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Hemozoin is a natural biomarker formed during the hemoglobin metabolism of Plasmodiumparasites, the causative agents of malaria. The rotating-crystal magneto-optical detection (RMOD) has been developed for its rapid and sensitive detection both in cell cultures and patient samples. In the current article we demonstrate that, besides quantifying the overall concentration of hemozoin produced by the parasites, RMOD can also track the size distribution of the hemozoin crystals. We establish the relations between the magneto-optical signal, the mean parasite age and the median crystal size throughout one erythrocytic cycle of Plasmodium falciparum parasites, where the latter two are determined by optical and scanning electron microscopy, respectively. The significant correlation between the magneto-optical signal and the stage distribution of the parasites indicates that the RMOD method can be utilized for species-specific malaria diagnosis and for the quick assessment of drug efficacy.

We investigate gold-4,4'-bipyridine-gold single-molecule junctions with the mechanically controllable break junction technique at cryogenic temperature ($T=4.2\,\text{K}$). We observe bistable probabilistic conductance switching between the two molecular binding configurations, influenced both by the mechanical actuation, and the applied voltage. We demonstrate that the relative dominance of the two conductance states is tunable by the electrode displacement, whereas the voltage manipulation induces an exponential speedup of both switching times. The detailed investigation of the voltage-tunable switching rates provides an insight into the possible switching mechanisms.

Graphene nanogaps are considered as essential building blocks of two-dimensional electronic circuits, as they offer the possibility to interconnect a broad range of atomic-scale objects. Here we provide an insight into the microscopic processes taking place during the formation of graphene nanogaps through the detailed analysis of their low-frequency noise properties. Following the evolution of the noise level, we identify the fundamentally different regimes throughout the nanogap formation. By modeling the resistance and bias dependence of the noise, we resolve the major noise-generating processes: atomic-scale junction-width fluctuations in the nanojunction regime and sub-atomic gap-size fluctuations in the nanogap regime. As a milestone toward graphene-based atomic electronics, our results facilitate the automation of an optimized electrical breakdown protocol for high yield graphene nanogap fabrication., npj 2D Materials and Applications, 5 (1), ISSN:2397-7132

In this study, the possibilities of noise tailoring in filamentary resistive switching memory devices are investigated. To this end, the resistance and frequency scaling of the low-frequency 1/f-type noise properties are studied in representative mainstream material systems. It is shown that the overall noise floor is tailorable by the proper material choice, as demonstrated by the order-of-magnitude smaller noise levels in Ta2O5 and Nb2O5 transition-metal oxide memristors compared to Ag-based devices. Furthermore, the variation of the resistance states allows orders-of-magnitude tuning of the relative noise level in all of these material systems. This behavior is analyzed in the framework of a point-contact noise model highlighting the possibility for the disorder-induced suppression of the noise contribution arising from remote fluctuators. These findings promote the design of multipurpose resistive switching units, which can simultaneously serve as analog-tunable memory elements and tunable noise sources in probabilistic computing machines., ACS Applied Materials & Interfaces, 13 (6), ISSN:1944-8244, ISSN:1944-8252

We study the vulnerability of single-molecule nanowires against a temporary disconnection of the junction. To this end, we compare the room and low-temperature junction formation trajectories along the opening and closing of gold-4,4'-bipyridine-gold single-molecule nanowires. In the low-temperature measurements, the cross-correlations between the opening and subsequent closing conductance traces demonstrate a strong structural memory effect: around half of the molecular opening traces exhibit similar, statistically dependent molecular features as the junction is closed again. This means that the junction stays rigid and the molecule remains protruding from one electrode even after the rupture of the junction, and therefore, the same single-molecule junction can be reestablished if the electrodes are closed again. In the room-temperature measurements, however, weak opening-closing correlations are found, indicating a significant rearrangement of the junction after the rupture and the related loss of structural memory effects.

