David D. Lawrie

Boron isotope effect in Ni and Pd based borocarbide superconductors

Abstract The boron partial isotope effect (10B-11B) was investigated in YNi2B2C and in YPd5B3C0.3. Samples with pure 10B and 11B, as well as natural boron, and a 1 : 1 mixture of 10B and 11B were investigated. We find a shift of the magnetic transition temperature of ΔT c = T c ( 10 B )−T c ( 11 B ) = 690 mK for the Pd based samples, and of 380 mK for the Ni based samples. This corresponds to partial isotope exponents αB = 0.25 ± 0.04 (Ni based samples) and αB = 0.32 ± 0.04 (Pd based samples). This supports the classification of these materials as electron-phonon mediated superconductors. Preliminary investigations of the carbon isotope effect (12C-13C) did not show a change in Tc.

The copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

The copper isotope effect (63Cu —65Cu) was investigated for oxygen-deficient YBa2Cu3O7−δ with transition temperatures between 40 and 91 K. The isotope exponent αCu=−ΔℓnTc/ΔℓnmCu is negative for all transition temperatures. It is small, αCu≈−0.14, in the 60 K plateau, and unobservable for Tc=91 K. In regions away from these special conditions, αCu reaches large values of up to αCu=−0.4.

Doping dependence of the critical field Hc2 and the transition temperature in Zn doped YBa2(Cu1−xZnx)3O7−δ

We present data on the doping dependence of Tc in Zn doped YBa2(Cu1−xZnx)3O6.94, for x up to 10.6%. We find re-entrant behaviour in the range between 9.0 and 10.6% Zn. Data on the magneto-resistance of a sample with extremely low Tc (9% Zn, zero field resistive onset near 3.5 K) show linear normal resistance in all fields down to 1.8 K in 17.5 T. A resistively defined critical field H*, related to Hc2, shows divergent behaviour. The temperature dependence of H* is well represented by expressions derived for the Bose condensation of pre-formed pairs.

High-resolution thermal expansion of MgB2

The thermal expansion of polycrystalline MgB2 from 5–300 K is studied using high-resolution capacitance dilatometry. The thermal expansivity exhibits a small jump of −5.8×10−8 K−1 at Tc (in accord with expectations from the Ehrenfest relationship and published specific heat and pressure data) and a negative peak-like feature close to 5 K. No indications of any structural instabilities are observed.

The copper isotope effect in oxygen depleted YBa2Cu3O7−δ

Abstract The copper isotope effect ( 63 Cu 65 Cu) was investigated for oxygen depleted YBCO with transition temperatures between 61.2 and 69.5K. Values of the copper isotope exponent α Cu vary between −0.34 and −0.10. The absolute value of α Cu decreases as the transition temperature moves from above to the 60 K plateau.

Search for an isotope effect in the antiferromagnetic transitions of cupric oxide CuO

Abstract The incommensurate antiferromagnetic transition near 230 K, and the commensurate antiferromagnetic transition near 210 K of CuO were investigated for isotopically pure 63 CuO, 65 CuO, Cu 16 O , and Cu 18 O samples. No isotopic shift was observed within the experimental uncertainty at either of the two transitions.

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (63Cu-65Cu), αCu =-ΔℓnTc/ΔℓnmCu, for two isotopic pairs of oxygen-deficient YBa2Cu3O7−δ, where δ varies between 0.06 and 0.52. αCu is below 0.01 at δ=0.06 (fully oxygenated), it takes values between −0.14 and −0.34 in the “60 K plateau”. Larger negative values of αCu are observed away from the plateau. The δ dependence of αCu is similar to that of the pressure effect dℓnTc/dP.

An investigation of the Boron isotope effect in Ni and Pd based boride carbide superconductors

Abstract The boron isotope effect (10B-11B) was investigated for the boride carbide superconductors YNi2B2C and YPd2B2C. A sizable isotope effect was observed for both systems and values for the boron isotope exponent were similar in the two systsems (αB≈0.27).