arXiv subjects

Andreas Bauer

Publications and source records attributed to Andreas Bauer.

84 records · Page 5Linked to original sources

arXiv2014

Scanning probe microscopy in an ultra-low vibration closed-cycle cryostat

We report on state-of-the-art scanning probe microscopy measurements performed in a pulse tube based top-loading closed-cycle cryostat with a base temperature of 4 K and a 9 T magnet. We decoupled the sample space from the mechanical and acoustic noise from the cryocooling system to enable scanning probe experiments. The extremely low vibration amplitudes in our system enabled successful imaging of 0.39 nm lattice steps on single crystalline SrTiO$_{3}$ as well as magnetic vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ superconductor. Fine control over sample temperature and applied magnetic field further enabled us to probe the helimagnetic and the skyrmion-lattice phases in Fe$_{0.5}$Co$_{0.5}$Si with unprecedented signal-to-noise ratio of 20:1. Finally, we demonstrate for the first time quartz-crystal tuning fork shear-force microscopy in a closed-cycle cryostat.

cond-mat.mtrl-sci↗

arXiv2013

Event based classification of Web 2.0 text streams

Web 2.0 applications like Twitter or Facebook create a continuous stream of information. This demands new ways of analysis in order to offer insight into this stream right at the moment of the creation of the information, because lots of this data is only relevant within a short period of time. To address this problem real time search engines have recently received increased attention. They take into account the continuous flow of information differently than traditional web search by incorporating temporal and social features, that describe the context of the information during its creation. Standard approaches where data first get stored and then is processed from a peristent storage suffer from latency. We want to address the fluent and rapid nature of text stream by providing an event based approach that analyses directly the stream of information. In a first step we want to define the difference between real time search and traditional search to clarify the demands in modern text filtering. In a second step we want to show how event based features can be used to support the tasks of real time search engines. Using the example of Twitter we present in this paper a way how to combine an event based approach with text mining and information filtering concepts in order to classify incoming information based on stream features. We calculate stream dependant features and feed them into a neural network in order to classify the text streams. We show the separative capabilities of event based features as the foundation for a real time search engine.

cs.IR↗

arXiv2013

Specific heat of the skyrmion lattice phase and field-induced tricritical point in MnSi

We report high-precision measurements of the temperature and magnetic field dependence of the specific heat, C(T,H), across the magnetic phase diagram of MnSi. Clear anomalies establish the skyrmion lattice unambiguously as a thermodynamic phase. The evolution of the specific heat anomalies, the field dependence of the entropy released at the phase transitions, and the temperature versus field dependence of crossover lines provide striking evidence of a tricritical point at an internal magnetic field of 340mT and temperature of 28.5K. The existence of this tricritical point represents strong support of a helimagnetic Brazovskii transition, i.e., a fluctuation-induced first order transition at H = 0.

cond-mat.str-el↗

arXiv2013

From propositional to first-order monitoring

The main purpose of this paper is to introduce a first-order temporal logic, LTLFO, and a corresponding monitor construction based on a new type of automaton, called spawning automaton. Specifically, we show that monitoring a specification in LTLFO boils down to an undecidable decision problem. The proof of this result revolves around specific ideas on what we consider a "proper" monitor. As these ideas are general, we outline them first in the setting of standard LTL, before lifting them to the setting of first-order logic and LTLFO. Although due to the above result one cannot hope to obtain a complete monitor for LTLFO, we prove the soundness of our automata-based construction and give experimental results from an implementation. These seem to substantiate our hypothesis that the automata-based construction leads to efficient runtime monitors whose size does not grow with increasing trace lengths (as is often observed in similar approaches). However, we also discuss formulae for which growth is unavoidable, irrespective of the chosen monitoring approach.

cs.LO↗

arXiv2012

Reasoning with Data-Centric Business Processes

We describe an approach to modelling and reasoning about data-centric business processes and present a form of general model checking. Our technique extends existing approaches, which explore systems only from concrete initial states. Specifically, we model business processes in terms of smaller fragments, whose possible interactions are constrained by first-order logic formulae. In turn, process fragments are connected graphs annotated with instructions to modify data. Correctness properties concerning the evolution of data with respect to processes can be stated in a first-order branching-time logic over built-in theories, such as linear integer arithmetic, records and arrays. Solving general model checking problems over this logic is considerably harder than model checking when a concrete initial state is given. To this end, we present a tableau procedure that reduces these model checking problems to first-order logic over arithmetic. The resulting proof obligations are passed on to appropriate "off-the-shelf" theorem provers. We also detail our modelling approach, describe the reasoning components and report on first experiments.

