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Porphyrin cyclophane 1, consisting of two rigidly fixed but still movable cofacial porphyrins and exposing acetate-masked thiols in opposed directions of the macrocycle, is designed, synthesized, and characterized. The functional cyclophane 1, as pioneer of mechanosensitive 3D materials, forms stable single-molecule junctions in a mechanically controlled break-junction setup. Its reliable integration in a single-molecule junction is a fundamental prerequisite to explore the potential of these structures as mechanically triggered functional units and devices.

Intra- and intermolecular interactions are dominating chemical processes, and their concerted interplay enables complex nonequilibrium states like life. While the responsible basic forces are typically investigated spectroscopically, a conductance measurement to probe and control these interactions in a single molecule far out of equilibrium is reported here. Specifically, by separating macroscopic metal electrodes, two π-conjugated, bridge-connected porphyrin decks are peeled off on one side, but compressed on the other side due to the covalent mechanical fixation. We observe that the conductance response shows an exceptional exponential rise by two orders of magnitude in individual breaking events during the stretching. Theoretical studies atomistically explain the measured conductance behavior by a mechanically activated increase in through-bond transport and a simultaneous strengthening of through-space coupling. Our results not only reveal the various interacting intramolecular transport channels in a molecular set of levers, but also the molecules' potential to serve as molecular electro-mechanical sensors and switches.

The possibility to study quantum interference phenomena at ambient conditions is an appealing feature of molecular electronics. By connecting two porphyrins in a cofacial cyclophane, we create an attractive platform for mechanically controlling electric transport through the intramolecular extent of π-orbital overlap of the porphyrins facing each other and through the angle of xanthene bridges with regard to the porphyrin planes. We analyze theoretically the evolution of molecular configurations in the pulling process and the corresponding changes in electric conduction by combining density functional theory (DFT) with Landauer scattering theory of phase-coherent elastic transport. Predicted conductances during the stretching process show order of magnitude variations caused by two robust destructive quantum interference features that span through the whole electronic gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Mechanically-controlled break junction (MCBJ) experiments at room temperature verify the mechanosensitive response of the molecular junctions. During the continuous stretching of the molecule, they show conductance variations of up to 1.5 orders of magnitude over single breaking events. Uncommon triple-and quadruple-frequency responses are observed in periodic electrode modulation experiments with amplitudes of up to 10 Å. This further confirms the theoretically predicted double transmission dips caused by the spatial and energetic rearrangement of molecular orbitals, with contributions from both through-space and through-bond transport. This journal is

Direct measurement of single-molecule electrical transparency by break junction experiments has become a major field of research over the two last decades. This review specifically and comprehensively highlights the use of porphyrins as molecular components and discusses their potential use for the construction of future devices. Throughout the review, the features provided by porphyrins, such as low level misalignments and very low attenuation factors, are shown with numerous examples, illustrating the potential and limitations of these molecular junctions, as well as differences emerging from applied integration/investigation techniques., Porphyrins have unique properties in electronic circuits. This review summarizes single molecule junction experiments and encourages the development of next generation molecular devices based on such building blocks.

By studying transport through seven structurally related porphyrin derivatives with a machine learning algorithm we could identify and distinguish three different electronic paths., Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.

