Table of contents

Low-dimensional systems have always deserved attention due to the peculiarity of their physics, which is different from or even at odds with three-dimensional expectations. This is precisely the case for many-body effects, as electron–electron correlation or electron–phonon coupling are behind many intriguing problems in condensed matter physics. These interesting phenomena at low dimensions can be studied in one of the paradigms of two dimensionality—the surface of crystals. The maturity of today's surface science techniques allows us to perform thorough experimental studies that can be complemented by the current strength of state-of-the-art calculations. Surfaces are thus a natural two-dimensional playground for studying correlation and many-body effects, which is precisely the object of this special section.

This special section presents a collection of eight invited articles, giving an overview of the current status of selected systems, promising techniques and theoretical approaches for studying many-body effects at surfaces and low-dimensional systems. The first article by Hofmann investigates electron–phonon coupling in quasi-free-standing graphene by decoupling graphene from two different substrates with different intercalating materials. The following article by Kirschner deals with the study of NiO films by electron pair emission, a technique particularly well-adapted for studying high electron correlation. Bovensiepen investigates electron–phonon coupling via the femtosecond time- and angle-resolved photoemission spectroscopy technique. The next article by Malterre analyses the phase diagram of alkalis on Si(111):B and studies the role of many-body physics. Biermann proposes an extended Hubbard model for the series of C, Si, Sn and Pb adatoms on Si(111) and obtains the inter-electronic interaction parameters by first principles. Continuing with the theoretical studies, Bechstedt analyses the influence of on-site electron correlation in insulating antiferromagnetic surfaces. Ortega reports on the gap of molecular layers on metal systems, where the metal–organic interaction affects the organic gap through correlation effects. Finally, Cazalilla presents a study of the phase diagram of one-dimensional atoms or molecules displaying a Kondo-exchange interaction with the substrate.

Acknowledgments

The editors are grateful to all the invited contributors to this special section of Journal of Physics: Condensed Matter. We also thank the IOP Publishing staff for handling the administrative matters and the refereeing process.

Correlation and many-body effects at surfaces contents

The dimensionality reduction at surfaces as a playground for many-body and correlation effectsA Tejeda, E G Michel and A Mascaraque

Electron–phonon coupling in quasi-free-standing grapheneJens Christian Johannsen, Søren Ulstrup, Marco Bianchi, Richard Hatch, Dandan Guan, Federico Mazzola, Liv Hornekær, Felix Fromm, Christian Raidel, Thomas Seyller and Philip Hofmann

Exploring highly correlated materials via electron pair emission: the case of NiO/Ag(100)F O Schumann, L Behnke, C H Li and J Kirschner

Coherent excitations and electron–phonon coupling in Ba/EuFe₂As₂ compounds investigated by femtosecond time- and angle-resolved photoemission spectroscopyI Avigo, R Cortés, L Rettig, S Thirupathaiah, H S Jeevan, P Gegenwart, T Wolf, M Ligges, M Wolf, J Fink and U Bovensiepen

Understanding the insulating nature of alkali-metal/Si(111):B interfacesY Fagot-Revurat, C Tournier-Colletta, L Chaput, A Tejeda, L Cardenas, B Kierren, D Malterre, P Le Fèvre, F Bertran and A Taleb-Ibrahimi

What about U on surfaces? Extended Hubbard models for adatom systems from first principlesPhilipp Hansmann, Loïg Vaugier, Hong Jiang and Silke Biermann

Influence of on-site Coulomb interaction U on properties of MnO(001)2 × 1 and NiO(001)2 × 1 surfacesA Schrön, M Granovskij and F Bechstedt

On the organic energy gap problemF Flores, E Abad, J I Martínez, B Pieczyrak and J Ortega

Easy-axis ferromagnetic chain on a metallic surfaceMiguel A Cazalilla

Quasi-free-standing monolayer graphene can be produced by intercalating species like oxygen or hydrogen between epitaxial graphene and the substrate crystal. If the graphene was indeed decoupled from the substrate, one would expect the observation of a similar electronic dispersion and many-body effects, irrespective of the substrate and the material used to achieve the decoupling. Here we investigate the electron–phonon coupling in two different types of quasi-free-standing monolayer graphene: decoupled from SiC via hydrogen intercalation and decoupled from Ir via oxygen intercalation. The two systems show similar overall behaviours of the self-energy and a weak renormalization of the bands near the Fermi energy. The electron–phonon coupling is found to be so weak that it renders the precise determination of the coupling constant λ through renormalization difficult. The estimated value of λ is 0.05(3) for both systems.