cs.LO↗

arXiv2012

Fluctuation-induced first-order phase transition in Dzyaloshinskii-Moriya helimagnets

Two centuries of research on phase transitions have repeatedly highlighted the importance of critical fluctuations that abound in the vicinity of a critical point. They are at the origin of scaling laws obeyed by thermodynamic observables close to second-order phase transitions resulting in the concept of universality classes, that is of paramount importance for the study of organizational principles of matter. Strikingly, in case such soft fluctuations are too abundant they may alter the nature of the phase transition profoundly; the system might evade the critical state altogether by undergoing a discontinuous first-order transition into the ordered phase. Fluctuation-induced first-order transitions have been discussed broadly and are germane for superconductors, liquid crystals, or phase transitions in the early universe, but clear experimental confirmations remain scarce. Our results from neutron scattering and thermodynamics on the model Dzyaloshinskii-Moriya (DM) helimagnet (HM) MnSi show that such a fluctuation-induced first-order transition is realized between its paramagnetic and HM state with remarkable agreement between experiment and a theory put forward by Brazovskii. While our study clarifies the nature of the HM phase transition in MnSi that has puzzled scientists for several decades, more importantly, our conclusions entirely based on symmetry arguments are also relevant for other DM-HMs with only weak cubic magnetic anisotropies. This is in particular noteworthy in light of a wide range of recent discoveries that show that DM helimagnetism is at the heart of problems such as topological magnetic order, multiferroics, and spintronics.

cond-mat.str-el↗

arXiv2012

Subsonic turbulence in smoothed particle hydrodynamics and moving-mesh simulations

Highly supersonic, compressible turbulence is thought to be of tantamount importance for star formation processes in the interstellar medium. Likewise, cosmic structure formation is expected to give rise to subsonic turbulence in the intergalactic medium, which may substantially modify the thermodynamic structure of gas in virialized dark matter halos and affect small-scale mixing processes in the gas. Numerical simulations have played a key role in characterizing the properties of astrophysical turbulence, but thus far systematic code comparisons have been restricted to the supersonic regime, leaving it unclear whether subsonic turbulence is faithfully represented by the numerical techniques commonly employed in astrophysics. Here we focus on comparing the accuracy of smoothed particle hydrodynamics (SPH) and our new moving-mesh technique AREPO in simulations of driven subsonic turbulence. To make contact with previous results, we also analyze simulations of transsonic and highly supersonic turbulence. We find that the widely employed standard formulation of SPH yields problematic results in the subsonic regime. Instead of building up a Kolmogorov-like turbulent cascade, large-scale eddies are quickly damped close to the driving scale and decay into small-scale velocity noise. Reduced viscosity settings improve the situation, but the shape of the dissipation range differs compared with expectations for a Kolmogorov cascade. In contrast, our moving-mesh technique does yield power-law scaling laws for the power spectra of velocity, vorticity and density, consistent with expectations for fully developed isotropic turbulence. We show that large errors in SPH's gradient estimate and the associated subsonic velocity noise are ultimately responsible for producing inaccurate results in the subsonic regime. In contrast, SPH's performance is much better for supersonic turbulence. [Abridged]

astro-ph.CO↗

arXiv2012

Decentralised LTL Monitoring

Users wanting to monitor distributed or component-based systems often perceive them as monolithic systems which, seen from the outside, exhibit a uniform behaviour as opposed to many components displaying many local behaviours that together constitute the system's global behaviour. This level of abstraction is often reasonable, hiding implementation details from users who may want to specify the system's global behaviour in terms of an LTL formula. However, the problem that arises then is how such a specification can actually be monitored in a distributed system that has no central data collection point, where all the components' local behaviours are observable. In this case, the LTL specification needs to be decomposed into sub-formulae which, in turn, need to be distributed amongst the components' locally attached monitors, each of which sees only a distinct part of the global behaviour. The main contribution of this paper is an algorithm for distributing and monitoring LTL formulae, such that satisfac- tion or violation of specifications can be detected by local monitors alone. We present an implementation and show that our algorithm introduces only a minimum delay in detecting satisfaction/violation of a specification. Moreover, our practical results show that the communication overhead introduced by the local monitors is considerably lower than the number of messages that would need to be sent to a central data collection point.

