Quantum statistics of interacting dimer spin systems

The compound TlCuCl3 represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Wurtz [Phys. Rev. B 50, 13 515 (1994)] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important

New Perspectives on the X-ray Emission of HD 104237 and Other Nearby Herbig Ae/Be Stars from XMM-Newton and Chandra

We present new X-ray observations of the nearby Herbig Ae star HD 104237 (= DX Cha) with XMM-Newton, whose objective is to clarify the origin of the emission. Several X-ray emission lines are clearly visible in the CCD spectra, including the high-temperature Fe K-alpha complex. The emission can be accurately modeled as a multi-temperature thermal plasma with cool (kT < 1 keV) and hot (kT > 3 keV) components. The presence of a hot component is compelling evidence that the X-rays originate in magnetically confined plasma, either in the Herbig star itself or in the corona of an as yet unseen late-type companion. The X-ray temperatures and luminosity (log Lx = 30.5 ergs/s) are within the range expected for a T Tauri companion, but high resolution Chandra and HST images constrain the separation of a putative companion to less than 1 arcsec. We place these new results into broader context by comparing the X-ray and bolometric luminosities of a sample of nearby Herbig stars with those of T Tauri stars and classical main-sequence Be stars. We also test the predictions of a model that attributes the X-ray emission of Herbig stars to magnetic activity that is sustained by a shear-powered dynamo.Comment: To appear in ApJ (part 1); 43 pages, 8 figures, 5 table

Vibronic Coupling in Trimers - Statics - the Effects of 2nd-Order Vibronic Coupling

The effect of second-order vibronic coupling on the adiabatic potential energy surfaces of the symmetric trimer is investigated. Regions of parameter space are found where one, three, four or six minima can coexist on the lower potential energy surface. These minima correspond approximately to electronic wavefunctions either localized on one of the trimer units or delocalized over all centers. Formulae for barrier heights, geometries and delocalization parameters are generalized to include the effects of second-order vibronic coupling

Excitation Avalanche in Ni-2+-Doped Cscdcl3

Nonlinear optical behaviour is observed in CsCdCl3:Ni2+ and compared with a phenomenon reported from lanthanide doped compounds called a "photon avalanche". Laser power and time dependences are correlated to an appropriate model. An excited-state excitation spectrum is shown

A Trimer Vibronic Coupling Model for Triptycene - the Jahn-Teller and Barnett Effects

The first singlet electronic excited state of triptycene, as measured by resonant two-photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E' x e' Jahn-Teller effect. Here it is shown that this E' x e' vibronic coupling can be profitably viewed as a subset of a (A1' + E') x (a2' + e') vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e' mode as well as an unusually large second-order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a2' vibration which couples through nonzero momentum rather than coordinate matrix elements. A coupling of this type has been termed the ''molecular Barnett effect'' and has been predicted to appear in Jahn-Teller systems when certain conditions are fulfilled. This effect has not been previously observed in molecular spectroscopy. It indicates that crude adiabatic basis functions are inadequate to describe the electronic states of the present system

Mechanism of ground-state selection in the frustrated molecular spin cluster V-15

Chaboussant G, Ochsenbein ST, Sieber A, et al. Mechanism of ground-state selection in the frustrated molecular spin cluster V-15. EUROPHYSICS LETTERS. 2004;66(3):423-429.We report an inelastic neutron scattering (INS) study under a magnetic field on the frustrated molecular spin cluster V-15. Several field-dependent transitions are observed and provide a comprehensive understanding of the low-energy quantum spin states. The energy gap 2Delta(o) approximate to 27(3) mueV between the two lowest S = 1/2 Kramers doublets is unambiguously attributed to a symmetry lowering of the cluster. The INS data are mapped onto an S = 1/2 Antiferromagnetic Heisenberg triangle with scalene distortion. A quantitative description of the wave function mixing within the ground state is derived