G.R. Stewart

Itinerant ferromagnetism in the C-15 Laves phase - TiBe2−xCux

Abstract By substituting copper for part of the beryllium in the C-15 Laves phase, TiBe2, the phase is converted into an itinerant electron ferromagnet. The stable C-15 phase forms over the general composition range, TiBe2−xCux where 0 ⩽ x

Specific heat of single phase Nb3Ge

Abstract For the first time, the specific heat of single phase, stoichiometric, high transition temperature (21.8 K) A-15 Nb 3 Ge has been measured. From the data between 4 and 29 K, the linear term coefficient, γ, of the specific heat is found to be 30.3±1. mJ/mole-K 2 and the Debye temperature, ∅ D , is 302±2 K. The bulk energy gap parameter, 2Δ/kT c , is found to be 4.2±0.2, in agreement with tunneling measurements.

Enhancement of Superconductivity through Lattice Softening

The superconducting transition temperature of an iridium-yttrium eutectic is enhanced extraordinarily through lattice softening. This is shown by a drastically reduced Debye temperature.

The specific heat of two cubic laves compounds: TiBe2 and ThMg2

Abstract The low temperature specific heats of two isomorphous Laves compounds, TiBe2 and ThMg2, have been measured. The shape of the transition for the itinerant antiferromagnet TiBe2 is characteristic of magnetic ordering; the entropy of this transition is quite small, approximately 107 mJ/mole-K, a further indication of itinerancy. The γ, proportional to the density of states, is 42 mJ/mole-K2 for TiBe2, and the Debye temperature, θD, is 650 K. These numbers are compared to those for the itinerant ferromagnet, ZrZn2. The results for ThMg2 are γ=9.2 mJ/ mole-K2 and θD = 217 K. Results for both compounds are discussed in regards to the theory of Enz and Matthias.

The specific heat of C-phase Gd2O3☆

Abstract The specific heat of C-phase (almost cubic) Gd2O3 has been measured between 1.4 and 18 K. It shows a broad peak at 2.0 K. While the peak can be fit by a Schottky specific heat curve, the weight of all the experimental evidence to date indicates that the specific heat is due to magnetic ordering of the Gd+++ ions. The implications of the results on the technological applications of C-phase Gd2O3 are discussed.

Discovery of a superconducting A-15 phase in the V-Re system

Abstract A superconducting A-15 type phase of a very unusual stochiometry has been discovered in the vanadium-rhenium system. The phase is stable only above 1500°C and exists over a very limited homogeneity range near 29 atom percent vanadium. The lattice parameters range from 4.8774 to 4.8802 A with the corresponding superconducting transition temperatures varying from 8.4 K to 7.6 K. X-ray diffraction shows that both Re and V atoms occupy the A-sites, only Re atoms are in the B-sites and there is no evidence of any superstructure. Specific heat measurements give values of γ = 4.03 mJ/g-atom K2, β = 0.0285 mJ/g-atom K4 and θD = 409 K.

A search for strong coupling superconductivity

Abstract The specific heats of Mo0.6Re0.4 (BCC-phase), Mo0.4Re0.6 (Sigma-Phase), Mo0.18Tc0.82 (HCP-phase), and W0.65Re0.35 (BCC-phase) have been measured as part of the investigation of the basis for the extreme strong coupling previously found in the BCC and A-15 phase of Mo0.4Tc0.6. The normalized energy gaps, 2Δ kT c , of the four compounds are found to be, respectively, 5.0, 3.4, 3.6, and 3.1. Thus, of these four compounds between VIB and VIIB elements, only the BCC phase, Mo.6Re.4 has strong coupling similar to the Mo.4Tc.6 materials. The implications of this result for defining the important parameters that cause strong coupling in Mo0.4Tc0.6 are discussed.

Specific heat of A-15 Nb3Al0.83Ge0.21

Abstract The specific heat of A-15 Nb3Al.83Ge.21 with a transition temperature, Tc, of 20.0 K has been measured from 6 to 25 K. Values of the parameters derived from the data are γ=35.0±2 mJ/mole-K2; the Debye temperature, θD=278±5 K; the energy gap, △, normalized as 2△/kTc, is 4.9±.3, and the thermodynamic critical field, Hc(0), is 4700±300 Gauss.

Phason interpretation of the specific heat anomaly in TiBe2−xCux

Data of the specific heat C for TiBe2.06,TiBe1.95Cu0.05 and TiBe1.79Cu0.21 are analyzed under the assumption that the anomaly which peaks at T≅2K is due to thermal phasons, the excitations of a spin density wave. Consistent fits are obtained. A careful analysis of the anomaly below the peak is found important for the determination of CT at T = 0 and gives evidence for the existence, in TiBe2.06, of a phason gap.

The specific heat of Y0.7Th0.3C1.58

Abstract The specific heat of the novel high temperature superconductor Y0.7Th0.3C1.58 (Tc = 17.0 K) has been measured between 4 and 22 K. Unlike the other known high temperature superconductors (Tc > 16 K) which have either an A-15 or a NaCl-type structure, this material forms in the b.c.c., Pu2C3-type, structure. The Debye temperature, θD, is 346 K and the linear term coefficient, γ, of the specific heat has the value 4.66 mJ/mole-K2. Thus the electronic density of states, N(0), which is proportional to γ, is quite low. The energy gap, 2Δ/kTc, on the other hand has an anomalously high value of 5.8. Comparisons between these parameters of Y0.7Th0.3C1.58 and those for some A-15 and NaCl-type superconductors are made.