ESR- und ENDOR-Untersuchungen vonU 2-Zentren in CsCl- und CsBr-Kristallen

We report on ESR and ENDOR investigations ofU ₂ centers in CsCl and CsBr. The results of the measurements are explained by a center model, where the interstitial hydrogen atom is situated between two anions and surrounded by four Cs ions.     

Elektronenspinresonanz vonU 2-Folgezentren in Alkalihalogenid-Kristallen

UV-irradiation at low temperatures may bring hydrogen atoms into interstitial sites (produceU ₂ centers) in alkali halides. At about 100 °K the hydrogen atoms begin to leave their sites and may react with any diamagnetic centerX to a paramagnetic center HX. In this paper CN^? and NCO^? ions are used for X. The paramagnetic centers are investigated by the method of ESR. The spectra and thermal properties of¹H¹²C¹⁴N^?,²H¹²C¹⁴N^?,¹H¹³C¹⁴N^?,¹HNCO^? and²HNCO^? are described. As well signals are found, which may be attributed to the²H¹³C¹⁴N^? center. A simple cryostat is described for use at temperatures continously variable between 25 °K and 500 °K.     

Paraelastisches Verhalten von S2 −-Zentren in Alkalihalogeniden

The reorientation of S₂ ^? molecule ions on anion sites in alkali halide crystals under mechanical stress has been investigated by means of electron spin resonance and optical measurements. The reorientation rate is given by an Arrhenius relation. Tunneling in the ground state of libration is unimportant contrary to the case of the O₂ ^? center. The stress-induced dichroism permitted identification and assignment of two optical absorption bands of the S₂ ^? center.     

U2-Zentren in CsCl-, CsBr- und CsJ-Kristallen

U₂ Centers are investigated in cesium halide crystals. Optical absorption measurements show the hydrogen atom being square planarly surrounded by four anions. Refering to this center model the energy of the U₂ band is calculated. A dichroic absorption is found when the tetragonal U₂ centers are aligned by irradiation with U₂ light.     

Die optische Absorption der U2-Zentren in Alkalihalogeniden des NaCl-Typs

The UV absorption of the U₂ center in nine alkalihalides at 20 °K is presented. In the bromides and iodides the U₂ band consists of three components. This splitting is interpreted as a combined effect of spin-orbit coupling and dynamic lattice distortion. The energy of the strongest U₂ band is calculated by using the effective charge of the H atom in the U₂ ground state as a parameter. This comes out to be ?0.17 e in chlorides, ?0.20 e in bromides, and ?0.25 e in iodides. In mixed crystals of KBr+Δ KJ U₂ centers of the general kind HBr _n J_4?n (n=0, ..., 4) are investigated. They form three classes of U₂ bands, which are distinguished by thep-orbital in the excited state. This can bep _BrBr,p _BrJ,p _JJ depending on what halide ions the defect electron is located in the excited state. Dichroic bleaching of each band with polarized light allows to orient the complexes. This demonstrates that by optical stimulation the H atom can diffuse through the interstitial lattice. By warming up to 70 °K HBr₄ changes to HBr₃J which becomes unstable above 80 °K.     

Die elastische Wechselwirkung zwischen O 2 − -Zentren in Alkalihalogeniden-Zentren in Alkalihalogeniden

The interaction between two elastic dipoles has been studied directly by means of electron spin resonance. The investigated system consists of pairs of O ₂ ^? centers on nearest neighbor anion sites in KCl, KBr and KI. Since the O ₂ ^? centers reorient at temperatures as low as 1°K a pair assumes the configuration of lowest energy at these temperatures. The investigation of the EPR spectrum of the pairs and its response to externally applied uniaxial stresses has yielded the orientations of the axes of the pair partners in the configuration of lowest energy as well as some information on configurations of higher energy. The experiments are in agreement with the predictions based on continuum elasticity theory. Electric quadrupole interaction and covalency effects are less important. In particular the magnetic interaction is small compared to the elastic interaction.     

Die Fluoreszenz von Z1-Zentren in KCl

Zeitschrift für Physik A Hadrons and nuclei - Detailed measurements on the absorption, emission and excitation spectra of Z1 centers in KCl doped with Ca, Sr or Ba are reported. Beyond the...     