We present experimental and theoretical studies of single-molecule conductance through nonplanar fullerocurcuminoid molecular dyads in ambient conditions using the mechanically controllable break junction technique. We show that molecular dyads with bare fullerenes form configurations with conductance features related to different transport channels within the molecules, as identified with filtering and clustering methods. The primary channel corresponds to charge transport through the methylthio-terminated backbone. Additional low-conductance channels involve one backbone side and the fullerene. In fullerenes with four additional equatorial diethyl malonate groups attached to them, the latter transport pathway is blocked. Density functional theory calculations corroborate the experimental observations. In combination with nonequilibrium green functions, the conductance values of the fullerocurcuminoid backbones are found to be similar to those of a planar curcuminoid molecule without a fullerene attached. In the nonplanar fullerocurcuminoid systems, the highest-conductance peak occurs partly through space, compensating for the charge delocalization loss present in the curcuminoid system., Financial support from the European Commission (COST Action MOLSPIN CA15128 and EU RISE (DAFNEOX) project SEP-210165479) is gratefully acknowledged. The work at the University of Chile was supported by Fondecyt Regular Project 1181080 (D.D.), Fondecyt Regular Project 1161775 (M.S.), Fondecyt Regular Project 1170524 (D.A.), and Fondequip EQM140055 and EQM180009 (D.D). Powered@NLHPC: this research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02). E.R. acknowledges MICIIN for grant PGC2018-093863-B-C21 and the Maria de Maeztu Excellence grant MDM-2017-0767, for the computer resources, technical expertise, and assistance provided by the Barcelona Supercomputing Centre and CSUC and to the Generalitat de Catalunya for an ICREA Academia award and grant 2017SGR1289. H.S.J.v.d.Z. acknowledges support from the Dutch Science foundation (NWO). N.A.-A. thanks MEC for grant MAT2016-77852-C2-1-R, to the Generalitat de Catalunya for the grant 2017SGR1277, and the Severo Ochoa Program for Centers of Excellence in R&D (SEV-2015-0496). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement 724981). L.E. thanks the US National Science Foundation (NSF) for generous support of this work under the CHE-1801317 program. The Robert A. Welch Foundation is also gratefully acknowledged for an endowed chair to L.E. (grant AH-0033).

We report on the fabrication and measurements of platinum-self-aligned nanogap devices containing cubed iron (core)/iron oxide (shell) nanoparticles (NPs) with two average different sizes (13 and 17 nm). The nanoparticles are deposited by means of a cluster gun technique. Their trapping across the nanogap is demonstrated by comparing the current vs voltage characteristics (I-Vs) before and after the deposition. At low temperature, the I-Vs can be well fitted to the Korotkov and Nazarov Coulomb blockade model, which captures the coexistence of single-electron tunneling and tunnel barrier suppression upon a bias voltage increase. The measurements thus show that Coulombblockaded devices can be made with a nanoparticle cluster source, which extends the existing possibilities to fabricate such devices to those in which it is very challenging to reduce the usual NP agglomeration given by a solution method., This study was supported by the EU Horizon 2020 research and innovation program under the Marie-Sklodowska-Curie Grant Agreement No. 645658 (DAFNEOX Project), by two FONDECYT REGULAR Grant Nos. 1181080 and 1161775, and by two FONDEQUIP Grant Nos. EQM140055 and EQM180009. We thank the Spanish Ministry of Science, Innovation and Universities (Project Nos. MAT2015-71664-R and RTI2018-099960-B-I00) and the Serbian Ministry of Education, Science and Technological Development (Project No. III45018) for their support. A.P. and Z.K. thank Senzor-INFIZ (Serbia) for the cooperation provided during their respective secondments.

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.

The bowl-shaped, 3-fold interlinked porphyrin dimer 2 was obtained in respectable yields during macrocyclization attempts. Its bicyclic structure, consisting of a macrocycle made of a pair of acetylene interlinked tetraphenylporphyrins which are additionally linked by a C-C bond interlinking two pyrrole subunits, has been confirmed spectroscopically (2D-NMR, UV/vis, HR-MALDI-ToF MS). Late-stage functionalization provided the structural analogue 1 with acetyl-protected terminal thiol anchor groups enabling single molecule transport investigations in a mechanically controlled break junction experiment. The formation of single-molecule junctions was observed, displaying large variations in the observed conductance values pointing at a rich diversity in the molecular junctions.

