Crystal growth and magnetic susceptibility of some rare-earth compounds

Magnetic susceptibility measurements have been carried out on five series of rare-earth compounds using an ac mutual inductance technique. The compounds satisfy the general formulae RNbO₄, R₃NbO₇, R₂TiO₅, R₂Ti₂O₇, or RAIO₃ where R is a rare-earth ion. The measurements were mainly carried out in the helium range 1 to 4.2° K, and the results analysed in terms of the general Van Vleck formula = C/(T+) +. Many of the compounds were observed to undergo antiferromagnetic transitions, and in some cases showed evidence of extremely anisotropic behaviour above the Néel temperature.     

Non-linear magnetic susceptibility of some vanadate glasses

The first measurements of the anomalous variation of d.c. magnetic susceptibility of the (50 – x) P₂O₅-xM-50V₂O₅ (M = Bi₂O₃ and Sb₂O₃, and x=0 to 40 mol% M) oxide glasses around 20 to 30 mol% M are reported here. Similar anomalous behaviour was also observed in the electrical and other physical properties of the glasses (reported in the previous paper). Like electrical and dielectric properties, this anomaly in the magnetic properties was also found to be mostly associated with the anomalous variation of the V⁴⁺/V⁵⁺ ratio (around 20 to 30 mol% M) with the concentration of M. From the temperature (80 to 300 K)-dependent magnetic susceptibility data the V⁴⁺ ion concentrations have also been calculated, which agree quite well with those obtained from chemical analysis, and the small discrepancy is attributed to the presence of V³⁺ and/or V²⁺ ions in the glass.     

Magnetic susceptibility measurements on some new cerium compounds for adiabatic demagnetization

Magnetic susceptibility measurements to temperatures below 5 mK were made on powdered samples of two cerium compounds: a complex of cerium thiocyanate and triphenyl phosphine oxide, and a complex of cerium iodide and antipyrine. In addition, an adiabatic demagnetization experiment was performed on the cerium thiocyanate complex which resulted in an observed minimum magnetic temperature of 1.42 mK. Also discussed is an interesting design feature of the³He–⁴He dilution refrigerator which makes possible the attainment of temperatures as low as 3.5 mK.Work supported by National Science Foundation and Research Corporation.     

Electronic specific heat and magnetic susceptibility of Ti3P-type compounds

A number of Ti₃P-type compounds, mainly based on Zr and Nb, were investigated for their low-temperature specific heat and magnetic susceptibility. Results are presented for Nb₃X (X=Si, P, As), Zr₃X (X=Si, P, Ge, As), and V₃P. We found an interesting correlation between the two electronic properties in the Zr₃X (X=Si, P, Ge, As) series. A tentative interpretation of this behavior is proposed.     

The effect of lattice contraction and charge transfer on the magnetic susceptibility of Chevrel compounds

The magnetic susceptibilities of SnMo₆S₈, PbMo_6.35S₈, and PbMo_6.2S₈ have been measured. There appear to be transitions at about 40 and 100 K. The behavior of the susceptibility is believed to be caused by changes in the lattice, affecting the charge transfer from the tin or lead atoms to the Mo₆S₈ clusters.     

Magnetic susceptibility observation of removing oxygen from some Chevrel compounds

A refined method for preventing the incorporation of oxygen into Chevrel compounds is presented. This method has been used in producing Cu_1.8Mo₆S₈, SnMo₆S₈, and PbMo₆S₈. The removal of oxygen enhances the magnetic susceptibility of these materials as well as raising their superconducting transition temperatures. The curves of magnetic susceptibility versus temperature show structures that may indicate lattice deformations.     

The effect of magnetic ordering on superconducting pairing in some rare-earth compounds

The effect of Coqblin-Schrieffer-type interaction between conduction and rare-earth ion localized electrons on the superconducting order parameter of some ternary compounds is studied. A reentrant behavior of the order parameter is found. Superconductivity can be destroyed at two temperatures: at T _c1 due to thermal pair breaking, and at T _C2(< T_c1), when = , being related to magnetic ordering of rare-earth ions and spin polarization of conduction electrons.A brief account of this work was presented at the 16th Solid State Physics Conference held at Warwick, England, January 3–5, 1979.     

Theory of superconductivity in heavy-fermion compounds

A single-order-parameter and two-gap model (a modified Suhl two-gap model) is proposed for heavy-fermion superconductors. A simple relation between two gaps is found. The thermodynamic quantities in the superconducting state are calculated in terms of this model. It is shown that the critical values of the physical quantities are quantitatively in agreement with experiment. The behavior of the specific heat, of the ultrasonic attenuation, and of the NMR rate are also in good agreement with those of CeCu₂Si₂ and UBe₁₃ at all temperatures.     