Metal oxides like NiO are usually termed 'highly correlated', because the material properties are decisively determined by the electron–electron interaction. This makes them interesting candidates for electron pair spectroscopy which is particularly sensitive to the electron correlation. We have prepared ultrathin NiO/Ag(100) films and studied the electron pair emission upon electron impact. Compared to the metal substrate we observe an increase of the coincidence intensity by a factor of 8 for NiO. Thickness dependent measurements prove that this enhancement is an intrinsic effect rather than due to a mean free path increase of the oxide. The Néel temperature T_N of NiO films displays a thickness dependence which allows us to tune T_N. We performed temperature dependent measurements and observed no temperature dependence of the coincidence spectra. This proves that the electron pair emission probes the local correlation rather than long range order. An enhanced coincidence intensity was also found for other oxide phases compared to their corresponding metal phases.

We employed femtosecond time- and angle-resolved photoelectron spectroscopy to analyze the response of the electronic structure of the 122 Fe-pnictide parent compounds Ba/EuFe₂As₂ and optimally doped BaFe_1.85Co_0.15As₂ near the Γ point to optical excitation by an infrared femtosecond laser pulse. We identify pronounced changes of the electron population within several 100 meV above and below the Fermi level, which we explain as a combination of (i) coherent lattice vibrations, (ii) a hot electron and hole distribution, and (iii) transient modifications of the chemical potential. The responses of the three different materials are very similar. In the coherent response we identify three modes at 5.6, 3.3, and 2.6 THz. While the highest frequency mode is safely assigned to the A_1g mode, the other two modes require a discussion in comparison to the literature. Employing a transient three temperature model we deduce from the transient evolution of the electron distribution a rather weak, momentum-averaged electron–phonon coupling quantified by values for λ〈ω²〉 between 30 and 70 meV². The chemical potential is found to present pronounced transient changes reaching a maximum of 15 meV about 0.6 ps after optical excitation and is modulated by the coherent phonons. This change in the chemical potential is particularly strong in a multiband system like the 122 Fe-pnictide compounds investigated here due to the pronounced variation of the electron density of states close to the equilibrium chemical potential.

We have recently revisited the phase diagram of alkali-metal/Si(111):B semiconducting interfaces previously suggested as the possible realization of a Mott–Hubbard insulator on a triangular lattice. The insulating character of the $2\sqrt{3}\times 2\sqrt{3}\mathrm{R}3 0$ surface reconstruction observed at the saturation coverage, i.e. 0.5 ML, has been shown to find its origin in a giant alkali-metal-induced vertical distortion. Low energy electron diffraction, photoemission spectroscopy and scanning tunneling microscopy and spectroscopy experiments coupled with linear augmented plane-wave density functional theory calculations allow a full understanding of the k-resolved band structure, explaining both the inhomogeneous charge transfers into an Si–B hybridized surface state and the opening of a band gap larger than 1 eV. Moreover, $\sqrt{3}\times \sqrt{3}\mathrm{R}3 0$, 3 × 3 and $2\sqrt{3}\times 2\sqrt{3}\mathrm{R}3 0$ surface reconstructions observed as a function of coverage may reveal a filling-controlled transition from a half-filled correlated magnetic material to a strongly distorted band insulator at saturation.

Electronic correlations together with dimensional constraints lead to some of the most fascinating properties known in condensed matter physics. As possible candidates where these conditions are realized, semiconductor (111) surfaces and adatom systems on surfaces have been under investigation for quite some time. However, state-of-the-art theoretical studies on these materials that include many-body effects beyond the band picture are rare. First principles estimates of inter-electronic Coulomb interactions for the correlated states are missing entirely, and usually these interactions are treated as adjustable parameters. In this work, we report on calculations of the interaction parameters for the group IV surface–adatom systems in the α-phase series of Si(111):C, Si, Sn, Pb. For all systems investigated, the inter-electronic Coulomb interactions are indeed large compared to the kinetic energies of the states in question. Moreover, our study reveals that intersite interactions cannot be disregarded. We explicitly construct an extended Hubbard model for the series of group IV surface–adatom systems on silicon, which can be used for further many-body calculations.

We investigate the non-polar and antiferromagnetic (001) surfaces of MnO and NiO crystallizing in the antiferromagnetic type-II phase by means of spin-polarized density functional theory. Results are presented for surface energy, magnetization, and electronic structure. Besides the comparison of the two materials with differently filled minority-spin t_2g shells we study the influence of exchange–correlation treatment within two schemes, the generalized gradient approximation GGA and the GGA + U scheme including an effective on-site d–d interaction U.

In conjugated organic molecules, the difference between the HOMO and LUMO Kohn–Sham eigenvalues is significantly smaller than the transport gap measured experimentally. We discuss here, within a local-orbital formulation of DFT, how this problem can be corrected using appropriate hybrid potentials, that add a fraction of Hartree–Fock exchange interaction in the DFT calculation. We illustrate this approach presenting calculations for two simple systems: H₂ and C₆H₆; then, we discuss how to implement this hybrid approach in a general local-orbital calculation, adjusting the hybrid contribution to yield the correct experimental HOMO/LUMO energy gap for the molecule. We also consider the case of an organic molecule on a metal and analyze the effect of the molecule–metal interaction on the organic energy gap. In particular, we discuss how to introduce in this hybrid-potential scheme the effect of the image potential, and present results for the organic molecules PTCDA, TTF, benzene and pentacene on the metal surfaces Au(111), Ag(111) and Cu(111).

