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Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for studying fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening with an effective electron mass ratio of $m^e_X/m^e_Y \sim 50$. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability and a Fano resonance from a van Hove singularity of similar strength of metallic carbon nanotubes. Moreover, due to the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D correlated many-body physics in the presence of magnetic order., main: 16 pages, 5 figures; SI: 15 pages, 9 figures

ACS nano 14(11), 15766-15775 (2020). doi:10.1021/acsnano.0c06798, Published by American Chemical Society, Washington, DC

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $\rm{X_D^-}$ 19 meV $\textit{below}$ the lowest bright trion, $\rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $\rm{X_D^-}$ lies 6 meV $\textit{above}$ $\rm{X}_1^-$, while $\rm{X_D^-}$ and $\rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $\textit{ab-initio}$ calculations and quantitatively explain the optical response of doped monolayers with temperature., Comment: 16 pages including supporting information, 4 figures in the main text, 10 figures in the supporting information

The effect of a magnetic field on the optical absorption in semiconductors has been measured experimentally and modeled theoretically for various systems in previous decades. We present a new first-principles approach to systematically determine the response of excitons to magnetic fields, i.e., exciton $g$ factors. By utilizing the $GW$-Bethe-Salpeter equation methodology we show that $g$ factors extracted from the Zeeman shift of electronic bands are strongly renormalized by many-body effects which we trace back to the extent of the excitons in reciprocal space. We apply our approach to monolayers of transition metal dichalcogenides (${\mathrm{MoS}}_{2}$, ${\mathrm{MoSe}}_{2}$, ${\mathrm{MoTe}}_{2}$, ${\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$) with strongly bound excitons for which $g$ factors are weakened by about 30%.

We present an extension of the tunneling theory for scanning tunneling microcopy (STM) to include different types of vibrational-electronic couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronic active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupled system comprising the molecule and the substrate to arbitrary order including the anti-adiabatic strong coupling regime as well as the Kondo effect on a free electron spin of the molecule. The tunneling current is derived in second order of the tunneling matrix element which is expanded in powers of the relevant vibrational displacements. We use the results of an ab-initio calculation for the single-particle electronic properties as an adapted material-specific input for a numerical renormalization group approach for accurately determining the electronic properties of a NTCDA molecule on Ag(111) as a challenging sample system for our theory. Our analysis shows that the mismatch between the ab-initio many-body calculation of the tunnel current in the absence of any electron-phonon coupling to the experiment scanning tunneling spectra can be resolved by including two mechanisms: (i) a strong unconventional Holstein term on the local substrate orbital leads to reduction of the Kondo temperature and (ii) a different electron-vibrational coupling to the tunneling matrix element is responsible for inelastic steps in the $dI/dV$ curve at finite frequencies., 34 pages, 26 figure

Weakly bound systems, like noble-gas dimers or two-dimensional layered materials (graphite, hexagonal boron nitride, or transition-metal dichalcogenides), exhibit excited electronic states of a particular nature. These so-called exciplex states combine on-site (or intralayer) and charge-transfer (or interlayer) configurations in a well-balanced way. We show by ab initio many-body perturbation theory that the energy and composition of the exciplex states depend sensitively on the bond length or interlayer distance of the material. When the constituents approach each other, the charge-transfer contribution increases and the excitation is redshifted to lower energy. If the system is excited into the exciplex state, then a covalent-like bond results. In consequence, noble-gas dimers form excimer complexes, while layered materials exhibit interlayer contraction.

Even if individual two-dimensional materials own various interesting and unexpected properties, the stacking of such layers leads to van der Waals solids which unite the characteristics of two dimensions with novel features originating from the interlayer interactions. In this topical review, we cover fabrication and characterization of van der Waals heterosructures with a focus on heterobilayers made of monolayers of semiconducting transition metal dichalcogenides. Experimental and theoretical techniques to investigate those heterobilayers are introduced. Most recent findings focusing on different transition metal dichalcogenides heterostructures are presented and possible optical transitions between different valleys, appearance of moire patterns and signatures of moire excitons are discussed. The fascinating and fast growing research on van der Waals hetero-bilayers provide promising insights required for their application as emerging quantum-nano materials., 14 pages, 6 figures

This article belongs to the Special Issue Low Dimensional Materials for Environmental and Biomedical Applications., The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2., This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research, innovation programme (grant agreement No. 755655, ERC-StG 2017 project 2D-TOPSENSE), the EU Graphene Flagship funding (Grant Graphene Core 2, 785219), the Netherlands Organisation for Scientific Research (NWO) through the research program Rubicon with project number 680-50-1515, the MINECO (program FIS2015-67367-C2-1-P), and the China Scholarship Council (File NO. 201506120102).

