S. Blumenröder

Elastic and magnetoelastic effects in CeB6

Elastic constants of CeB6 were redetermined by ultrasonic and Brillouin scattering measurements. We found all elastic constants to be positive, especiallyc12. The temperature and magnetic field dependence of the elastic modesc44 and (c11-c12)/2 is explained on the basis of the newly proposed crystalline-electric-field level scheme for CeB6 with theΓ7 level 545 K above theΓ8 ground state. The deduced quadrupolar coupling constants give an antiferroquadrupolar ordering temperature of 3.3 K in agreement with experiment.

Raman spectroscopy as a function of the oxygen content in YBa2Cu3O7-x

Abstract We report on Raman spectroscopy of YBa 2 Cu 3 O 7-x samples with different oxygen contents. Besides other phonon modes we reproducibly observe a lattice mode at 641 cm −1 , which appears for superconducting samples and disappears if the samples are rapidly quenched from 800 °C. We conclude that the 641 cm −1 peak has to be related to vibrations of the Cuue5f8O chains in YBa 2 Cu 3 O 7-x .

Crystalline-electric-field level scheme of NdB6

We have determined the crystalline-electric-field (CEF) level scheme of NdB6 by means of Raman spectroscopy. We could resolve two CEF-transitions at 92 cm−1 and 98 cm−1 due to the spectral resolution of 2 cm−1 (=0.25 meV). The temperature dependent Raman scattering reveals the two transitions to be on top of one another, in agreement with fits of the temperature dependence of the magnetic susceptibility. The spectroscopically determined CEF level scheme yields a good fit of the elastic constants and hence the mean field prediction of a ferroquadrupolar ordering near 6 K. This result sheds new light on the phase transition in NdB6 nearTN=8.6 K which has so far been interpreted only in terms of antiferromagnetic ordering.

Relation of phonon anomalies to charge fluctuation rates in intermediate valence compounds

Abstract The charge fluctuation rate ( c ) of intermediate valence (IV) compounds is identified experimentally by the different behavior of the phonon mode frequencies for h ω ⪢ c and h ω ⪅ c . As an example IV SmB 6 is discussed. The phenomenological concept allows an understanding of the phonon anomalies in practically all so far spectroscopically investigated IV compounds.

Tunneling and point-contact spectroscopy of high-Tc superconductors M−Ba2Cu3O7 (M=Y, La, Eu)

Tunneling and point-contact measurements have been performed on different high-Tc superconductors M−Ba2Cu3O7 with M=Y, La, Eu. The average energy gaps deduced from the tunneling spectra are about 2Δ=33 meV for the Y- and La-type samples and about 2Δ=38 meV for the Eu-type samples, which give ratios 2Δ/kBTc in the range from 4 to 6. The point-contact characteristics show a distinct minimum in the differential resistance about zero bias and additional sharp spikes up to 40 mV. From these pointcontacts we can give an estimate of the critical current, yielding values in the range from 0.2 to 3 mA.

Inelastic electronic excitations in CePd3 observed by Raman scattering and luminescence emission

Abstract In intermediate valent CePd 3 the intraionic J = 5 2 → J = 7 2 excitation of the 4f 1 ( 2 F J ) configuration and the interconfigurational 4f 1 → 4f 0 + e - excitation energy have been observed near 256 and 325 meV, respectively, by means of electronic Raman scatterring. A narrow d-like band is identified near 2.4 eV above E F by d → f luminescence emission.

Resonant electron-phonon coupling in CeAl2

Abstract The resonant coupling of the Γ + 5 phonon to the Γ 7 -Γ 8 CEF excitations in CeAl 2 is probed by Raman scattering via the simultaneous observation of the renormalized (Γ + 6 ) phonon and the phonon bound state. This represents the “phononic analogue” to the instantaneous observation of electronic charge fluctuations by XPS in intermediate valence compounds.

Magnetostriction of REBa2Cu3O7−δ

The magnetostriction (MS) of REBa2Cu3O7−δ (RE=Y, Nd, Eu, Gd, Ho, Er, Yb) is reported as a function of magnetic field and temperature forH<5 T and 1.5<T<40 K. With nonmagnetic rare earths the MS is small and a linear function of the field atH>1 T. The MS is much larger when the rare earth carries a magnetic moment, as usual, but the volume MS observed with moment carrying REBa2Cu3O7−δ is larger than that of nonsuperconducting intermetalics. This effect is explained by the shielding currents which are induced by the 4f magnetic moments in the superconducting CuO-lattice. The thermal expansion of GdBa2Cu3O7−δ shows a large peak at the ordering temperature of 2.2 K in both the superconducting and the nonsuperconducting samples.

Raman scattering from spin fluctuations and phonons in the heavy-fermion superconductor UPt3

Abstract Quasielastic scattering from spin fluctuations has been observed in UPt3 by Raman spectroscopy. The experiments for wave vectors q≈0 show a nearly temperature independent linewidth for 5 K ⩽ T ⩽ 300 K Complementary to neutron scattering results this establishes the q independence of the spin relaxation rate, indicating the localized nature of the spin fluctuations. A Raman-active phonon near 79 cm-1 (10 meV) shows a drastic increase in line-width with decreasing temperature, demonstrating strong electron-phonon coupling.

Electronic and magnetic properties of the high-T c superconductor EuBa2Cu3O x

Electrical resistivity and magnetic susceptibility show that single phase EuBa2Cu3Ox becomes superconducting at 94 K. Magnetization and magnetoresistivity yield a lower critical field of Hc1≃500 G and a variation ofTcwith magnetic field of dTc/dH≃1 K/T. The high temperature resistivity shows that at about 700 K due to oxygen loss the sample is driven into a semiconducting state, in which the superconductivity is destroyed. The normal state magnetic susceptibility measured in the superconducting and in the oxygen degased, semiconducting sample indicates that the sample is paramagnetic in the normal state. This paramagnetism is larger than the one expected for trivalent Eu and therefore has to be attributed in part to a magnetic moment on Cu. Eu is found to be stable trivalent between 300 and 4.2 K through Mößbauer-spectroscopy.