Scott A. Baily

Pseudoisotropic Upper Critical Field in Cobalt-Doped SrFe2As2 Epitaxial Films

We present resistivity measurements of the complete superconducting upper critical field (H{c2}) phase diagram as a function of angle (theta) and temperature (T) for cobalt-doped SrFe2As2 epitaxial films to 0.5 K and 50 T. Although H{c2}(theta) at 10 K is indistinguishable from that derived from a single-band anisotropy model, the apparent anisotropy H{c2}{ perpendicularc}/H{c2};{ parallelc} linearly decreases to 1 at low T, with H{c2}(0)=47 T. The data are well described by a two-band model with small, opposing anisotropies for the bands. This unusual relationship is confirmed by the observation of a local maximum for H{c2};{ parallelc} at low T.

Liquid vortex phase and strong c-axis pinning in low anisotropy BaCoxFe2 − xAs2 pnictide films

We present linear and nonlinear electronic transport measurements as a function of magnetic field strength and orientation at different temperatures for BaCoxFe2 ? xAs2 biaxially oriented films grown on (La, Sr)(Al, Ta)O3 single crystals and determine the critical current density, melting line and upper critical field (Jc, Hm and Hc2). Unlike observations on single crystals of the same composition with similar Tc ~ 20?K and anisotropy ? ~ 2, we observe here a vortex liquid phase as a broadening of the resistive transition with increasing H. We find evidence of a high density of correlated defects that generate a very large Jc peak as a function of field orientation centered near the c-axis orientation with Jc > 1?MA??cm ? 2 at 4?K and 1?T and reduce the dissipation in the vortex liquid state along the defect direction. These studies indicate that these correlated defects could be more effective than the correlated defects found in more anisotropic cuprate superconductors.

Highly aligned carbon nanotube forests coated by superconducting NbC

The formation of carbon nanotube and superconductor composites makes it possible to produce new and/or improved functionalities that the individual material does not possess. Here we show that coating carbon nanotube forests with superconducting niobium carbide (NbC) does not destroy the microstructure of the nanotubes. NbC also shows much improved superconducting properties such as a higher irreversibility and upper critical field. An upper critical field value of ~5 T at 4.2 K is much greater than the 1.7 T reported in the literature for pure bulk NbC. Furthermore, the aligned carbon nanotubes induce anisotropy in the upper critical field, with a higher upper critical field occurring when the magnetic field is parallel to the carbon nanotube growth direction. These results suggest that highly oriented carbon nanotubes embedded in superconducting NbC matrix can function as defects and effectively enhance the superconducting properties of the NbC.

Ultrathin epitaxial superconducting niobium nitride films grown by a chemical solution technique

Ultrathin epitaxial superconducting NbN (18 nm) films, exhibiting a superconducting transition temperature of 14 K and a critical current density as high as 5.2 MA cm(-2) at 5 K under zero magnetic field, were grown on SrTiO(3) (STO) by a chemical solution technique, polymer assisted deposition (PAD).

Angular and field properties of the critical current and melting line of Co-doped SrFe2As2 epitaxial films

We present measurements of the field and angular dependence of the critical current density (Jc) and melting line made using transport and magnetization techniques for SrFe1.8Co0.2As2 (nominal composition) biaxially oriented films. At high magnetic fields the angular dependence of the melting line and Jc (for ?0H>2?T) can be successfully scaled using an anisotropic scaling with ? = 2, indicating a random-point-like pinning contribution. At lower fields, angular and field Jc dependences strongly depend on the magnetic field history. We find a hysteretic behaviour of Jc with H with higher Jc in the descending branch, similar to that attributed to magnetic pinning and granularity effects. Furthermore this allows us to tune Jc(?) such that a Jc peak along the c axis can be enhanced up to 30%. The low values of Jc measured suggest that granularity depresses Jc but does not rule out the possibility of dilute magnetic pinning.