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques also opened the door to study the optical properties of these nanomaterials. We present a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2 and WSe2, with thickness ranging from one layer up to six layers. We analyze the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provides a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2 and WSe2.

Niehues, Iris et al., Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton–phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe2, WSe2, WS2, and MoS2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS2. For MoS2 monolayers, the line width increases. These effects are due to a modified exciton–phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton–phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale., A.K., M.S., and D.C. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG) through SFB 951 (to A.K.) and SFB 910 (to D.C.) and the School of Nanophotonics SFB 787 (to M.S.). E.M. and G.B. were supported by funding from the European Unions Horizon 2020 research and innovation program under grant agreement No. 696656 (Graphene Flagship) and the Swedish Research Council (VR).

We present a physically intuitive model of molecular quantum dots beyond the constant interaction approximation. It accurately describes their charging behavior and allows the extraction of important molecular properties that are otherwise experimentally inaccessible. The model is applied to data recorded with a noncontact atomic force microscope on three different molecules that act as a quantum dot when attached to the microscope tip. The results are in excellent agreement with first-principles simulations.

Monolayers of transition metal dichalcogenides (TMDCs) have unique optoelectronic properties. Density functional theory allows only for a limited description of the electronic excitation energies in these systems, while a more advanced treatment within many-body perturbation theory employing the $\mathit{GW}/\mathrm{BSE}$ approximation is often rather time consuming. Here, we show that the recently developed $\mathrm{LDA}+\mathit{GdW}$ approach provides an efficient and at the same time reliable description of one-particle energies, as well as optical properties including bound excitons in TMDCs. For five exemplary materials (${\mathrm{MoSe}}_{2}, {\mathrm{MoS}}_{2}, {\mathrm{WSe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{ReSe}}_{2}$), we discuss the numerical convergence, in particular with respect to k-point sampling, and show that the $\mathit{GdW}/\mathrm{BSE}$ approximation gives results similar to common $\mathit{GW}/\mathrm{BSE}$ calculations. Such efficient approaches are essential to treat larger multilayer systems or defects.

Bound electron–hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose–Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments and ab initio calculations for 2H-MoTe2, we explain their salient features: the positive sign of the g-factor and the large diamagnetic shift. Our investigations solve the long-standing puzzle of positive g-factors in transition metal dichalcogenides, and pave the way for studying collective phenomena in these materials at elevated temperatures., Excitons, quasi-particles of bound electron-hole pairs, are at the core of the optoelectronic properties of layered transition metal dichalcogenides. Here, the authors unveil the presence of interlayer excitons in bulk van der Waals semiconductors, arising from strong localization and spin-valley coupling of charge carriers.

Since their discovery single-layer semiconducting transition metal dichalcogenides have attracted much attention thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper we demonstrate that biaxial strain, both tensile and compressive, can be applied and released in a timescale of a few seconds in a reproducible way on transition metal dichalcogenides monolayers deposited on polymeric substrates. We can control the amount of biaxial strain applied by letting the substrate expand or compress. To do this we change the substrate temperature and choose materials with a large thermal expansion coefficient. After the investigation of the substrate-dependent strain transfer, we performed micro-differential spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2, WS2, WSe2) under the application of biaxial strain and measured their optical properties. For tensile strain we observe a redshift of the bandgap that reaches a value as large as 95 meV/% in the case of single-layer WS2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe2 < MoS2 < WSe2 < WS2. Theoretical calculations of these four materials under biaxial strain predict the same trend for the material-dependent rates of the shift and reproduce well the features observed in the measured reflectance spectra., 10 pages, 5 figures, 2 tables, supporting information

Optická kontrola spinových prúdov v topologických izolátoroch je nová možnosť v spintronike. Na porozumenie týchto procesov je nutné zistiť elektrónovú štruktúru nad fermiho hladinou za použitia jednokrokového modelu a inverznej ARPES techniky The optical control of spin currents in topological surface states opens new perspectives in (opto-) spintronics. To understand these processes, a profound knowledge about the dispersion and the spin polarization of both the occupied and the unoccupied electronic states is required. We present a joint experimental and theoretical study on the unoccupied electronic states of the topological insulator Bi2Se3 . We discuss spin- and angle-resolved inverse-photoemission results in comparison with calculations for both the intrinsic band structure and, within the one-step model of (inverse) photoemission, the expected spectral intensities. This allows us to unravel the intrinsic spin texture of the unoccupied bands at the surface of Bi2 Se3.