Single-molecule break junction measurements deliver a huge number of conductance vs.\ electrode separation traces. Along such measurements the target molecules may bind to the electrodes in different geometries, and the evolution and rupture of the single-molecule junction may also follow distinct trajectories. The unraveling of the various typical trace classes is a prerequisite of the proper physical interpretation of the data. Here we exploit the efficient feature recognition properties of neural networks to automatically find the relevant trace classes. To eliminate the need for manually labeled training data we apply a combined method, which automatically selects training traces according to the extreme values of principal component projections or some auxiliary measured quantities, and then the network captures the features of these characteristic traces, and generalizes its inference to the entire dataset. The use of a simple neural network structure also enables a direct insight to the decision making mechanism. We demonstrate that this combined machine learning method is efficient in the unsupervised recognition of unobvious, but highly relevant trace classes within low and room temperature gold-4,4' bipyridine-gold single molecule break junction data.

Nanometer-scale resistive switching devices operated in the metallic conductance regime offer ultimately scalable and widely reconfigurable hardware elements for novel in-memory and neuromorphic computing architectures. Moreover, they exhibit high operation speed at low power arising from the ease of the electric-field-driven redistribution of only a small amount of highly mobile ionic species upon resistive switching. We investigate the memristive behavior of a so-far less explored representative of this class, the Ag/AgI material system in a point contact arrangement established by the conducting PtIr tip of a scanning probe microscope. We demonstrate stable resistive switching duty cycles and investigate the dynamical aspects of non-volatile operation in detail. The high-speed switching capabilities are explored by a custom-designed microwave setup that enables time-resolved studies of subsequent set and reset transitions upon biasing the Ag/AgI/PtIr nanojunctions with sub-nanosecond voltage pulses. Our results demonstrate the potential of Ag-based filamentary memristive nanodevices to serve as the hardware elements in high-speed neuromorphic circuits., Beilstein Journal of Nanotechnology, 11, ISSN:2190-4286

Atomic synapses represent a special class of memristors whose operation relies on the formation of metallic nanofilaments bridging two electrodes across an insulator. Due to the magnifying effect of this narrowest cross section on the device conductance, a nanometer-scale displacement of a few atoms grants access to various resistive states at ultimately low energy costs, satisfying the fundamental requirements of neuromorphic computing hardware. However, device engineering lacks the complete quantum characterization of such filamentary conductance. Here we analyze multiple Andreev reflection processes emerging at the filament terminals when superconducting electrodes are utilized. Thereby, the quantum PIN code, i.e., the transmission probabilities of each individual conduction channel contributing to the conductance of the nanojunctions, is revealed. Our measurements on Nb2O5 resistive switching junctions provide profound experimental evidence that the onset of the high conductance ON state is manifested via the formation of truly atomic-sized metallic filaments., Nano Letters, 20 (2), ISSN:1530-6984, ISSN:1530-6992

Break-junction measurements are typically aimed at characterizing electronic properties of single molecules bound between two metal electrodes. Although these measurements have provided structure-function relationships for such devices, there is little work that studies the impact of molecule-molecule interactions on junction characteristics. Here, we use a scanning tunneling microscope based break-junction technique to study pi-stacked dimer junctions formed with two amine-terminated conjugated molecules. We show that the conductance, force and flicker noise of such dimers differ dramatically when compared with the corresponding monomer junctions and discuss the implications of these results on intra- and inter-molecular charge transport.

The microscopic origins and technological impact of 1/f type current fluctuations in Ag based, filamentary type resistive switching devices have been investigated upon downscaling toward the ultimate single atomic limit. The analysis of the low-frequency current noise spectra revealed that the main electronic noise contribution arises from the resistance fluctuations due to internal dynamical defects of Ag nanofilaments. The resulting 0.01-1% current noise ratio, i.e. the total noise level with respect to the mean value of the current, is found to be universal: its magnitude only depends on the total resistance of the device, irrespective of the materials aspects of the surrounding solid electrolyte and of the specific filament formation procedure. Moreover, the resistance dependence of the current noise ratio also displays the diffusive to ballistic crossover, confirming that stable resistive switching operation utilizing Ag nanofilaments is not compromised even in truly atomic scale junctions by technologically impeding noise levels.