A conformationally flexible calix[4]pyrrole possessing a conjugated electronic structure (an N‐substituted oxoporphyrinogen (OxP) related to porphyrin) was used to investigate the influence of mechanical stretching on the single‐molecule conductance of these molecules using the mechanically‐controlled break junction (MCBJ) technique. The results show that the molecule can be immobilized in a single‐molecule break junction configuration, giving rise to different behaviours. These include step‐like features in the conductance vs. displacement traces as well as conductance traces that exhibit a slower decay (‘downhill’ traces) than measured for direct tunneling. The latter class of traces can be associated with the mechanical manipulation (i. e., stretching) of the molecule with inter‐electrode distances as long as 2 nm. Density functional theory (DFT) calculations reveal that OxP molecules are stable under stretching in the length regime studied.

Switching of conductance between conformers can be explained by the rotation of one methyl group., We have studied the single-molecule conductance of a family of curcuminoid molecules (CCMs) using the mechanically controlled break junction (MCBJ) technique. The CCMs under study contain methylthio (MeS–) as anchoring groups: MeS-CCM (1), the free-ligand organic molecule, and two coordination compounds, MeS-CCM-BF2 (2) and MeS-CCM-Cu (3), where ligand 1 coordinates to a boron center (BF2 group) and to a CuII moiety, respectively. We found that the three molecules present stable molecular junctions allowing detailed statistical analysis of their electronic properties. Compound 3 shows a slight increase in the conductance with respect to free ligand 1, whereas incorporation of BF2 (compound 2) promotes the presence of two conductance states in the measurements. Additional experiments with control molecules point out that this bistability is related to the combination of MeS– anchoring groups and the BF2 moiety within the structure of the molecules. Theoretical calculations show that this can be explained by the presence of two conformers once compound 2 is anchored between the gold electrodes. An energy minimum is found for a flat structure but there is a dramatic change in the magnitude and orientation of dipole moment (favouring a non-flat conformer in the presence of an external electric field) due to a conformational change of one of the terminal MeS– groups. The results thus point to an intricate interplay between the applied bias voltage and the molecule dipole moment which could be the basis for designing new molecules aiming at controlling their conformation in devices.

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks(1). To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes(2) and the electronic coupling with those electrodes(3,4). Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each others(5,6) and molecules in which quantum interference effects are possible(7,8). Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Green's functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

et al., Fullerenes have attracted interest for their possible applications in various electronic, biological, and optoelectronic devices. However, for efficient use in such devices, a suitable anchoring group has to be employed that forms well-defined and stable contacts with the electrodes. In this work, we propose a novel fullerene tetramalonate derivate functionalized with trans-1 4,5-diazafluorene anchoring groups. The conductance of single-molecule junctions, investigated in two different setups with the mechanically controlled break junction technique, reveals the formation of molecular junctions at three conductance levels. We attribute the conductance peaks to three binding modes of the anchoring groups to the gold electrodes. Density functional theory calculations confirm the existence of multiple binding configurations and calculated transmission functions are consistent with experimentally determined conductance values., The work at TUDelft was supported by the EU through an advanced ERC grant (Mols@Mols); device fabrication was done at the Kavli Nanolab at Delft. The work at University of Chile was supported by Fondecyt Regular Project 1161775 (M.S. and J.L.M.), Fondecyt Regular Project 1140770, EU RISE (DAFNEOX) project SEP-210165479 (D.D.), and CONICYT/Fondecyt Postdoctoral Project 3150674 (C.G.C.). D.A. thanks CONICYT + PAI ‘Concurso nacional de apoyo al retorno de investigadores/as desde el extranjero, convocatoria 2014 82140014’ for financial support. Powered@NLHPC: This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02). L.E. thanks the National Science Foundation [grant CHE-1408865] and the PREM Program [grant DMR-1205302] as well as the Robert A. Welch Foundation [grant AH-0033] for generous financial support.

