arXiv subjects

R. Berndt

Publications and source records attributed to R. Berndt.

At least 19 recordsLinked to original sources

arXiv2012

Local Gating of an Ir(111) Surface Resonance by Graphene Islands

The influence of graphene islands on the electronic structure of the Ir(111) surface is investigated. Scanning tunneling spectroscopy (STS) indicates the presence of a two-dimensional electron gas with a binding energy of -160meV and an effective mass of -0.18m_e underneath single-layer graphene on the Ir(111) surface. Density functional calculations reveal that the STS features are predominantly due to a holelike surface resonance of the Ir(111) substrate. Nanometer-sized graphene islands act as local gates, which shift and confine the surface resonance.

cond-mat.mes-hall↗

arXiv2011

Two-Site Kondo Effect in Atomic Chains

Linear CoCu_nCo clusters on Cu(111) are fabricated by means of atomic manipulation. They represent a two-site Kondo system with tunable interaction. Scanning tunneling spectroscopy reveals oscillations of the Kondo temperature T_K with the number n of Cu atoms for n>=3. Density functional calculations show that the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the Cu chains causes the oscillations. Calculations find ferromagnetic and antiferromagnetic interaction for n=1 and 2, respectively. Both interactions lead to a decrease of T_K as experimentally observed.

cond-mat.mtrl-sci↗

arXiv2010

Kondo effect of a Co atom on Cu(111) in contact with an Fe tip

Single Co atoms, which exhibit a Kondo effect on Cu(111), are contacted with Cu and Fe tips in a low-temperature scanning tunneling microscope. With Fe tips, the Kondo effect persists with the Abrikosov-Suhl resonance significantly broadened. In contrast, for Cu-covered W tips, the resonance width remains almost constant throughout the tunneling and contact ranges. The distinct changes of the line width are interpreted in terms of modifications of the Co d state occupation owing to hybridization with the tip apex atoms.

cond-mat.mtrl-sci↗

arXiv2009

Surface reconstruction and energy gap of superconducting V3Si(001)

A yet unknown surface reconstruction of V3Si(001), which is most likely induced by carbon, is used to investigate the quasi-particle energy gap at the atomic scale by a cryogenic scanning tunneling microscope. The width of the gap was virtually not altered at and close to carbon impurities, nor did it change at different sites of the reconstructed surface lattice. A remarkable modification of the spectroscopic signature of the gap was induced, however, upon moving the tip of the microscope into controlled contact with the superconductor. Spectroscopy of the resulting normal-metal -- superconductor junction indicated the presence of Andreev reflections.

cond-mat.supr-con↗

arXiv2008

Scanning tunneling microscopy and kinetic Monte Carlo investigation of Cesium superlattices on Ag(111)

Cesium adsorption structures on Ag(111) were characterized in a low-temperature scanning tunneling microscopy experiment. At low coverages, atomic resolution of individual Cs atoms is occasionally suppressed in regions of an otherwise hexagonally ordered adsorbate film on terraces. Close to step edges Cs atoms appear as elongated protrusions along the step edge direction. At higher coverages, Cs superstructures with atomically resolved hexagonal lattices are observed. Kinetic Monte Carlo simulations model the observed adsorbate structures on a qualitative level.

cond-mat.mtrl-sci↗

arXiv2008

Controlling the Kondo Effect in CoCu_n Clusters Atom by Atom

Clusters containing a single magnetic impurity were investigated by scanning tunneling microscopy, spectroscopy, and ab initio electronic structure calculations. The Kondo temperature of a Co atom embedded in Cu clusters on Cu(111) exhibits a non-monotonic variation with the cluster size. Calculations model the experimental observations and demonstrate the importance of the local and anisotropic electronic structure for correlation effects in small clusters.

cond-mat.mtrl-sci↗

arXiv2008

Quantized Conductance of a Single Magnetic Atom

A single Co atom adsorbed on Cu(111) or on ferromagnetic Co islands is contacted with non-magnetic W or ferromagnetic Ni tips in a scanning tunneling microscope. When the Co atom bridges two non-magnetic electrodes conductances of 2e^2/h are found. With two ferromagnetic electrodes a conductance of e^2/h is observed which may indicate fully spin-polarized transport.

cond-mat.mtrl-sci↗

arXiv2007

Rotation of C60 in a single-molecule contact

The orientation of individual C60 molecules adsorbed on Cu(100) is reversibly switched when the tip of a scanning tunneling microscope is approached to contact the molecule. The probability of switching rises sharply upon displacing the tip beyond a threshold. A mechanical mechanism is suggested to induce the rotation of the molecule.

cond-mat.mtrl-sci↗

arXiv2007

Unoccupied states of individual silver clusters and chains on Ag(111)

Size-selected silver clusters on Ag(111) were fabricated with the tip of a scanning tunneling microscope. Unoccupied electron resonances give rise to image contrast and spectral features which shift toward the Fermi level with increasing cluster size. Linear assemblies exhibit higher resonance energies than equally sized compact assemblies. Density functional theory calculations reproduce the observed energies and enable an assignment of the resonances to hybridized atomic 5s and 5p orbitals with silver substrate states.

