arXiv subjects

M. Muller

Publications and source records attributed to M. Muller.

9 recordsLinked to original sources

arXiv2022

Data-driven design of new catalytic materials in methane oxidation based on a site isolation concept

The conversion of natural gas (methane) to ethane and ethylene (OCM: oxidative coupling of methane) facilitates its transportation and provides a way to synthesize higher value chemicals. The search for high-performance catalysts to achieve this conversion is the main scope of most corresponding studies in the field of OCM. Here, we present a general data-driven strategy for the search of novel catalytic materials, focusing particularly on materials useful for the OCM reaction. Our strategy is based on consistent experimental measurements and includes ab initio thermodynamics calculations and active screening. Based on our experiments, which showed unique volcano-type dependence of the performance on the stability of formed carbonates attributed to the site isolation concept, we developed a method for efficient and inexpensive DFT calculations of the formation energies of carbonates with prediction accuracy 0.2 eV. This method was implemented into a high-throughput screening scheme, which includes both general requirements for catalyst candidates and an actively done artificial intelligence part. Experimental validation of some of the candidates obtained during the screening showed successful reproduction of the initial volcano dependence. Moreover, several new materials were found to outperform standard OCM catalysts, specifically at lower temperatures.

cond-mat.mtrl-sci↗

arXiv2018

Ga$_{x}$Cu$_{4-x}$(OD)$_{6}$Cl$_{2}$: Insulating ground state in an electron doped kagome system

We present a successful synthesis method of the series Ga$_{x}$Cu$_{4-x}$(OD)$_{6}$Cl$_{2}$ with substitutions up to $x=0.8$. The compound remains a frustrated kagome system with an insulating ground state similar to herbertsmithite for these substitutions, as the additional charge of Ga$^{3+}$ is most likely bound to additional OH$^{-}$ and Cl$^{-}$. Besides infrared measurements, we present magnetic and specific-heat data down to 2 K for selected samples with $0\leq x\leq0.8$. With increasing $x$ the long-range magnetic order is suppressed, similar to what was observed in the series Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$Cl$_{2}$, indicating that Ga goes predominantly to the inter-plane position of the layered crystal structure. The reduction of the frozen magnetic fraction with increasing substitution was followed by $μ$SR measurements. $^{69,71}$Ga nuclear magnetic resonance (NMR) was applied as a local probe for Ga induced disorder. One well resolved Ga NMR line of moderate width is found for each isotope across the phase diagram which indicates a rather homogeneous distribution of the Ga isotopes on a single site in the lattice.

cond-mat.str-el↗

arXiv2017

THz electrical writing speed in an antiferromagnetic memory

The speed of writing of state-of-the-art ferromagnetic memories is physically limited by an intrinsic GHz threshold. Recently, an alternative research direction has been initiated by realizing memory devices based on antiferromagnets in which spin directions periodically alternate from one atomic lattice site to the next. In our work we experimentally demonstrate at room temperature that the speed of reversible electrical writing in a memory device can be scaled up to THz using an antiferromagnet. Efficient current-induced spin-torque mechanism is responsible for the switching in our memory devices throughout the twelve orders of magnitude range of writing speeds from Hz to THz. Our work opens the path towards the development of memory-logic technology reaching the elusive THz band.

physics.app-ph↗

arXiv2017

Emission of Linearly Polarized Single Photons from Quantum Dots Contained in Nonpolar, Semipolar, and Polar Sections of Pencil-Like InGaN/GaN Nanowires

A pencil-like morphology of homoepitaxially grown GaN nanowires is exploited for the fabrication of thin conformal intrawire InGaN nanoshells which host quantum dots in nonpolar, semipolar and polar crystal regions. All three quantum dot types exhibit single photon emission with narrow emission line widths and high degrees of linear optical polarization. The host crystal region strongly affects both single photon wavelength and emission lifetime, reaching subnanosecond time scales for the non- and semipolar quantum dots. Localization sites in the InGaN potential landscape, most likely induced by indium fluctuations across the InGaN nanoshell, are identified as the driving mechanism for the single photon emission. The hereby reported pencil-like InGaN nanoshell is the first single nanostructure able to host all three types of single photon sources and is, thus, a promising building block for tunable quantum light devices integrated into future photonic circuits.

cond-mat.mtrl-sci↗

arXiv2017

Blue-to-green single photons from InGaN/GaN dot-in-a-wire nanowire ordered arrays