Almost all experiments and future applications of transition metal dichalcogenide monolayers rely on a substrate for mechanical stability, which can significantly modify the optical spectra of the monolayer. Doping from the substrate might lead to the domination of the spectra by trions. Here we show by ab initio many-body theory that the negative trion (A−) splits into three excitations, with both inter- and intra-valley character, while the positive counterpart (A+) consists of only one inter-valley excitation. Furthermore, the substrate enhances the screening, which renormalizes both band gap and exciton as well as the trion-binding energies. We verify that these two effects do not perfectly cancel each other, but lead to red-shifts of the excitation energies for three different substrates ranging from a wide-bandgap semiconductor up to a metal. Our results explain recently found experimental splittings of the lowest trion line as well as excitation red-shifts on substrates., The optical and electrical properties of atomically thin transition metal dichalcogenides critically depend on the underlying substrate. Here, the authors develop an abinitio many-body formalism to investigate the full spectrum of negative and positive trions in these layered semicondutors.

Due to their unique band structure, single layers of transition metal dichalcogenides are promising for new atomic-scale physics and devices. It has been shown that the band structure and the excitonic transitions can be tuned by straining the material. Recently, the discovery of single-photon emission from localized excitons has put monolayer WSe2 in the spotlight. The localized light emitters might be related to local strain potentials in the monolayer. Here, we measure strain-dependent energy shifts for the A, B, C, and D excitons for uniaxial tensile strain up to 1.4% in monolayer WSe2 by performing absorption measurements. A gauge factor of and is derived for the A, B, C, and D exciton, respectively. These values are in good agreement with ab initio GW-BSE calculations. Furthermore, we examine the spatial strain distribution in the WSe2 monolayer at different applied strain levels. We find that the size of the monolayer is crucial for an efficient transfer of strain from the substrate to the monolayer.

We investigate the adsorption of a single tetracyanoethylene (TCNE) molecule on the silver (001) surface. Adsorption structures, electronic properties, and scanning tunneling microscopy (STM) images are calculated within density-functional theory. Adsorption occurs most favorably in on-top configuration, with the C=C double bond directly above a silver atom and the four N atoms bound to four neighboring Ag atoms. The lowest unoccupied molecular orbital of TCNE becomes occupied due to electron transfer from the substrate. This state dominates the electronic spectrum and the STM image at moderately negative bias. We discuss and employ a spatial extrapolation technique for the calculation of STM and scanning tunneling spectroscopy (STS) images. Our calculated images are in good agreement with experimental data.

Electronic excitations are often accompanied by displacement of the ions from their ground-state equilibrium positions. This leads to line broadening of optical spectra, Stokes shifts, conformational changes, and photoinduced reactions. Here we discuss approaches to these features within ab-initio methods, in particular within many-body perturbation theory. A number of various systems, including molecules, point defects, polymers, and surfaces, are discussed to illustrate issues like localization and self-trapping that are relevant for a detailed understanding of the interrelation between excited states and geometrical structure. To cite this article: M. Rohlfing, C. R. Physique 10 (2009).

Using a scanning tunneling microscope we have measured the quantum conductance through a PTCDA molecule for different configurations of the tip-molecule-surface junction. A peculiar conductance resonance arises at the Fermi level for certain tip to surface distances. We have relaxed the molecular junction coordinates and calculated transport by means of the Landauer/Keldysh approach. The zero bias transmission calculated for fixed tip positions in lateral dimensions but different tip substrate distances show a clear shift and sharpening of the molecular chemisorption level on increasing the STM-surface distance, in agreement with experiment., accepted for publication in Applied Physics A

The dispersion of quasi-one-dimensional dangling-bond electrons in pi-bonded chains at the Si(111)-2 x 1 and Ge(111)-2 x 1 surfaces has been experimentally investigated by angle-resolved photoemission spectroscopy, in the direction perpendicular to the chains, with a high energy and angle precision. The results show a very small dispersion in the case of Si(111)-2 x 1 and instead a much larger (downward) dispersion (156 meV) in the case of Ge(111)-2 x 1. Accurate density-functional calculations with GW corrections are in very good agreement with the experimental results. Then the surface chains are somewhat interacting in Ge(111)-2 x 1 - the coupling occurring mainly through the subsurface region - while in Si(111)-2 x 1 they are essentially decoupled. Therefore the one-dimensional character of electrons in surface chains is enhanced in Si(111)-2 x 1 with respect to Ge(111)-2 x 1. (c) 2008 Elsevier B. V. All rights reserved.

