M. S. Torikachvili

Superconducting and normal state properties of Y1−xMxBa2Cu3O7−δ (M=Pr, Na)

Superconducting and normal state properties of compounds in the series Y1−xMxBa2Cu3O7−δ for 0 ≤ × ≤ 1 and Y1−xMxBa2Cu3O7−δ in the limited region of solubility 0 ≤ × ≤ 0.5 have been investigated by means of X-ray diffraction, electrical resistivity ϱ, and magnetic susceptibility χ measurements. For both systems, the superconducting transition temperature Tc decreases monotonically with the Pr or Na concentration x. The temperature T coefficient of the normal state electrical resistivity α ≡ (1/ϱ)∂ϱ/∂T changes from positive to negative at x ≈ 0.4 for Pr and x ≈ 0.2 for Na. Magnetic susceptibility χ vs T data indicate a mixed valent state for the Pr substitutions with an effective magnetic moment μeff≈ 2.7 μB per Pr ion and the gradual development of magnetic moment for Na substitutions that attains a value μeff ≈ 3 μB per Na ion in the range 0.2 ≤ × ≤ 0.5, although the Na+ ions are presumably nonmagnetic. The depression of Tc in both systems is probably due to the fact that the Pr and Na ions are not trivalent in the orthorhombic YBa2Cu3O7− δ structure (the valence of Pr apparently lies between 3+ and 4+, while the valence of Na is 1+).

UFe4P12 and CeFe4P12: Nonmetallic isotypes of superconducting LaFe4P12

The new compound UFe4P12, which was found to be isostructural to superconducting LaFe4P12 and have a lattice constant of 7.7729 A, is a semiconductor and shows ferromagnetic order below 3.15 K. CeFe4P12 is also a semiconductor, and its magnetic susceptibility is unusually small in comparison to LaFe4P12. The semiconducting behaviors of both UFe4P12 and CeFe4P12 seem anomalous and may arise from strong f‐electron hybridization.

Electronic structure of cerium and light rare-earth intermetallics

Abstract We give an overview of the use of the impurity Anderson Hamiltonian to describe the spectroscopic and low-energy thermodynamic properties of cerium intermetallics, with emphasis on interpreting 4f photoemission spectra. We show Ce valence-band resonant photoemission, Bremsstrahlung isochromat and 3d X-ray photoemission spectra for CeRu2, CeNi2, CeIr2 and CeAl, and give a complete theoretical analysis of the spectra. We summarize the relation between the large and small energy scale properties. For each system, all the spectra, as well as the static magnetic susceptibility and Kondo temperature, can be described by the model using essentially the same parameters. We also present details of resonant photoemission spectra for five other cerium compounds, CeSi2, CeOs2, CePd3, CeCo2 and CeNi5, and discuss generally the problem of obtaining the experimental 4f spectrum. Alternative theories of 4f photoemission are examined critically and we give applications of the Anderson Hamiltonian theory to CeAl2,...

High temperature superconductivity in rare-earth (R)-barium copper oxides (RBa2)Cu3O9−σ

Abstract The series of rare-earth (R)-barium copper oxides with nominal composition (RBa2)Cu3O9−σ and the prototype compound (YBa2)Cu3O9−σ have been prepared from the R (except Pm), Y, and Cu oxides and Ba carbonate by sintering, followed by arc-melting and annealing in pure oxygen. X-ray diffraction measurements show that, except for R = Ce, Tb, and Lu, the compounds have the perovskite-type structure reported by Cava et al. for high superconducting transition temperature Tc (YBa2)Cu3O9−σ. Electrical resistivity and magnetic susceptibility vs temperature measurements reveal super-conductive onsets above 90 K for all R elements, except La, Ce, Pr, and Tb, for which no superconductivity was observed down to 4.2 K. Light (heavy) rare-earth compounds tend to have relatively little (large) orthorhombic splitting in their X-ray patterns, relatively high (low) normal-state resistivities with negative (positive) temperature coefficients, and broad (sharp) superconductive transitions. The negligible destructive effect of the R magnetic moments on superconductivity indicates that the superconducting electrons are primarily associated with the Cu-O polyhedral clusters and interact only weakly with the R ions, suggesting that the (RBa2)Cu3O9−σ compounds may constitute an interesting new class of “high Tc” magnetic superconductors.

