N. Cavadini, Ch. Rüegg, W. Henggeler, A. Furrer, H.-U. Güdel, K. Krämer and H. Mutka
Temperature renormalization of the magnetic excitations in S=1/2 KCuCl₃
Eur. Phys. J. B 18, 565-571 (2000)                 

Abstract: A complete temperature characterization of the spin dynamics in the unconventional S=1/2 antiferromagnet KCuCl₃ is presented from single crystal inelastic neutron scattering studies. KCuCl₃ features a quantum disordered singlet ground state and a finite spin gap to triplet excitations of dimer origin. Three dimensional magnetic correlations support the dispersive propagation of the excitations in the whole reciprocal space. Upon increasing the temperature, a renormalization in the energy, in the intensity and in the damping rate of the triplet modes is reported. The experimental observations can be described within the framework of a selfconsistent dimer RPA theory, with no free parameters. The driving mechanism behind the model is the thermally activated decrease of the occupation difference n₀-n₁ between singlet and triplet dimer states. This is the expression of kinematic constraints which are of minor importance for classical magnons in Néel ordered antiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed.

Daniel R. Gamelin, Stefan R. Lüthi and Hans U. Güdel
The Role of Laser Heating in the Intrinsic Optical Bistability of Yb³⁺-Doped Bromide Lattices
J. Phys. Chem. B 104, 11045-11057 (2000)                 

Abstract: Materials displaying intrinsic optical bistability (IOB), i.e., allowing the coexistence of two stable steady-state excitation rates for a single given excitation power, are of interest for potential technological applications related to optical data switching and manipulation. The properties of the unusual IOB previously observed in Yb³⁺-doped Cs₃Lu₂Br₉ and CsCdBr₃ host materials are studied here using absorption and luminescence spectroscopies. The IOB phenomenon is concluded to derive ultimately from laser heating effects. When combined with a strongly increasing and nonlinear dependence of the material's absorbance on internal temperature, laser heating leads to a positive-feedback absorption amplification process showing a hysteresis in both power- and temperature-sweep experiments. A simple model describing this effect in terms of rates of sample heating and cooling in the irradiated volume reproduces the power, temperature, concentration, and excitation-energy dependence of the Yb³⁺ IOB using only the experimental absorption data as input. The temperature dependence of the absorption cross section is correlated with thermal changes in the monomeric YbBr₆^3- geometry, which becomes more asymmetric as the temperature is elevated. The IOB observed in Yb³⁺-doped Cs₃Lu₂Br₉ and CsCdBr₃ host lattices is therefore a property of the monomeric Yb³⁺ ion in these materials, and not a dimer property as was previously believed. These results also emphasize the more general conclusion that laser heating may contribute significantly to the shape ore slope of an excitation power dependence curve, and may even be the dominant aspect of that curve when absorption cross sections are strongly dependent on temperature.

Hanspeter Andres, Reto Basler, Hans-Ulrich Güdel, Guillem Aromi, George Christou, Herma Büttner and Benoit Rufflé
Inelastic Neutron Scattering and Magnetic Susceptibilities of the Single-Molecule Magnets [Mn₄O₃X(OAc)₃(dbm)₃] (X=Br, Cl, OAc and F): Variation of the Anisotropy along the Series.
J. Am. Chem. Soc. 122, 12469-12477 (2000)                 

Abstract: The single-molecule magnets (SMMs) [Mn₄O₃X(OAc)₃(dbm)₃] (X=Br, Cl, OAc and F) were investigated by a detailed inelastic neutron scattering (INS) study. Up to four magnetic excitations between the zero-field split levels of the lowest S=9/2 cluster ground-state have been resolved. From the determined energy-level diagrams and the relative INS intensities we can show that the inclusion of a rhombic term in the zero-field splitting (ZFS) Hamiltonian is essential in these compounds. On the basis of the Hamiltonian: H_ZFS = D[S_z²-1/3S(S+1)] + E(S_x²-S_y²) + B₄⁰O₄⁰, the following sets of parameters are derived: For X = Cl: D = -0.529cm^–1, |E| = 0.022cm^–1, and B₄⁰ = -6.5e-5cm^–1; for X = Br: D = -0.502cm^–1, |E| = 0.017cm^–1, and B₄⁰ = -5.1e-5cm^–1; for X = OAc: D = -0.4.69cm^–1, |E| = 0.017cm^–1, and B₄⁰ = -7.9e-5cm^–1; and for X = F: D = -0.379cm^–1 and B₄⁰ = -11.1e-5cm^–1. The wave functions derived from the energy analysis are in excellent agreement with the relative intensities of the observed INS transitions. The observed temperature maxima of the out-of-phase component of the variable freqency AC magnetic susceptibility T_max[chi''] correlate well with the energy splittings determined by INS. Direct information about the rate of quantum tunneling is contained in the cluster wave functions derived in this study. The difference in the quantum tunneling between X = Cl and Br is shown to be directly related to differences in the rhombic anisotropy parameter |E|.

Daniel R. Gamelin and Hans U. Güdel
Excited-State Dynamics and Sequential Two-Photon Upconversion Excitation of Mo³⁺-Doped Chloro- and Bromo-elpasolites
J. Phys. Chem. B 104, 10222-10234 (2000)                 

Abstract: The photophysical properties of Mo³⁺-doped chloro- and bromo-elpasolites at 10 K and as a function of temperature are presented. Visible upconverted luminescence is observed with near-infrared laser excitation, and the properties of this two-photon excitation process are experimentally characterized by a variety of luminescence and time-dependent measurements. Three upconversion mechanisms are active in these Mo³⁺-doped materials. Two involve the sequential absorption of two near-infrared photons of the same color, and a third involves the sequential absorption of two near-infrared photons of different colors. The two one-color upconversion mechanisms are distinguishable on the basis of time dependence measurements and simulations. Of the three mechanisms, the two-color sequential two-photon absorption process is found to be more efficient than the corresponding one-color mechanism by a factor of >10², and this is related to the relatively small effective spin-orbit coupling magnitude in this ion. These excited-state processes are assisted by an extremely long 10 K ²E lifetime of 67.5 ms in 2.5% Mo³⁺:Cs₂NaYCl₆, and 48.5 ms in 2.5% Mo³⁺:Cs₂NaYBr₆. The upconversion properties of Mo³⁺ presented here reflect the rich photophysics of this ion, and represent a significant new addition to the emerging area of transition-metal upconversion.

E. V. D. van Loef, P. Dorenbos and C. W. E. van Eijk, K. Krämer and H. U. Güdel
High-energy-resolution scintillator: Ce³⁺ activated LaCl₃
Appl. Phys. Lett. 77, 1467-1468 (2000)                 

Abstract: The scintillation properties of LaCl₃ doped with 10% Ce³⁺ are presented. Under optical and gamma ray excitation, Ce³⁺ emission is observed to peak at 330 and 352 nm. The scintillation light output is 46000+/-1000 photons/MeV at 662 keV. Forty percent is emitted with a decay time of 26 ns, 30% with 210 ns, and 30% with about 1000 ns. An energy resolution (full width at half maximum over the peak position) of 3.3+/-0.3% was observed for the 662 keV full absorption peak.

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