Trion states of three correlated particles (e.g., two electrons and one hole) are essential to understand the optical spectra of doped or gated nanostructures, like carbon nanotubes or transition-metal dichalcogenides. We develop a theoretical many-body description for such correlated states using an ab-initio approach. It can be regarded as an extension of the widely used $GW$ method and Bethe-Salpeter equation, thus allowing for a direct comparison with excitons. We apply this method to a semiconducting (8,0) carbon nanotube, and find that the lowest optically active trions are red-shifted by $\sim 130$ meV compared to the excitons, confirming experimental findings for similar tubes. Moreover, our method provides detailed insights in the physical nature of trion states. In the prototypical carbon nanotube we find a variety of different excitations, discuss the spectra, energy compositions, and correlated wave functions., main text, supplement

By means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), we have investigated the adsorption of single Au atoms on a PTCDA monolayer physisorbed on the Au(111) surface. A chemical reaction between the Au atom and the PTCDA molecule leads to the formation of a radical that has an unpaired electron in its highest occupied orbital. This orbital is a $\ensuremath{\pi}$ orbital that extends over the whole Au-PTCDA complex. Because of the large Coulomb repulsion in this orbital, the unpaired electron generates a local moment when the molecule is adsorbed on the Au(111) surface. We demonstrate the formation of the radical and the existence of the local moment after adsorption by observing a zero-bias differential conductance peak that originates from the Kondo effect. By temperature dependent measurements of the zero-bias differential conductance, we determine the Kondo temperature to be ${T}_{\mathrm{K}}=(38\ifmmode\pm\else\textpm\fi{}8)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. For the theoretical description of the properties of the Au-PTCDA complex we use a hierarchy of methods, ranging from density functional theory (DFT) including a van der Waals correction to many-body perturbation theory (MBPT) and the numerical renormalization group (NRG) approach. Regarding the high-energy orbital spectrum, we obtain an excellent agreement with experiments by both spin-polarized DFT/MBPT and NRG. Moreover, the NRG provides an accurate description of the low-energy excitation spectrum of the spin degree of freedom, predicting a Kondo temperature very close to the experimental value. This is achieved by a detailed analysis of the universality of various definitions of ${T}_{\mathrm{K}}$ and by taking into account the full energy dependence of the coupling function between the molecule-metal complex and the metallic substrate.

The topological crystalline insulator SnTe belongs to the recently discovered class of materials in which a crystalline symmetry ensures the existence of topologically protected surface states. The bulk band structure of SnTe is characterized by a band inversion at the four equivalent $L$ points, giving rise to a mirror Chern number of ${n}_{\mathcal{M}}=\ensuremath{-}2$. The (001) surface exhibits four Dirac cones which lie at non-time-reversal-invariant points close to $\overline{X}$ and $\overline{Y}$ and are protected by the $(\overline{1}10)$ and (110) mirror symmetries. In contrast to topological insulators, this symmetry can be broken via deformations of the crystal. This opens up new possibilities for manipulating the Dirac states and inducing a controllable gap. Here, we have employed density-functional theory to investigate the response of the Dirac states to applied distortions from first principles. Our calculations show that a local gap of up to $\ensuremath{\approx}30\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ can be introduced via lattice deformations that break at least one of the underlying mirror symmetries. It is formed at either all four or just two cones, depending on the direction of the displacement vector, making it possible to create either a global gap or a state where opened and intact Dirac cones coexist. Notably, applying these deformations at the surface only can already induce the gap. If the complete slab is distorted, bulk bands are pushed into the gap making the whole system metallic.