The use of porphyrin molecules as building blocks of functional molecular devices has been widely investigated. The structural flexibility and well-developed synthetic chemistry of porphyrins allows their physical and chemical properties to be tailored by choosing from a wide library of macrocycle substituents and central metal atoms. Nature itself offers magnificent examples of processes that utilize porphyrin derivatives, such as the activation and the transport of molecular oxygen in mammals and the harvesting of sunlight in plant photosynthetic systems. In order to exploit the highly desirable functionality of porphyrins in artificial molecular devices, it is imperative to understand and control the interactions that occur at the molecule–substrate interface. Such interactions largely depend on the electronic and conformational structures of the adsorbed molecules, which can be studied using techniques such as scanning tunneling microscopy, UV photoemission spectroscopy, and X-ray photoemission spectroscopy, and on a theoretical level with density functional calculations. Recent studies on conjugated rod-like molecules have shown that molecular conductance measurements can be significantly affected by the binding geometry, coupling of the p orbitals to the leads, or p–p stacking between adjacent molecules. Herein, we present the results of a study of the interaction of laterally extended p-conjugated porphyrin molecules with electrodes by means of timeand stretching-dependent conductance measurements on molecular junctions. We further investigate strategies to reduce interactions of the molecular p electrons with the metal electrodes by modifying the chemical structure of the porphyrin molecules. We used the series of molecules represented in Figure 1a– c to examine the influence of the molecular structure on the formation of porphyrin single-molecule junctions. Since the thiol group is most commonly used to contact rod-like molecules to form straight molecular bridges, we first compared 5,10,15,20-tetraphenylporphyrin without thiol termination (H2-TPP; Figure 1a) to a nearly identical molecule with two thiol groups on opposite sides of the molecule (5,15di(p-thiophenyl)-10,20-di(p-tolyl)porphyrin (H2-TPPdT); Figure 1b). To investigate the influence of the molecular backbone geometry on the junction formation we further studied a thiol-terminated porphyrin molecule with two bulky pyridine axial groups attached through an octahedral Ru ion ([Ru{5,15-di(p-thiophenyl)-10,20-diphenylporphyrin}(py)2] (Ru-TPPdT); Figure 1c). As a consequence of steric hindrance, the pyridine groups in Ru-TPPdT reduce the direct interaction of the metal electrodes with the p face of the porphyrin. A similar strategy was used previously. Prior to electrical characterization, the molecules were deposited using self-assembly from solution. To study the conductance of these molecules we used lithographic mechanically controllable break junctions (MCBJs) in vacuum at room temperature. The layout of an MCBJ device in a threepoint bending mechanism is shown in Figure 1d. Details concerning the synthesis of the molecules and the experimental procedures are given in the Supporting Information. Sets of 1000 consecutive breaking traces from individual junctions were analyzed numerically to construct “trace histograms” of the conductance (log10G versus the electrode displacement d). This statistical method maps the breaking dynamics of the junctions beyond the point of rupture of the last monatomic gold contact (defined as d= 0), which has a conductance of one quantum unit G0= 2e h. Areas of high counts represent the most typical breaking behavior of the molecular junctions. Figure 2 presents trace histograms as well as examples of individual breaking traces for acetone as reference, H2-TPP, H2-TPPdT, and Ru-TPPdT. For all three porphyrin molecules as well as for the reference sample several junctions were measured (see the Supporting Information). Herein, we only show a typical set of junctions. In the junction which was exposed to pure acetone (Figure 2a), the Au bridge initially gets stretched until a plateau around the conductance quantum (G G0) is observed (only visible in the individual traces shown in [*] M. L. Perrin, F. Prins, Dr. C. A. Martin, Prof. Dr. H. S. J. van der Zant, Dr. D. Dulic Kavli Institute of Nanoscience, Delft University of Technology Lorentzweg 1, 2628 CJ Delft (The Netherlands) E-mail: d.dulic@tudelft.nl

