Tycho Brahe's supernova: light from centuries past

The light curve of SN 1572 is described in the terms used nowadays to characterize SNeIa. By assembling the records of the observations done in 1572--74 and evaluating their uncertainties, it is possible to recover the light curve and the color evolution of this supernova. It is found that, within the SNe Ia family, the event should have been a SNIa with a normal rate of decline, its stretch factor being {\it s} $\sim$ 0.9. Visual light curve near maximum, late--time decline and the color evolution sustain this conclusion. After correcting for extinction, the luminosity of this supernova is found to be M$_{V}$ $=$ --19.58 --5 log (D/3.5 kpc) $\pm$ 0.42.Comment: 28 pages, 3 figures, 3 tables. submitted to ApJ (Main Journal

Magnons in real materials from density-functional theory

We present an implementation of the adiabatic spin-wave dynamics of Niu and Kleinman. This technique allows to decouple the spin and charge excitations of a many-electron system using a generalization of the adiabatic approximation. The only input for the spin-wave equations of motion are the energies and Berry curvatures of many-electron states describing frozen spin spirals. The latter are computed using a newly developed technique based on constrained density-functional theory, within the local spin density approximation and the pseudo-potential plane-wave method. Calculations for iron show an excellent agreement with experiments.Comment: 1 LaTeX file and 1 postscript figur

Giant anharmonicity and non-linear electron-phonon coupling in MgB2_{2}; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $\sigma$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$Comment: In this revised version (to appear in PRL) we also discuss the boron isotope effect. Please visit http://www.ncnr.nist.gov/staff/taner/mgb2 for detail

Magnetism, Spin-Orbit Coupling, and Superconducting Pairing in UGe2_2

A consistent picture on the mean-field level of the magnetic properties and electronic structure of the superconducting itinerant ferromagnet UGe$_2$ is shown to require inclusion of correlation effects beyond the local density approximation (LDA). The "LDA+U" approach reproduces both the magnitude of the observed moment, composed of strongly opposing spin and orbital parts, and the magnetocrystalline anisotropy. The largest Fermi surface sheet is comprised primarily of spin majority states with orbital projection $m_{\ell}$=0, suggesting a much simpler picture of the pairing than is possible for general strong spin-orbit coupled materials. This occurrence, and the quasi-two-dimensional geometry of the Fermi surface, support the likelihood of magnetically mediated p-wave triplet pairing.Comment: accepted for publication in Phys. Rev. Lett; URL for better quality image of Fig.3 (2MB) at http://yammer.ucdavis.edu/public/UGe2/fig3.ep

Linear Response Calculations of Spin Fluctuations

A variational formulation of the time--dependent linear response based on the Sternheimer method is developed in order to make practical ab initio calculations of dynamical spin susceptibilities of solids. Using gradient density functional and a muffin-tin-orbital representation, the efficiency of the approach is demonstrated by applications to selected magnetic and strongly paramagnetic metals. The results are found to be consistent with experiment and are compared with previous theoretical calculations.Comment: 11 pages, RevTex; 3 Figures, postscript, high-resolution printing (~1200dpi) is desire

Structure and stability of finite gold nanowires

Finite gold nanowires containing less than 1000 atoms are studied using the molecular dynamics simulation method and embedded atom potential. Nanowires with the face-centered cubic structure and the (111) oriented cross-section are prepared at T=0 K. After annealing and quenching the structure and vibrational properties of nanowires are studied at room temperature. Several of these nanowires form multi-walled structures of lasting stability. They consist of concentrical cylindrical sheets and resemble multi-walled carbon nanotubes. Vibrations are investigated by diagonalization of the dynamical matrix. It was found that several percents of vibrational modes are unstable because of uncompleted restructuring of initial fcc nanowires.Comment: 4 figures in gif forma

A Transport and Microwave Study of Superconducting and Magnetic RuSr2EuCu2O8

We have performed susceptibility, thermopower, dc resistance and microwave measurements on RuSr2EuCu2O8. This compound has recently been shown to display the coexistence of both superconducting and magnetic order. We find clear evidence of changes in the dc and microwave resistance near the magnetic ordering temperature (132 K). The intergranular effects were separated from the intragranular effects by performing microwave measurements on a sintered ceramic sample as well as on a powder sample dispersed in an epoxy resin. We show that the data can be interpreted in terms of the normal-state resistivity being dominated by the CuO2 layers with exchange coupling to the Ru moments in the RuO2 layers. Furthermore, most of the normal-state semiconductor-like upturn in the microwave resistance is found to arise from intergranular transport. The data in the superconducting state can be consistently interpreted in terms of intergranular weak-links and an intragranular spontaneous vortex phase due to the ferromagnetic component of the magnetization arising from the RuO2 planes.Comment: 20 pages including 6 figures in pdf format. To be published in Phys. Rev.

Glass Transition in the Polaron Dynamics of CMR Manganites

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polarons correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T* 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (Tc=114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.Comment: 4 pages, 4 figures. Physical Review Letters (in Press

The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7

A system of strongly-interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by a crystallographic analysis of broad satellite maxima observed in diffuse X-ray and neutron scattering data. The resulting q=(0.3,0,1) modulation is a longitudinal octahedral-stretch mode, consistent with an incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the directions.Comment: Reformatted with RevTe

Information and Discrimination from b Quark Production on Z Resonance

We introduce and define operatively in a model independent way a new ``heavy" b-vertexparameter, $\eta_b$, that can be derived from the measurement of a special polarization asymmetry for production of b-quarks on Z resonance. We show that the combination of the measurement of $\eta_b$ with that of a second and previously defined ``heavy" b-vertex parameter $\delta_{bV}$ can discriminate a number of models of New Physics that remain associated to different ``trajectories" in the plane of the variations of the two parameters. This is shown in particular for some popular SUSY and technicolor-type models. In general, this discrimination is possible if a measurement of \underline{both} parameters is performed.Comment: 22 pages, 6 figures available by air mail upon request, (e-mail [email protected] PM/94-04, UTS-DFT-94-02 .( revised version with corrected references