We report on specific heat measurements in Ba$_{1-x}$K$_x$Fe$_{2}$As$_{2}$ ($xle 0.6$). For the underdoped sample with $x=0.2$ both the spin-density-wave transition at $T = 100$ K and the superconducting transition at 23 K can be identified. The elec
tronic contribution to the specific heat in the superconducting state for concentrations in the vicinity of optimal doping $x=0.4$ can be well described by a full single-gap within the BCS limit.
We report on infrared, Raman, magnetic susceptibility, and specific heat measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the nearest-neighbor and next-nearest neighbor exchange constants from the magnetic susceptibility and extract t
he spin-spin correlation functions obtained from the magnetic susceptibility and the magnetic contribution to the specific heat. By comparing with the frequency shift of the infrared optical phonons above TN , we derive estimates for the spin-phonon coupling constants in these systems. The observation of phonon modes which are both Raman and infrared active suggest the loss of inversion symmetry below the Neel temperature in CdCr2O4 in agreement with theoretical predictions by Chern and coworkers [Phys. Rev. B 74, 060405 (2006)]. In ZnCr2O4 several new modes appear below TN, but no phonon modes could be detected which are both Raman and infrared active indicating the conservation of inversion symmetry in the low temperature phase.
We analyzed the magnetic susceptibilities of several Cr spinels using two recent models for the geometrically frustrated pyrochlore lattice, the Quantum Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both models can describe
the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd) satisfactorily, with the former yielding a somewhat better agreement with experiment for A = Zn, Mg. The obtained exchange constants for nearest and next-nearest neighbors are discussed.
The reflectivity of single-crystalline CoO has been studied by optical spectroscopy for wave numbers ranging from 100 to 28,000wn and for temperatures 8 $< T <$ 325 K@. A splitting of the cubic IR-active phonon mode on passing the antiferromagnetic p
hase transition at $T_N$ = 289 K has been observed. At low temperatures the splitting amounts to 15.0wn. In addition, we studied the splitting of the cubic crystal field ground state of the Co$^{2+}$ ions due to spin-orbit coupling, a tetragonal crystal field, and exchange interaction. Below $T_N$, magnetic dipole transitions between the exchange-split levels are identified and the energy-level scheme can be well described with a spin-orbit coupling $lambda = 151.1wn$, an exchange constant $J = 17.5wn$, and a tetragonal crystal-field parameter $D = -47.8wn$. Already in the paramagnetic state electric quadrupole transitions between the spin-orbit split level have been observed. At high frequencies, two electronic levels of the crystal-field-split $d$-manifold were identified at 8,000 and 18,500wn.