et al., We studied the electronic and conductance properties of two thiophene–curcuminoid molecules, 2-thphCCM (1) and 3-thphCCM (2), in which the only structural difference is the position of the sulfur atoms in the thiophene terminal groups. We used electrochemical techniques as well as UV/Vis absorption studies to obtain the values of the HOMO–LUMO band gap energies, showing that molecule 1 has lower values than 2. Theoretical calculations show the same trend. Self-assembled monolayers (SAMs) of these molecules were studied by using electrochemistry, showing that the interaction with gold reduces drastically the HOMO–LUMO gap in both molecules to almost the same value. Single-molecule conductance measurements show that molecule 2 has two different conductance values, whereas molecule 1 exhibits only one. Based on theoretical calculations, we conclude that the lowest conductance value, similar in both molecules, corresponds to a van der Waals interaction between the thiophene ring and the electrodes. The one order of magnitude higher conductance value for molecule 2 corresponds to a coordinate (dative covalent) interaction between the sulfur atoms and the gold electrodes., This work has been funded by the FONDECYT REGULAR grants 1110206,1140770, and 1140199, PAI-CONICYT 79150041, Conicyt PFCHA 21140734, ANILLO project Act 1117, Millenium Project RC120001, the Dutch funding agencies FOM and NWO/OCW, by the EU through a RISE (DAFNEOX) project, SEP-210165479 by the MICINN of Spain (projects CTQ2012-32247 and MAT2013-47869- C4-2-P). N.A.-A. and R.D.-T. also acknowledge financial support from MINECO through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496).

We present a method for incorporating image-charge effects into the description of charge transport through molecular devices. A simple model allows us to calculate the adjustment of the transport levels, due to the polarization of the electrodes as charge is added to and removed from the molecule. For this, we use the charge distributions of the molecule between two metal electrodes in several charge states, rather than in gas phase, as obtained from a density-functional theory-based transport code. This enables us to efficiently model level shifts and gap renormalization caused by image-charge effects, which are essential for understanding molecular transport experiments. We apply the method to benzene di-amine molecules and compare our results with the standard approach based on gas phase charges. Finally, we give a detailed account of the application of our approach to porphyrin-derivative devices recently studied experimentally by Perrin et al. [Nat. Nanotechnol. 8, 282 (2013)], which demonstrates the importance of accounting for image-charge effects when modeling transport through molecular junctions.

We investigated the temperature dependence and c-axis magnetic field dependence of the Josephson plasma resonance in optimally doped Tl2Ba2CaCu2O8 thin films and underdoped YBa2Cu3O6.5 (YBCO) ortho-II single crystals using infrared spectroscopy. We obtained the c-axis penetration depths, at low temperature, in zero fields of about 20 and 7 microm, respectively. While the temperature dependencies of the resonances in the two compounds are very similar, the magnetic field dependence in YBCO is much weaker. We attribute this weak magnetic field dependence to the lower anisotropy of YBCO and interpret the observed behaviors in terms of thermal fluctuations and pinning of pancake vortices.

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-delta, which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)2O2-delta block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes.

The c-axis Josephson plasmon in optimally doped single-layer and bi-layer high Tc cuprates Tl2201 and Tl2212 have been investigated using infrared spectroscopy. We observed the plasma frequencies for these two compounds at 27.8 and 25.6 cm-1 respectively, which we interpret as a Josephson resonance across the TlO blocking layers. No maximum in the temperature dependence of the c-axis conductivity was observed below Tc, indicating that even in the superconducting state a coherent quasi-particle contribution to the c-axis conductivity is absent or very weak, in contrast to the behaviour of the ab-plane conductivity., Comment: 4 pages, 3 figures

In this paper we present the partial and total DOS of the plasmons in a layered electron gas system consisting of two layers per unit cell using the Random Phase Approximation. The description of a lattice in terms of conducting bilayers surrounded by a non-conducting medium can be seen as a first approximation to oxide superconductors like Bi2212 and Tl2212. The relevancy of the partial density of states for experimental probes like reflection EELS will be briefly discussed. Preceding the exact description of the partial DOS we will give a short summary of the derivation of the plasmon dispersion in a two layer/unit cell system [A. Griffin, A.J. Pindor, Phys. Rev. B 39 (1989) 11 503] and describe the two different branches that emerge. Finally, the total DOS is calculated numerically and is compared to the one layer/unit cell case.

