R.W. McCallum

Superconducting and normal state properties of ErRh4B4 and LuRh4B4

Detailed heat capacity measurements of the ternary compounds ErRh4B4 and LuRh4B4 have been performed between 0.5 and 36 K and in magnetic fields up to 4 kG, yielding new information on crystal field effects in these materials and on the influence of externally applied magnetic fields on the coupled superconducting-ferromagnetic reentrant transition in ErRh4B4. Static magnetic susceptibility data on LuRh4B4 are presented which allow qualitative conclusions to be drawn regarding the magnitude of exchange enhancement and orbital paramagnetic effects in the RERh4B4 compounds. The electrical resistivity of ErRh4B4 has also been determined between 4 K and room temperature.

Superconducting and normal state properties of Li1+x Ti2?x O4 spinel compounds. II. Low-temperature heat capacity

Heat capacity data are reported which confirm as a bulk effect the previously reported superconductivity in LiTi2O4. These data also establish LiTi2O4 as a weak couplingd-band superconductor with superconducting state properties well described by the Bardeen—Cooper—Schrieffer theory of superconductivity. The properties of LiTi2O4 are compared with those of other superconducting spinel compounds, and the composition dependence ofTc for Li1+xTi2−xO4 is discussed. The disappearance of superconductivity forx≳0.1 was found to be correlated with a rapid decrease in the normal-state linear heat capacity coefficient.

Coexistence of superconductivity and long-range antiferromagnetic order in the compound ErxMo6Se8

Abstract Low temperature specific heat and magnetic susceptibility measurements indicate that superconductivity and long-range antiferromagnetic order coexist in the compound Er x Mo 6 Se 8 ( x = 1.0 and 1.2).

Phase transition in the superconducting state in Gd-rich pseudoternary compounds (La1-xGdx)1.0Mo6Se8☆

Abstract A pronounced lambda-type specific heat anomaly in the superconducting state is reported for Gd-rich pseudoternary compounds (La1-xGdx)1.0Mo6Se8. The anomaly is very similar to that previously reported for the compound Gd1.2Mo6Se8. Both the temperature of the lambda anomaly (≈ 3.5 K) and the Curie-Weiss temperature were found to be nearly independent of x. In addition, evidence for the occurence of magnetic ordering at 0.8 K in Gd1.2Mo6Se8 is presented. These observations support our earlier conjecture that the lambda anomaly at 3.5 K does not arise from magnetic ordering of the Gd3+ ions. The anomaly appears instead to be due to a different type of phase transition which is associated with the presence of the localized 4f electrons of the Gd3+ ions, but the exact nature of the transition remains to be established.

Observation of a Kondo effect for a dilute alloy containing Sm impurities

Abstract Measurements of the depressions of the superconducting transition temperature T c with Sm impurity concentration and the specific heat jump at T c as a function of T c , and the temperature dependences of the normal state specific heat and magnetic susceptibility are reported for the matrix impurity system ( La Sm)Sn 3 . The results constitute the first definitive evidence of a Kondo effect for a dilute alloy containing Sm impurities.

Kondo Anomalies due to Rare Earth Impurity Ions

Rare earth (RE) ions are capable of producing “Kondo-like” anomalies in the physical properties of both concentrated and dilute RE metallic systems which are qualitatively similar. These temperature dependent anomalies scale with a characteristic temperature T0, above which the RE metallic system behaves magnetically and below which it behaves nonmagnetically. The physical properties in which Kondo-like anomalies are found include the electrical resistivity, magnetic susceptibility, thermoelectric power, specific heat, and, in systems where appropriate, superconducting properties such as the critical temperature Tc and the jump in specific heat which occurs at Tc. The anomalies can be viewed as originating from admixture of the localized 4f electron states with the extended states of the conduction band, which can be traced to the unstable valence of certain RE ions.

Anomalous pressure dependences of the superconducting transition temperature of dilute alloys of LaSn3 containing light rare earth impurities

The initial depressions of the superconducting transition temperature Tc of LaSn3 by rare earth (RE) impurities are known to be anomalous. In an attempt to clarify this behavior, we have measured the pressure dependence of the Tc of (LaRE)Sn3 alloys (RE = Y, Ce, Pr, Nd, Sm, Eu, Gd, and Lu) to pressures up to 25 kbar. The magnitude of the pressure dependence of the Tc of (LaRE)Sn3 alloys containing RE impurities with partially filled 4f electron shells decreases monotonically with increasing RE atomic number. A general discussion of existing zero- and high-pressure data for various physical properties of (LaRE)Sn3 alloys demonstrates that the association of large pressure dependences of Tc with the occurrence of unstable impurity 4f shells (the Kondo effect) should be made with caution. The experimental high-pressure results are analyzed in terms of appropriate theoretical models for magnetic and nonmagnetic localized impurity states in superconductors.

Electron spin resonance on GdxMo6Se8 (x = 1.0 and 1.2)

Abstract Electron spin resonance of Gd3+ in GdxMo6Se8 (x = 1.0 and 1.2) has been measured in X-band and in K-band between 1.4 and 290 K. The results indicate that the conduction electron - Gd3+ spin exchange coupling constant is positive and equal to +0.011 eV, in agreement with previous determinations based on superconductivity and static magnetic susceptibility measurements.

Specific heat of a new metastable phase of scandium-chromium☆

Abstract Specific heat and magnetic susceptibility measurements are reported for a new metastable phase of scandium-chromium with the E9 3 crystal structure. The results show that this phase exhibits bulk type II superconductivity with an onset near 6.7 K.

Free energy and thermodynamical critical fieldH c (T) of a superconductor with crystalline-field-split magnetic impurities: (La/itPr)Sn3

An expression for the free energy of a superconductor containing magnetic impurities with crystalline-field-split energy levels is derived. It is used to calculate the thermodynamic critical fieldHc(T) of(LaPr)Sn3. ExperimentallyHc(T) is determined from measurements of the heat capacity of(LaPr)Sn3 in the normal and superconducting states. The two results are in general agreement and demonstrate the role of the inelastic pair-breaking mechanism in this nonmagnetic ground-state system.