cs.SE↗

arXiv2011

Magnetohydrodynamics on an unstructured moving grid

Magnetic fields play an important role in astrophysics on a wide variety of scales, ranging from the Sun and compact objects to galaxies and galaxy clusters. Here we discuss a novel implementation of ideal magnetohydrodynamics (MHD) in the moving mesh code AREPO which combines many of the advantages of Eulerian and Lagrangian methods in a single computational technique. The employed grid is defined as the Voronoi tessellation of a set of mesh-generating points which can move along with the flow, yielding an automatic adaptivity of the mesh and a substantial reduction of advection errors. Our scheme solves the MHD Riemann problem in the rest frame of the Voronoi interfaces using the HLLD Riemann solver. To satisfy the divergence constraint of the magnetic field in multiple dimensions, the Dedner divergence cleaning method is applied. In a set of standard test problems we show that the new code produces accurate results, and that the divergence of the magnetic field is kept sufficiently small to closely preserve the correct physical solution. We also apply the code to two first application problems, namely supersonic MHD turbulence and the spherical collapse of a magnetized cloud. We verify that the code is able to handle both problems well, demonstrating the applicability of this MHD version of AREPO to a wide range of problems in astrophysics.

astro-ph.IM↗

arXiv2010

Monitorability of $ω$-regular languages

Arguably, omega-regular languages play an important role as a specification formalism in many approaches to systems monitoring via runtime verification. However, since their elements are infinite words, not every omega-regular language can sensibly be monitored at runtime when only a finite prefix of a word, modelling the observed system behaviour so far, is available. The monitorability of an omega-regular language, L, is thus a property that holds, if for any finite word u, observed so far, it is possible to add another finite word v, such that uv becomes a "finite witness" wrt. L; that is, for any infinite word w, we have that uvw \in L, or for any infinite word w, we have that uvw \not\in L. This notion has been studied in the past by several authors, and it is known that the class of monitorable languages is strictly more expressive than, e.g., the commonly used class of so-called safety languages. But an exact categorisation of monitorable languages has, so far, been missing. Motivated by the use of linear-time temporal logic (LTL) in many approaches to runtime verification, this paper first determines the complexity of the monitorability problem when L is given by an LTL formula. Further, it then shows that this result, in fact, transfers to omega-regular languages in general, i.e., whether they are given by an LTL formula, a nondeterministic Buechi automaton, or even by an omega-regular expression.

cs.FL↗

arXiv2009

A decidable policy language for history-based transaction monitoring

Online trading invariably involves dealings between strangers, so it is important for one party to be able to judge objectively the trustworthiness of the other. In such a setting, the decision to trust a user may sensibly be based on that user's past behaviour. We introduce a specification language based on linear temporal logic for expressing a policy for categorising the behaviour patterns of a user depending on its transaction history. We also present an algorithm for checking whether the transaction history obeys the stated policy. To be useful in a real setting, such a language should allow one to express realistic policies which may involve parameter quantification and quantitative or statistical patterns. We introduce several extensions of linear temporal logic to cater for such needs: a restricted form of universal and existential quantification; arbitrary computable functions and relations in the term language; and a "counting" quantifier for counting how many times a formula holds in the past. We then show that model checking a transaction history against a policy, which we call the history-based transaction monitoring problem, is PSPACE-complete in the size of the policy formula and the length of the history. The problem becomes decidable in polynomial time when the policies are fixed. We also consider the problem of transaction monitoring in the case where not all the parameters of actions are observable. We formulate two such "partial observability" monitoring problems, and show their decidability under certain restrictions.

cs.LO↗

arXiv2007

AutoMoDe - Model-Based Development of Automotive Software

This paper describes first results from the AutoMoDe (Automotive Model-Based Development) project. The overall goal of the project is to develop an integrated methodology for model-based development of automotive control software, based on problem-specific design notations with an explicit formal foundation. Based on the existing AutoFOCUS framework, a tool prototype is being developed in order to illustrate and validate the key elements of our approach.

cs.SE↗