Approaching the ground state of the kagome antiferromagnet

Y{0.5}$Ca{0.5}BaCo4O7 contains kagome layers of Co ions, whose spins are strongly coupled according to a Curie-Weiss temperature of -2200 K. At low temperatures, T = 1.2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three dimensional long-range AF order proves negligible coupling between the kagome layers. The scattering intensities are consistent with high spin S=3/2 states of Co2+ in the kagome layers and low spin S=0 states for Co3+ ions at interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of only short range chiral order.Comment: 4 pages, 4 figures, contact author: [email protected]

Incorporating interactive 3-dimensional graphics in astronomy research papers

Most research data collections created or used by astronomers are intrinsically multi-dimensional. In contrast, all visual representations of data presented within research papers are exclusively 2-dimensional. We present a resolution of this dichotomy that uses a novel technique for embedding 3-dimensional (3-d) visualisations of astronomy data sets in electronic-format research papers. Our technique uses the latest Adobe Portable Document Format extensions together with a new version of the S2PLOT programming library. The 3-d models can be easily rotated and explored by the reader and, in some cases, modified. We demonstrate example applications of this technique including: 3-d figures exhibiting subtle structure in redshift catalogues, colour-magnitude diagrams and halo merger trees; 3-d isosurface and volume renderings of cosmological simulations; and 3-d models of instructional diagrams and instrument designs.Comment: 18 pages, 7 figures, submitted to New Astronomy. For paper with 3-dimensional embedded figures, see http://astronomy.swin.edu.au/s2plot/3dpd

XYZ-polarisation analysis of diffuse magnetic neutron scattering from single crystals

Studies of diffuse magnetic scattering largely benefit from the use of a multi-detector covering wide scattering angles. Therefore, the different contributions to the diffuse scattering that originate from magnetic, nuclear coherent, and nuclear spin-incoherent scattering can be separated by the so-called XYZ-polarization analysis. In the past this method has been successfully applied to the analysis of diffuse scattering by polycrystalline samples of magnetic disordered materials. Single crystal studies that exploit the vector properties of spin correlations are of particular interest for furthering our understanding of frustration effects in magnetism. Based on the symmetry properties of polarised scattering a suitable extension of the conventional XYZ method has been derived, which allows for the complete separation and the analysis of features of diffuse magnetic scattering from single crystals.Comment: 6 pages 2 figures, revised as published, one Eq. removed, minor corrections, typos correcte

Blower Characteristic Measurement and Design of an Experiment to Measure Parameters of the Blower Model Plane Inlet

Pro dané dmychadlo s neznámými parametry bylo potřeba změřit tlakovou charakteristiku. Pro tento účel byl navržen a postaven měřící stand. Měřící stand byl navržen na základě možností které jsou v laboratoři. Další parametry měřícího standu byly zvoleny podle normy pro měření průtoku pomocí clony, která bude v měření použita.For a given blower with unknown parameters, it was necessary to measure the pressure characteristic. For this purpose it was necessary to design and construct a measuring stand. The laboratory equipment was taken into consideration when designing the measuring stand. Other parameters of measuring stand were selected according to the volume flow rate measurement using orifice plate

Temperature dependence of the diffuse scattering fine structure in equiatomic CuAu

The temperature dependence of the diffuse scattering fine structure from disordered equiatomic CuAu was studied using {\it in situ} x-ray scattering. In contrast to Cu$_3$Au the diffuse peak splitting in CuAu was found to be relatively insensitive to temperature. Consequently, no evidence for a divergence of the antiphase length-scale at the transition temperature was found. At all temperatures studied the peak splitting is smaller than the value corresponding to the CuAuII modulated phase. An extended Ginzburg-Landau approach is used to explain the temperature dependence of the diffuse peak profiles in the ordering and modulation directions. The estimated mean-field instability point is considerably lower than is the case for Cu$_3$Au.Comment: 4 pages, 5 figure

Chiral Spin Liquid Ground State in YBaCo3FeO7

A chiral spin liquid state is discovered in the highly frustrated, noncentrosymmetric swedenborgite compound YBaCo3FeO7, a layered kagome system of hexagonal symmetry, by advanced polarized neutron scattering from a single domain crystalline sample. The observed diffuse magnetic neutron scattering has an antisymmetric property that relates to its specific chirality, which consists of three cycloidal waves perpendicular to the c axis, forming an entity of cylindrical symmetry. Chirality and symmetry agree with relevant antisymmetric exchanges arising from broken spatial parity. Applying a Fourier analysis to the chiral interference pattern, with distinction between kagome sites and the connecting trigonal interlayer sites of threefold symmetry, the chiral spin correlation function is determined. Characteristic chiral waves originate from the trigonal sites and extend over several periods in the kagome planes. The chiral spin liquid is remarkably stable at low temperatures despite strong antiferromagnetic spin exchange. The observation raises a challenge, since the commonly accepted ground states in condensed matter either have crystalline long-range order or form a quantum liquid. We show that, within the classical theory of magnetic order, a disordered ground state may arise from chirality. The present scenario, with antisymmetric exchange acting as a frustrating gauge background that stabilizes local spin lumps, is similar to the avoided phase transition in coupled gauge and matter fields for subnuclear particles