The dynamical aspects of bipolar resistive switchings have been investigated in Nb/Nb2O5/PtIr nanojunctions. We found that the widely tuneable ON and OFF state resistances are well separated at low bias. On the other hand, the high-bias regime of the resistive switchings coincides with the onset of a high nonlinearity in the current–voltage characteristics, where the impedance of both states rapidly decreases and becomes equivalent around 50 Ω. This phenomenon enables the overriding of the RC limitations of fast switchings between higher resistance ON and OFF states. Consequently, nanosecond switching times between multiple resistance states due to subnanosecond voltage pulses are demonstrated. Moreover, this finding provides the possibility of impedance engineering by the appropriate choice of voltage signals, which facilitates that both the set and reset transitions take place in an impedance matched manner to the surrounding circuit, demonstrating the merits of ultra-fast operation of Nb2O5 based neuromorphic networks., Nanoscale, 10 (41), ISSN:2040-3364, ISSN:2040-3372

The dynamics of resistive switchings in nanometer-scale metallic junctions formed between an inert metallic tip and an Ag film covered by a thin Ag2S layer are investigated. Our thorough experimental analysis and numerical simulations revealed that the resistance change upon a switching bias voltage pulse exhibits a strongly non-exponential behaviour yielding markedly different response times at different bias levels. Our results demonstrate the merits of Ag2S nanojunctions as nanometer-scale non-volatile memory cells with stable switching ratios, high endurance as well as fast response to write/erase, and an outstanding stability against read operations at technologically optimal bias and current levels., Nanoscale, 7 (10), ISSN:2040-3364, ISSN:2040-3372

We report on the characterization of the electrical breakdown (EB) process for the formation of tunneling nanogaps in single-layer graphene. In particular, we investigated the role of oxygen in the breakdown process by varying the environmental conditions (vacuum and ambient conditions). We show that the density of oxygen molecules in the chamber is a crucial parameter that defines the physical breakdown process: at low density, the graphene lattice is sublimating, whereas at high density, the process involved is oxidation, independent of the substrate material. To estimate the activation energies of the two processes, we use a scheme which consists of applying voltage pulses across the junction during the breakdown. By systematically varying the voltage pulse length, and estimating the junction temperature from a 1D thermal model, we extract activation energies which are consistent with the sublimation of graphene under high vacuum and the electroburning process under air. Our study demonstrates that, in our system, a better control of the gap formation is achieved in the sublimation regime.

We investigate the formation and evolution of Au–CO single-molecule break junctions. The conductance histogram exhibits two distinct molecular configurations, which are further investigated by a combined statistical analysis. According to conditional histogram and correlation analysis these molecular configurations show strong anticorrelations with each other and with pure Au monoatomic junctions and atomic chains. We identify molecular precursor configurations with somewhat higher conductance, which are formed prior to single-molecule junctions. According to detailed length analysis two distinct types of molecule-affected chain-formation processes are observed, and we compare these results to former theoretical calculations considering bridge- and atop-type molecular configurations where the latter has reduced conductance due to destructive Fano interference.

The interaction of carbon monoxide molecules with atomic-scale platinum nanojunctions is investigated by low temperature mechanically controllable break junction experiments. Combining plateaus' length analysis, two dimensional conductance-displacement histograms and conditional correlation analysis a comprehensive microscopic picture is proposed about the formation and evolution of Pt-CO-Pt single-molecule configurations. Our analysis implies that before pure Pt monoatomic chains would be formed a CO molecule infiltrates the junction, first in a configuration being perpendicular to the contact axis. This molecular junction is strong enough to pull a monoatomic platinum chain with the molecule being incorporated in the chain. Along the chain formation the molecule can either stay in the perpendicular configuration, or rotate to a parallel configuration. The evolution of the single-molecule configurations along the junction displacement shows quantitative agreement with theoretical predictions, justifying the interpretation in terms of perpendicular and parallel molecular alignment. Our analysis demonstrates that the combination of two dimensional conductance-displacement histograms with conditional correlation analysis is a useful tool to separately analyze fundamentally different types of junction trajectories in single molecule break junction experiments.

