Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated ͑295-4 K͒. The resulting full phononic characterization of the R 2 Ti 2 O 7 pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb 2 Ti 2 O 7 in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb 3+ sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13 cm −1 is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb 2 Ti 2 O 7 . The additional crystal field information on Tb 2 Ti 2 O 7 adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.

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In this paper we investigate the acoustic-to-optical up-conversion phonon processes in a multicomponent system. These processes take place during heat transport and limit the efficiency of heat flow. By combining time-resolved optical and heat capacity experiments we quantify the thermal coupling constant to be g ∼ 0.4 10 17 W/Km 3. The method is based on selective excitation of a part of a multicomponent system, and the measurement of the thermalization dynamics by probing the linear birefringence of the sample with femtosecond resolution. In particular, we study a layered multiferroic organic-inorganic hybrid, in the vicinity of the ferroelectric phase transition. A diverging term of the heat capacity is associated to soft-mode dynamics, in agreement with previous spectroscopy measurements.

By comparing the photoinduced magnetization dynamics in simple layered systems we show how light-induced modifications of the magnetic anisotropy directly enhance the magnetization. It is observed that the spin precession in (CH 3 NH 3 ) 2 CuCl 4 , initiated by a light pulse, increases in amplitude at the critical temperature T C . The phenomenon is related to the dependence of the critical temperature on the axial magnetic anisotropy. The present results underline the possibility and the importance of the optical modifications of the anisotropy, opening different paths toward the control of the magnetization state for ultrafast memories.

Organic-inorganic hybrid materials have shown a remarkable and rapid development during the past decade because they can be tailored to obtain new device concepts with controlled physical properties. Here, we report on the electronic and magnetic properties of multilayer organic-inorganic hybrid films. Electrical transport properties arising from the π electrons in the organic layer are characteristic of a metallic state at high temperature and evolve into a state described by two-dimensional variable range hopping when temperature decreases below 150 K. The intrinsic electronic behavior of the hybrid films was further studied via the optical properties in the IR range. The optical response confirms the metallic character of the hybrid films. In the second part, the magnetic properties are discussed. A long-range ferromagnetic order with an ordering temperature of ∼ 1 K is revealed in the Gd-based hybrid film. The Cu-based hybrid film, however, shows more extended ferromagnetic exchange interactions than the Gd-based hybrid LB film.