Inverse Isotope Effect on Kondo Temperature in Electron-Rattling System

In an electron system coupled with anharmonic phonons, i.e., { rattling}, inverse isotope effect on the Kondo temperature TK is found to occur by the numerical evaluation of the Sommerfeld constant γ of the Anderson-Holstein model. For the anharmonic potential of an oscillator with mass M in which large γ has been found to be almost independent of an applied magnetic field, γ is significantly suppressed when M is increased, i.e., TK is enhanced due to the relation of γ⁻¹ in the Kondo problem, leading to the inverse isotope effect on TK. Since this phenomenon does not occur for harmonic phonons, it can be a key experiment to prove the relevance of rattling to magnetically robust heavy electron state.