Library

Notes: An inelastic electronic excitation has been observed in CeB6 near 47 meV (545 K) by means of neutron and Raman spectroscopy. This excitation has been identified as the Γ8→Γ7 crystalline-electric-field (CEF) transition. From the anomalous energy shift of this excitation at low temperatures, detectable due to the high resolution of Raman spectroscopy (±5 K), we deduce a Γ8 ground-state split by about 30 K. With this new CEF level scheme the first consistent interpretation of so far seemingly unrelated thermal, elastic, and magnetic data is achieved. In spite of the large body of thermal, magnetic, and elastic data of CeB6 accumulated over the past years, various diverging proposals for the CEF level scheme have been reported. In all schemes a Γ7 ground state was assumed, but the CEF splittings ranged from 10 K6 to more than 400 K. The absence of CEF excitations in direct spectroscopic measurements up to 400 K has been puzzling, and pointed to the necessity of high-energy neutron experiments. We have performed inelastic magnetic neutron scattering experiments using high-energy incident neutrons up to 185 meV from the Intense Pulsed Neutron Source at Argonne National Laboratory. An inelastic peak at 46 meV (530 K) is clearly identified as magnetic scattering by its Q dependence. The absolute intensity corresponds to the value of a Γ8→Γ7 transition. In addition, we have carried out Raman measurements on (100) faces of CeB6. At room temperature we found an inelastic excitation at 372 cm−1 (530 K). It is absent in the reference compound LaB6. From the polarization analysis we found that the transition has Γ3+ and Γ5+ symmetry. Both facts characterize this excitation as the Γ8→Γ7 CEF transition, which is the first CEF excitation seen in a metal by means of Raman spectroscopy. The high resolution of Raman spectroscopy enabled us to detect a 10-cm−1 shift of the Γ8→Γ7 transition energy to 382 cm−1 for temperatures below 20 K. This can only be explained by assuming a Γ8 ground state, which is split by 30 K. From the spectroscopic data we can establish a completely new CEF level scheme for CeB6 with a Γ8 ground state, split by about 30 K and a Γ7 state 545 K above. This allows a straightforward interpretation of various other experimental data, such as magnetic entropy, static magnetic susceptibility, high-field magnetization, magnetic form factor,8 temperature, and magnetic-field-dependent elastic constants and the antiferroquadrupolar ordering below TQ =3.3 K. A recent theoretical investigation of magnetic ordering of a periodic Anderson Hamiltonian with orbital degeneracy has been applied to CeB6. On the basis of entropy and magnetization data it was assumed that the quartet Γ8 is the ground state. Hence it was predicted that the low-temperature phase II (2.1 K〈T〈3.3 K) is due to an orbital antiferromagnetic ordered state, contrary to the conclusions of an antiferroquadrupolar ordering. This work was in part supported by Deutsche Forschungsgemeinschaft, SFB 125 and SFB 126.

Notes: We report on the first observation of standing spin waves in thin evaporated Fe films by means of Raman scattering (RS). In earlier investigations using Brillouin scattering (BS) these modes could only be observed down to a thickness d≈200 A(ring). We were now able to extend this range down to d≈120 A(ring) using BS and to 40 A(ring) using RS. We find that for values of d comparable to the penetration depth of the light the scattering intensity decreases together with d. This is explained by the assumption that the scattering intensity is determined by the net magnetic moment resulting within the probing depth of the light. The mode frequencies ν observed by RS as a function of d follow very well a ν∼D@B|exd−2 relationship from which the value of the exchange parameter Dex has been determined.

Notes: Abstract Recent progress of light scattering in metals is reviewed by selected highlights: Raman scattering in CeB6 has established a new crystalline electric field (CEF) level scheme with aΓ 8 ground state. The resonant CEF-phonon coupling has been identified in CeAl2. The ionic model description of intermediate valence of rare earth (RE) ions in metals could directly be proved by Raman spectroscopy in EuPd2Si2. Brillouin scattering from elastic surface wave excitations of bulk metals or supported thin layers can now be used for the reliable determination of their elastic constants. For very thin metallic layers light scattering has been very successful in observing the dispersion of standing spin waves and their elastic analogue, the Sezawa waves. From the standing spin waves the exchange constant can be determined most directly. The investigation of Au clusters deposited on NaCl has revealed a new localized acoustic surface mode (“cluster mode”). Its frequency crosses with increasing Au deposition the frequency of the transverse acoustic phonon mode of the substrate at the percolation threshold.

Notes: Abstract 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 nearT N =8.6 K which has so far been interpreted only in terms of antiferromagnetic ordering.

Notes: Abstract Tunneling and point-contact measurements have been performed on different high-T c 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/k B T c 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.

Notes: Abstract An anomalous splitting of theJ=0→J=1 multiplet transition has been observed by magnetic neutron scattering from intermediate valent Sm0.75Y0.25S. The splitting is discussed in terms of a resonant coupling of optical phonons to the magnetic exciton built from the localizedJ=0→J=1 multiplet transition by magnetic intersite coupling.

Notes: Abstract Electrical resistivity and magnetic susceptibility show that single phase EuBa2Cu3O x becomes superconducting at 94 K. Magnetization and magnetoresistivity yield a lower critical field of Hc1≃500 G and a variation ofT c with magnetic field of dT c /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.

Notes: Abstract 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.

Notes: Abstract Elastic constants of CeB6 were redetermined by ultrasonic and Brillouin scattering measurements. We found all elastic constants to be positive, especiallyc 12. The temperature and magnetic field dependence of the elastic modesc 44 and (c 11-c 12)/2 is explained on the basis of the newly proposed crystalline-electric-field level scheme for CeB6 with theΓ 7 level 545 K above theΓ 8 ground state. The deduced quadrupolar coupling constants give an antiferroquadrupolar ordering temperature of 3.3 K in agreement with experiment.