Magnetic Cluster Excitations

Magnetic clusters, i.e., assemblies of a finite number (between two or three and several hundred) of interacting spin centers which are magnetically decoupled from their environment, can be found in many materials ranging from inorganic compounds, magnetic molecules, artificial metal structures formed on surfaces to metalloproteins. The magnetic excitation spectra in them are determined by the nature of the spin centers, the nature of the magnetic interactions, and the particular arrangement of the mutual interaction paths between the spin centers. Small clusters of up to four magnetic ions are ideal model systems to examine the fundamental magnetic interactions which are usually dominated by Heisenberg exchange, but often complemented by anisotropic and/or higher-order interactions. In large magnetic clusters which may potentially deal with a dozen or more spin centers, the possibility of novel many-body quantum states and quantum phenomena are in focus. In this review the necessary theoretical concepts and experimental techniques to study the magnetic cluster excitations and the resulting characteristic magnetic properties are introduced, followed by examples of small clusters demonstrating the enormous amount of detailed physical information which can be retrieved. The current understanding of the excitations and their physical interpretation in the molecular nanomagnets which represent large magnetic clusters is then presented, with an own section devoted to the subclass of the single-molecule magnets which are distinguished by displaying quantum tunneling of the magnetization. Finally, some quantum many-body states are summarized which evolve in magnetic insulators characterized by built-in or field-induced magnetic clusters. The review concludes addressing future perspectives in the field of magnetic cluster excitations.Comment: 59 pages, 64 figures, to appear in Rev. Mod. Phy

High-Frequency Electron-Spin-Resonance Study of the Octanuclear Ferric Wheel CsFe8_8

High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 T as well as Q-band ($f$ = 34.1 GHz) EPR were performed on single crystals of the molecular wheel CsFe$_8$. In this molecule, eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic (AF) Heisenberg exchange interactions, form a nearly perfect ring. The angle-dependent EPR data allow for the accurate determination of the spin Hamiltonian parameters of the lowest spin multiplets with $S \leq$ 4. Furthermore, the data can well be reproduced by a dimer model with a uniaxial anisotropy term, with only two free parameters $J$ and $D$. A fit to the dimer model yields $J$ = -15(2) cm$^{-1}$ and $D$ = -0.3940(8) cm$^{-1}$. A rhombic anisotropy term is found to be negligibly small, $E$ = 0.000(2) cm$^{-1}$. The results are in excellent agreement with previous inelastic neutron scattering (INS) and high-field torque measurements. They confirm that the CsFe$_8$ molecule is an excellent experimental model of an AF Heisenberg ring. These findings are also important within the scope of further investigations on this molecule such as the exploration of recently observed magnetoelastic instabilities.Comment: 21 pages, 8 figures, accepted for publication in Inorganic Chemistr

Strict Wick-type deformation quantization on Riemann surfaces: Rigidity and Obstructions

Let $X$ be a hyperbolic Riemann surface. We study a convergent Wick-type star product $\star_X$ on $X$ which is induced by the canonical convergent star product $\star_{\mathbb{D}}$ on the unit disk $\mathbb{D}$ via Uniformization Theory. While by construction, the resulting Fr\'echet algebras $(\mathcal{A}(X),\star_X)$ are strongly isomorphic for conformally equivalent Riemann surfaces, our work exhibits additional severe topological obstructions. In particular, we show that the Fr\'echet algebra $(\mathcal{A}(X),\star_X)$ degenerates if and only if the connectivity of $X$ is at least $3$, and $(\mathcal{A}(X),\star_X)$ is noncommutative if and only if $X$ is simply connected. We also explicitly determine the algebra $\mathcal{A}_X$ and the star product $\star_X$ for the intermediate case of doubly connected Riemann surfaces $X$. As a perhaps surprinsing result, we deduce that two such Fr\'echet algebras are strongly isomorphic if and only if either both Riemann surfaces are conformally equivalent to an (not neccesarily the same) annulus or both are conformally equivalent to a punctured disk.Comment: References update

TETRA: a web-service and a stand-alone program for the analysis and comparison of tetranucleotide usage patterns in DNA sequences

BACKGROUND: In the emerging field of environmental genomics, direct cloning and sequencing of genomic fragments from complex microbial communities has proven to be a valuable source of new enzymes, expanding the knowledge of basic biological processes. The central problem of this so called metagenome-approach is that the cloned fragments often lack suitable phylogenetic marker genes, rendering the identification of clones that are likely to originate from the same genome difficult or impossible. In such cases, the analysis of intrinsic DNA-signatures like tetranucleotide frequencies can provide valuable hints on fragment affiliation. With this application in mind, the TETRA web-service and the TETRA stand-alone program have been developed, both of which automate the task of comparative tetranucleotide frequency analysis. Availability: RESULTS: TETRA provides a statistical analysis of tetranucleotide usage patterns in genomic fragments, either via a web-service or a stand-alone program. With respect to discriminatory power, such an analysis outperforms the assignment of genomic fragments based on the (G+C)-content, which is a widely-used sequence-based measure for assessing fragment relatedness. While the web-service is restricted to the calculation of correlation coefficients between tetranucleotide usage patterns of submitted DNA sequences, the stand-alone program generates a much more detailed output, comprising all raw data and graphical plots. The stand-alone program is controlled via a graphical user interface and can batch-process a multitude of sequences. Furthermore, it comes with pre-computed tetranucleotide usage patterns for 166 prokaryote chromosomes, providing a useful reference dataset and source for data-mining. CONCLUSIONS: Up to now, the analysis of skewed oligonucleotide distributions within DNA sequences is not a commonly used tool within metagenomics. With the TETRA web-service and stand-alone program, the method is now accessible in an easy to use manner for a broad audience. This will hopefully facilitate the interrelation of genomic fragments from metagenome libraries, ultimately leading to new insights into the genetic potentials of yet uncultured microorganisms