We have investigated charge transport in ZnTPPdT–Pyr (TPPdT: 5,15-di(p-thiolphenyl)-10,20-di(p-tolyl)porphyrin) molecular junctions using the lithographic mechanically controllable break-junction (MCBJ) technique at room temperature and cryogenic temperature (6 K). We combined low-bias statistical measurements with spectroscopy of the molecular levels in the form of I(V) characteristics. This combination allows us to characterize the transport in a molecular junction in detail. This complex molecule can form different junction configurations, having an observable effect on the trace histograms and the current–voltage (I(V)) measurements. Both methods show that multiple, stable single-molecule junction configurations can be obtained by modulating the interelectrode distance. In addition we demonstrate that different ZnTPPdT–Pyr junction configurations can lead to completely different spectroscopic features with the same conductance values. We show that statistical low-bias conductance measurements should be interpreted with care, and that the combination with I(V) spectroscopy represents an essential tool for a more detailed characterization of the charge transport in a single molecule.

have been demonstrated. Photoswitching effects are based on the reorganization of the p-conjugated backbone of the molecule that consequently leads to different physical properties. Knowledge of the vital factors governing photochromic processes is essential for future utilization of diarylethenes as smart materials as well as for a fundamental understanding of the photoprocesses involved. Hence, temperature-dependent studies are necessary to reveal new aspects and limits of the switching process, as previously shown by measurements at elevated temperatures. [19] Additionally, determining possible switching restrictions at low temperatures is particularly important in view of carrying out conductance measurements at the single-molecular level, which are restricted at room temperature because of the high mobility of the gold atoms constituting the electrodes in this type of experiment. In order to achieve stability of the metal/molecule/metal system it is often necessary to perform measurements at low temperatures. [20] Herein, we explore the temperature dependence of the photochemical behavior of dithienylcyclopentenes. We report the suppression of the ring-opening process with decreasing temperature, leading to the complete absence of the photoreaction below a cutoff temperature. By contrast, we demonstrate that the reverse ring-closure process shows no significant temperature dependence above 115 K. These observations of the temperature dependence for the ring-opening process are essential to understand the behavior of these molecular switches before they can be used in advanced technological applications. The theoretical description of the ringopening process is not completely understood. Furthermore, the observed suppression of the ring opening might have important implications for gating of photochromic processes. Like for other photoreactions that have a photostationary state with less than 100% product, the ring-closure process for diarylethenes is accompanied by ring opening, because both forms have absorption peaks in the UV region, hence decreasing total conversions. [21] On the other hand, if ring opening can be blocked, total conversion increases. [22]

The Intlerlayer Josephson coupling between the planes of Tl2Ba2CuO6 was determined using infrared spectroscopy and magnetic flux vortex imaging. These methods give a consistent value of $\omega_J$= 28 cm$^{-1}$ which, when combined with the condensation energy produces a discrepancy of at least an order of magnitude with deductions based on the interlayer tunneling model.

We investigate the mechanics of lithographically defined mechanically controllable break junctions, both theoretically and experimentally. It is shown that the relationship between controlled deflection and junction opening depends on the details of the break junction geometry. As a result the generally used formula for the ``attenuation factor'' $r$ needs to be corrected by a factor $\ensuremath{\zeta}$. For typical break junction geometries, we obtain $2\ensuremath{\lesssim}\ensuremath{\zeta}\ensuremath{\lesssim}4$.

Two series of novel diarylethenes, the first containing one and the second two acetyl protected thiol groups, were synthesised in two and three steps from an easily available known intermediate. Newly synthesised compounds present ideal examples of model substrates for photochromic studies on metal surfaces.

