H. C. Ku

Superconducting and magnetic properties of new ternary borides with the CeCo3B2-type structure

Abstract Four new ternary boride systems with the CeCo 3 B 2 -type structure are reported with the general formulae: MRu 3 B 2 (M = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Th or U), MRh 3 B 2 (M = La, Ce, Pr, Nd, Sm, Eu or Gd), MOs 3 B 2 (M = Lu or U) and MIr 3 B 2 (M = La, Th or U). Most members of these systems were found to become either superconducting or magnetically ordered. The structure and properties of these materials are discussed in relation to those of other ternary systems previously reported to exhibit superconductivity and/or magnetic order.

Field-induced crossover from cluster-glass to ferromagnetic state in La0.7Sr0.3Mn0.7Co0.3O3

The thermodynamics and kinetics of the magnetic states of the semiconductor-like compound La0.7Sr0.3Mn0.7Co0.3O3 were studied by detailed magnetic measurements. The spontaneous magnetic state was found to be a cluster-glass phase which can cross over to a ferromagnetic state under an external magnetic field H. The defreezing temperature Tf for the cluster-glass phase and the merging temperature Tm for zero field cooled and field cooled magnetizations are related to the field H by Hj=H0j(1−Tj/Tc)nj, where H0j and nj are constants and j=f,m. Codoping tends to destroy the double exchange in Mn4+–O–Mn3+ and broadens the coexistence region of the cluster-glass and ferromagnetic states. The magnetic relaxation was found to deviate from the usual logarithmic time dependence and follow a power law.

Crystal structure and physical properties of new ternary rare earth borides

Abstract Two ternary boride systems with a new type of structure are reported for MRh 3 B 2 (M ≡ Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) and MIr 3 B 2 (M ≡ Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y). These materials were found to crystallize in a base-centered monoclinic structure with space group C 2/ m and two formula units per unit cell. The proposed atomic positions were determined by a structural study of the ErIr 3 B 2 compound. The magnetic and superconducting properties are discussed by comparisons with those of other related binary and ternary compounds.

Intermediate valence behavior of ternary cerium and uranium transition metal borides

Low-temperature specific heat, electrical resistivity, ac magnetic susceptibility, and dc magnetization measurements were made on ternary cerium and uranium transition metal borides with the general formula CeT3B2 (T=Co, Ru, Rh, and Ir) and UT3B2 (T=Co, Ru, and Ir). The compound CeRu3B2 was found to exhibit superconductivity below 0.68 K. The values of the electronic specific heat coefficient range from 9.7 mJ/mole-K2 for CeCo3B2 to 64 mJ/mole-K2 for UIr3B2. The electrical resistivity versus temperature curves of all of the compounds exhibit negative curvature and are reminiscent of valence fluctuation behavior. In the case of CeIr3B2, the electrical resistivity attains a maximum value at 180 K, while the dc magnetic susceptibility has a temperature dependence that is typical of intermediate valence Ce compounds, approaching a finite value at zero temperature. The electrical resistivity of the ferromagnetic compound CeRh3B2 reveals a rapid decrease in spin disorder resistivity below 120 K. The dc magnetic susceptibility of this material can be described as the sum of a constant term and a Curie-Weiss contribution with an effective magnetic moment of 1.01 µB per formula unit and a Curie-Weiss temperature of 119 K. Magnetization measurements on CeRh3B2 yield a saturation magnetic moment of 0.37 µB per formula unit and a Curie temperature of 113 K.

