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Register a new userQuantum modulation of the Kondo resonance of Co adatoms on Cu/Co/Cu(100)
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Low-temperature scanning tunneling spectroscopy reveals that the Kondo temperature T_K of Co atoms adsorbed on Cu/Co/Cu(100) multilayers varies between 60 K and 134 K as the Cu film thickness decreases from 20 to 5 atomic layers. The observed change of T_K is attributed to a variation of the density of states at the Fermi level rho_F induced by quantum well states confined to the Cu film. A model calculation based on the quantum oscillations of rho_F at the belly and the neck of the Cu Fermi surface reproduces most of the features in the measured variation of T_K.
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The Kondo zero bias anomaly of Co adatoms probed by scanning tunneling microscopy is known to depend on the height of the tip above the surface, and this dependence is different on different low index Cu surfaces. On the (100) surface, the Kondo temperature first decreases then increases as the tip approaches the adatom, while on the (111) surface it is virtually unaffected. These trends are captured by combined density functional theory and numerical renormalization group (DFT+NRG) calculations. The adatoms are found to be described by an S = 1 Anderson model on both surfaces, and ab initio calculations help identify the symmetry of the active d orbitals. We correctly reproduce the Fano lineshape of the zero bias anomaly for Co/Cu(100) in the tunneling regime but not in the contact regime, where it is probably dependent on the details of the tip and contact geometry. The lineshape for Co/Cu(111) is presumably affected by the presence of surface states, which are not included in our method. We also discuss the role of symmetry, which is preserved in our model scattering geometry but most likely broken in experimental conditions.
We report on the magnetic properties of Fe and Co adatoms on a Cu$_{2}$N/Cu(100)-$c(2 times 2)$ surface investigated by x-ray magnetic dichroism measurements and density functional theory (DFT) calculations including the local coulomb interaction. We compare these results with properties formerly deduced from STM spin excitation spectroscopy (SES) performed on the individual adatoms. In particular we focus on the values of the local magnetic moments determined by XMCD compared to the expectation values derived from the description of the SES data.The angular dependence of the projected magnetic moments along the magnetic field, as measured by XMCD, can be understood on the basis of the SES Hamiltonian. In agreement with DFT, the XMCD measurements show large orbital contributions to the total magnetic moment for both magnetic adatoms.
Linear atomic chains containing a single Kondo atom, Co, and several nonmagnetic atoms, Cu, were assembled atom by atom on Cu(111) with the tip of a scanning tunneling microscope. The resulting one-dimensional wires, Cu$_m$CoCu$_n$ ($0leq m, nleq 5$), exhibit a rich evolution of the single-Co Kondo effect with the variation of $m$ and $n$, as inferred from changes in the line shape of the Abrikosov-Suhl-Kondo resonance. The most striking result is the quenching of the resonance in CuCoCu$_2$ and Cu$_2$CoCu$_2$ clusters. State-of-the-art first-principles calculations were performed to unravel possible microscopic origins of the remarkable experimental observations.
We revisit the theory of the Kondo effect observed by a scanning-tunneling microscope (STM) for transition-metal atoms (TMAs) on noble-metal surfaces, including $d$ and $s$ orbitals of the TMA, surface and bulk conduction states of the metal, and their hoppingto the tip of the STM. Fitting the experimentally observed STM differential conductance for Co on Cu(111) including both, the Kondo feature near the Fermi energy and the resonance below the surface band, we conclude that the STM senses mainly the Co $s$ orbital and that the Kondo antiresonance is due to interference between states with electrons in the $s$ orbital and a localized $d$ orbital mediated by the conduction states.
We investigate the electronic structure of cobalt atoms on a copper surface and in a copper host by combining density functional calculations with a numerically exact continuous-time quantum Monte Carlo treatment of the five-orbital impurity problem. In both cases we find low energy resonances in the density of states of all five Co $d$-orbitals. The corresponding self-energies indicate the formation of a Fermi liquid state at low temperatures. Our calculations yield the characteristic energy scale -- the Kondo temperature -- for both systems in good agreement with experiments. We quantify the charge fluctuations in both geometries and suggest that Co in Cu must be described by an Anderson impurity model rather than by a model assuming frozen impurity valency at low energies. We show that fluctuations of the orbital degrees of freedom are crucial for explaining the Kondo temperatures obtained in our calculations and measured in experiments.
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