The magnetic field and temperature dependence of the transverse and longitudinal plasmons in SmLa(0.8)Sr(0.2)CuO(4-delta) have been investigated. A transition between a vortex-glass and a vortex-liquid regime, which revealed different field dependencies of the resonance frequencies, could be clearly observed. The positions and the spectral weights of the plasmons were successfully described using the multilayer model [Phys. Rev. B 64, 024530 (2001)], which takes the compressibility of the electronic liquid into account. The absolute value of the compressibility is close to that of a two-dimensional noninteracting electron gas.

We present grazing incidence reflectivity measurements in the far infrared region at temperatures above and below Tc for a series of thallium (Tl2Ba2CuO6, Tl2Ba2CaCu2O8) and bismuth (Bi2Sr2CuO6, Bi2Sr2CaCu2O8, and Bi(2-x)Pb(x)Sr2CaCu2O8) based cuprate superconductors. From the spectra, which are dominated by the c-axis phonons, longitudinal frequencies (LO) are directly obtained. The reflectivity curves are well fitted by a series of Lorentz oscillators. In this way the transverse (TO) phonon frequencies were accurately determined. On the basis of the comparative study of the Bi and Tl based cuprates with different number of CuO2 layers per unit cell, we suggest modifications of the assignment of the main oxygen modes. We compare the LO frequencies in Bi2Sr2CaCu2O8 and Tl2Ba2Ca2Cu3O10 obtained from intrinsic Josephson junction characteristics with our measurements, and explain the discrepancy in LO frequencies obtained by the two different methods., 8 pages Revtex, 6 eps figures, 3 tables, to appear in Phys. Rev. B

A high-current annealing technique is used to fabricate nanogaps and hybrid diamond/graphite structures in boron-doped nanocrystalline diamond films. Nanometer-sized gaps down to 1 nm are produced using a feedback-controlled current annealing procedure. The nanogaps are characterized using scanning electron microscopy and electronic transport measurements. The structural changes produced by the elevated temperature, achieved by Joule heating during current annealing, are characterized using Raman spectroscopy. The formation of hybridized diamond/graphite structure is observed at the point of maximum heat accumulation., Comment: 14 pages 4 figures + 1 supplemental figure

We present a study of electronic transport in short (12-base-pair) DNA duplexes covalently bonded (via thiol groups) to two gold electrodes obtained by a mechanically controllable break junction (MCJB) technique in dry conditions. A large number of DNA junctions have been repeatedly formed in order to obtain a conductance histogram that reveals a peak which corresponds to the conductance of a single DNA molecule. We observed that the conductivity of a DNA increases upon increasing the content of G:C base pairs in the duplex. With our method we are able to obtain a reliable value of a single DNA conductance and subsequently measure its current-voltage (I-V) characteristics. In contrast to the electronic transport measurements performed with long DNA sequences (hundreds of base pairs) where the obtained conductance values vary a lot with environmental conditions, our values obtained for the short DNA sequences are consistent with the values reported for comparable sequences in aqueous solution.

We investigate photochromic molecular switches that are self-assembled on gold. We use two experimental techniques; namely, the mechanically controllable break-junction technique to measure electronic transport, and UV/Vis spectroscopy to measure absorption. We observe switching of the molecules from the conducting to the insulating state when illuminated with visible light (lambda=546 nm), in spite of the gold surface plasmon absorption present around this wavelength. However, we fail to observe the reverse process which should occur upon illumination with UV light (lambda=313 nm). We attribute this to quenching of the excited state of the molecule in the open form by the presence of gold.

We study the formation and stability of nanometer‐sized gaps, created by electromigration, in thin gold wires. After a wire breaks due to a high local current density, nanogaps of random size are found, all exhibiting tunneling behavior at low bias. Surprisingly, we find that small gaps (< appr. 0.5 nm) can be closed again when a voltage of about 2 V is applied. For larger gaps this is not possible, but the gold does become unstable, leading to an apparent negative differential resistance. We relate these effects to field evaporation and field‐enhanced diffusion, respectively.