Y. Dalichaouch

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+).

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.

Non fermi liquid ground states in strongly correlated f-electron materials

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M1−xUxPd3 (M = Sc, Y), U1−xThxPd2Al3, and UCu5−xPdx. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility χ for T ≪ T0, where To is a characteristic temperature: P(T) ∼ 1 –aT/T0, where a < 0 or > 0, C(T)/T ∼ (-1/To) In (T/bT0), and χ(T) ∼ 1 −c(T/To)1/2. In several of the f-electron systems, the characteristic temperature To can be identified with the Kondo temperature Tk.

Non fermi liquid behavior in strongly correlated f-electron materials

Evidence for non Fermi liquid (NFL) behavior in y1−x UxPd3 and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Low temperature electrical resistivity, specific heat, and magnetic susceptibility measurements on the Th1−xUxPd2Al3 system reveal unconventional Kondo behavior with NFL low temperature characteristics. Magnetic susceptibility measurements on UCu3.5Pd1.5, which has previously been shown to exhibit NFL behavior, are presented. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity ϱ ∼ 1−aT with either positive or negative coefficient a, a logarithmically diverging specific heat C/T ∼−lnT, and T1/2 asymptotic behavior of the magnetic susceptibility χ ∼ 1 − T1/2.

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.

On narrowing of the resistive superconducting transition in applied magnetic fields and positive curvature in Hc2(T) in the (Gd1−xPrx)Ba2Cu3O7−δ system

Abstract The superconducting transition temperature T c and the temperature dependence of the upper critical field H c2 were measured resistively on polycrystalline samples in the (Gd 1− x Pr x )Ba 2 Cu 3 O 7−δ system. The experimental results reveal that T c decreases monotonically with increasing x and vanishes at x ≈0.49. Whereas H c2 of a sample with x =0.10 has an almost linear temperature dependence and the superconducting transition broadens with increasing magnetic field, a sample with x =0.45 shows strong positive curvature in the H c2 versus temperature curve and narrowing of the superconducting transition width with increasing magnetic field. Both phenomena observed in the sample with x =0.45 can be explained qualitatively in terms of a negative exchange interaction between the superconducting charge carriers and the Pr 4f electrons.

Normal and superconducting state magnetic properties of RBa2Cu3O7−δ compounds

Abstract Magnetic properties have been investigated for polycrystalline samples of RBa 2 Cu 2 O 7−δ with R = Y and all the rare earth elements except Pm. The normal-state magnetic susceptibilities of these compounds can be described as χ ( T )= χ 0 + Nμ 2 eff /[3 k b ( T − ϑ )], with μ eff comparable to the values of free R 3+ ions. The superconducting-state magnetizations for all measured superconducting compounds are very different for different R while the field and temperature dependence of critical current densities J c ( H , T ) inferred from the hysteresis of M ( H ) isotherms at different temperatures are similar. J c decreases with H , rapidly in low fields and slowly in high fields, and decreases by two orders of magnitude as the temperatures increase from 4.2 to 65 K. At 4.2 K and 180 kOe, the critical current densities of Gd, Yb, and Tm compounds range from 1.6 to 17 kA/cm 2 .

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.

Positive muon Knight shift and spin relaxation in heavy fermion superconductors UPt3 and UBe13

Abstract We report muon spin rotation/relaxation (μSR) measurements of the heavy fermion superconductors UBe 13 and UPt 3 . In both materials we find that the muon Knight shift is unchanged in the superconducting state, consistent with odd-parity pairing (such as p-wave). The magnetic field penetration depths in UPt 3 and UBe 13 are extremely long, greater than 10000 A. We find no evidence of a magnetic transition in UBe 13 below 10 K.

Advances in photoemission spectroscopy of f-electron materials

Abstract The recent capability to perform angle-resolved photoemission spectroscopy (ARPES) with improved energy and angle resolution over a wide photon energy range has provided new information about f-electron systems. Results for two rather different systems, 5f-electron heavy fermion metal URu2Si2 and 4f-electron mixed-valent SmB6 are compared and contrasted.