Invertible and Non-invertible Alloy Ising Models

Physical properties of alloys are compared as computed from ``direct'' and ``inverse'' procedures. The direct procedure involves Monte Carlo simulations of a set of local density approximation (LDA)-derived pair and multibody interactions {\nu_f}, generating short-range order (SRO), ground states, order- disorder transition temperatures, and structural energy differences. The inverse procedure involves ``inverting'' the SRO generated from {\nu_f} via inverse-Monte-Carlo to obtain a set of pair only interactions {\tilde{\nu}_f}. The physical properties generated from {\tilde{\nu}_f} are then compared with those from {\nu_f}. We find that (i) inversion of the SRO is possible (even when {\nu_f} contains multibody interactions but {\tilde{\nu}_f} does not) but, (ii) the resulting interactions {\tilde{\nu}_f} agree with the input interactions {\nu_f} only when the problem is dominated by pair interactions. Otherwise, {\tilde{\nu}_f} are very different from {\nu_f}. (iii) The same SRO pattern can be produced by drastically different sets {\nu_f}. Thus, the effective interactions deduced from inverting SRO are not unique. (iv) Inverting SRO always misses configuration-independent (but composition- dependent) energies such as the volume deformation energy G(x); consequently, the ensuing {\tilde{\nu}_f} cannot be used to describe formation enthalpies or two-phase regions of the phase diagram, which depend on G(x).Comment: 4 pages, ReVTeX galley format, 1 eps figures embedded using epsf, to be published in Solid State Communication

Excited states of quantum many-body interacting systems: A variational coupled-cluster description

We extend recently proposed variational coupled-cluster method to describe excitation states of quantum many-body interacting systems. We discuss, in general terms, both quasiparticle excitations and quasiparticle-density-wave excitations (collective modes). In application to quantum antiferromagnets, we reproduce the well-known spin-wave excitations, i.e. quasiparticle magnons of spin $\pm 1$. In addition, we obtain new, spin-zero magnon-density-wave excitations which has been missing in Anserson's spin-wave theory. Implications of these new collective modes are discussed.Comment: 17 pages, 4 figure

Optisch detektierte paramagnetische Resonanz-Spektroskopie am Rubin

The optically detected paramagnetic resonance has been proven to be a valuable method for the investigation of the electronic structure of transition metal in metallo-proteins. Up to now a model has been used for the interpretation of the experimental spectra which described this experiment on the basis of circular magnetic dichroism, which is induced by a rotating transverse magnetization. This model of magnetic circular dichroism induced by a rotating magnetization is just applicable to special kinds of transition metal systems and a special optical configuration of this experiment.In this thesis it will be shown, that a theory, based on the model of coherent Raman-scattering, provides a much more general description of this experiment. This will be demonstrated on the experimental spectra at the R-lines of the ruby cristal. These spectra can not be interpreted by the model of transverse MCD and show a very complex structure. The optical and magnetic properties of the chromium ions in ruby have been investigated in the last century extensively. Thus ruby is an ideal testing system, to proof the advantages of the model of coherent Raman scattering and to obtain a deeper insight into the nature of this spectroscopic method.Die optisch detektierte paramagnetische Resonanz hat sich als eine wertvolle Methode zur Untersuchung der elektronischen Struktur von Übergangsmnetallen in Metallo-Proteinen bewährt. Zur Interpretation der experimentellen Spektren wurde dazu bisher ein Modell benutzt, welches das Experiment auf der Basis eines magnetischen zirkularen Dichroismus beschreibt, der durch eine rotierende transversale Magnetisierung induziert wird.Dieses Modell des magnetischen zirkularen Dichroismus an einer rotierenden Magnetisierung ist nur auf bestimmte Arten von Übergangsmetall-Systemen und nur auf eine bestimmte optische Konfiguration des Eyperimentes anwendbar.In dieser Arbeit wird gezeigt, dass eine Theorie, basierend auf dem Modell der kohärenten Raman-Streuung, eine wesentlich allgemeinere Beschreibung dieses Experimentes zulässt. Dieses wird an experimentellen Spektren bei den R-Linien des Rubins demonstriert. Diese Spektren lassen sich grundsätzlich nicht mehr mit dem Modell der transversalen MCD beschreiben und zeigen eine sehr komplexe Struktur. Die optischen und magnetischen Eigenschaften der Chrom-Ionen im Rubin sind im letzten Jahrhundert ausgiebig erforscht worden. Aus diesen Gründen ist der Rubin ein ideales Test-System, um die Vorzüge des Modells der kohärenten Raman-Streuung zu belegen und einen tieferen Einblick in die Natur dieser spektroskopischen Methode zu gewähren