Prevailing models of resistive switching arising from electrochemical formation of conducting filaments across solid state ionic conductors commonly attribute the observed polarity of the voltage-biased switching to the sequence of the active and inert electrodes confining the resistive switching memory cell. Here we demonstrate stable switching behaviour in metallic Ag-Ag2S-Ag nanojunctions at room temperature exhibiting similar characteristics. Our experimental results and numerical simulations reveal that the polarity of the switchings is solely determined by the geometrical asymmetry of the electrode surfaces. By the lithographical design of a proof of principle device we demonstrate the merits of simplified fabrication of atomic-scale, robust planar Ag2S memory cells., Scientific Reports, 6, ISSN:2045-2322

We study the behavior of hydrogen molecules between atomic-sized metallic electrodes using the mechanically controllable break junction technique. We focus on the interaction H2 with monoatomic gold chains demonstrating the possibility of a hydrogen molecule being incorporated in the chain. We also show that niobium is strongly reactive with hydrogen, which enables molecular transport studies between superconducting electrodes. This opens the possibility for a full characterization of the transmission properties of molecular junctions with superconducting subgap structure measurements.

Resistive switchings in nanometer-scale metallic junctions formed between an inert metallic tip and an Ag film covered by a thin Ag2S layer are investigated as a function of temperature at different biasing conditions. The observed switching threshold voltages along with the ON and OFF state resistances are quantitatively understood by taking the local overheating of the junction volume and the resulting structural phase transition of the Ag2S matrix into account. Our results demonstrate that the essential characteristics of the resistive switching in Ag2S based nanojunctions can be routinely optimized by suitable sample preparation and biasing schemes., Nanoscale, 7 (26), ISSN:2040-3364, ISSN:2040-3372

Contains fulltext : 58476.pdf (Publisher’s version ) (Open Access) We used the mechanically controllable break junction technique to discriminate between the electric field and temperature effects in the process of surface modification. The electric field strength at the surface of electrodes was accurately determined using the field emission resonance spectra and was gradually raised to the point where surface modification starts. We found that only a limited number of metals with large values of the work function, greater than or similar to4.5-5 eV, and evaporation fields, less than or similar to1.5-2 V/Angstrom, can be modified in this way. Adsorption of He on the surface drastically increases the local work function of material and enables the field induced nanostructuring of the electrode surface practically for all metals. (C) 2004 American Institute of Physics.

Interaction of the physically adsorbed molecular hydrogen with a breaking gold nanowire results in additional stable atomic configurations in few atom contacts and appearance of fractional peaks in the conductance histogram. This effect is explained by peculiar dynamic evolution of the hydrogen-embedded nanoconstriction due to competition between tensile and capillary forces. Dimerization within the atomic wire and hydrogen-assisted stabilization of gold dimers results in preferable atomic arrangements with conductivity close to 0.5 and 1.5 of quantum conductance unit ${G}_{0}=2{e}^{2}/h$.

Ag/Fe/Ag and Cr/Fe/Cr trilayers with a single 25 nm thick ferromagnetic layer exhibit giant magnetoresistance type behavior. The resistance decreases for parallel and transversal magnetic field alignments with a Langevin type magnetic field dependence up to B=12 T . The phenomenon is explained by a granular interface structure. Results on Fe/Ag multilayers are also interpreted in terms of a granular interface magnetoresistance.

To analyze the rhizosphere bacterial communities in wetlands, the total lipid content was extracted from a peat soil and 4 abundant wetland plant roots ( Typha angustifolia L., Salix cinerea L., Carex pseudocyperus L., Thelypteris palustris Salisb.). The separated phospholipid fraction was further fractionated and deriva­tized prior to gas chromatography-mass spectrometry (GC-MS) measurement. In the evaluation only the bacteria-specific fatty acids were used in order to neglect fatty acid information derived from plant root cells. Based on these analyses, a high level bacterial concentration was demonstrated in the rhizosphere, and the relative occurrence of aerobe and anaerobe, Gram positive and negative bacteria, methanotrophs, sulphate reducers and Actinobacteria was determined. Through the PLFA analysis the study of bacteria regardless of culturability was possible.

