arXiv subjects

Michael Huth

Publications and source records attributed to Michael Huth.

54 records · Page 3Linked to original sources

arXiv2014

Disorder-induced gap in the normal density of states of the organic superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br

The density of states of the organic superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, measured by scanning tunneling spectroscopy on \textit{in-situ} cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature $T_\mathrm{c}$. A soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder exactly describes the experimentally observed suppression. The electronic disorder also explains the diminished coherence peaks of the quasiparticle density of states below $T_\mathrm{c}$.

cond-mat.str-el↗

arXiv2014

Fatal Attractors in Parity Games: Building Blocks for Partial Solvers

Attractors in parity games are a technical device for solving "alternating" reachability of given node sets. A well known solver of parity games - Zielonka's algorithm - uses such attractor computations recursively. We here propose new forms of attractors that are monotone in that they are aware of specific static patterns of colors encountered in reaching a given node set in alternating fashion. Then we demonstrate how these new forms of attractors can be embedded within greatest fixed-point computations to design solvers of parity games that run in polynomial time but are partial in that they may not decide the winning status of all nodes in the input game. Experimental results show that our partial solvers completely solve benchmarks that were constructed to challenge existing full solvers. Our partial solvers also have encouraging run times in practice. For one partial solver we prove that its run-time is at most cubic in the number of nodes in the parity game, that its output game is independent of the order in which monotone attractors are computed, and that it solves all Buechi games and weak games. We then define and study a transformation that converts partial solvers into more precise partial solvers, and we prove that this transformation is sound under very reasonable conditions on the input partial solvers. Noting that one of our partial solvers meets these conditions, we apply its transformation on 1.6 million randomly generated games and so experimentally validate that the transformation can be very effective in increasing the precision of partial solvers.

cs.LO↗

arXiv2014

Probing near-interface ferroelectricity by conductance modulation of a nano-granular metal

The electronic functionality of thin films is governed by their interfaces. This is very important for the ferroelectric (FE) state which depends on thin-film clamping and interfacial charge transfer. Here we show that in a heterostructure consisting of a nano-granular metal and an organic FE layer of [tetrathiafulvalene]$^{+δ}$[p-chloranil]$^{-δ}$ the nano-granular layer's conductance provides a sensitive and non-invasive probe of the temperature-dependent dielectric properties of the FE layer. We provide a theoretical framework that is able to qualitatively reproduce the observed conductance changes taking the anisotropy of the dielectric anomaly at the paraelectric(PE)-FE phase transition into account. The approach is also suitable for observing dynamical effects close to the phase transition. Focused electron beam induced deposition as fabrication method for the nano-granular metal guarantees excellent down-scaling capabilities, so that monitoring the FE state on the lateral scale down to 20--30\,nm can be envisioned.

cond-mat.mes-hall↗

arXiv2014

Dielectric sensing by charging energy modulation in a nano-granular metal

Several sensing concepts using nanostructures prepared by focused electron beam induced deposition have been developed over the last years. Following work on highly miniaturized Hall sensors for magnetic sensing with soft magnetic Co, strain and force sensing based on nano-granular platinum-carbon structures (Pt(C)) was introduced. Very recently the capability of nano-granular Pt(C) structures to detect the presence of adsorbate water layers by conductance modulations was demonstrated. For magnetic and strain sensing the underlying physical mechanisms of the sensing effect have been analyzed in detail and are now quite well understood. This is not the case for the adsorbate layer induced conductance modulation effect. Here we provide a theoretical framework that allows for a semi-quantitative understanding of the observed water-sensing effect. We show how the near-interface renormalization of the Coulomb charging energy in the nano-granular metal caused by the dielectric screening of the polarizable adsorbate layer leads to a conductance modulation. The model can account for the conductance modulation observed in the water adsorbate experiments and can also be applied to understand similar effects caused by near-interface dielectric anomalies of ferroelectric thin films on top of nano-granular Pt(C). Our findings provide a pathway towards optimized nano-granular layer structures suitable for a wide range of dielectric or local potential sensing applications.