Interactions between atomic and molecular objects are to a large extent defined by the nanoscale electrostatic potentials which these objects produce. We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with sub-nanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we quantitatively measure the quadrupole field of a single molecule and the dipole field of a single metal adatom, both adsorbed on a clean metal surface. Because of its high sensitivity, the technique can record electrostatic potentials at large distances from their sources, which above all will help to image complex samples with increased surface roughness., Comment: main text: 5 pages, 4 figures, supplementary information file: 4 pages, 2 figures

In investigations of the proteins which are responsible for the surface adhesion of the blue mussel Mytilus edulis, an unusually frequent appearance of the otherwise rare amino acid 3-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA) has been observed. This amino acid is thought to play a major role in the mechanism of mussel adhesion. Here we report a detailed structural and spectroscopic investigation of the interface between L-DOPA and a single-crystalline Au(110) model surface, with the aim of understanding fundamentals about the surface bonding of this amino acid and its role in mussel adhesion. Molecular layers are deposited by organic molecular beam deposition (OMBD) in an ultrahigh-vacuum environment. The following experimental techniques have been applied: ex situ Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). Vibrational spectra of isolated L-DOPA molecules and the zwitterionic bulk have been calculated using density functional theory (DFT). The predicted modes are assigned to observed spectra, allowing conclusions regarding the molecule-substrate and molecule-molecule interactions at the L-DOPA/Au(110) interface. We find that zwitterionic L-DOPA forms a monochiral, one-domain commensurate monolayer on Au(110), with the catechol rings on top of [110] gold rows, oriented parallel to the surface. The (2 x 1)-Au(110) surface reconstruction is not lifted. The carboxylate group is found in a bidentate or bridging configuration, the amino group is tilted out of the surface plane, and the hydroxyl groups do not dehydrogenate on Au(110). Similar to the case for the bulk, molecules form dimers on Au(110). However, the number of hydrogen bridge bonds between L-DOPA molecules is reduced as compared to the bulk. Thicker layers which are deposited onto the commensurate interface do not order in the bulk structure. In conclusion, our study shows that the aromatic ring system of L-DOPA functions as a surface anchor. Since it is also known that the hydroxyl groups support cross-link reactions between L-DOPA residues in the mussel glue protein, we can conclude that the catechol ring supports surface adhesion of mussel proteins via two independent functions.

Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory based. Here we show measurements of the attractive potential between differently sized organic molecules and a metal surface using an atomic force microscope. Our choice of molecules and the large molecule-surface separation cause this attraction to be purely of vdW type. The experiment allows testing the asymptotic vdW force law and its validity range. We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules. Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction., Van der Waals interactions are difficult to calculate at an atomistic level for moderate sized structures due to the many distinct atoms involved. Here, the authors measure the van der Waals force between an organic molecule and a metal surface, examining the non-additive part of these interactions.

Two state-of-the-art computational approaches: quantum Monte Carlo (QMC), based on accurate total energies, and GW with exciton effects (GW-BSE), based on perturbation theory are employed to calculate ionization potentials, electron affinities, and first excited singlet and triplet energies for the silane and methane molecules. Results are in excellent agreement between these dramatically different approaches and with available experiment. The optically forbidden triplet excitation in silane is predicted to lie roughly 1 eV higher than previously reported. For methane, the impact of geometry relaxation is shown to be $\sim$ 2 eV for excited states. Further, in the GW-BSE method, we demonstrate that inclusion of off-diagonal matrix elements in the self-energy operator is crucial for an accurate picture., Comment: Accepted to Physical Review Letters (11/15/00)

Optical excitations of molecular systems can be modified by their physical environment. We analyze the underlying mechanisms within many-body perturbation theory, which is particularly suited to study non-local polarizability effects on the electronic structure. Here we focus on the example of a semiconducting carbon nanotube, which observes redshifts of its excitons when the tube is touched by another nanotube or other physisorbates. We show that the redshifts mostly result from the polarizability of the attached ad-system. Electronic coupling may enhance the redshifts, but depends very sensitively on the structural details of the contact.

We discuss an efficient approach to excited electronic states within ab initio many-body perturbation theory (MBPT). Quasiparticle corrections to density-functional theory result from the difference between metallic and nonmetallic dielectric screening. They are evaluated as a small perturbation to the density-functional theory (within the local-density approximation) band structure rather than fully calculating the self-energy and evaluating its difference from the exchange-correlation potential. The dielectric screening is described by a model, which applies to bulk crystals, as well as, to systems of reduced dimension, like molecules, surfaces, interfaces, and more. The approach also describes electron-hole interaction. The resulting electronic and optical spectra are slightly less accurate but much faster to calculate than a full MBPT calculation. We discuss results for bulk silicon and argon, for the $\text{Si}(111)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface, the ${\text{SiH}}_{4}$ molecule, an argon-aluminum interface, and liquid argon.

