Far infrared study of the two dimensional dimer spin system SrCu_2(BO_3)_2

Using far-infrared spectroscopy in magnetic fields up to 12T we have studied a two-dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several infrared active modes in the dimerized state (below 10K) in the frequency range from 3 to 100cm^-1. The measured splitting from the ground state to the excited triplet M_S=0 sublevel is Delta_1=24.2cm^-1 and the other two triplet state sublevels in zero magnetic field are 1.4cm^-1 below and above the M_S=0 sublevel. Another multiplet is at Delta_2=37.6cm^-1 from the ground state. A strong electric dipole active transition polarized in the (ab)-plane is activated in the dimer spin system below 15K at 52cm^-1.Comment: 4 pages including 5 figures, submitted to PRB, instrumental arte facts remove

Reynolds stresses and mean fields generated by pure waves: applications to shear flows and convection in a rotating shell

A general reformulation of the Reynolds stresses created by two-dimensional waves breaking a translational or a rotational invariance is described. This reformulation emphasizes the importance of a geometrical factor: the slope of the separatrices of the wave flow. Its physical relevance is illustrated by two model systems: waves destabilizing open shear flows; and thermal Rossby waves in spherical shell convection with rotation. In the case of shear-flow waves, a new expression of the Reynolds–Orr amplification mechanism is obtained, and a good understanding of the form of the mean pressure and velocity fields created by weakly nonlinear waves is gained. In the case of thermal Rossby waves, results of a three-dimensional code using no-slip boundary conditions are presented in the nonlinear regime, and compared with those of a two-dimensional quasi-geostrophic model. A semi-quantitative agreement is obtained on the flow amplitudes, but discrepancies are observed concerning the nonlinear frequency shifts. With the quasi-geostrophic model we also revisit a geometrical formula proposed by Zhang to interpret the form of the zonal flow created by the waves, and explore the very low Ekman-number regime. A change in the nature of the wave bifurcation, from supercritical to subcritical, is found

Strong damping of phononic heat current by magnetic excitations in SrCu_2(BO_3)_2

Measurements of the thermal conductivity as a function of temperature and magnetic field in the 2D dimer spin system SrCu$_2$(BO$_3$)$_2$ are presented. In zero magnetic field the thermal conductivity along and perpendicular to the magnetic planes shows a pronounced double-peak structure as a function of temperature. The low-temperature maximum is drastically suppressed with increasing magnetic field. Our quantitative analysis reveals that the heat current is due to phonons and that the double-peak structure arises from pronounced resonant scattering of phonons by magnetic excitations.Comment: a bit more than 4 pages, 2 figures included; minor changes to improve the clarity of the presentatio

Extended moment formation and magnetic ordering in the trigonal chain compound Ca3Co2O6

The results of electronic structure calculations for the one-dimensional magnetic chain compound Ca3Co2O6 are presented. The calculations are based on density functional theory and the local density approximation and used the augmented spherical wave (ASW) method. Our results allow for deeper understanding of recent experimental findings. In particular, alternation of Co 3d low- and high-spin states along the characteristic chains is related to differences in the oxygen coordination at the inequivalent cobalt sites. Strong hybridization of the d states with the O 2p states lays ground for polarization of the latter and the formation of extended localized magnetic moments centered at the high-spin sites. In contrast, strong metal-metal overlap along the chains gives rise to intrachain ferromagnetic exchange coupling of the extended moments via the d_{3z^2-r^2} orbitals of the low-spin cobalt atoms.Comment: 10 pages, 4 figures more information at http://www.physik.uni-augsburg.de/~eyert

Collective Singlet Excitations and Evolution of Raman Spectral Weights in the 2D Spin Dimer Compound SrCu2(BO3)2

We present a Raman light scattering study of the two-dimensional quantum spin system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich structure, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)2 is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound. Based on our analysis of the Heisenberg model relevant for this material, we argue that the collective excitations involving two and three-particle singlet bound states have large binding energies and are observed as well-defined peaks in the Raman spectrum.Comment: 5 pages, 2 figures. Revised version, to appear in Phys. Rev. Lett. (2000

Outcrossing rates and inbreeding coefficients in rubber trees (Hevea brasiliensis (Willd. ex Adr. de Juss.) Müeller. Arg.).

Electrophoretic analyses were carried out in starch gel with young leaves of rubber trees of two native populations from the state of Acre, Brazil

Multi-Triplet Magnons in SrCu2_2(BO3_3)2_2 Studied by Thermal Conductivity Measurements in Magnetic Fields

We have measured the thermal conductivity parallel to the a-axis of the Zn-free and 1% Zn-substituted SrCu$_{2-x}$Zn$_x$(BO$_3$)$_2$ in magnetic fields up to 14 T, in order to examine the thermal conductivity due to the multi-triplet magnons. It has been found that the thermal conductivity peak observed in the spin gap state is suppressed by the substitution of Zn for Cu in high magnetic fields above 6 T, while it is not changed in low magnetic fields below 6 T. The results suggest that the thermal conductivity peak in the spin-gap state of SrCu$_2$(BO$_3$)$_2$ is composed of not only thermal conductivity due to phonons but also that due to the multi-triplet magnons in high fields above 6 T.Comment: 7 pages, 2 figure

X-Band ESR Determination of Dzyaloshinsky-Moriya Interaction in 2D SrCu2_2(BO3_3)2_2 System

X-band ESR measurements on a single crystal of SrCu$_2$(BO$_3$)$_2$ system in a temperature range between 10 K and 580 K are presented. The temperature and angular dependence of unusually broad ESR spectra can be explained by the inclusion of antisymmetric Dzyaloshinsky-Moriya (DM) interaction, which yields by far the largest contribution to the linewidth. However, the well-accepted picture of only out-of-plane interdimer DM vectors is not sufficient for explanation of the observed angular dependence. In order to account for the experimental linewidth anisotropy we had to include sizable in-plane components of interdimer as well as intradimer DM interaction in addition to the out-of-plane interdimer one. The nearest-neighbor DM vectors lie perpendicular to crystal anisotropy c-axis due to crystal symmetry. We also emphasize that above the structural phase transition occurring at 395 K dynamical mechanism should be present allowing for instantaneous DM interactions. Moreover, the linewidth at an arbitrary temperature can be divided into two contributions; namely, the first part arising from spin dynamics governed by the spin Hamiltonian of the system and the second part due to significant spin-phonon coupling. The nature of the latter mechanism is attributed to phonon-modulation of the antisymmetric interaction, which is responsible for the observed linear increase of the linewidth at high temperatures.Comment: 17 pages, 4 figures, submitted to PR