cond-mat.mes-hall↗

arXiv2014

Superconductivity in the system $\rm Mo_xC_yGa_zO_δ$ prepared by focused ion beam induced deposition

We have prepared the new amorphous superconductor $\rm Mo_xC_yGa_zO_δ$ with a maximum critical temperature $T_c$ of 3.8\,K by the direct-write nano-patterning technique of focused (gallium) ion beam induced deposition (FIBID) using $\rm Mo(CO)_6$ as precursor gas. From a detailed analysis of the temperature-dependent resistivity and the upper critical field we found clear evidence for proximity of the samples to a disorder-induced metal-insulator transition. We observed a strong dependence of $T_c$ on the deposition parameters and identified clear correlations between $T_c$, the localization tendency visible in the resistance data and the sample composition. By an in-situ feedback-controlled optimization process in the FIB-induced growth we were able to identify the beam parameters which lead to samples with the largest $T_c$-value and sharpest transition into the superconducting state.

cond-mat.supr-con↗

arXiv2013

In-situ growth optimization in focused electron-beam induced deposition

We present the application of an evolutionary genetic algorithm for the in-situ optimization of nanostructures prepared by focused electron-beam-induced deposition. It allows us to tune the properties of the deposits towards highest conductivity by using the time gradient of the measured in-situ rate of change of conductance as fitness parameter for the algorithm. The effectiveness of the procedure is presented for the precursor W(CO)6 as well as for post-treatment of Pt-C deposits obtained by dissociation of MeCpPt(Me)3. For W(CO)6-based structures an increase of conductivity by one order of magnitude can be achieved, whereas the effect for MeCpPt(Me)3 is largely suppressed. The presented technique can be applied to all beam-induced deposition processes and has great potential for further optimization or tuning of parameters for nanostrucures prepared by FEBID or related techniques.

cond-mat.mtrl-sci↗

arXiv2013

Magnetoresistance of granular Pt-C nanostructures close to the metal-insulator-transition

We investigate the electrical and magneto-transport properties of Pt-C granular metals prepared by focused-electron-beam induced deposition. In particular, we consider samples close to the metal-insulator-transition obtained from as-grown deposits by means of a low- energy electron irradiation treatment. The temperature dependence of the conductivity shows a lnT behavior with a transition to square root of T at low temperature, as expected for systems in the strong-coupling tunneling regime. The magnetoresistance is positive and is described within the wave-function shrinkage model, normally used for disordered system in the weak-coupling regime. In order to fit the experimental data spin-dependent tunneling has to be taken into account. In the discussion we attribute the origin of the spin-dependency to confinement effects of Pt nano-grains embedded in the carbon matrix.

cond-mat.mes-hall↗

arXiv2013

The Rabin index of parity games

We study the descriptive complexity of parity games by taking into account the coloring of their game graphs whilst ignoring their ownership structure. Colored game graphs are identified if they determine the same winning regions and strategies, for all ownership structures of nodes. The Rabin index of a parity game is the minimum of the maximal color taken over all equivalent coloring functions. We show that deciding whether the Rabin index is at least k is in PTIME for k=1 but NP-hard for all fixed k > 1. We present an EXPTIME algorithm that computes the Rabin index by simplifying its input coloring function. When replacing simple cycle with cycle detection in that algorithm, its output over-approximates the Rabin index in polynomial time. Experimental results show that this approximation yields good values in practice.

cs.GT↗

arXiv2012

Spontaneous Dissociation of Co2(CO)8 and Autocatalytic growth of Co on SiO2 : A Combined Experimental and Theoretical Investigation

We present experimental results and theoretical simulations of the adsorption behavior of the metal-organic precursor Co2(CO)8 on SiO2 surfaces after application of two different pre-treatment steps, namely by air plasma cleaning or a focused electron beam pre-irradiation. We observe a spontaneous dissociation of the precursor molecules as well as auto-deposition of cobalt on the pre-treated SiO2 surfaces. We also find that the differences in metal content and relative stability of these deposits depend on the pre-treatment conditions of the substrate. Transport measurements of these deposits are also presented. We are led to assume that the degree of passivation of the SiO2 surface by hydroxyl groups is an important controlling factor in the dissociation process. Our calculations of various slab settings using dispersion corrected density functional theory support this assumption. We observe physisorption of the precursor molecule on a fully hydroxylated SiO2 surface (untreated surface) and chemisorption on a partially hydroxylated SiO2 surface (pre-treated surface) with a spontaneous dissociation of the precursor molecule. In view of these calculations, we discuss the origin of this dissociation and the subsequent autocatalysis.