RBa2Cu3O7−δ (R = rare earth) high-Tc magnetic superconductors

Electrical resistivity p, magnetization M, and specific heat C measurements have been made on polycrystalline RBa2Cu3O7–δ compounds (R = Y or a rare earth element) which, except for R = Ce, Pr and Tb, exhibit superconductivity with TC ≈ 90K. Measurements of C(T) below Tc reveal (1) a contribution linear in T and Debye and Einstein lattice contributions for R = Y, and (2) electronic Schottky anomalies due to crystalline electric field effects and magnetic ordering for many R ions with partially-filled 4f electron shells. Measurements of ρ(T, H) and M(T, H) yield upper critical field slopes near Tc of ≥ 3T/K and critical current densities ≥3.5 × 104 A/cm2 at 4.2 K in zero field.

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.

Low temperature specific heat and magnetoresistance of the ferromagnetic semiconductor UFe4P12

Abstract Low temperature (T) specific heat and magnetoresistance measurements have been performed on the ferromagnetic semiconducting compound UFe4P12 in the vicinity of its Curie temperature Tc=3.15 K. The specific heat data reveal a pronounced peak near Tc. The magnetoresistance above Tc is large and negative, whereas near Tc, it is initially positive, passes through a maximum, then decreases and becomes large and negative.

Heavy electron behavior in a new uranium-based ternary compound: UPd2Sn

Abstract Electrical resistivity, magnetic susceptibility and specific heat measurements on a new compound, UPd 2 Sn, reveal that this material exhibits valence fluctuation or Kondo lattice phenomena below a characteristic temperature ∼ 10 K. In particular, the electronic specific heat coefficient appears to be strongly temperature dependent with a maximum of ∼ 270 mJ/mole U−K 2 at 9.7 K and an extrapolated value of ∼ 70 mJ/mole U−K 2 at 0 K. The compound UPd 2 Sn was expected to crystallize in the same structure as the family of cubic Heusler alloys, but instead, crystallized in a more complicated structure which appears to be orthorhombic. The compound can be characterized as a nonmagnetic, nonsuperconducting heavy electron material.

High temperature superconductivity in (M1−xM′x)2CuO4−δ and related compounds (M = Y, La, Eu, Sm; M′ = Ba, Sr)

Abstract Compounds of the form (M 1−x M′ x ) 2 CuO 4−δ and related compounds where M and M′ are Y, various rare earths from La to Lu, and the alkaline earths Sr and Ba, have been investigated in connection with high temperature superconductivity. High temperature superconductivity is confirmed for the system (La 1−x Ba x ) 2 CuO 4−δ , (La 1−x Sr x ) 2 CuO 4−δ and (Y 1−x Ba x ) 2 CuO 4−δ with superconducting transition temperature T c onsets of 30 K, 38 K and 90 K, respectively. We have found that the related systems (Eu 1−x Ba x ) 2 CuO 4−δ and (Sm 1−x Ba x ) 2 CuO 4−δ also exhibit high temperature superconductivity with T c onsets of 95 K and 65 K, respectively. The highest T c onset observed in this investigation was 97 K for a sample with the nominal composition of the spinel structure Y 0.33 Ba 0.67 Cu 2 O 4−δ . Measurements of the specific heat C as a function of temperature T on a La 0.8 Sr 0.2 CuO 4−δ sample reveal a break in slope in the C/T vs T curve at the T c midpoint, but no clearly discernable jump in C at T c . A linear term ≈ λ′T in C was observed at low temperature in the superconducting state.

Observation of the coexistence of superconductivity and long-range magnetic order in TmRh4B4

The ac electrical resistance, heat capacity, static magnetic susceptibility, thermal conductivity, and linear thermal expansion coefficient have been measured for the superconducting ternary compound TmRh4B4. The results indicate that the Tm3+ magnetic moments order at about 0.4 K, while bulk superconductivity, which occurs at 9.8 K, persists to temperatures below 60 mK, which was the low-temperature limit of the apparatus. Various possibilities for the type of magnetic order are examined.