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment., 31 pages, 10 figures

The dynamics of a molecular junction consisting of a PTCDA molecule between the tip of a scanning tunneling microscope and a Ag(111) surface have been investigated experimentally and theoretically. Repeated switching of a PTCDA molecule between two conductance states is studied by low-temperature scanning tunneling microscopy for the first time, and is found to be dependent on the tip-substrate distance and the applied bias. Using a minimal model Hamiltonian approach combined with density-functional calculations, the switching is shown to be related to the scattering of electrons tunneling through the junction, which progressively excite the relevant chemical bond. Depending on the direction in which the molecule switches, different molecular orbitals are shown to dominate the transport and thus the vibrational heating process. This in turn can dramatically affect the switching rate, leading to non-monotonic behavior with respect to bias under certain conditions. In this work, rather than simply assuming a constant density of states as in previous works, it was modeled by Lorentzians. This allows for the successful description of this non-monotonic behavior of the switching rate, thus demonstrating the importance of modeling the density of states realistically., 20 pages, 6 figures, 1 table

10 páginas, 6 figuras, 4 tablas.-- PACS number(s): 71.15.Ap, 71.20.-b.-- et al., We present the calculations of quasiparticle bandstructures and lifetimes for noble metals Cu and Ag within the GW approximation based on localized Gaussian orbital basis sets. For Cu, both the calculated positions of the d bands and the width of the d bands are within 0.1 eV compared to the experimental results. For Ag, partial core correction should be included in the pseudopotential to get reliable positions of the d bands. The calculated lifetimes agree with the experiment in the energy region away from the Fermi level, but deviate from the experimental results near the Fermi level where short-range interactions which GW approach fails to describe play an important role. For a better description of the lifetime near the Fermi level, terms beyond the GW approximation in the many-body perturbation theory need to be considered., This work has been supported by the Niedersachsen Ph. D. program ”Synthesis and Characterization of Surfaces and Interfaces assembled from Clusters and Molecules” and by the Deutsche Forschungsgemeinschaft (Bonn, Germany) by Grant No. Ro 1318/4–3.

Molecular self-assembly is employed for creating unidirectional molecular nanostructures on a truly insulating substrate, namely the (10 (1) over bar4) cleavage plane of calcite. The molecule used is racemic heptahelicene-2-carboxylic acid, which forms structures, well-aligned along the [010] crystallographic direction and stable at room temperature. Precise control of both molecule-substrate and molecule-molecule interaction is required, leading to the formation of such wire-like structures of well-defined width and lengths exceeding 100 nm. This subtle balance is governed by the heptahelicene-2-carboxylic acid used in this study, allowing for both hydrogen bond formation as well pi-pi stacking.

The electronic spectrum of a chemically contacted molecule in the junction of a scanning tunneling microscope can be modified by tip retraction. We analyze this effect by a combination of density functional, many-body perturbation and numerical renormalization group theory, taking into account both the non-locality and the dynamics of electronic correlation. Our findings, in particular the evolution from a broad quasiparticle resonance below to a narrow Kondo resonance at the Fermi energy, correspond to the experimental observations., Comment: 4 pages, 3 figures

Structure formation of the highly polar molecule cytosine on the (111) cleavage plane of calcium fluoride is investigated in ultrahigh vacuum using noncontact atomic force microscopy at room temperature. Molecules form well-defined trimer structures, covering the surface as homogeneously distributed stable structures. Density-functional theory calculations yield a diffusion barrier of about 0.5 eV for individual molecules suggesting that they are mobile at room temperature. Furthermore, it is predicted that the molecules can form trimers in a configuration allowing all molecules to attain their optimum adsorption position on the substrate. As the trimer geometry facilitates hydrogen bonding between the molecules within the trimer, we conclude that the stabilization of individual diffusing molecules into stable trimers is due to a cooperative mechanism involving polar interactions between molecules and substrate as well as hydrogen bonding between molecules.