Amorphous Silica Nanoparticles Embedded in Epitaxial SrTiO3 and CoFe2O4 Matrices

Silica (SiO2)–metal oxide composites have a wide range of potential applications in many fields such as catalysis, sensors, optics, magnetism, and electronics. For example, SiO2 or mesoporous SiO2 are often used as insulating matrices or supports for metal oxide nanoparticles for catalytic and sensor applications. The addition of SiO2 can enhance the coercivity of CoFe2O4 (CFO) in SiO2-CFO powders or CFO thin films grown on silicon or silica substrates. Furthermore, SiO2 can enhance the magnetoresistance in SiO2– La0.7Sr0.3MnO3 composites. [14] SiO2 not only affects the magnetic properties of magnetic materials but also modifies the electrical properties of oxides. For instance, SiO2 has been used to enhance the dielectric constants of ZrSiO4 and HfSiO4. [15] In addition, SiO2–ZrO2, SiO2–HfO2, and SiO2– SrTiO3 (STO) composites have been investigated as possible replacements for SiO2 as the gate dielectric materials in standard complementary metal-oxide-semiconductor (CMOS) transistors. It should be noted that most efforts in this field have involved incorporating SiO2 into metal oxide powders or metal oxide polycrystalline films. The composites are commonly prepared by the sol–gel method using tetraethoxysilane (TEOS, Si(OC2H5)4) as the silica source. Herein we report SiO2 nanoparticles with grain sizes as small as 10 nm embedded in epitaxial STO and CFOmatrices prepared by a solution approach involving polymer-assisted deposition (PAD). The water-soluble polymer controls the desired viscosity and binds the metal ions to prevent premature precipitation, which results in a homogeneous distribution of the metal ions in solution and the formation of uniform metal oxide thin films. Solutions of different metals can be mixed to control the stoichiometry when preparing complex metal oxides. We found that SiO2 nanoparticles bind to polyethyleneimine (PEI) polymer presumably by a surfactant-like interaction of the polymer with the surface of the nanoparticles. The ease of forming the soluble SiO2 nanoparticle solution makes it very attractive as a precursor to SiO2-metal oxide composite films. Herein we have selected STO and CFO to explore this unique synthetic route to nanocomposite materials. As compared to LAO (pseudo-cubic with a lattice constant a of 0.3789 nm), STO is cubic with a= 0.3905 nm, while CFO possesses a face-centered-cubic inverse spinel structure with a= 0.838 nm (a/2= 0.419 nm). Such small lattice mismatches make it possible to grow both STO and CFO epitaxially on LAO substrates. An analysis of the X-ray diffraction (XRD) q–2q scans indicates that only STO and CFO (001) peaks are present, with no peaks attributable to silica, thereby indicating that silica retains its amorphous nature in the composite films. Figure 1 displays the f-scans

Doping dependence of the upper critical field and Hall resistivity of LaFeAsO1-x Fx (x=0, 0.025, 0.05, 0.07, 0.11, and 0.14)

The electrical resistivity ({rho}{sub xx}) and Hall resistivity ({rho}{sub xy}) have been measured over wide composition range using 60 T pulsed magnets. While the superconducting phase diagram (T{sub c}, x) displays the classic dome-shaped structure, we find that the upper critical field (H{sub c2}) increases monotonically with decreasing fluorine concentration (x), with the largest H{sub c2} {>=} 75 T for x = 0.05. {rho}{sub xx} and {rho}{sub xy} data provide evidence for the multiband electronic structure for all x {<=} 0.14 in LaFeAsO{sub 1-x}F{sub x}, and demonstrate the quantum phase transition near x {approx} 0.05. Both the multiband structure and quantum phase transition play crucial roles in the large H{sub c2}.

Nanoscale disorder in pure and doped MgB2 thin films

MgB2 thin films have superior superconducting properties compared to bulk MgB2 and demonstrate the potential for further improving the performances of MgB2 wires and tapes. Using transmission electron microscopy, we have characterized the microstructure of pure and C-doped MgB2 using various carbon sources grown by hybrid physical?chemical vapor deposition (HPCVD), and cold-grown?annealed film deposited by molecular beam epitaxy (MBE). The MgB2 HPCVD films increase in crystal quality in the order (MeCp)2Mg-sourced films, CH4-sourced films, B(CH3)3-sourced films, pure films, while the Hc2 values of these films follow the opposite order. The cold-grown?annealed MgB2 MBE film contains non-epitaxial ? 10?nm MgB2 grains and MgO nanoparticles. The microstructural origins of electron scattering and flux pinning in both films are discussed. We also show the structure and chemistry of the degraded phase in HPCVD films and its effects on superconducting properties.

The role of thermally and chemically stable composite Y2O3:Al2O3 in the development of YBa2Cu3O7?x films on metal substrates

We have developed Y2O3:Al2O3 (YAlO) composites to simplify the architecture of superconducting YBa2Cu3O7?x (YBCO) thick films on polycrystalline metal substrates. By implementing the use of YAlO, we have reduced the total number of non-superconducting layers between the polycrystalline metal substrate and the YBCO film from five (as in the standard architecture used by industry) to three. The YBCO films grown on this simplified platform exhibited an in-plane mosaic spread of less than 4? in full width at half-maximum, correlated pinning centered at , and an ? value (the proportionality factor of the critical current density H ? ?) of around 0.38 over the field range of 0.1?1.0?T. We believe that the excellent structural stability at high temperatures and the exceptional chemical inertness in an oxidizing environment make YAlO a good choice for use in the growth of biaxially oriented MgO and subsequent buffer and superconducting layers.

Inversion of the upper critical field anisotropy in FeTeS films

We present the complete superconducting upper critical field (Hc2)—temperature (T) diagram of FeTeS films measured at three crystalline orientations (H ∥ c, 45° and ab). We find that Hc2 is almost isotropic in magnetic field orientation with μ0Hc2(T = 0) ~ 30 T and a transition temperature of Tc ~ 7 K. A small but clear Hc2 angular anisotropy is observed, with a crossover around T = 0.7Tc, from Hc2(∥ c) 0.7Tc to Hc2(∥ c) > Hc2(∥ ab) for T < 0.7Tc. This change in the anisotropy is similar to that observed in FeTeS and FeTeSe single crystals but occurs at a higher T/Tc for our film. We analyze the Hc2(T) in terms of pair-breaking mechanisms and two-band superconductor theory. Understanding the inversion of Hc2 opens the possibility to adjust the effective anisotropy of superconductors for different applications.