The Kondo effect at the atomic and molecular level:
Historical remarks and novel developments
Wolf-Dieter Schneider
Federal Institute of Technology (EPFL), 1015 Lausanne, Switzerland,
Institute of Basic Science (IBS), Center for Quantum NanoScience (QNS), Ewha Womans University, Seoul, Republic of Korea
wolf-dieter [dot] schneider [at] epfl [dot] ch (wolf-dieter[dot]schneider[at]epfl[dot]ch)
In 1930, a resistance minimum observed in dilute magnetic alloys [1, 2] was the first experimental evidence for a new scattering mechanism of conduction electrons at magnetic impurities at low temperatures. More than 30 years later, J. Kondo developed a theory which described the effect as a consequence of the spin-flip scattering of conduction electrons at a localised magnetic moment [3, 4]. About the same time P. W. Anderson developed a single impurity model [5] where he calculated the implications of this scattering mechanism for the local density of electronic states. Anderson, Suhl [6], Nagaoka [7], and Abrikosov [8] predicted a strong singularity at the Fermi level, later termed Kondo resonance. This conjecture triggered numerous experimental physicists to search for and finally to discover this resonance in transition metal, rare earth elements and in their alloys using techniques such as photoelectron [9], point contact [10], electronic transport [11, 12], and scanning tunneling spectroscopies (STS) [13, 14, 15]. The latter technique revealed the presence of Kondo resonances also in molecular systems containing no magnetic atoms but unpaired electron spins [16]. These discoveries opened the door for a detailed study of electronic transport and magnetism on the atomic and molecular scale.
For example, a magnetic sensor has been fabricated by attaching a single nickelocene molecule to the tip apex of a scanning tunneling microscope (STM) [17, 18]. Similarly, a tip that has its apex functionalized with a Kondo screened spin system, a small Ce cluster [19], is able to sense the spin of an individual Ce adatom adsorbed on a CuN ultrathin film. No external magnetic field has been applied. The technique may lead to applications in ultra-dense magnetic storage, spintronics, quantum computers, and quantum information technologies.
A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces, as detected by STS, emerged recently. These features have been proposed to originate from gapped spin-excitations induced by a finite magnetic anisotropy energy, in contrast to the usual widespread interpretation relating them to Kondo resonances [20]. Experimental evidence for the existence of these theoretically predicted spin excitations, termed spinarons, has been reported for Co adatoms in a recent publication [21].
In a recent study, a combination of STM manipulation of the adsorption geometry of molecules, high-resolution lowtemperature STS at 1 K, as well as coadsorption and manipulation of nearest neighbor molecules has allowed us to obtain a microscopic control of a transition from Kondo physics to spin-excitations for a Fe-phthalocyanine (FePc) molecule adsorbed on Au(111)[22]. However, this transition driven by engineering the spin interaction channel can alternatively be described as a topological quantum phase transition arising from a non-Landau Fermi liquid state of a two-channel spin-1 Kondo model with anisotropy [23]. Distinguishing between these two mechanisms will require additional experiments under applied magnetic fields. In any case, the possibility to obtain spin control of magnetic molecules on metallic substrates in addition to insulating ones [24] comes al little closer.
References
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