Magnetoresistance and magnetic behaviour of a vacuum evaporated multilayer consisting of 1.4 nm Fe and 2 nm Ag layers were studied. The magnetisation of the multilayer shows normal ferromagnetic behaviour while the resistance decreases both in parallel and transversal geometry up to 12 T magnetic field and can be well described by squared Langevin functions. The giant magnetoresistance (GMR) is attributed to a small amount of Fe clusters and/or single impurities in the Ag matrix. This explanation is supported by similar results on a 8 nm Ag/25 nm Fe/8 nm Ag trilayer. The concentration distribution due to interface mixing was studied by Rutherford backscattering spectrometry (RBS) measurements.

We review data analysis techniques that can be used to study temporal correlations among conductance traces in break junction measurements. We show that temporal histograms are a simple but efficient tool to check the temporal homogeneity of the conductance traces, or to follow spontaneous or triggered temporal variations, like structural modifications in trained contacts, or the emergence of single-molecule signatures after molecule dosing. To statistically analyze the presence and the decay time of temporal correlations, we introduce shifted correlation plots. Finally, we demonstrate that correlations between opening and subsequent closing traces may indicate structural memory effects in atomic-sized metallic and molecular junctions. Applying these methods on measured and simulated gold metallic contacts as a test system, we show that the surface diffusion induced flattening of the broken junctions helps to produce statistically independent conductance traces at room temperature, whereas at low temperature repeating tendencies are observed as long as the contacts are not closed to sufficiently high conductance setpoints. Applying opening-closing correlation analysis on Pt-CO-Pt single-molecule junctions, we demonstrate pronounced contact memory effects and recovery of the molecule for junctions breaking before atomic chains are formed. However, if chains are pulled the random relaxation of the chain and molecule after rupture prevents opening-closing correlations.

Experimental correlation analysis and first-principles theory are used to probe the structure and evolution of Ag-CO-Ag single-molecule junctions, both before the formation, and after the rupture of the junctions. Two dimensional correlation histograms and conditional histograms demonstrate that prior to the single-molecule bridge configuration the CO molecule is already bound parallel to the Ag single-atom contact. This molecular precursor configuration is accompanied by the opening of additional conductance channels compared to the single-channel transport in pure Ag monoatomic junctions. To investigate the post-rupture evolution of the junction we introduce a cross-correlation analysis between the opening and the subsequent closing conductance traces. This analysis implies that the molecule is bound rigidly to the apex of one electrode, and so the same single-molecule configuration is re-established as the junction is closed. The experimental results are confirmed by ab initio simulations of the evolution of contact geometries, transmission eigenvalues and scattering wavefunctions.

The nonlinear transport properties of nanometer-scale junctions formed between an inert metallic tip and an Ag film covered by a thin Ag2S layer are investigated. Suitably prepared samples exhibit memristive behavior with technologically optimal ON and OFF state resistances yielding to resistive switching on the nanosecond time scale. Utilizing point contact Andreev reflection spectroscopy, we studied the nature of electron transport in the active volume of memristive junctions showing that both the ON and OFF states correspond to truly nanometer-scale, highly transparent metallic channels. Our results demonstrate the merits of Ag2S nanojunctions as nanometer-scale memory cells which can be switched by nanosecond voltage pulses., Nanoscale, 6 (5), ISSN:2040-3364, ISSN:2040-3372

Herein we demonstrate the controlled and reproducible fabrication of sub-5 nm wide gaps in single-layer graphene electrodes. The process is implemented for graphene grown via chemical vapor deposition using an electroburning process at room temperature and in vacuum. A yield of over 95 percent for the gap formation is obtained. This approach allows producing single-layer graphene electrodes for molecular electronics at a large scale. Additionally, from Raman spectroscopy and electroburning carried out simultaneously, we can follow the heating process and infer the temperature at which the gap formation happens.