cond-mat.mtrl-sci↗

arXiv2012

Josephson effect in a $CeCoIn_5$ microbridge

We report DC Josephson effects observed in a microbridge prepared from an individual crystalline growth domain of $CeCoIn_5$ thin film. Josephson effects were observed by periodic voltage modulations under external magnetic field $ΔV(B)$ with the expected periodicity and by the temperature dependence of the Josephson critical current $I_c(T)$. The shape of $ΔV(B)$ was found to be asymmetric, as it is expected for microbridges. The dependence $I_c(T)$ follows the Ambegaokar-Baratoff relation, which is unexpected for microbridges. Features in the dynamical resistance curves were attributed to the periodic motion of Abricosov vortices within the microbridge.

cond-mat.str-el↗

arXiv2011

Evidence for universal conductance correction in a tunable strongly coupled nanogranular metal

We present temperature-dependent conductivity data obtained on a sample set of nanogranular Pt-C with finely tuned inter-grain tunnel coupling strength g. For samples in the strong-coupling regime g > g_C, characterized by a finite conductivity for T -> 0, we find a logarithmic behavior at elevated temperatures and a crossover to a sqrt(T)-behavior at low temperatures over a wide range of coupling strengths g_C = 0.25 < g <= 3. The experimental observation for g > 1 is in very good agreement with recent theoretical findings on ordered granular metals in three spatial dimensions. The results indicate a validity of the predicted universal conductivity behavior that goes beyond the immediate range of the approach used in the theoretical derivation.

cond-mat.mes-hall↗

arXiv2011

Josephson effect in $CeCoIn_5$ microbridges as seen via quantum interferometry

A superconducting quantum interference device (SQUID) was prepared on a micron-sized single crystal using a selected growth domain of a thin film of $CeCoIn_5$ grown by molecular beam epitaxy. SQUID voltage oscillations of good quality were obtained as well as interference effects stemming from the individual Josephson microbridges. The transport characteristics in the superconducting state exhibited several peculiarities which we ascribe to the periodic motion of vortices in the microbridges. The temperature dependence of the Josephson critical current shows good correspondence to the Ambegaokar-Baratoff relation, expected for the ideal Josephson junction. The results indicate a promising pathway to identify the type of order parameter in $CeCoIn_5$ by means of phase-sensitive measurements on microbridges.

cond-mat.str-el↗

arXiv2011

Influence of the substrate-induced strain and irradiation disorder on the Peierls transition in TTF-TCNQ microdomains

The influence of the combined effects of substrate-induced strain, finite size and electron irradiation-induced defects have been studied on individual micron-sized domains of the organic charge transfer compound tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) by temperature-dependent conductivity and current-voltage measurements. The individual domains have been isolated by focused ion beam etching and electrically contacted by focused ion and electron beam induced deposition of metallic contacts. The temperature-dependent conductivity follows a variable range hopping behavior which shows a crossover of the exponent as the Peierls transition is approached. The low temperature behavior is analyzed within the segmented rod model of Fogler, Teber and Shklowskii, as originally developed for a charge-ordered quasi one-dimensional electron crystal. The results are compared with data obtained on as-grown and electron irradiated epitaxial TTF-TCNQ thin films of the two-domain type.

