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A. Halbritter

Publications and source records attributed to A. Halbritter.

At least 19 recordsLinked to original sources
arXiv2024

Picosecond Femtojoule Resistive Switching in Nanoscale VO$_{2}$ Memristors

Beyond-Moore computing technologies are expected to provide a sustainable alternative to the von Neumann approach not only due to their down-scaling potential but also via exploiting device-level functional complexity at the lowest possible energy consumption. The dynamics of the Mott transition in correlated electron oxides, such as vanadium dioxide, has been identified as a rich and reliable source of such functional complexity. However, its full potential in high-speed and low-power operation has been largely unexplored. We fabricated nanoscale VO$_{2}$ devices embedded in a broad-band test circuit to study the speed and energy limitations of their resistive switching operation. Our picosecond time-resolution, real-time resistive switching experiments and numerical simulations demonstrate that tunable low-resistance states can be set by the application of 20~ps long, $<$1.7~V amplitude voltage pulses at 15~ps incubation times and switching energies starting from a few femtojoule. Moreover, we demonstrate that at nanometer-scale device sizes not only the electric field induced insulator-to-metal transition, but also the thermal conduction limited metal-to-insulator transition can take place at timescales of 100's of picoseconds. These orders of magnitude breakthroughs open the route to the design of high-speed and low-power dynamical circuits for a plethora of neuromorphic computing applications from pattern recognition to numerical optimization.

cond-mat.mes-hall↗
arXiv2020

Unsupervised feature recognition in single molecule break junction data

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.

cond-mat.mes-hall↗
arXiv2016

Temporal correlations and structural memory effects in break junction 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.

cond-mat.mes-hall↗
arXiv2016

Asymmetry-induced resistive switching in Ag-Ag$_{2}$S-Ag memristors enabling a simplified atomic-scale memory design

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 equivalent, stable switching behavior in metallic Ag-Ag$_{2}$S-Ag nanojunctions at room temperature. 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 Ag$_{2}$S memory cells.

cond-mat.mes-hall↗
arXiv2013

GHz Operation of Nanometer-Scale Metallic Memristors: Highly Transparent Conductance Channels in Ag$_{2}$S Devices

The nonlinear transport properties of nanometer-scale junctions formed between an inert metallic tip and an Ag film covered by a thin Ag$_{2}$S 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 the 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 Ag$_{2}$S nanojunctions as nanometer-scale memory cells with GHz operation frequencies.

cond-mat.mes-hall↗
arXiv2011

Regular atomic narrowing of Ni, Fe and V nanowires resolved by 2D correlation analysis

We present a novel statistical method for the study of stable atomic configurations in breaking nanowires based on the 2D cross-correlation analysis of conductance versus electrode separation traces. Applying this method, we can clearly resolve the typical evolutions of the conductance staircase in some transition metal nanojunctions (Ni, Fe, V) up to high conductance values. In these metals our analysis demonstrates a very well ordered atomic narrowing of the nanowire, indicating a very regular, stepwise decrease of the number of atoms in the minimal cross section of the junction, in contrast to the majority of the metals. All these features are hidden in traditional conductance histograms.

cond-mat.mes-hall↗
arXiv2011

From stochastic single atomic switch to nanoscale resistive memory device

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.

cond-mat.mes-hall↗
arXiv2011

Transition from coherent mesoscopic single particle transport to proximity Josephson-current

The transport through a metal-superconductor interface is governed by a special charge conversion process, the Andreev reflection, where each incident electron drags another electron with itself to form a Cooper pair. At the normal side a hole is left behind dressed by superconducting correlations. For a low transparency interface the simultaneous transfer of two charges is strongly suppressed leading to a reduced conductance. Here we demonstrate that this reduced conductance can be turned to an infinite one by tuning the nanoscale geometry. 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 initially the transmission through the interface is continuously enhanced. However, as the phase-coherent volume reaches the opposite surface of the thin film a resonator is formed, and a robust transition is induced due to Cooper pair condensation.

cond-mat.supr-con↗
arXiv2008

Huge negative differential conductance in Au-H2 molecular nanojunctions

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.

cond-mat.mes-hall↗
arXiv2008

Interaction of hydrogen molecules with superconducting nanojunctions

In this paper the interaction of hydrogen molecules with atomic-sized superconducting nanojunctions is studied. It is demonstrated by conductance histogram measurements that the superconductors niobium, tantalum and aluminum show a strong interaction with hydrogen, whereas for lead a slight interaction is observed, and for tin and indium no significant interaction is detectable. For Nb, Ta and Pb subgap method is applied to determine the transmission eigenvalues of the nanojunctions in hydrogen environment. It is shown, that in Nb and Ta the mechanical behavior of the junction is spectacularly influenced by hydrogen reflected by extremely long conductance traces, but the electronic properties based on the transmission eigenvalues are similar to those of pure junctions. Evidences for the formation of a single-molecule bridge between the electrodes -- as in recently studied platinum hydrogen system -- were not found.

cond-mat.mes-hall↗
arXiv2008

Nanoscale spin-polarization in dilute magnetic semiconductor (In,Mn)Sb

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.

cond-mat.other↗
arXiv2005

Pulling gold nanowires with a hydrogen clamp

In this paper an experimental study of the interaction of hydrogen molecules with gold nanowires is presented. Our results show, that chains of Au atoms can also be pulled in hydrogen environment, however in this case the conductance of the chain is strongly reduced compared to the perfect transmission of pure Au chains. The comparison of our experiments with recent theoretical prediction for the hydrogen welding of Au nanowires implies that a hydrogen molecule can even be incorporated in the gold nanocontact, and this hydrogen clamp is strong enough to pull a chain of gold atoms.

cond-mat.mes-hall↗
arXiv2004

Slow two-level systems in point contacts

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.

cond-mat.mes-hall↗
arXiv2004

Conductance of Pd-H nanojunctions

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.

cond-mat.mes-hall↗
arXiv2004

Hydrogen-assisted distortion of gold nanowires

In this paper the influence of adsorbed hydrogen on the behavior of gold nanojunctions is investigated. It is found, that the hydrogen environment has a strong effect on the conductance of atomic-sized gold junctions, which is markedly reflected by the growth of an additional peak in the conductance histogram at 0.5 quantum unit, 2e^2/h. The statistical analysis of the conductance traces implies that the new peak arises due to the hydrogen-assisted distortion of atomic gold chains.

cond-mat.mtrl-sci↗
arXiv2004

Quantum interference structures in the conductance plateaus of gold nanojunctions

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.

cond-mat.mes-hall↗
arXiv2003

Transition from tunneling to direct contact in tungsten nanojunctions

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.

cond-mat.mes-hall↗