Tl0.5Pb0.5Sr2CuO5: a nonsuperconductor despite an “optimum” hole concentration in the Cu-O sheets

Abstract Doping of La 3+ for Ba 2+ in TlBa 2 CuO 5 leads to superconductivity in the material TlBa 1.2 La 0.8 CuO 5 (with T ≈52 K). An increase in the Cu-O in-plane bond distance [1.9240(1) A] from that in TlBa 2 CuO 5 (1.915 A) is consistent with a lowering of the average hole concentration, p , in the Cu-O planes from 1.0 to approximately +0.4. We have doped Pb 4+ for Tl 3+ in TlSr 2 CuO 5 to give an average hole concentration of +0.53, but find that Tl 0.5 Pb 0.5 Sr 2 CuO 5 is not superconducting down to 5 K. The structure is similar to TlBa 2 CuO 5 and TlSr 2 CuO 5 , belonging to the space group P4/mmm. Structural refinement using neutron powder diffraction data collected at 300 K show that the planar lattice constant [ a =3.7309(3) A] is considerably smaller than that for TlBa 1.2 La 0.8 CuO 5 . This leads to the substantially smaller Cu-O in-plane bond distance of 1.8654(15) A. The atoms in the Tl/Pb-O(3) planes are again disordered, and both the cation and the oxygen site occupancies are close to stoichiometry. This stoichiometry and the Tl/Pb oxidation states suggest that the hole concentration is close to the optimum value for these materials. Several reasons for the absence of superconductivity are discussed.

Superconductivity of new ternary borides with the LuRuB2-type structure

Abstract A new group of superconducting ternary borides has been found with the general formula MTB 2 , where M is Sc, Y or Lu and T is Ru or Os. The superconducting transition temperatures range from 1.3K for ScOsB 2 to 10.0K for LuRuB 2 . Additionally, ismorphous compounds with M=Tb, Dy, Ho, Er or Tm order magnetically with critical temperatures in the range 2K to 46K. The occurrence of superconductivity and magnetism in this new ternary boride structure is discussed with respect to analogous data in MT 4 B 4 systems.

Superconductivity, magnetism and metastability of new ternary iridium borides

Abstract Superconductivity has been found in the rare earth iridium borides with the general formula MIr 4 B 4 and in the pseudoternary (Y x Lu 1−x )−Ir 4 B 4 system, M being a rare earth element. The pure phase in itself is only metastable at best. The compounds were found to crystallize with the primitive tetragonal CeCo 4 B 4 -type structure. The close correlation between superconductivity, magnetism and structural metastability is demonstrated with the study of the pseudoternary systems M(Ir x Rh 1−x ) 4 B 4 ( x = 0.1 and 0.5) and Er(Ir x Rh 1−x ) 4 B 4 .

Transport and magnetic properties in perovskite compound

Magnetic and transport properties of the series (x = 0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1.0) have been investigated. It was found that with increasing Co content, the Curie temperature decreases, while the spin glass transition temperature decreases first then increases. As for the sample with x = 0.3, the relation of and the merging point (for the zero-field-cooled and field-cooled magnetization curves starting to merge) with the applied magnetic field H follows a relation , ), where and are constants. The resistivity increases drastically with Co doping and a semiconductor-like behaviour is displayed in the sample x = 0.3. These facts strongly suggest that the double-exchange interaction in the perovskite materials is very fragile since a small Co content can change the magnetic and relevant transport properties of the system.

Superconductivity and magnetic ordering in the pseudoternary Ho(IrxRh1−x)4B4 system

Abstract The boundaries between the paramagnetic, superconducting and magnetically ordered phases in the tetragonal pseudoternary system Ho(IrxRh1-x)4B4 have been established by means of magnetic susceptibility measurements down to 1.2 K. Antiferromagnetic ordering occurs for x ⩾ 0.60 with Neel temperatures higher than the superconducting transition temperatures, showing a new aspect of the coexistence of magnetic order and superconductivity.

Crystal structure of LuRuB2

Ternary metal borides of the formula MTB/sub 2/ have been synthesized in a new structure for M=Sc,Y,Tb,Dy,Ho,Er,Tm,Lu, and T=Ru,Os. Single crystal x-ray diffraction data on LuRuB/sub 2/ were refined on an orthorhombic unit cell, space group Pnma, Z=4, and yielded an R factor equal to 0.085. All metal atoms are coplanar with short bond distances of 3.10A between rare earth atoms which are arranged in zig-zag chains. 1 figure, 4 tables.