Single-photon emitters (SPEs) are at the basis of many applications for quantum information management. Semiconductor-based SPEs are best suited for practical implementations because of high design flexibility, scalability and integration potential in practical devices. Single-photon emission from ordered arrays of InGaN nano-disks embedded in GaN nanowires is reported. Intense and narrow optical emission lines from quantum dot-like recombination centers are observed in the blue-green spectral range. Characterization by electron microscopy, cathodoluminescence and micro-photoluminescence indicate that single photons are emitted from regions of high In concentration in the nano-disks due to alloy composition fluctuations. Single-photon emission is determined by photon correlation measurements showing deep anti-bunching minima in the second-order correlation function. The present results are a promising step towards the realization of on-site/on-demand single-photon sources in the blue-green spectral range operating in the GHz frequency range at high temperatures.

cond-mat.mtrl-sci↗

arXiv2017

Open quantum generalisation of Hopfield neural networks

We propose a new framework to understand how quantum effects may impact on the dynamics of neural networks. We implement the dynamics of neural networks in terms of Markovian open quantum systems, which allows us to treat thermal and quantum coherent effects on the same footing. In particular, we propose an open quantum generalisation of the celebrated Hopfield neural network, the simplest toy model of associative memory. We determine its phase diagram and show that quantum fluctuations give rise to a qualitatively new non-equilibrium phase. This novel phase is characterised by limit cycles corresponding to high-dimensional stationary manifolds that may be regarded as a generalisation of storage patterns to the quantum domain.

cond-mat.dis-nn↗

arXiv2014

The Next Generation Virgo Cluster Survey. XV. The photometric redshift estimation for background sources

The Next Generation Virgo Cluster Survey is an optical imaging survey covering 104 deg^2 centered on the Virgo cluster. Currently, the complete survey area has been observed in the u*giz-bands and one third in the r-band. We present the photometric redshift estimation for the NGVS background sources. After a dedicated data reduction, we perform accurate photometry, with special attention to precise color measurements through point spread function-homogenization. We then estimate the photometric redshifts with the Le Phare and BPZ codes. We add a new prior which extends to iAB = 12.5 mag. When using the u*griz-bands, our photometric redshifts for 15.5 \le i \lesssim 23 mag or zphot \lesssim 1 galaxies have a bias |Δz| < 0.02, less than 5% outliers, and a scatter σ_{outl.rej.} and an individual error on zphot that increase with magnitude (from 0.02 to 0.05 and from 0.03 to 0.10, respectively). When using the u*giz-bands over the same magnitude and redshift range, the lack of the r-band increases the uncertainties in the 0.3 \lesssim zphot \lesssim 0.8 range (-0.05 < Δz < -0.02, σ_{outl.rej} ~ 0.06, 10-15% outliers, and zphot.err. ~ 0.15). We also present a joint analysis of the photometric redshift accuracy as a function of redshift and magnitude. We assess the quality of our photometric redshifts by comparison to spectroscopic samples and by verifying that the angular auto- and cross-correlation function w(θ) of the entire NGVS photometric redshift sample across redshift bins is in agreement with the expectations.

astro-ph.GA↗

arXiv1999

Topological effects in ring polymers (II): Influence of persistence length

The interplay of topological constraints and persistence length of ring polymers in their own melt is investigated by means of dynamical Monte Carlo simulations of a three dimensional lattice model. We ask if the results are consistent with an asymptotically regime where the rings behave like (compact) {\em lattice animals} in a self-consistent network of topological constraints imposed by neighbouring rings. Tuning the persistence length provides an efficient route to increase the ring overlap required for this mean-field picture to hold: The {\em effective} Flory exponent for the ring size decreases down to $ν\stackrel{<}{\sim} 1/3$ with increasing persistence length. Evidence is provided for the emergence of one additional characteristic length scale $\dtop \propto N^0$, only weakly dependent on the persistence length and much larger than the excluded volume screening length $ξ$. At distances larger than \dtop the conformational properties of the rings are governed by the topological interactions, at smaller distances rings and their linear chain counterparts become similar. (At distances smaller than $ξ$ both architectures are identical.) However, the crossover between both limits is intricate and broad as a detailed discussion of the local fractal dimension (e.g., obtained from the static structure factor) reveals.

cond-mat.stat-mech↗

arXiv1999

Measurements of the radiation hardness of selected scintillating and light guide fiber materials

Radiation hardness studies of KURARAY SCSF-78M scintillating fibers and clear fibers from KURARAY and pol.hi.tech. performed under different dose rate conditions in proton and electron beams are summarized. For high dose rates in-situ measurements of the fiber light output were done. During several months after irradiation all fibers were measured concerning light emission and transparency. Fibers irradiated at high rates to about 1 Mrad are clearly damaged but recover within a few hours up to several weeks. Using smaller rates up to the same integral dose a decrease of the light output of scintillating fibers of up to 30% can not be excluded. Clear fibers seem to be uneffected up to 400 krad. No significant influence of fiber coverage and atmosphere during irradiation was found.

physics.ins-det↗