Bulk resistivity and point contact spectra of ${\mathrm{Ni}}_{x}{\mathrm{Nb}}_{1\ensuremath{-}x}$ metallic glasses have been investigated as functions of temperature (0.3--300 K) and magnetic field (0--12 T). Metallic glasses in this family undergo a superconducting phase transition determined by the Nb concentration. When superconductivity was suppressed by a strong magnetic field, both the bulk sample $R(T)$ and the point contact differential resistance curves of ${\mathrm{Ni}}_{x}{\mathrm{Nb}}_{1\ensuremath{-}x}$ showed logarithmic behavior at low energies, which is explained by a strong electron-``two level system'' coupling. We studied the temperature, magnetic field, and contact resistance dependence of ${\mathrm{Ni}}_{44}{\mathrm{Nb}}_{56}$ point-contact spectra in the superconducting state and found telegraphlike fluctuations superimposed on superconducting characteristics. These $R(V)$ characteristics are extremely sensitive detectors for slow relaxing ``two level system'' motion.

We demonstrate a self-contained methodology for predicting conductance histograms of atomic and molecular junctions. Fast classical molecular-dynamics simulations are combined with accurate density functional theory calculations predicting both quantum transport properties and molecular-dynamics force field parameters. The methodology is confronted with experiments on atomic-sized indium nanojunctions. Beside conductance histograms the distribution of individual channel transmission eigenvalues is also determined by fitting the superconducting subgap features in the I-V curves. The remarkable agreement in the evolution of the channel transmissions demonstrates that the simulated ruptures are able to reproduce a realistic statistical ensemble of contact configurations, whereas simulations on selected ideal geometries show strong deviations from the experimental observations.

Creating variable size nanojunctions between a thin metallic film and a superconducting tip we study how multiple phase-coherent scatterings enhance the superconducting correlations at the normal side. By increasing the coherent volume of carriers the transmission through the interface is smoothly enhanced as reflected in the zero-bias conductance. As the phase-coherent volume reaches the opposite surface of the thin film a resonator is formed, the conductance of the interface is dramatically enhanced, and finally a proximity induced Josephson supercurrent is established.

Solid state ionic conductors are good candidates for the next generation of nonvolatile computer memory elements. Such devices have to show reproducible resistance switching at reasonable voltage and current values even if scaled down to the nanometer sizes. Here we study the switching characteristics of nanoscale junctions created between a tungsten tip and a silver film covered by a thin ionic conductor layer. Atomic-sized junctions show spectacular current induced switching characteristics, but both the magnitude of the switching voltage and the direction of the switching vary randomly for different junctions. In contrast, for somewhat larger junctions with diameters of a few nanometers a well defined, reproducible switching behavior is observed which is associated with the formation and destruction of nanoscale channels in the ionic conductor surface layer. Our results define a low size limit of 3 nm for reliable ionic nano-switches, which is well below the resolution of recent lithographic techniques.

Transport through a molecular device coupled to a vibrational mode is studied. By mapping it to the Yu-Anderson model in the large contact broadening limit, the zero bias electric and heat conductances are evaluated non-perturbatively. These exhibit a step from their T=0 value to half of its value as T increases due to the opening of the inelastic scattering channel. The spectral function exhibits the Franck-Condon suppressed multiphonon steps. The Wiedemann-Franz law is satisfied at low and high temperatures, but is violated in between. Relations to experiments are discussed., Comment: 5 pages, 2 figures

Results of point contact Andreev reflection (PCAR) experiments on (In,Mn)Sb are presented and analyzed in terms of current models of charge conversion at a superconductor-ferromagnet interface. We investigate the influence of surface transparency, and study the crossover from ballistic to diffusive transport regime as contact size is varied. Application of a Nb tip to a (In,Mn)Sb sample with Curie temperature Tc of 5.4 K allowed the determination of spin-polarization when the ferromagnetic phase transition temperature is crossed. We find a striking difference between the temperature dependence of the local spin polarization and of the macroscopic magnetization, and demonstrate that nanoscale clusters with magnetization close to the saturated value are present even well above the magnetic phase transition temperature., 4 pages

Experimental results showing huge negative differential conductance in gold-hydrogen molecular nanojunctions are presented. The results are analyzed in terms of two-level system (TLS) models: It is shown that a simple TLS model cannot produce peaklike structures in the differential conductance curves, whereas an asymmetrically coupled TLS model gives perfect fit to the data. Our analysis implies that the excitation of a bound molecule to a large number of energetically similar loosely bound states is responsible for the peaklike structures. Recent experimental studies showing related features are discussed within the framework of our model.