The phases and excitation spectrum of an easy-axis ferromagnetic chain of S = 1/2 magnetic impurities built on the top of a clean metallic surface are studied. As a function of the (Kondo) coupling to the metallic surface and at low temperatures, the spin chain exhibits a quantum phase transition from an Ising ferromagnetic phase with long-range order to a paramagnetic phase where quantum fluctuations destroy the magnetic order. In the paramagnetic phase, the system consists of a chain of Kondo singlets where the impurities are completely screened by the metallic host. In the ferromagnetic phase, the excitations above the Ising gap are damped magnons, with a finite lifetime arising due to the coupling to the substrate. We discuss the experimental consequences of our results to spin-polarized electron energy loss spectroscopy, and we finally analyze possible extensions to spin chains with S > 1/2.

We have studied the atomic structure and energetic stability of helium (He) and He–vacancy clusters in an iron (Fe) Σ5(310)/[001] grain boundary (GB) using a first-principles method. The He and He–vacancy clusters in the Fe GB are shown to exhibit high-symmetry structures. The equilibrium He–He distance in the clusters is ∼1.70 Å, much smaller than 2.80 Å in the vacuum or 2.94 Å in a face centred cubic (fcc) crystal, indicating the attractive interaction between the He atoms due to the presence of Fe. The charge density surrounding He is demonstrated to decrease with an increasing number of He atoms in the clusters, leading to a positive binding energy of a He atom to the clusters. This suggests He and He–vacancy clusters can energetically trap more He atoms, which is responsible for the growth of the He-related clusters (He and He–vacancy clusters) and thus the He bubbles in the GB. The binding energy of an interstitial He atom to the He-related clusters is found generally lower in the GB than in a bcc crystal. Besides, the binding strengths of small He clusters to the GB and to a vacancy in a bcc matrix are compared, and the latter shows greater trapping strength to an interstitial He and a He₂ cluster. The magnetism of the Fe atoms near the GB as well as its variation caused by the He-related clusters is also investigated. The local magnetic moment variation of the Fe atoms in the system is enhanced to a different extent, depending on the size of the He-related clusters.

We report on the preparation of large-scale uniform bilayer graphenes on nominally flat Si-polar 6H-SiC(0001) substrates by flash annealing in ultrahigh vacuum. The resulting graphenes have a single thickness of one bilayer and consist of regular terraces separated by the triple SiC bilayer steps on the 6H-SiC(0001) substrates. In situ scanning tunneling microscopy reveals that suppression of pit formation on terraces and uniformity of SiC decomposition at step edges are the key factors to the uniform thickness. By studying the surface morphologies prepared under different annealing rates, it is found that the annealing rate is directly related to SiC decomposition, diffusion of the released Si/C atoms and strain relaxation, which together determine the final step structure and density of defects.

The formation of ripple structures on ion bombarded semiconductor surfaces is examined theoretically. Previous models are discussed and a new nonlinear model is formulated, based on the infinitesimal local atomic relocation induced by elastic nuclear collisions in the early stages of collision cascades and an associated density change in the near surface region. Within this framework ripple structures are shown to form without the necessity to invoke surface diffusion or large sputtering as important mechanisms. The model can also be extended to the case where sputtering is important, and it is shown that in this case certain 'magic' angles can occur at which the ripple patterns are most clearly defined. The results are in very good agreement with experimental observations.

Epitaxial thin oxide layers were grown by simultaneous aluminum deposition and oxidation on a Co(0001) single crystal, and the metal–oxide interface between the substrate and the grown layer was studied using photoelectron spectroscopy. The oxide layers were composed of two kinds of chemically different layers. Angle-resolved measurements were used to determine the compositions of oxide sub-layers and to reveal their respective thicknesses. The topmost oxide layers were up to 0.23 nm thick, determined by analysis of O 1s and Co 2p_3/2 photoelectron spectra. The results of the analysis show that the interface layer is composed of a mixture of oxygen and cobalt atoms and its thickness is approximately 0.6 nm. The analysis of Co 2p_3/2, Al 2p_3/2 and O 1s core level binding energies confirmed the presence of CoO in the interface layer and Al₂O₃ in the topmost oxide layer.

