Per Hedegård

Single-electron transistor of a single organic molecule with access to several redox states.

A combination of classical Coulomb charging, electronic level spacings, spin, and vibrational modes determines the single-electron transfer reactions through nanoscale systems connected to external electrodes by tunnelling barriers. Coulomb charging effects have been shown to dominate such transport in semiconductor quantum dots, metallic and semiconducting nanoparticles, carbon nanotubes, and single molecules. Recently, transport has been shown to be also influenced by spin—through the Kondo effect—for both nanotubes and single molecules, as well as by vibrational fine structure. Here we describe a single-electron transistor where the electronic levels of a single π-conjugated molecule in several distinct charged states control the transport properties. The molecular electronic levels extracted from the single-electron-transistor measurements are strongly perturbed compared to those of the molecule in solution, leading to a very significant reduction of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We suggest, and verify by simple model calculations, that this surprising effect could be caused by image charges generated in the source and drain electrodes resulting in a strong localization of the charges on the molecule.

How curved membranes recruit amphipathic helices and protein anchoring motifs

Lipids and several specialized proteins are thought to be able to sense the curvature of membranes (MC). Here we used quantitative fluorescence microscopy to measure curvature-selective binding of amphipathic motifs on single liposomes 50-700 nm in diameter. Our results revealed that sensing is predominantly mediated by a higher density of binding sites on curved membranes instead of higher affinity. We proposed a model based on curvature-induced defects in lipid packing that related these findings to lipid sorting and accurately predicted the existence of a new ubiquitous class of curvature sensors: membrane-anchored proteins. The fact that unrelated structural motifs such as alpha-helices and alkyl chains sense MC led us to propose that MC sensing is a generic property of curved membranes rather than a property of the anchoring molecules. We therefore anticipate that MC will promote the redistribution of proteins that are anchored in membranes through other types of hydrophobic moieties.

Amphipathic motifs in BAR domains are essential for membrane curvature sensing

BAR (Bin/Amphiphysin/Rvs) domains and amphipathic α‐helices (AHs) are believed to be sensors of membrane curvature thus facilitating the assembly of protein complexes on curved membranes. Here, we used quantitative fluorescence microscopy to compare the binding of both motifs on single nanosized liposomes of different diameters and therefore membrane curvature. Characterization of members of the three BAR domain families showed surprisingly that the crescent‐shaped BAR dimer with its positively charged concave face is not able to sense membrane curvature. Mutagenesis on BAR domains showed that membrane curvature sensing critically depends on the N‐terminal AH and furthermore that BAR domains sense membrane curvature through hydrophobic insertion in lipid packing defects and not through electrostatics. Consequently, amphipathic motifs, such as AHs, that are often associated with BAR domains emerge as an important means for a protein to sense membrane curvature. Measurements on single liposomes allowed us to document heterogeneous binding behaviour within the ensemble and quantify the influence of liposome polydispersity on bulk membrane curvature sensing experiments. The latter results suggest that bulk liposome‐binding experiments should be interpreted with great caution.

A simple tight-binding model of spin-orbit splitting of sp-derived surface states

Abstract In crystals with inversion symmetry, splitting of states due to the spin–orbit interaction is not allowed. At the surface, however, the symmetry is broken, and spin–orbit splitting becomes possible. Spin–orbit splitting has been observed experimentally for the sp-derived surface state on Au(111) [S. LaShell, B.A. McDougall, E. Jensen, Phys. Rev. Lett. 77 (1996) 3419]. We present a simple tight-binding model, which shows that the spin–orbit splitting can be described within a nearly-free-electron model, and the splitting depends on (1) the spin–orbit splitting of the atomic levels and (2) the surface potential. Furthermore, we show that the splitting of the surface state is unobservable in the surface charge density as probed with scanning tunneling microscopy.

Electrical manipulation of spin states in a single electrostatically gated transition-metal complex.

We demonstrate an electrically controlled high-spin (S = 5/2) to low-spin (S = 1/2) transition in a three-terminal device incorporating a single Mn(2+) ion coordinated by two terpyridine ligands. By adjusting the gate-voltage we reduce the terpyridine moiety and thereby strengthen the ligand-field on the Mn-atom. Adding a single electron thus stabilizes the low-spin configuration and the corresponding sequential tunnelling current is suppressed by spin-blockade. From low-temperature inelastic cotunneling spectroscopy, we infer the magnetic excitation spectrum of the molecule and uncover also a strongly gate-dependent singlet-triplet splitting on the low-spin side. The measured bias-spectroscopy is shown to be consistent with an exact diagonalization of the Mn-complex, and an interpretation of the data is given in terms of a simplified effective model.

Auger Energy Shifts for Metallic Elements

A previous treatment of core-level binding-energy shifts for metals is extended to Auger energy shifts. In comparison to single core-hole shifts, the new information given by the Auger shift is contained in the shift of the two core-hole final state. An accurate expression for the shift of the two core-hole ionization energy between the free atom and the metal is derived. In the actual calculation of the shift a (Z + 2) equivalent core approximation is applied, but where important corrections are included. Comparison between theory and experiment is performed for the metallic elements Na-Al, Ni-Ga, Pd-Sn, Au, Tl-Bi, Ba and Yb, and in all cases a good agreement is obtained. A rather detailed compilation of atomic and solid phase double-hole energies is presented in the Appendices. To elucidate the break-down of the concept of metallic screening, non-conducting elements like Si, Ge-Se, Sb and Te are also considered. In addition, atom-metal two core-hole energy shifts are calculated for the 4d transition series and an abrupt change in the energy shift is found when we proceed from the d elements to the elements beyond. The influence of the atomic structure on the shift is considered for the 4dn5s2, 4dn+15s and 4dn+2 configurations. The metallic renormalization of the atomic core-hole Coulomb correlation energy is calculated for the 4d elements and the chemical shift of this quantity is shown to be closely related to the change of the Auger parameter. The present formalism provides a most suitable framework for treating chemical shifts in metallic systems. As a special example the situation at the surface of a metal is considered and surface shifts are predicted both for the two-hole and the Auger energies. As for the single core-holes, these surface shifts change sign as we proceed through a transition series due to the bonding-antibonding division of the d-band.

Quantum interference in off-resonant transport through single molecules

We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance. (Less)

Single Enzyme Studies Reveal the Existence of Discrete Functional States for Monomeric Enzymes and How They Are “Selected” upon Allosteric Regulation

Allosteric regulation of enzymatic activity forms the basis for controlling a plethora of vital cellular processes. While the mechanism underlying regulation of multimeric enzymes is generally well understood and proposed to primarily operate via conformational selection, the mechanism underlying allosteric regulation of monomeric enzymes is poorly understood. Here we monitored for the first time allosteric regulation of enzymatic activity at the single molecule level. We measured single stochastic catalytic turnovers of a monomeric metabolic enzyme (Thermomyces lanuginosus Lipase) while titrating its proximity to a lipid membrane that acts as an allosteric effector. The single molecule measurements revealed the existence of discrete binary functional states that could not be identified in macroscopic measurements due to ensemble averaging. The discrete functional states correlate with the enzymes major conformational states and are redistributed in the presence of the regulatory effector. Thus, our data support allosteric regulation of monomeric enzymes to operate via selection of preexisting functional states and not via induction of new ones.

Interaction-induced negative differential resistance in asymmetric molecular junctions

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a nonconducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in, e.g., oligo(phenyleneethynylene)-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.

Laserlike Vibrational Instability in Rectifying Molecular Conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.