Structure and superconductivity in ternary systems of compounds

A survey of the known highT _c ternary systems of compounds is presented. Special emphasis has been placed on the part played by the crystal structure and bonding in the exhibition of highT _c superconductivity. The superconducting and other physical properties of the ternary borides have been discussed in relatively more detail. Finally, the available guidelines from crystal and solid state chemistry points of view, have been reiterated for a search for new highT _c materials.     

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Atomic ordering and superconductivity in A-15 compounds

Twenty-six different binary A-15 compounds were investigated to determine if superconductivity could be related to the degree of long-range atomic ordering on the crystallographic lattice sites. Significant changes in the superconducting transition temperatures and critical fields were produced by quenching from high temperatures followed by low-temperature annealing. These changes inT _cwere accompanied by changes in the degree of long-range ordering as determined using x-ray diffraction methods. The results can be interpreted within the framework of Weger's linear-chain model only when the B-element is a nontransition element. When both the A-and B-components are transition elements, however, the superconducting behavior will apparently depend on the nature of d-electron interactions between the component atoms. Complete ordering is not always an essential requirement for optimizing the superconducting properties.     

Possible coexistence of superconductivity and magnetism in layered compounds

The possibility of a coexistent superconducting and magnetic phase in layered transition metal dichalcogenides of the type MX ₂ A _x is investigated (M =transition metal; X = S, Se; A = magnetic ion; x 0.25). Describing such systems with a model in which a highly anisotropic electron gas interacts with a quasi-two-dimensional Heisenberg magnet, we find the following conditions for coexistence: (1) The coupling between adjacent layers of ferromagnetically ordered magnetic ions needs to be antiferromagnetic (S ^z = 0) in order to have no pair-breaking internal fields. (2) The exchange interaction between conduction electrons and magnetic ions must be very small ( 0.003 eV), since otherwise, due to the high concentration of localized magnetic moments, spin-flip scattering processes and spin fluctuations would destroy superconductivity. The theoretical prediction for the persistence of superconductivity up to concentrations of x 0.25 of magnetic ions is compared with recent experiments on Eu-intercalated TaS ₂ and NbS ₂ and related compounds showing a tendency for ferromagnetically ordered layers of Eu impurities and antiferromagnetic coupling between neighboring layers but no superconductivity for x exceeding a few percent. Reasons for the quick disappearance of superconductivity in these systems and criteria for possible observation of high-magnetic-impurity-concentration superconductivity in other layered compounds are given.     

Influence of superconductivity on magnetic exchange interactions

Using the formulation due to Fedro and Robinson of magnetic exchange in the presence of superconductivity, the mean field magnetic interaction energy has been calculated for rare earth (RE) ions in the RERh₄B₄ lattice. It is shown that increasing the strength of the superconducting interaction relative to the magnetic interaction results in a decrease of ferromagnetic coupling, and in some cases may drive ferromagnetic systems into antiferromagnetism.     

Structural and magnetic investigations of some new boron containing rare earth intermetallic compounds

By alloying boron with RPd₃ (R=rare earth) compounds, new ternary alloys of the formula RPd₃B _x (0 ≤x ≤ 1) have been prepared. The parent RPd₃ compounds crystallise in the cubic AuCu₃ type structure. The addition of boron does not change the structure but results in an expansion of the lattice. Therefore, it is likely that the small boron atoms occupy the vacant body centred position in AuCu₃ type structure. It is also observed that compounds of the composition RRh₃B can be formed in the cubic structure for all rare earths R though the parent RRh₃ compounds exist only for R=Ce (AuCu₃ type) and La, Sm, Nd and Gd (all hexagonal CeNi₃ type). This points out the role of boron in stabilising new crystallographic phases. The results of susceptibility measurements on some of the RPd₃B and RRh₃B compounds are presented. In particular, it is noted that while GdPd₃ orders antiferromagnetically withT _N=6 K, GdPd₃B does not order magnetically down to 4.2 K.     

Spin dynamics and implications for superconductivity: some problems with thed-wave scenario

We review the spin dynamics of the normal state of the cuprates with special emphasis on neutron data in both the YBa₂Cu₃O_7?δ and La_2?x Sr _x CuO₄ systems. When realistic models of the Fermi surface shapes are incorporated, along with a moderate degree of spin fluctuations, we find good semiquantitative agreement with experiment for both cuprates. Building on the success of this Fermi-liquid-based scheme, we explore the implications ford-wave pairing from a number of vantage points. We conclude that our present experimental and theoretical understanding is inadequate to confirm or refute thed-wave scenario.