S. E. Lambert

Superconductivity, magnetism and valence fluctuations in rare earth-transition metal borides☆

Abstract Experiments on rare earth (R)-transition metal (T) borides of the type RRh4B4 and CeT3B2 are described. The interaction between superconductivity and competing types of magnetic order produce complex and interesting low temperature phase diagrams in pseudoternary RRh4B4 systems, formed by substituting another R element at the R sites, or a different T element at the Rh sites. The compounds CeT3B2, with T  Ru, Rh and Ir, display superconductivity, ferromagnetism and valence fluctuations, respectively. The low temperature phase diagram of the pseudoternary system formed between CeRu3B2 and CeRh3B2, and resonant photoemission measurements on these two compounds are discussed in connection with the anomalous ferromagnetism of CeRh3B2.

Upper critical magnetic field of the superconducting heavy fermion system (U1−x Thx)Be13

Measurements of the ac electrical resistance as a function of temperature and applied magnetic field R(T,H) were made for various compositions x≲0.03 in the mixed binary system (U1−xThx)Be13. All samples within this range of x exhibit heavy Fermion superconductivity with a superconducting transition temperature Tc that is rapidly suppressed by the substitution of Th for U. The shape of the upper critical field curve Hc2(T) determined from the R(T,H) data is similar for all compositions with a very large initial slope (−dHc2/dT)Tc.

Magnetic and superconducting properties of rare earth osmium stannides

Abstract We present data on the magnetic and superconducting properties of rare earth osmium stannides. The compounds of Tb and Ho are superconducting only, those of Er and Tm are reentrant superconductors, and those of Gd and Dy appear to exhibit some type of short range magnetic order at low temperatures.

SUPERCONDUCTIVITY OF HEAVY-ELECTRON URANIUM-COMPOUNDS

Abstract The current understanding of the superconductivity of heavy-electron uranium compounds is reviewed. Both the pairing and the mechanism appear to have been established as unusual in UPt 3 and UBe 13 . Small-moment magnetic ordering is now known to coexist with superconductivity in URu 2 Si 2 and UPt 3 . The understanding of the properties of UBe 13 may be viewed as the central problem in heavy-electron physics.

Superconductivity and normal‐state electronic properties of URu3 and UCo: New behavior for U‐compound superconductors

The compound URu3 has been found to exhibit superconductivity below 0.15 K. Polycrystalline specimens of URu3 and two other U‐compound superconductors, UPt3 and UCo, have been investigated by means of ac magnetic susceptibility measurements in zero and finite magnetic fields. Electrical resistivity, dc magnetic susceptibility, and specific‐heat measurements on URu3 and UCo reveal non‐heavy‐fermion behavior for both compounds and evidence for relatively large exchange enhancement of the Pauli paramagnetic susceptibility for UCo.

Normal and superconducting properties of EuMo6S8 in high magnetic fields

The variation of the electrical resistanceR of EuMo6S8 with temperatureT and applied magnetic fieldH was determined at pressuresP to 19 kbar. At ambient pressure, the results are in agreement with previous work on samples of similar quality, where it was determined that variations of the carrier concentration were responsible for the very strong dependence ofR onT andH. AtP=8.5 kbar, the magnetoresistance is determined by the freezing out of the Eu2+ spins asH increases. ForP>13 kbar, EuMo6S8 exhibits bulk superconductivity, and we investigated the upper critical magnetic fieldHc2(T). The analysis of these results shows that our EuMo6S8 samples are clean superconductors. We observe thatHc2 is strongly reduced by pressure, perhaps due to an increase of the carrier concentration with pressure.

Magnetic and superconducting properties of single crystals of rare earth rhodium boride compounds RRh4B4

Abstract Small single crystals of RRh 4 B 4 compounds where R = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu, and Y have been prepared by precipitation from a molten copper flux. Measurements of the low frequency ac electrical resistance R are reported with particular emphasis on the behavior of R for temperatures T less than the magnetic ordering temperature. The upper critical magnetic field of ErRh 4 B 4 determined from R ( T ) measurements in applied magnetic fields is presented for parallel and perpendicular orientations of the tetragonal c -axis of the sample with respect to the applied magnetic field. Comparisons are made with previous work on polycrystalline samples of all the compounds as well as single crystals of ErRh 4 B 4 .

Influence of high magnetic fields (10 T) on paramagnons in rare-earth intermetallic compounds

Abstract Low temperature (0.5–20 K) high field (0–10T) heat capacity and low field (0–2T) magnetic susceptibility (60 μK−300K) measurements were used to study and characterize the quenching of spin fluctuations (paramagnons) in the highly exchange enhanced RCo2 phases, where R = Sc, Y and Lu. These RCo2 compounds are characterized by large many-body mass enhancements (2.54 to 5.38) and large Stoner factors (15.5 to 25.3). The application of magnetic fields lowers their electronic specific heat constants by 7 to 10% at 10 T. For YCo2 the coefficient of the T3 term (β) in the normal heat capacity equation is found to increase with increasing field, but for ScCo2 and LuCo2 β remains constant.

Superconductivity and magnetic interactions in Yb1.2?x Eu x Mo6S8 pseudoternary compounds

The superconducting and magnetic properties of the Yb1.2−xEuxMo6S8 series of compounds have been investigated by means of ac magnetic susceptibility, electrical resistivity, and upper critical magnetic fieldHc2 measurements. The superconducting transition temperature at ambient pressure is depressed by Eu additions, until superconductivity disappears completely atx∼0.7. For Eu concentrationsx<0.45,Hc2 is enhanced at low temperatures and exceeds the values for pure Yb1.2Mo6S8. The temperature dependence ofHc2 for the various Yb1.2−xEuxMo6S8 compounds is analyzed using a multiple pair-breaking theory that includes the compensation of the applied magnetic field by the negative exchange field produced by antiferromagnetic interactions between the conduction electron spins and the Eu2+ spins (Jaccarino-Peter effect). Hydrostatic pressure was found to induce superconductivity in all Eu-rich compositions that were not superconducting at ambient pressure.

Anisotropic magnetic and superconducting properties of single crystals of RERh4B4 compounds

We report the synthesis of small single crystals of several RERh4B4 compounds where RE=Sm, Gd, Tb, Dy, Ho, Er, and Tm using a flux‐growth technique. Measurements of the ac electrical resistance R in zero and applied magnetic fields and dc magnetic susceptibility are reported. The upper critical magnetic field Hc2(T) determined from the resistance measurements for RE=Sm, Er, and Tm are also presented. Considerable anisotropy of Hc2(T) is observed for Er and Tm, while very little is found for Sm.