mechanism in a bulk material might verify some of the aspects of the TLS Kondo model and open a new route to the nFL problem.…”

Orbital Kondo behavior from dynamical structural defects

“…For a special class of parameters the renormalized energy splitting between the lowest two levels is around T K implying that such dynamical defects may give rise to some of the Kondo-like anomalies observed [1,3,6]. Our calculations also indicate that to obtain a measurable T K one needs resonant scattering on the defect.…”

“…The best candidates are therefore atoms with resonant d or f-scattering at the Fermi energy or possibly small groups of atoms [25]. Thus dynamical local defects with special realistic model parameters can explain the Kondo-like anomalies observed in some experiments for T > T K [1,3,6]. Note that in amorphous materials the positions of the defect f -or d-orbitals have usually a distribution, and therefore many of the defects may have resonant scattering at the Fermi energy.…”

“…There is a great number of experimental anomalies observed by point contact [1][2][3], dephasing [4,5] and transport [6,7] measurements in metals at low temperature which have not been theoretically explained in a satisfactory way. Sensitivity on annealing [1,5], sample preparation [5], electro-migration [2] support that these anomalies may originate from some kind of dynamical defects.…”

“…Sensitivity on annealing [1,5], sample preparation [5], electro-migration [2] support that these anomalies may originate from some kind of dynamical defects. The concept of two-level systems (TLS's), i.e.…”