arXiv subjects

U. Landman

Publications and source records attributed to U. Landman.

7 recordsLinked to original sources

arXiv2006

Quantum dots based on parabolic quantum wells: importance of electronic correlations

We present measurements and theoretical interpretation of the magnetic field dependent excitation spectra of a two-electron quantum dot. The quantum dot is based on an Al$_x$Ga$_{1-x}$As parabolic quantum well with effective $g^\star$-factor close to zero. Results of tunneling spectroscopy of the four lowest states are compared to exact diagonalization calculations and a generalized Heitler--London approximation and good agreement is found. Electronic correlations, associated with the formation of an H$_2$-type Wigner molecule, turn out to be very important in this system.

cond-mat.mes-hall↗

arXiv2005

Excitation spectrum of two correlated electrons in a lateral quantum dot with negligible Zeeman splitting

The excitation spectrum of a two-electron quantum dot is investigated by tunneling spectroscopy in conjuction with theoretical calculations. The dot made from a material with negligible Zeeman splitting has a moderate spatial anisotropy leading to a splitting of the two lowest triplet states at zero magnetic field. In addition to the well-known triplet excitation at zero magnetic field, two additional excited states are found at finite magnetic field. The lower one is identified as the second excited singlet state on the basis of an avoided crossing with the first excited singlet state at finite fields. The measured spectra are in remarkable agreement with exact diagonalization calculations. The results prove the significance of electron correlations and suggest the formation of a state with Wigner-molecular properties at low magnetic fields.

cond-mat.mes-hall↗

arXiv2002

Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface

Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001) surface F-center (FC) was studied by spin-density-functional molecular dynamics simulations. The incident clusters are steered by an attractive "funnel" created by the FC, resulting in adsorption of the cluster, with one of its atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their gas-phase structures, while for N>6 surface-commensurate isomers are energetically more favorable. Adsorbed clusters with N > 3 are found to remain magnetic at the surface.

cond-mat.mtrl-sci↗

arXiv2002

Instability driven fragmentation of nanoscale fractal islands

Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.

cond-mat.mtrl-sci↗

arXiv2001

CO oxidation on a single Pd atom supported on magnesia

The oxidation of CO on single Pd atoms anchored to MgO(100) surface oxygen vacancies is studied with temperature-programmed-reaction mass-spectrometry and infrared spectroscopy. In one-heating-cycle experiments CO$_2$, formed from O$_2$ and CO preadsorbed at 90 K, is detected at 260 K and 500 K. Ab-initio simulations suggest two reaction routes, with Pd(CO)$_2$O$_2$ and Pd(CO$_3$)CO found as precursors for the low and high temperature channels, respectively. Both reactions result in annealing of the vacancy and induce migration and coalescence of the remaining Pd-CO to form larger clusters.

cond-mat.mtrl-sci↗

arXiv2001

Structure and Magnetism of Neutral and Anionic Palladium Clusters

The properties of neutral and anionic Pd_N clusters were investigated with spin-density-functional calculations. The ground state structures are three-dimensional for N>3 and they are magnetic with a spin-triplet for 2<=N<=7 and a spin nonet for N=13 neutral clusters. Structural- and spin-isomers were determined and an anomalous increase of the magnetic moment with temperature is predicted for a Pd_7 ensemble. Vertical electron detachment and ionization energies were calculated and the former agree well with measured values for anionic Pd_N clusters.

physics.atm-clus↗

arXiv2000

Nanowire Gold Chains: Formation Mechanisms and Conductance

Structural transformations, electronic spectra and ballistic transport in pulled gold nanowires are investigated with ab initio simulations, and correlated with recent measurements. Strain-induced yield of an initial double-strand wire results first in formation of a bent-chain which transforms upon further elongation to a linear atomic chain exhibiting dimerized atomic configurations. These structures are stabilized by directional local bonding with spd-hybridization. The conductance of the initial double-stranded contact is close to 2g_0 and it drops sharply to 1g_0 during the transformation to a single chain, exhibiting subsequently a 1g_0 plateau extending over an elongation well above typical Au-Au distances.

cond-mat.mtrl-sci↗