A great variety of experiments, like heat release measurements, acoustic measurements, and transport measurements on mesoscopic samples have proved that two level systems (TLSs) have a crucial role in the low temperature thermal and electric properties of disordered systems. This paper is aimed at reviewing the role of slow TLSs in point contacts. First the theory of point contacts is summarized, concentrating on the discussion of different point contact models, and on the different regimes of electron flow in the contact, mainly focusing on the ballistic and diffusive limit. The Boltzmann equation is solved in both regimes, and the position dependence of the electrical potential is determined. Then the scattering processes in point contacts are investigated, particularly concentrating on the scattering on slow TLSs. If the the electron assisted transitions between the two states are negligible the electron-two level system interaction can be treated with a simplified Hamiltonian. The scattering on such slow TLSs causes nonlinearity in the current-voltage characteristics of the point contact, which can be determined using Fermi's golden role. These calculations are presented showing both the contribution of elastic and inelastic scattering, and including the dependence on the position of the TLS, and on the effect of high frequency irradiation. These results are used to discuss the differences between these slow TLSs and the fast centers which may be described by the two channel Kondo model. The available experimental results are analyzed, distinguishing between the effects due to the different types of TLSs., 53 pages, 40 figures

The conductance of breaking metallic nanojunctions shows plateaus alternated with sudden jumps, corresponding to the stretching of stable atomic configurations and atomic rearrangements, respectively. We investigate the structure of the conductance plateaus both by measuring the voltage dependence of the plateaus' slope on individual junctions and by a detailed statistical analysis on a large amount of contacts. Though the atomic discreteness of the junction plays a fundamental role in the evolution of the conductance, we find that the fine structure of the conductance plateaus is determined by quantum interference phenomenon to a great extent., 4 pages, 4 figures

We apply the mechanically controllable break junctions technique to investigate the transition from tunneling to direct contact in tungsten. This transition is quite different from that of other metals and is determined by the local electronic properties of the tungsten surface and the relief of the electrodes at the point of their closest proximity. The conductance traces show a rich variety of patterns from the avalanche-like jump to a mesoscopic contact to the completely smooth transition between direct contact and tunneling. Due to the occasional absence of an adhesive jump the conductance of the contact can be continuously monitored at ultra-small electrode separations. The conductance histograms of tungsten are either featureless or show two distinct peaks related to the sequential opening of spatially separated groups of conductance channels. The role of surface states of tungsten and their contribution to the junction conductance at sub-Angstrom electrode separations are discussed., 6 pages, 6 figures

Results of an experimental study of palladium nanojunctions in hydrogen environment are presented. Two new hydrogen-related atomic configurations are found, which have a conductances of ~0.5 and ~1 quantum unit (2e^2/h). Phonon spectrum measurements demonstrate that these configurations are situated between electrodes containing dissolved hydrogen. The crucial differences compared to the previously studied Pt-H_2 junctions, and the possible microscopic realizations of the new configurations in palladium-hydrogen atomic-sized contacts are discussed., 4 pages, 4 figures

Dynamic evolution of the connective neck in Al and Pb mechanically controllable break junctions was studied during continuous approach of electrodes at bias voltages V_b up to a few hundred mV. A high level of power dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in the constriction lead to overheating of the contact area, electromigration and current-enhanced diffusion of atoms out of the "hot spot". At a low electrode approach rate (10 - 50 pm/s) the transverse dimension of the neck and the conductance of the junction depend on V_b and remain nearly constant over the approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists of a few atoms only and the quantum nature of conductance manifests itself in abrupt steps and reversible jumps between two or more levels. These features are related to an ever changing number of individual conductance channels due to the continuous rearrangement in atomic configuration of the neck, the recurring motion of atoms between metastable states, the formation and breaking of isolated one-atom contacts and the switching between energetically preferable neck geometries., 21 pages 10 figures