Quantifying the effect of environment stability on the transcription factor repertoire of marine microbes

Background: DNA-binding transcription factors (TFs) regulate cellular functions in prokaryotes, often in response to environmental stimuli. Thus, the environment exerts constant selective pressure on the TF gene content of microbial communities. Recently a study on marine Synechococcus strains detected differences in their genomic TF content related to environmental adaptation, but so far the effect of environmental parameters on the content of TFs in bacterial communities has not been systematically investigated. Results: We quantified the effect of environment stability on the transcription factor repertoire of marine pelagic microbes from the Global Ocean Sampling (GOS) metagenome using interpolated physico-chemical parameters and multivariate statistics. Thirty-five percent of the difference in relative TF abundances between samples could be explained by environment stability. Six percent was attributable to spatial distance but none to a combination of both spatial distance and stability. Some individual TFs showed a stronger relationship to environment stability and space than the total TF pool. Conclusions: Environmental stability appears to have a clearly detectable effect on TF gene content in bacterioplanktonic communities described by the GOS metagenome. Interpolated environmental parameters were shown to compare well to in situ measurements and were essential for quantifying the effect of the environment on the TF content. It is demonstrated that comprehensive and well-structured contextual data will strongly enhance our ability to interpret the functional potential of microbes from metagenomic data

3D-MuPPET: 3D Multi-Pigeon Pose Estimation and Tracking

Markerless methods for animal posture tracking have been developing recently, but frameworks and benchmarks for tracking large animal groups in 3D are still lacking. To overcome this gap in the literature, we present 3D-MuPPET, a framework to estimate and track 3D poses of up to 10 pigeons at interactive speed using multiple-views. We train a pose estimator to infer 2D keypoints and bounding boxes of multiple pigeons, then triangulate the keypoints to 3D. For correspondence matching, we first dynamically match 2D detections to global identities in the first frame, then use a 2D tracker to maintain correspondences accross views in subsequent frames. We achieve comparable accuracy to a state of the art 3D pose estimator for Root Mean Square Error (RMSE) and Percentage of Correct Keypoints (PCK). We also showcase a novel use case where our model trained with data of single pigeons provides comparable results on data containing multiple pigeons. This can simplify the domain shift to new species because annotating single animal data is less labour intensive than multi-animal data. Additionally, we benchmark the inference speed of 3D-MuPPET, with up to 10 fps in 2D and 1.5 fps in 3D, and perform quantitative tracking evaluation, which yields encouraging results. Finally, we show that 3D-MuPPET also works in natural environments without model fine-tuning on additional annotations. To the best of our knowledge we are the first to present a framework for 2D/3D posture and trajectory tracking that works in both indoor and outdoor environments

Megx.net: integrated database resource for marine ecological genomics

Megx.net is a database and portal that provides integrated access to georeferenced marker genes, environment data and marine genome and metagenome projects for microbial ecological genomics. All data are stored in the Microbial Ecological Genomics DataBase (MegDB), which is subdivided to hold both sequence and habitat data and global environmental data layers. The extended system provides access to several hundreds of genomes and metagenomes from prokaryotes and phages, as well as over a million small and large subunit ribosomal RNA sequences. With the refined Genes Mapserver, all data can be interactively visualized on a world map and statistics describing environmental parameters can be calculated. Sequence entries have been curated to comply with the proposed minimal standards for genomes and metagenomes (MIGS/MIMS) of the Genomic Standards Consortium. Access to data is facilitated by Web Services. The updated megx.net portal offers microbial ecologists greatly enhanced database content, and new features and tools for data analysis, all of which are freely accessible from our webpage http://www.megx.net

Modifying the properties of 4f single-ion magnets by peripheral ligand functionalisation

We study the ligand-field splittings and magnetic properties of three Er-III single-ion magnets which differ in the peripheral ligand sphere but exhibit similar first coordination spheres by inelastic neutron scattering (INS) and SQUID magnetometry. The INS spectra of the three compounds are profoundly different pointing at a strong response of the magnetic behavior tominor structural changes, as they are e. g. encountered when depositing molecules on surfaces. The observation of several magnetic excitations within the J = 15/2 ground multiplet together with single-crystal magnetic measurements allows for the extraction of the sign and magnitude of all symmetry-allowed Stevens parameters. The parameter values and the energy spectrum derived from INS are compared to the results of state-of-the-art ab initio CASSCF calculations. Temperature-dependent alternating current (ac) susceptibility measurements suggest that the magnetisation relaxation in the investigated temperature range of 1.9 K < T < 5 K is dominated by quantum tunnelling of magnetisation and two-phonon Raman processes. The possibility of observing electron paramagnetic resonance transitions between the ground-state doublet states, which can be suppressed in perfectly axial single-ion magnets, renders the studied systems interesting as representations of quantum bits