The adsorption of 3,4,9,10-perylene tetracarboxylic diimide derivative molecules on the rutile ${\text{TiO}}_{2}(110)$ surface was investigated by noncontact atomic force microscopy and density-functional theory (DFT) calculations. After submonolayer deposition, individual molecules are observed to adsorb with their main axis aligned along the [001] direction and centered on top of the bridging oxygen rows. Depending on the tip termination, two distinctly different molecular contrasts are achieved. In the first mode, the molecules are imaged as bright elongated features, while in another mode the molecules appear with a bright rim and a dark bow-shaped center. Comparison with the defect density on the bare ${\text{TiO}}_{2}(110)$ surface suggests that the molecules preferentially anchor to surface defects. Our DFT calculations reveal details of the molecular adsorption position, confirming the experimentally observed adsorption on top of the bridging oxygen rows. The DFT results indicate that diffusion along the rows should be quite easily possible, while diffusion perpendicular to the rows seems to be hindered by a significant energy barrier.

We present a tight-binding approach for the calculation of quasiparticle energy levels in confined systems such as molecules. The method is based on Hedin's GW approximation, in which the self-energy is given as the product of the Green's function $(G)$ and the screened Coulomb interaction $(W)$. Key quantities in the GW formalism such as the microscopic dielectric function are expressed in a minimal basis of spherically averaged atomic orbitals. All necessary integrals are either precalculated or approximated without resorting to empirical data. The method is validated against first-principles results for benzene and anthracene, where good agreement is found for levels close to the frontier orbitals. Further, the size dependence of the quasiparticle gap is studied for conformers of the polyacenes $({\mathrm{C}}_{4n+2}{\mathrm{H}}_{2n+4})$ up to $n=30$.

Structural relaxations in electronically excited poly(para-phenylene) are studied using many-body perturbation theory and density-functional-theory methods. A sophisticated description of the electron-hole interaction is required to describe the energies of the excitonic states, but we show that the structural relaxations associated with exciton formation can be obtained quite accurately within a constrained density-functional-theory approach. We find that the structural relaxations in the low-energy excitonic states extend over about 8 monomers, leading to an energy reduction of 0.22 eV and a Stokes shift of 0.40 eV., Comment: 4 pages, 3 figures

We report a photoemission study on high-quality single-crystal graphite epitaxially grown on SiC. The results are interpreted using independent information on the final states obtained by very-low-energy electron diffraction. Significant intrinsic photoemission and surface effects are identified, which distort the photoemission response and narrow the observed dispersion range of the $\ensuremath{\pi}$ state. We assess its true dispersion range using a model photoemission calculation. A significant dependence of the excited-state self-energy effects on the wave-function character is found. The experimental results are compared with a $\mathrm{GW}$ calculation.

Experimental and theoretical studies have shown that excitonic effects play an important role in the optical properties of conjugated polymers. The optical absorption spectrum of trans-polyacetylene, for example, can be understood as completely dominated by the formation of exciton bound states. We review a recently developed first-principles method for computing the excitonic effects and optical spectrum, with no adjustable parameters. This theory is used to study the absorption spectrum of two conjugated polymers: trans-polyacetylene and poly-phenylene-vinylene(PPV).

We present the quasiparticle band structure and the optical excitation spectrum of bulk LiCl, using many-body perturbation theory. Density-functional theory is used to calculate the ground-state geometry of the system. The quasiparticle band structure is calculated within the GW approximation. Taking the electron-hole interaction into consideration, electron-hole pair states and optical excitations are obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function. The calculated band gap is 9.5 eV, which is in good agreement with the experimental result of 9.4 eV. And the calculated optical absorption spectrum, which contains an exciton peak at 8.8 eV and a resonant-exciton peak at 9.8 eV, is also in good agreement with experimental data.

The adsorption behavior of trimesic acid (TMA) on rutile TiO(2)(110) is studied by means of non-contact atomic force microscopy (NC-AFM) and density-functional theory (DFT). Upon low-coverage adsorption at room temperature, NC-AFM imaging reveals individual molecules, centered above the surface titanium rows. Based on the NC-AFM results alone it is difficult to deduce whether the molecules are lying flat or standing upright on the surface. To elucidate the detailed adsorption geometry, we perform DFT calculations, considering a large number of different adsorption positions. Our DFT calculations suggest that single TMA molecules adsorb with the benzene ring parallel to the surface plane. In this configuration, two carboxylic groups can anchor to the surface in a bidentate fashion with the oxygen atoms binding to surface titanium atoms while the hydrogen atoms approach oxygen atoms within the bridging oxygen rows. The most favorable adsorption position is obtained in the presence of a hydroxyl defect, allowing for additional binding of the third carboxylic group.