Jeffrey O. Willis

Specific heat and electrical resistivity of CeCu6 below 1 k

Abstract Results of measurements of the electrical resistivity ϱ between 0.04 and 1 K and the specific heat cp between 0.06 and 1 K of annealed polycrystalline CeCu6 are reported. ϱ varies proportional to T2 below 0.1 K but is linear in T above 0.6 K. The specific heat is proportional to T below 0.5 K and the electronic specific-heat parameter γ = 1.53 J mole K 2 .

Influence of microstructures and crystalline defects on the superconductivity of MgB2

This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at higher fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10 000 A/cm2 in 50 000 Oe (5 T) at 5 K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H>45 000 Oe(4.5 T) and T=5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2...

Heavy fermion state and pressure-induced resistive behaviour in CeRu2Si2

Abstract Pressure-induced variation in the resistive behaviour of CeRu 2 Si 2 , a Kondo-lattice system, has been studied in the temperature interval 1.5K – 300K. At low temperatures, resistivity exhibits (i) a linear temperature dependence at low applied pressures (P ⩽2.26 kbar) and (ii) a quadratic temperature dependence (T) ∼ AT 2 at higher pressures (6.5 kbar ⩽ P ⩽ 16.8 kbar). The coefficient A decrease, which in turn implies that the spin fluctuation temperature T sf increases, with increasing applied pressure. The temperature coefficient γ of specific heat C = γ T at low temperature (0.3K ⩽ T ⩽ 1.6K) is quite large (385 mJ/mole-K 2 ) confirming the heavy fermi liquid nature of the material.

Impurities in the heavy‐fermion superconductor UBe13 (invited)

Small amounts of Sc, Lu, Gd, Np, Ce, Th, La, and Ba have been substituted for uranium in UBe13 to observe their effects on the superconducting and normal state properties. The thorium, which was the most complete study, resulted in an extremely unusual nonmonotonic depression of the transition temperature for a nonmagnetic impurity. This comes from an interplay that exists between the lowest temperature resistivity peak and the transition temperature, as the peak is depressed. These results suggest that heavy Fermion superconductivity is only one of the possible ground states for heavy mass electron systems. All of the impurities tested resulted in a transition temperature depression.

A new finite-element method simulation model for computing AC loss in roll assisted biaxially textured substrate YBCO tapes

This paper presents a new finite-element simulation model for computing the electromagnetic properties and AC losses in systems of YBCO (yttrium barium copper oxide) conductors on roll assisted biaxially textured substrates (RABiTS). In this model, the magnetic field dependent permeability and ferromagnetic loss of the substrates in RABiTS YBCO tapes are taken into account. The simulations were employed to simulate the AC loss in stacks of two parallel connected YBCO tapes. The simulation results are compared with the experimental data to check the validity of the simulation model. The result reveals an effective way of significantly reducing AC loss in YBCO tapes by stacking two RABiTS YBCO coated conductors with the appropriate relative tape orientation.

Search for magnetism in hcp ε‐Fe

We report the results of our recent experimental investigation which extends the search for magnetic ordering in the high‐pressure, hcp phase of iron (e‐phase) to a temperature of 0.030 K. Mossbauer effect measurements were performed in the pressure range from atmospheric pressure to 21.5 GPa on a foil of natural iron (doped with 57Co). Measurements were made of the properties of both the α‐ and e‐phase constituents of the sample. No measurable magnetic hyperfine field was observed in the e‐phase from any of the spectra at pressures as high as 21.5 GPa and temperatures as low as 0.030 K.

Advances in YBCO-coated conductor technology

Processes for producing both the YSZ template film by IBAD and the YBCO superconducting film by PLD at 1 m lengths have achieved Ic values of 122 A and Jc values of 1 MA/cm2. Improvements have been made in all stages of the process. Variations of Ic along the length of the 1-m samples stimulated development of a new in-field Ic measurement capability. The use of MgO as an IBAD template film has made great progress and can potentially decrease the time to produce the template film by more than an order of magnitude. A combination of electrical and microstructural investigations are being made to understand and improve the properties of the YBCO coated

AC loss study of antiparallel connected YBCO coated conductors

Some applications of high temperature superconducting conductors require a non-inductive winding, which may be constructed from antiparallel connected YBCO (yttrium barium copper oxide) tapes. In the case of AC applications, this antiparallel winding changes the AC losses from that of an isolated conductor. This study focuses on the effect of the spatial separation and misalignment between conductors on their AC loss behavior for YBCO conductors on both rolling assisted biaxially textured substrate (RABiTS) and ion beam assisted deposition templates in an effort to fully understand the behavior of these conductors in real world applications. For RABiTS samples, the study was carried out for all three possible configurations (the so-called back-to-back, front-to-front and same-way configurations) to clarify the effect of the ferromagnetic substrate on the AC loss behavior in these conductor configurations. Numerical simulations were also employed in some cases to compare with and elucidate experimental observations.

Combined X-ray and neutron powder diffraction study of the structure of La1.8Sr0.2CaCu2O6: Cation ordering and oxygen stoichiometry

Abstract La 1.8 Sr 0.2 CaCu 2 O 6 exhibits superconductivity around T c =60 K after a HIP (Hot Isostatic Pressing) treatment at 1020°C for 50 h under a total pressure of 1000 atm and an oxygen partial pressure of 200 atm. A joint neutron/X-ray powder diffraction study of this superconductor was carried out to analyze both cation ordering and oxygen stoichiometry. This revealed: (1) a preferential occupation of Ca for the 2a site and of both La and Sr for the 4e site, (2) a Cu-O(1) bond length which is the shortest among known 326 cuprates, and (3) that the O(2) site contains ∼3.5% vacancies and the O(3) site is occupied at a level of ∼2.3%. A comparison of the structure of La 1.8 Sr 0.2 CaCu 2 O 6 with those of four nonsuperconducting or weakly superconducting doped 326 cuprates suggests structural differences that may be responsible for the observed differences in behavior.

Restoration and testing of an HTS fault current controller

A three-phase, 1200 A, 12.5 kV fault current controller using three HTS 4 mH coils, was built by industry and tested in 1999 at the Center Substation of Southern California Edison in Norwalk, CA. During the testing, it appeared that each of the three single-phase units had experienced a voltage breakdown, one externally and two internally. Los Alamos National Laboratory (LANL) was asked by DOE to restore the operation of the fault current controller provided the HTS coils had not been damaged during the initial substation tests. When the internally-failed coil vacuum vessels were opened it became evident that in these two vessels, a flashover had occurred at the high voltage bus section leading to the terminals of the superconducting coil. An investigation into the failure mechanism resulted in six possible causes for the flashover. Based on these causes, the high voltage bus was completely redesigned. Single-phase tests were successfully performed on the modified unit at a 13.7 kV LANL substation. This paper presents the postulated voltage flashover failure mechanisms, the new high voltage bus design which mitigates the failure mechanisms, the sequence of tests used to validate the new design, and finally, the results of variable load and short-circuit tests with the single-phase unit operating on the LANL 13.7 kV substation.