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We present high-resolution photoemission experiments of the G-surface state of Be(0001). The spectral function near the Fermi wave vector reveals a strong quasiparticle peak due to coupling with surface phonon modes. The coupling constant l is estimated to be 1.18 6 0.07 based on the renormalization of the effective mass. No gap was observed down to 12 K. Any interpretation of the data based on charge density wave formation or surface superconductivity can therefore be discarded.It has been recognized for many years that the surfaces of Be have a behavior contrasting the bulk properties of this metal. The electronic structure of Be metal is dominated by the fraction of 2s electrons promoted to 2p states, leading to severe deviations from the freeelectron model. Some examples are an unusually high Debye temperature and Poisson's ratio [1], as well as a large diamagnetic susceptibility and a small electronic contribution to the specific heat [2]. From bulk band structure calculations it is known that the Fermi level is situated in a dip in the density of states [2,3], leading to a number of states at E F an order of magnitude less than for other simple metals, e.g., Mg [3].The (0001) surface of Be does not reconstruct, but drastic modification of the bulk properties can be anticipated from the reduction in coordination at the surface. Lowenergy electron diffraction (LEED) measurements reveal large deviations of the interplanar spacing at the surface compared to the bulk. Large values for the mean square displacement and thermal expansion are also unveiled [4], and all of these physical parameters are well described by density-functional theory [5]. The surface phonon modes investigated by electron-energy-loss spectroscopy (EELS) could only be reproduced by calculations which assume substantial variations of the nearest neighbor force constants, inducing in the surface layer a reduction of the interplanar bonding and an increase of the in-plane bonding [3,6]. A detailed calculation of the electronic structure of Be (0001) [7] predicts the existence of surface states in gaps of the projected bulk density of states (DOS), in good agreement with angle-resolved photoemission data [8,9]. The charge density originating from the surface states is essentially localized within the first two layers [7,10] and dominates the DOS at E F by a factor of 4 over the bulk density. As a consequence of the valence charge redistribution, the binding energy of the 1s core level, which probes the local charge density [10], is shifted with respect to its bulk value. Four distinct components resolved by x-ray photoemission spectroscopy could be related to emission from layers at different depths below the surface [11]. Finally, the electron inelastic mean free path of 2 Å at low kinetic energies (10-40 eV) is anomalously small compared to about 10 Å of the universal curve due to a high electronhole pair and/or surface plasmon creation rate [12].The G-surface state of Be(0001) is widely decoupled from the bulk states and forms a nearly ideal...

Antimonene, a novel group 15 two‐dimensional material, is functionalized with a tailormade perylene bisimide through strong van der Waals interactions. The functionalization process leads to a significant quenching of the perylene fluorescence, and surpasses that observed for either graphene or black phosphorus, thus allowing straightforward characterization of the flakes by scanning Raman microscopy. Furthermore, scanning photoelectron microscopy studies and theoretical calculations reveal a remarkable charge‐transfer behavior, being twice that of black phosphorus. Moreover, the excellent stability under environmental conditions of pristine antimonene has been tackled, thus pointing towards the spontaneous formation of a sub‐nanometric oxide passivation layer. DFT calculations revealed that the noncovalent functionalization of antimonene results in a charge‐transfer band gap of 1.1 eV.

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We present high-resolution photoemission data of the ⌫ -surface state on Be͑0001͒. Near the Fermi surface a narrow quasiparticle peak caused by strong electron-phonon coupling emerges. A many-body calculation is performed, which describes precisely the exceptional evolution of the experimental spectra. We demonstrate that all the necessary parameters can be directly deduced from the experiment. ͓S0163-1829͑99͒00236-2͔

We identify and characterize a two-dimensional phase transition in a layer of Sn on Cu͑100͒. The stable phase at room temperature has a ͑3 ͱ 2 ϫ ͱ 2͒R45°structure. Above ϳ360 K, a new phase with ͑ ͱ 2 ϫ ͱ 2͒R45°structure is formed. The high-temperature phase exhibits a quasi-two-dimensional free-electron surface band, with Fermi surface nesting in excellent agreement with the three-times larger periodicity of the low-temperature phase. A momentum-dependent band gap opens along the nested areas of the Fermi surface in the low-temperature phase. The phase transition is a clear experimental confirmation of the role of Fermisurface gapping and nesting in the stabilization of a commensurate two-dimensional phase, which is interpreted as a charge-density wave.

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2D materials have opened a new field in materials science with outstanding scientific and technological impact. A largely explored route for the preparation of 2D materials is the exfoliation of layered crystals with weak forces between their layers. However, its application to covalent crystals remains elusive. Herein, a further step is taken by introducing the exfoliation of germanium, a narrow‐bandgap semiconductor presenting a 3D diamond‐like structure with strong covalent bonds. Pure α‐germanium is exfoliated following a simple one‐step procedure assisted by wet ball‐milling, allowing gram‐scale fabrication of high‐quality layers with large lateral dimensions and nanometer thicknesses. The generated flakes are thoroughly characterized by different techniques, giving evidence that the new 2D material exhibits bandgaps that depend on both the crystallographic direction and the number of layers. Besides potential technological applications, this work is also of interest for the search of 2D materials with new properties.