Based on the recently constructed Ni–Zr–Al n-body potential, Monte Carlo simulations are performed to study the glass formation and associated structural evolutions in the system. The micro-chemical inhomogeneity (MCI) parameter and Honeycutt and Anderson (HA) pair analysis are employed to investigate both the chemical short-range orders and topological short-range orders for the ternary Ni–Zr–Al metallic glasses. Results reveal that remarkable chemical short-range orders (CSROs) exist in the ternary Ni–Zr–Al metallic glasses and are strongly influenced by the chemical interactions among the constituent elements. Moreover, topological short-range orders are clearly formed in the ternary Ni–Zr–Al metallic glasses, with the most remarkable characteristic being the icosahedral local packing. Similarly to CSRO, the extent of icosahedral short-range orders formed in the Ni–Zr–Al system varies distinctly with the chemical composition. In addition, simulation results reveal that chemical short-range orders and topological short-range orders turn out to be influenced by different factors. Unlike CSRO, both chemical interactions and geometrical constraints play important roles in forming the topological short-range orders.

Electrodes with highly porous morphologies are of great technological interest, as their exceptionally high specific surface areas make them ideal for use in capacitors, battery electrodes and electrochemical sensors. There is a large body of research focusing on the structure of confined electrolytes in these systems, but the majority of these studies focus on cases where the length scale of the porous domain is equal to or less than the Debye screening length of the electrolyte. In this work, we use a thermodynamic model to consider the structure of electrolytes in mesoscale domains, where the pore dimensions are significantly larger than the Debye screening length. In this limit, the interface is screened by the electrochemical double layer and the enclosed volume primarily consists of an electroneutral 'bulk liquid' domain. Despite the absence of direct interactions between ions in the bulk domain and the charged interface, we show that minimization of the free energy of the system leads to a reduction in the ionic strength of the electrolyte within the bulk liquid domain of the pore. Based on our model studies, we anticipate that this depletion will apply for porous domains with widths of the order of 50–200 nm even under mild experimental conditions and low applied voltages. The results imply relationships between electrolyte strength, surface morphology and applied voltage that may be important in device design.

We have measured the surface topography and calculated the surface roughness power spectrum for an asphalt road surface. For the same surface we have measured the friction for a tire tread compound for velocities 10⁻⁶ m s⁻¹ < v < 10⁻³ m s⁻¹ at three different temperatures (at −8 °C, 20 °C and 48 °C). The friction data was shifted using the bulk viscoelasticity shift factor a_T to form a master curve. We have measured the effective rubber viscoelastic modulus at large strain and calculated the rubber friction coefficient (and contact area) during stationary sliding and compared it to the measured friction coefficient. We find that for the low velocities and for the relatively smooth road surface we consider, the contribution to friction from the area of real contact is very important, and we interpret this contribution as being due to shearing of a very thin confined rubber smear film.

The incorporation of Li atoms into Si(111) is studied with the aid of density functional theory (DFT). We find that Li adsorption removes the (1 × 2) reconstruction on this surface. For the whole range of chemical potential values of Li at which Li adsorption occurs, only a high coverage structure (1 ML) is stabilized on unreconstructed Si(111). We find for this surface that the simultaneous penetration of two or more Li atoms into the subsurface is very unfavorable. However, the penetration barrier for a single Li atom into the subsurface on 1 ML-Li/Si(111) is very similar to that on clean Si(111). The present work can help us to better understand the process of Li insertion into Si anodes of Li-ion batteries.

We have studied the adsorption of copper on the clean Re$(1 0\bar {1}0)$ surface between 300 and 900 K by means of low- and medium-energy electron diffraction (LEED and MEED) and temperature-programmed thermal desorption (TPD). The persistence of a (1 × 1) LEED pattern during Cu deposition suggests the formation of pseudomorphic Cu islands. Accordingly, the intensity–voltage behaviour of the (1 × 1) LEED beams can be quantitatively superimposed by the coverage-weighed fractions of the I(V)-curves of uncovered Re areas and of Cu-covered (1 × 1) islands. At a coverage of 1.625 × 10¹⁹ Cu atoms m⁻² dynamical LEED I(V) calculations suggest a full hcp-oriented Cu bilayer (BL). Within this first BL, Cu wets the Re surface completely, while all following layers exhibit remnant roughness due to small Cu nuclei, as confirmed by in situ grazing-incidence MEED experiments. The completion of the first BL coincides with the saturation of a single β TPD state at 1180 K, whilst higher coverages produce an additional zero-order α state, which peaks at 1080 K at 2.6 BL. The energy of desorption rises from 320 kJ mol⁻¹ at small coverages to ∼360 kJ mol⁻¹ for a bilayer Cu film, pointing to attractive lateral Cu–Cu interactions. An analysis of the leading edge of the multilayer α state yields a desorption energy of ∼305 kJ mol⁻¹, somewhat lower than the sublimation enthalpy of bulk Cu. Our data are discussed and compared with previous results on related systems.