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We report on the formation of a two-dimensional supramolecular Kondo lattice made of organic molecules comprising only C, N, and H atoms, namely metal-free phthalocyanines 2HPc (C32H18N8), adsorbed on a Ag(111) surface. Low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS), ultraviolet photoemission spectroscopy (UPS), and density functional theory (DFT) are used to investigate the electronic structure of the system for low molecular density and commensurate self-assemblies. Abrikosov–Suhl resonances that are characteristic of the arising of a Kondo effect are observed by using both STS and UPS at low temperature. Whereas freestanding 2HPc is a no-spin-bearing molecule, it acquires an unpaired electron in its π-conjugated lowest unoccupied molecular orbital (LUMO) upon adsorption on Ag. As a result, the Kondo effect can be interpreted as originating from the interaction between this unpaired π-spin and the Ag Fermi sea. Extra side resonances are observed in STS spectra that are a manifestation of the coupling between the π electron and molecular vibrational excitations, qualifying the effect as the molecular vibrational Kondo effect. The variation of the Kondo temperature TKof single molecules and upon two-dimensional self-assembly is discussed. This study brings new insights into the Kondo-related physics in correlation with molecular spins and low dimensionality. Moreover, it may open new routes to the synthesis of organic molecular spintronic devices.

High resolution photoemission spectroscopy is exploited for the study of phase transitions in low-dimensional systems. We show that 1D-metals do not exhibit a finite spectral density at the Fermi level. Different mechanisms are invoked to explain this striking behaviour.

Low dimensional systems are characterized by at least one spatial dimension of only some atoms. Such size reduction has often important consequences for physical properties. Electronic correlation and electron–phonon coupling can originate Mott insulators or charge density waves (CDWs), both phenomena enhanced by dimensionality reduction. Interfaces offer a natural way of reducing the dimensionality. Among all the surfaces, semiconducting surfaces are particularly well adapted for electronic correlation studies. In them, correlation is enhanced because of the low dimension, the electronic localization in dangling bonds and the large inter‐orbital distances in reconstructions. Despite these factors favoring correlation, eventually stronger than in bulk systems, the field is by far much less developed. We review here the discovery of correlated surfaces, while studying the Schottky barrier of alkalis on Si or GaAs, and coetaneous studies on SiC. We summarize then the studies on K/Si(111):B, whose ${\left( {\sqrt {3} \times \sqrt {3} } \right)}R30^\circ $was considered a surface Mott insulator with an important electron‐phonon coupling. The recent discovery of a ${\left( {2\sqrt {3} \times 2\sqrt {3} } \right)}R30^\circ $symmetry has been first interpreted as evidence of a bipolaronic insulator, but new findings have finally proven it to be a band insulator. We will then focus on the model system Sn/Ge(111). The last unclear issue about the (3 × 3) reconstruction at 150 K was related to the Sn 4d core level. High‐resolution photoemission has clarified the core level deconvolution while refining the structural model. This metallic reconstruction is sensitive to electronic correlations which trigger a phase transition to a Mott phase below 30 K.

Abstract:: The electronic state of cerium has been investigated by core-level and valence band photoemission spectroscopy in ultra-thin films of a Ce-Pd monocrystalline compound obtained by epitaxial growth. Careful characterization of the surface leads us to the conclusion that the Ce configuration admixture is strongly reduced in the surface layer. By evaporating one extra monolayer of Pd, we were able to obtain core-level photoemission spectra without any surface contribution and to evidence an exceptional configuration mixing for the bulk ground state. The 4f spectral function obtained from photoemission measurements at the N4,5 threshold also exhibits a very singular behavior and shed some new light on the description of highly-hybridized 4f states in Ce compounds.

Temperature dependent photoelectron spectra of the heavy fermion YbAgCu4 show that the Kondo resonance is strongly depressed with increasing temperature. The observed reduction is fully consistent with the predictions of the Anderson impurity model, and reveals the transition from a Fermi liquid at low temperature to localized moments above the characteristic Kondo temperature. Despite this important experimental confirmation of the model, discrepancies exist between the experimental and calculated linewidth of the resonance. It is argued that a more realistic treatment of the photoemission process would improve the agreement between theory and experiment.

Magnetic properties of evaporated amorphous CexSi100−xalloys 28⩽×⩽40) are compared with those of the crystalline compounds of same composition. It is suggested that a inhomogeneous admixture of Ce ions in amorphous alloys substitutes for the homogeneous intermediate valence state observed in crystalline CeSi2.

Photoelectron spectroscopy can probe low energy electronic excitations in solids, with practical limitations set by the experimental energy and angular resolution. Appropriate conditions can now be achieved which allow us to visualize changes in the electronic states near the Fermi surface at phase transitions, namely in two-dimensional materials. Quasi one-dimensional conductors are unique in their apparent failure to exhibit a clear metallic signature. This unusual spectroscopic behavior might be the consequence of the singular nature of electronic correlations in 1D.

Abstract:: We exploited resonant photoemission at the Ce $$4d \to 4f$$ absorption edge to investigate the Ce 4f states in3. High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds. The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent $$({f^0})$$ ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations.

High resolution photoemission and bremsstrahlung isochromat spectroscopy experiments have been carried out on CeAg and LaAg. From a comparison between these two compounds, we extract the cerium 4f spectral function, we compare it with band structure calculations and with many-body model simulations based on the single-impurity Anderson Hamiltonian. We show that the localized 4f charge estimated from previous band structure calculations cannot be identified with the 4f occupation number deduced from the Anderson model.

The RE valence in series of amorphous Yb-Pd and Eu-Pd amorphous alloys was determined from bulk magnetization, LIIIabsorption edge and 151Eu isomer shift measurements. The RE electronic configuration was found to change from trivalent to divalent with increasing RE content. Same values for the average Eu valence were obtained from M⊙ssbauer and LIIIedge measurements in the Eu based alloys, indicating that the valence admixture is inhomogeneous and that the final state effects in the LIIIspectra are negligible for these alloys. Discrepancies observed about the Yb valence values as determined from magnetic and from LIIIedge measurements suggest that some amount of trivalent Yb ions are not magnetic.

Electronic correlation and electron‐phonon coupling can cause Mott insulators or charge density waves, both phenomena significantly enhanced by the reduction of dimensionality. Correlated surfaces were discovered in the 1980s during studies aimed at understanding the microscopic details of Schottky barriers. In the Feature Article on pp. 614–626, Tejeda et al. show the relevance of semiconducting surfaces for the study of correlation effects. Evidence is provided that the narrow bands associated with widely spaced dangling bonds increase the electronic localization and may drive metal‐insulator transitions. The localization can be tuned by studying similar reconstructions with different lattice parameters or different adsorbate species. For this purpose, the authors discuss the effects on K/Si(111):B and Sn/Ge(111) surfaces. Doping experiments with standard surface science techniques allow for further tailoring the systems and eventually relating the Mott insulators with two‐dimensional superconductors. Furthermore, electronic localization is related to spin localization, which may promote exotic magnetic phases, especially in triangular lattice systems prone to magnetic frustration. This opens up new prospects for the future research on electronic correlation and many‐body effects at surfaces.