Kondo Effect of an Adsorbed Cobalt Phthalocyanine (CoPc) Molecule: The Role of Quantum Interference

Abstract

A recent experimental study showed that, distorting a CoPc molecule adsorbed on a Au(111) surface, a Kondo effect is induced with a temperature higher than 200 K. We examine a model in which an atom with strong Coulomb repulsion (Co) is surrounded by four atoms on a square (molecule lobes), and two atoms above and below it representing the apex of the STM tip and an atom on the gold surface (all with a single, half-filled, atomic orbital). The Hamiltonian is solved exactly for the isolated cluster, and, after connecting the leads (STM tip and gold), the conductance is calculated by standard techniques. Quantum interference prevents the existence of the Kondo effect when the orbitals on the square do not interact (undistorted molecule); the Kondo resonance shows up after switching on that interaction. The weight of the Kondo resonance is controlled by the interplay of couplings to the STM tip and the gold surface, and between the molecule lobes. Coupling of localized spins to conduction electrons may lead to a transport anomaly known as the Kondo effect [1,2]. This effect, that usually shows up at low temperatures, consists of a sharp peak at the Fermi level, whose half-width is known as the Kondo temperature (T K ), and a conductance close to one conductance quantum G 0 = 2e 2 /h. The Kondo temperature in the case of magnetic impurities in non-magnetic metals is around 50 K [1], whereas in artificial atoms (quantum dots) is just a few hundred mK [3,4]. In a recent experiment [5,6] it has been shown that it is possible to control the characteristics, and even the existence, of the Kondo resonance by modifying the chemical surroundings of a magnetic atom. The experiments were carried out on a cobalt phthalocyanine molecule (CoPc) adsorbed on a Au(111) surface. Dehydrogenation of this molecule (d-CoPc) by means of voltage pulses from a Scanning Tunneling Microscope (STM) triggered a Kondo effect with a rather high Kondo temperature (T K ≈ 200 K). This temperature is even higher than that observed for bare Co adsorbed on a similar surface [6,7]. Besides such a high T K , one of the most remarkable results of [5] is the fact that the undistorted molecule does not show a Kondo effect, while it is readily promoted by distorting the molecule upon dehydrogenation. Topographic images taken by means of the STM [5] indicated that the CoPc molecule has four almost non-overlapping lobes symmetrically placed around the Co atom. Dehydrogenation distorts the molecule and forces those lobes to overlap. In addition it strongly decreases the distance from the molecule lobes to the gold surface and increases the Co/gold surface distance in approximately 30% [5].We hereby propose a simple model that accounts for some of the salient features of the experiment described above. We take a model Hamiltonian on a small atomic arrangement which is solved exactly, and subsequently connected to semi-infinite chains used to describe the STM tip and the gold surface. Fig. 1 depicts this atomic arrangement. A central site with a single atomic or...

Kondo effect in transport through molecules adsorbed on metal surfaces: From Fano dips to Kondo peaks

“…These features are in line with experimental observations indicating that our simple model contains the essential physics underlying the transport properties of such complex molecules. [2,3] in transport through a Co atom adsorbed on an Au(111) surface, a great deal of attention has been devoted to investigate, both theoretical and experimentally, such effect in either isolated Co atoms [4,5] or in Co-containing molecules [6,7,8,9] adsorbed on metal surfaces. The possibility of tuning the Kondo temperature [6,9] has been recently demonstrated, increasing considerably the interest of these systems.…”

“…The enormous complexity of the electronic structure of these molecules and the manybody physics involved hinder a detailed account of these systems. One of the main goals of the present work is to show, using a simple model that catches the most prominent features of both molecules, under which circumstances dips or peaks appear in the transmission.A model Hamiltonian was taken assuming the following small atomic arrangement [8]: a central site with a single atomic orbital, a strong Coulomb repulsion at the Co atom and four lobes of the molecule described by four atomic orbitals placed on a square, which center is the Co atom (see Fig. 1 of Ref.…”

“…A model Hamiltonian was taken assuming the following small atomic arrangement [8]: a central site with a single atomic orbital, a strong Coulomb repulsion at the Co atom and four lobes of the molecule described by four atomic orbitals placed on a square, which center is the Co atom (see Fig. 1 of Ref.…”

“…It is worth mentioning that in the present work U was taken always smaller than 3 eV, while in [8] a value of 8 eV was assumed. We note that the latter value is probably too large, as indicated by a recent study of atomic Co on a Cu(100) surface [5].…”

Dimensionality effects in the local density of states of ferromagnetic hosts probed via STM: Spin-polarized quantum beats and spin filtering

“…6,7 In the absence of a ferromagnetic host, the Fano-Kondo profile becomes doubly degenerate. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Away from the Kondo regime, a spin diode emerges. 24 In the condensed-matter literature on scanning microscopy, there is a profusion of work discussing spin-dependent phenomena employing ferromagnetic leads coupled to quantum dots or adatoms in the Kondo regime.…”