cond-mat.mtrl-sci↗

arXiv2006

Conductance and Kondo effect of a controlled single atom contact

The tip of a low-temperature scanning tunneling microscope is brought into contact with individual Kondo impurities (cobalt atoms) adsorbed on a Cu(100) surface. A smooth transition from the tunneling regime to a point contact with a conductance of $G\approx\text{G}_0$ occurs. Spectroscopy in the contact regime, {\it i. e.}, at currents in a $μ\text{A}$ range was achieved. A modified line shape is observed indicating a significant change of the Kondo temperature $T_{\text{K}}$ at contact. Model calculations indicate that the proximity of the tip shifts the cobalt $d$-band and thus affects $T_{\text{K}}$.

cond-mat.str-el↗

arXiv2006

Lifetimes of electrons in the Shockley surface state band of Ag(111)

We present a theoretical many-body analysis of the electron-electron (e-e) inelastic damping rate $Γ$ of electron-like excitations in the Shockley surface state band of Ag(111). It takes into account ab-initio band structures for both bulk and surface states. $Γ$ is found to increase more rapidly as a function of surface state energy E than previously reported, thus leading to an improved agreement with experimental data.

cond-mat.mtrl-sci↗

arXiv2006

Controlled Contact to a C60 Molecule

The conductance of C60 on Cu(100) is investigated with a low-temperature scanning tunneling microscope. At the transition from tunneling to the contact regime the conductance of C60 adsorbed with a pentagon-hexagon bond rises rapidly to 0.25 conductance quanta G0. An abrupt conductance jump to G0 is observed upon further decreasing the distance between the instrument's tip and the surface. Ab-initio calculations within density functional theory and non-equilibrium Green's function techniques explain the experimental data in terms of the conductance of an essentially undeformed C60. From a detailed analysis of the crossover from tunneling to contact we conclude that the conductance in this region is strongly affected by structural fluctuations which modulate the tip-molecule distance.

cond-mat.mtrl-sci↗

arXiv2006

Adsorption geometry of Cu(111)-Cs studied by scanning tunneling microscopy

Using scanning tunneling microscopy at low temperatures we investigated the adsorbate system Cu(111)-Cs for various cesium coverages. At low coverages we observe a hexagonally ordered Cs adsorption layer with a mutual adsorbate distance of 1.1 nm. This distance is discussed in terms of a commensurate adsorption superstructure which is stabilized by long-range adsorbate interactions mediated by Cu(111) surface state electrons. Intermediate coverages are characterized by incommensurate superstructures which are rotated with respect to the substrate lattice. The rotation behavior at 9K follows a general trend which is consistent with models of epitaxial rotation. With increasing coverage the adsorption layers are found to rotate toward alignment with the substrate.

cond-mat.mtrl-sci↗

arXiv2005

Theoretical analysis of STM-derived lifetimes of excitations in the Shockley surface state band of Ag(111)

We present a quantitative many-body analysis using the GW approximation of the decay rate $Γ$ due to electron-electron scattering of excitations in the Shockley surface state band of Ag(111), as measured using the scanning tunnelling microscope (STM). The calculations include the perturbing influence of the STM, which causes a Stark-shift of the surface state energy $E$ and concomitant increase in $Γ$. We find $Γ$ varies more rapidly with $E$ than recently found for image potential states, where the STM has been shown to significantly affect measured lifetimes. For the Shockley states, the Stark-shifts that occur under normal tunnelling conditions are relatively small and previous STM-derived lifetimes need not be corrected.

cond-mat.other↗

arXiv2005

Molecular Orbital Shift of Perylenetetracarboxylic-Dianhydride

Using low-temperature scanning tunneling microscopy we find that perylenetetracarboxylic-dianhydride on Au(788) exhibits three coexisting adsorption phases. Single-molecule tunneling spectroscopy reveals orbital energies, which differ in the different adsorption phases. Density functional theory calculations associate the experimentally observed submolecular corrugation to the spatial distribution of the second-to-lowest unoccupied molecular orbital. We tentatively attribute the orbital shifts to a varying number of hydrogen bonds.

cond-mat.other↗

arXiv2005

Role of elastic scattering in electron dynamics at ordered alkali overlayers on Cu(111)

Scanning tunneling spectroscopy of p(2x2) Cs and Na ordered overlayers on Cu(111) reveals similar line widths of quasi two-dimensional quantum well states despite largely different binding energies. Detailed calculations show that 50% of the line widths are due to electron-phonon scattering while inelastic electron-electron scattering is negligible. A frequently ignored mechanism for ordered structures, i.e., enhanced elastic scattering due to Brillouin zone back folding, contributes the remaining width.

cond-mat.mtrl-sci↗

arXiv2005

Resonator design for surface electron lifetime studies using scanning tunneling spectroscopy

We derive expressions for the lossy boundary-scattering contribution to the linewidth of surface electronic states confined with atomic corrals and island resonators. Correcting experimentally measured linewidths for these contributions along with thermal and intrumental broadening enables intrinsic many-body lifetimes due to electron-electron and electron-phonon scattering to be determined. In small resonators lossy-scattering dominates linewidths whilst different scaling of widths and separations cause levels to merge in large resonators. Our results enable the design of resonators suitable for lifetime studies.

cond-mat.other↗

arXiv2004

Atom Transfer and Single-Adatom Contacts

The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump-to-contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the homoepitaxial adatom and the surface atoms.

cond-mat.str-el↗