The Kondo Effect of a Molecular Tip As a Magnetic Sensor

Abstract

A single molecule offers to tailor and control the probing capability of a scanning tunneling microscope when placed on the tip. With the help of first-principles calculations, we show that on-tip spin sensitivity is possible through the Kondo ground state of a spin S = 1/2 cobaltocene molecule. When attached to the tip apex, we observe a reproducible Kondo resonance, which splits apart upon tuning the exchange coupling of cobaltocene to an iron atom on the surface. The spin-split Kondo resonance provides quantitative information on the exchange field and on the spin polarization of the iron atom. We also demonstrate that molecular vibrations cause the emergence of Kondo side peaks, which, unlike the Kondo resonance, are sensitive to cobaltocene adsorption. 1Introduction. The decoration of metal probe-tips by a molecule intentionally picked up from a surface has proven to be a powerful method to improve the measurement capabilities of a scanning tunneling microscope (STM). The degrees of freedom of the molecule introduce tip-surface interactions across the vacuum gap that are usually absent when using a metallic apex. These interactions can endow STM with an enhanced sub-molecular resolution, 1-3 and provide new chemical insight. [4][5][6][7] Detailed information can be gathered about single molecules on a surface and the way they interact among each other, which is of interest to chemistry and biology. 8To probe magnetic effects with a molecular tip it is necessary to have a magnetic molecule at the tip apex. Recently, a tip decorated with a nickelocene molecule 9 [Ni(C 5 H 5 ) 2 ] was used to monitor surface magnetism through the inelastic component of the tunneling current, 10,11 which provides an electrical access to the nickelocene spin states. 12 An exciting alternative could consist in using a magnetic molecule undergoing a Kondo effect, which is quite common for magnetic molecules adsorbed on metal surfaces. 13 This ubiquitous quantum phenomenon occurs when a magnetic impurity -molecule or atom-couples to a non-magnetic host metal. 14 The conduction electrons of the metal spin-screen the impurity to form a singlet below a temperature T K . The screening occurs coherently causing the emergence of

Richness of molecular junction configurations revealed by tracking a full pull-push cycle

“…For example, this approach is relevant when a metallocene molecule such as vanadocene is attached to the tip of a scanning tunneling microscope or as a bridge in a molecular junction. In these cases, the orientation of the molecule can significantly affect the magnetic characteristics and spin-transport properties of these systems 26,27,18 .…”

Harnessing the Quantum Behavior of Spins on Surfaces

“…[135,136], where the magnetic exchange interactions from individual magnetic atoms or molecules on surfaces were spatially mapped by tracking the variations in IETS spectra of the nickelocene molecule on the tip [135,136]. In a similar measurement scheme, a spin-1/2 cobaltocene molecule attached to an STM tip was found to exhibit a Kondo resonance, and the Kondo resonance splitting was used to extract the exchange field of an Fe atom on the surface [137].…”

Unravelling the robustness of magnetic anisotropy of a nickelocene molecule in different environments: a first-principles-based study

“…48–52 In particular, recent breakthroughs in low-temperature STM and STS have proven that such a class of molecules are suitable for functionalizing the tip apex to collect spin-related fingerprints in conductance measurements. 40–46 Several remarkable achievements were also accomplished recently. For instance, Ormaza et al have carried out a series of investigations of spin switching, spin-flip excitation and the Kondo effect with a NiCp 2 or CoCp 2 -terminated tip.…”

“…28,29 To realize advanced characterization of physical and chemical processes and properties of molecular structures on surfaces, a crucial step that enables the obtainment of the adsorbed molecular images is the decoration of the STM tips with extra atoms or molecules of varying chemical complexities. These include Xe or He atoms, [30][31][32][33] CO, [34][35][36][37] H 2 or D 2 , 38,39 CH 4 31 and transition-metallocene molecules (TMCp 2 where Cp = C 5 H 5 and TM = Fe, Co, Ni), [40][41][42][43][44][45][46] etc. Such an improvement has proven successful in collecting chemical and structural information about surface-supported atoms and molecules otherwise inaccessible with a metallic tip.…”

“…For instance, Ormaza et al have carried out a series of investigations of spin switching, spin-flip excitation and the Kondo effect with a NiCp 2 or CoCp 2 -terminated tip. 40,41,45 Later two groups independently proposed similar STM experiments to probe and image spin-spin exchange interactions, 43,44 and detect spin-vibration states 42,46 via a NiCp 2 -decorated tip.…”