Definierter Einbau und optische Absorption von O 2 − - und O−−-Zentren in KCl-Kristallen

A method is described how to grow single crystals of potassium chloride with high concentrations of O ₂ ^? by strictly excluding spurious amounts of water, which would react to OH^?. Reducing treatment with F-centers transforms O ₂ ^? quantitatively into O^?? and anion vacancies. For both oxygen centers the absorption spectra in the UV contain several bands. All the O^??-bands can be interpreted as electron transfer from O^?? to the surrounding K⁺-ions considering different excited states of the potassium atom. Another band is due to the α-centers. Photochemical reaction at 20 °C yields F-centers and O^?-centers. The concentration of the O^??-centers (and of the original O ₂ ^? -centers) can be determined from the produced F-band.     

Quantenausbeute für den photochemischen Abbau von O−−-Zentren in KCl

Potassium chloride crystals containing various amounts of O^?? centers are irradiated with ultraviolet light into their bands at 2.92 eV, 4.25 eV, and 5.85 eV. It is shown that the photochemical reaction

     

Optische Absorption und photochemisches Verhalten von SH−-, S−- und S−−-Zentren in KCl-Kristallen

SH^?-centers are investigated for the first time. New other centers can be produced by photochemical reaction or by reducing treatment with an excess of potassium (F-centers). They are interpreted as S^?- and S^??-centers.     

Polar optic phonons in Hg2Cl2 and Hg2Br2

Far infrared (30–430 cm^?1) reflectivity measurements of Hg₂Cl₂ and Hg₂Br₂ single crystals have been performed in polarized light. The spectra, which are in agreement with group-theoretical predictions, were analyzed by the oscillator fitting procedure and Kramers-Kronig method. The results are compared with the existing data from other measurements and the large anisotropy of polar modes is briefly discussed. The polarization vectors of all long-wavelength symmetry modes were determined group-theoretically.     

Superconductivity in LaPd_2Bi_2 with CaBe_2Ge_2-type structure

正Since the discovery of superconductivity in LaFeAsO_(1-x)F_x,the high-T_c iron-based superconductors have been extensively studied from both experimental and theoretical viewpoints [1-8]. However, the mechanism of the unconventional superconductivity is still to be resolved. To     

Magnetic ordering in HoRu2Si2, HoRh2Si2, TbRh2Si2 and TbIr2Si2 by neutron diffraction

Neutron diffraction study of polycrystalline HoRu₂Si₂, HoRh₂Si₂, TbRh₂Si₂, and TbIr₂Si₂ was performed in the temperature range between 4.2 and 300 K. For HoRu₂Si₂ the magnetic spin alignment of a linear transverse wave mode below the Néel temperature 19 K is observed. This static moment wave is propagating along the b-axis with k=(0, 0.2, 0) and is polarized in the c-axis. The root-mean-square and maximum saturation moments per Ho atom are 9.26 and 13.09μ_B, respectively. HoRh₂Si₂, TbRh₂Si₂ an TbIr₂Si₂ are simple collinear antiferromagnets of +-+- type with Néel temperatures of (27±1), (98±2) and (72±3) K, respectively. For TbRh₂Si₂ and TbIr₂Si₂ magnetic moments are localized on RE ions only and are aligned along the tetragonal axis, while for HoRh₂Si₂ they form an angle ø = (28±3)°.     

Photoexcited Eu2+ spin dynamics in EuFe2As2

Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu²⁺ spin relaxation dynamics in EuFe₂As₂ (EFA). As previously reported in other undoped iron-based pnictides, we observe the quasiparticle relaxation bottleneck due to the charge gap opening in the spin density wave (SDW) state below T _SDW = 189 K. Below the Eu²⁺ antiferromagnetic (AFM) spin ordering temperature, T _AFM = 19 K, we observe another slower relaxation component, which we attribute to the Eu²⁺ AFM order dynamics. The slow dynamics of this component suggests a weak coupling between the Eu²⁺ spins and the carriers in the Fe-d derived bands.     

Haploid (2, 2)-superfields in 2-dimensional space-time

Superfields in 2-dimensional (2,2)-superspace-time which are independent of (some) half of the fermionic coordinates are discussed in a hopefully both comprehensive and comprehensible manner. An embarrassing abundance of these simplest ‘building blocks’ makes it utterly impossible to write down the ‘most general Lagrangian’. With some ad hoc but perhaps plausible restrictions, a rather general Lagrangian is found, which exhibits many of the phenomena that have been studied recently, and harbors many more. In particular, it becomes patently obvious that the (2,2)-supersymmetric 2-dimensional field theory target space geometries (many of which are suitable for (super)string propagation) are far more general than Kähler manifolds with holomorphic bundles.