Raymond N. Enzweiler

Local chemistry and vibrational behavior of the rare-earth in the high TC superconductor EuBa2Cu3O7

Temperature dependent 151Eu Mossbauer spectroscopy experiments on the high Tc≧90K superconductor EuBa2Cu3O7 in the range of 4.2K < T < 300K display evidence of Eu3+ site only. No evidence of Eu2+ sites is observed. Variation of isomer-shift σ(T) and recoil-free-fraction f(T) shows that the mean square displacement (lnf) and mean square velocity (σ) of the Eu3+ site are both characterized by a Debye temperature θD=280(5)K. No discernible evidence of a phonon mediated structural transition as a precursor to Tc is observed in the temperature range examined.

Lamb-Mössbauer factors as a local probe of floppy modes in network glasses

Abstract The temperature dependence of the Lamb-Mossbauer factor, f(T), in a solid provides the first inverse, 〈 1 ω 〉 , and second inverse, 〈 1 ω 2 〉 , moments of the vibrational density of states. In network glasses, these moments serve as local probes of low-frequency vibrational excitations, such as floppy modes, and provide a means to establish the rigidity percolation threshold. Lamb-Mossbauer results on prototypical chalcogenide glasses (GexSe1−x) correlate well with those of Raman scattering, inelastic neutron scattering and Mossbauer hyperfine structure experiments in indicating that the rigidity percolation threshold occurs near 〈r〉c = 2.46(4). These observations provide experimental support for predictions of the Phillips-Thorpe constraint theory, when provision is made for a small but finite concentration of broken bond-bending constraints around chalcogen sites.

A general purpose cold finger using a vibration‐free mounted He closed‐cycle cryostata)

A method for mounting a He closed‐cycle cryostat which consists of an exchange gas envelope around the cold head to cool an independently supported sample mount as in model DE202 with a DMX‐20 interface from APD Cryogenics, Inc. is described. No detectable vibration of the sample mount is observed as evaluated using 57Fe Mossbauer spectroscopy and a piezoelectric accelerometer. Using a 25 μm thick α‐Fe foil a linewidth of 0.231(3) mm/s at 300 K with the refrigerator on and the same linewidth with the refrigerator off is observed. The easy optical access afforded by such a cold finger makes it an economical general purpose laboratory tool for performing low‐T spectroscopic investigations, such as microwave, optical, γ‐ray, x‐ray and neutron‐scattering measurements. Other applications include electrical transport, SIMS, RBS, and rare‐gas matrix isolation.

Fe-site assignments in Y1Ba2Cu3O7 revisited

A new symmetric Fe3+ site (labelled E) (δ=0.28(2) mm/s and Δ=0.15 mm/s) is observed in the R1Ba2Cu3O7 family of superconductors for R=Y and Eu. The decay and growth of site E are established in two seemingly unrelated experiments, one as a function of room temperature aging for R=Y, and the other as a function of oxygen loading at elevated temperatures and pressures for R=Eu. In each experiment, the results show that site E is genetically related to the well documented chainsites A(Δ=1.9 mm/s), C(Δ=1.1 mm/s) and D(Δ=1.6 mm/s). We propose that site E represents a quasi-octahedral chain-site having pairs of O(1), O(4) and O(5) nearestneighbor oxygen sites. The two remaining doublets, B(Δ≈0.4 mm/s) and B′(Δ=0.78 mm/s) represent Fe3+ dopant sites present in the all-important CuO2 planes, possessing coordination numbers of 6 and 5, respectively.

GeSnSe3 glass — A novel exception to the Ioffe-Regel rule

Abstract Glasses based on the Ge 2−2 x Sn 2 x Se 3 ternary have been examined using scanning calorimetry, Mossbauer Spectroscopy, and molar volume measurements. The origin of extensive glass formation, especially near x = 0.5, is traced to a floppy molecular fragment based on the structure of c - GeSnS 3 . c - GeSnSe 3 does not exist in a structure isomorphic to c - GeSnS 3 , thus showing g - GeSnSe 3 to be a novel exception to the Ioffe-Regel Rule.

The unusual glass forming tendency in the Ge2−2xSn2xSe3 ternary

Homogeneous melt-quenched alloy glasses in the indicated ternary have been prepared over a wide range of composition 0<x<0.72. The molecular structure has been examined by differential scanning calorimetry and Mossbauer spectroscopy. The microscopic origin of the unusual glass forming tendency is traced to the existence of a floppy molecular fragment of zig-zag chains of GeSe4 tetrahedral units, based on the structure of c-GeSnS3.