J. O. Willis

Bio: J. O. Willis is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Superconductivity & Flux pinning. The author has an hindex of 40, co-authored 162 publications receiving 5495 citations.

Possibility of Coexistence of Bulk Superconductivity and Spin Fluctuations in UPt3

Abstract: Convincing evidence has been discovered for bulk superconductivity in UPt3 at 0.54 K based on specific-heat, resistance, and ac susceptibility measurements. In addition, new evidence is presented that indicates that UPt3 is a spin-fluctuation system. If true, this is the first coexistent superconductor-spin-fluctuation system.

Dependence of flux-creep activation energy upon current density in grain-aligned YBa2Cu3O7-x.

Abstract: We have investigated magnetic relaxation on a grain-aligned sample of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} powder embedded in epoxy with the magnetic field along the {ital c} axis. Through a thermal-cycling procedure and through variations in the measuring temperature over the range 10--30 K, we manage to commence magnetic relaxation from a range of values of the magnetization, {ital M}, or current density {ital J}. Analysis of relaxation measurements at a magnetic field of 1.0 T over the temperature range 10--30 K reveals a nonlinear dependence of the effective activation energy {ital U}{sub {ital e}} on {ital M} or {ital J} that is consistent with the logarithmic dependence obtained by Zeldov {ital et} {ital al}. from transport measurements. This demonstration of a nonlinear dependence of {ital U}{sub {ital e}}({ital M}) is capable of resolving anomalies in the magnitudes and temperature dependence of flux-creep activation energies obtained from magnetic-relaxation studies.

Heavy-electron metals: new highly correlated States of matter.

Abstract: Heavy-electron metals exhibit highly correlated electronic behavior at liquid helium temperatures, with conduction-electron masses some hundred times the free-electron mass. Whether "normal," antiferromagnetic, or superconducting, their electronic behavior differs drastically from their ordinary metallic counterparts. The physical origin of the large mass and unusual superconducting and magnetic properties is the strong coupling between the conduction electrons and the local f-electron moment fluctuations characteristic of these materials.

Angular-dependent vortex pinning mechanisms in YBa2Cu3O7 coated conductors and thin films

Abstract: We compare the angular-dependent critical current density (Jc) in YBa2Cu3O7 films deposited on MgO templates grown by ion-beam-assisted deposition (IBAD), and on single-crystal substrates. We identify three angular regimes in which pinning is dominated by different types of correlated and uncorrelated defects. Those regimes are present in all cases, but their extension and characteristics are sample dependent, reflecting differences in texture and defect density. The more defective nature of the films on IBAD turns into an advantage as it results in higher Jc, demonstrating that the performance of the films on single crystals is not an upper limit for the IBAD coated conductors.

Angular dependent vortex pinning mechanisms in YBCO coated conductors and thin films

Abstract: We present a comparative study of the angular dependent critical current density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and SrTiO3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns into an advantage as it results in stronger vortex pinning, demonstrating that the critical current density of the films on single crystals is not an upper limit for the performance of the IBAD coated conductors.

Iron-Based Layered Superconductor La[O1-xFx]FeAs (x = 0.05−0.12) with Tc = 26 K

Abstract: We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ∼26 K at ∼11 atom %.

Magnetically mediated superconductivity in heavy fermion compounds

Abstract: In a conventional superconductor, the binding of electrons into the paired states that collectively carry the supercurrent is mediated by phonons — vibrations of the crystal lattice. Here we argue that, in the case of the heavy fermion superconductors CePd2Si2 and CeIn3, the charge carriers are bound together in pairs by magnetic spin–spin interactions. The existence of magnetically mediated superconductivity in these compounds could help shed light on the question of whether magnetic interactions are relevant for describing the superconducting and normal-state properties of other strongly correlated electron systems, perhaps including the high-temperature copper oxide superconductors.

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

Abstract: The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3

Abstract: There are numerous potential applications for superconducting tapes based on YBa(2)Cu(3)O(7-x) (YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5-5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.