cond-mat.mtrl-sci↗

arXiv2011

Vortex lattice symmetry break in nanostructured Nb thin films

An advanced mask-less nanofabrication technique, focused electron beam-induced deposition (FEBID), has been employed on epitaxial Nb thin films for their ferromagnetic decoration by an array of Co stripes. These substantially modify the non-patterned films' superconducting properties, providing a washboard-like pinning potential landscape for the vortex motion. At small magnetic fields B<0.1T, vortex lattice matching effects have been investigated by magneto-transport measurements. Peculiarities in the field dependencies of the films resistivity R(B) have been observed in particular for the vortex motion perpendicular to the Co stripes. The deduced field values correspond to the vortex lattice parameter matching the pinning structure's period, whereas no fields matching the stripe width have been observed, as it was reported previously [D. Jaque et al., Appl. Phys. Lett. 81, 2851 (2002)] for Nb films grown on periodically distributed submicrometric lines of Ni.

cond-mat.supr-con↗

arXiv2010

Defect-induced shift of the Peierls transition in TTF-TCNQ thin films

In this paper we investigate the influence of the substrate material and film thickness on the Peierls transition temperature in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) thin films, grown by physical vapor deposition. Our analysis shows that the substrate material and the growth conditions strongly influence the film morphology. In particular, we demonstrate that the Peierls transition temperature in thin films is lower than in TTF-TCNQ single crystals. We argue that this effect arises due to defects, which emerge in TTF-TCNQ thin films during the growth process.

cond-mat.mtrl-sci↗

arXiv2010

Magnetization dynamics of a CrO$_2$ grain studied by micro-Hall magnetometry

Micro-Hall magnetometry is employed to study the magnetization dynamics of a single, micron-size CrO$_2$ grain. With this technique we track the motion of a single domain wall, which allows us to probe the distribution of imperfections throughout the material. An external magnetic field along the grain's easy magnetization direction induces magnetization reversal, giving rise to a series of sharp jumps in magnetization. Supported by micromagnetic simulations, we identify the transition to a state with a single cross-tie domain wall, where pinning/depinning of the wall results in stochastic Barkhausen jumps.

cond-mat.str-el↗

arXiv2009

Organic charge-transfer phase formation in thin films of the BEDT-TTF/TCNQ donor-acceptor system

We have performed charge transfer phase formation studies on the donor/acceptor system bis(ethylendithio)tetrathiafulvalene,(BEDT-TTF)/tetracyanoquinodimethane,(TCNQ) by means of physical vapor deposition. We prepared donor/acceptor bilayer structures on glass and Si(100)/SiO_2 substrates held at room temperature and analyzed the layer structures by optical microscopy, X-ray diffraction and focused ion beam cross sectioning before and after annealing. We found clear evidence for the formation of a charge transfer phase during the annealing procedure. For the as-grown samples we could not detect the occurrence of a charge transfer phase. X-ray diffraction indicated that the monoclinic variant of the (BEDT-TTF)-TCNQ was formed. This was further corroborated by single-source evaporation experiments from pre-reacted (BEDT-TTF)-TCNQ obtained from solution growth, and in particular from co-evaporation experiments of (BEDT-TTF)-TCNQ and TCNQ. In the course of these experiments we found that (0,l,l)-oriented BEDT-TTF layers can be prepared on alpha-Al_2O_3 (1,1,-2,0) substrates at about 100 C using (BEDT-TTF)-TCNQ as source material. We speculate that due to its high vapor pressure the TCNQ component serves as a carrier gas for BEDT-TTF vapor phase transport.

cond-mat.mtrl-sci↗

arXiv2006

Labelled transition systems as a Stone space

A fully abstract and universal domain model for modal transition systems and refinement is shown to be a maximal-points space model for the bisimulation quotient of labelled transition systems over a finite set of events. In this domain model we prove that this quotient is a Stone space whose compact, zero-dimensional, and ultra-metrizable Hausdorff topology measures the degree of bisimilarity such that image-finite labelled transition systems are dense. Using this compactness we show that the set of labelled transition systems that refine a modal transition system, its ''set of implementations'', is compact and derive a compactness theorem for Hennessy-Milner logic on such implementation sets. These results extend to systems that also have partially specified state propositions, unify existing denotational, operational, and metric semantics on partial processes, render robust consistency measures for modal transition systems, and yield an abstract interpretation of compact sets of labelled transition systems as Scott-closed sets of modal transition systems.

cs.LO↗