M. S. Osofsky

Using metamaterial nanoengineering to triple the superconducting critical temperature of bulk aluminum.

Recent experiments have shown the viability of the metamaterial approach to dielectric response engineering for enhancing the transition temperature, Tc, of a superconductor. In this report, we demonstrate the use of Al2O3-coated aluminium nanoparticles to form the recently proposed epsilon near zero (ENZ) core-shell metamaterial superconductor with a Tc that is three times that of pure aluminium. IR reflectivity measurements confirm the predicted metamaterial modification of the dielectric function thus demonstrating the efficacy of the ENZ metamaterial approach to Tc engineering. The developed technology enables efficient nanofabrication of bulk aluminium-based metamaterial superconductors. These results open up numerous new possibilities of considerable Tc increase in other simple superconductors.

Experimental demonstration of superconducting critical temperature increase in electromagnetic metamaterials

A recent proposal that the metamaterial approach to dielectric response engineering may increase the critical temperature of a composite superconductor-dielectric metamaterial has been tested in experiments with compressed mixtures of tin and barium titanate nanoparticles of varying composition. An increase of the critical temperature of the order of ΔT ~ 0.15u2005K compared to bulk tin has been observed for 40% volume fraction of barium titanate nanoparticles. Similar results were also obtained with compressed mixtures of tin and strontium titanate nanoparticles.

Transformational dynamics of BZO and BHO nanorods imposed by Y2O3 nanoparticles for improved isotropic pinning in YBa2Cu3O7-δ thin films

An elastic strain model was applied to evaluate the rigidity of the c-axis aligned one-dimensional artificial pinning centers (1D-APCs) in YBa2Cu3O7-δ matrix films. Higher rigidity was predicted for BaZrO3 1D-APCs than that of the BaHfO3 1D-APCs. This suggests a secondary APC doping of Y2O3 in the 1D-APC/YBa2Cu3O7-δ nanocomposite films would generate a stronger perturbation to the c-axis alignment of the BaHfO3 1D-APCs and therefore a more isotropic magnetic vortex pinning landscape. In order to experimentally confirm this, we have made a comparative study of the critical current density Jc (H, θ, T) of 2 vol.% BaZrO3 + 3 vol.%Y2O3 and 2 vol.%BaHfO3 + 3 vol.%Y2O3 double-doped (DD) YBa2Cu3O7-δ films deposited at their optimal growth conditions. A much enhanced isotropic pinning was observed in the BaHfO3 DD samples. For example, at 65 K and 9.0 T, the variation of the Jc across the entire θ range from θ=0 (H//c) to θ=90 degree (H//ab) is less than 18% for BaHfO3 DD films, in contrast to about 100% for the ...

Deposition and spin polarization study of Fe4N thin films with (111) orientation

We have successfully deposited Fe4N thin films with (111) out-of-plane orientation on thermally oxidized Si substrates using a facing-target-sputtering system. A Ta/Ru composite buffer layer was adopted to improve the (111) orientation of the Fe4N thin films. The N2 partial pressure and substrate temperature during sputtering were optimized to promote the formation of the Fe4N phase. Furthermore, we measured the transport spin polarization of (111) oriented Fe4N by the point contact Andreev reflection (PCAR) technique. The spin polarization ratio was determined to be 0.50 using a modified BTK model. The film thickness dependence of the spin polarization was also investigated. The spin polarization of Fe4N measured by PCAR does not show degradation as the sample thickness was reduced to 10nm.

Induced Magnetic Anisotropy and Spin Polarization in Pulsed Laser-Deposited Co2MnSb Thin Films

Co2MnSb thin films were grown on glass and GaAs (001) substrates using pulsed laser deposition. The films were grown in magnetic fields (HGu2009=u2009500u2009Oe and 0u2009Oe) that were applied in the plane of the substrate during the deposition process. Angle-dependent magneto-optic Kerr effect measurements for films grown on glass revealed a uniaxial magnetic anisotropy in the direction of the applied growth field. Films grown on GaAs (001) exhibited more complicated magnetic anisotropy behavior, due to additional contributions from the substrate. Point contact Andreev reflection spectroscopy measurements indicated that the spin polarizations of the films were about Pu2009∼u200950%, with negligible difference between films grown in zero and non-zero applied fields.

Structural, magnetic and transport properties of SmBaxSr1-xCo2O5+δ (0.1≤ x ≤0.5)

The structural, transport, and magnetic properties of bulk SmBaxSr1-xCo2O5+δ samples were investigated as a function of Ba content. At room temperature the magnetization was observed to increase as a function of decreasing Ba composition. As the samples were cooled below room temperature, a ferromagnetic transition was observed for 0.1≤ x ≤0.5 with the Curie temperatures showing a linear dependence on the Ba-composition. Transport measurements showed that resistivity values increased with decreasing temperature indicating a semiconducting-like behavior.The structural, transport, and magnetic properties of bulk SmBaxSr1-xCo2O5+δ samples were investigated as a function of Ba content. At room temperature the magnetization was observed to increase as a function of decreasing Ba composition. As the samples were cooled below room temperature, a ferromagnetic transition was observed for 0.1≤ x ≤0.5 with the Curie temperatures showing a linear dependence on the Ba-composition. Transport measurements showed that resistivity values increased with decreasing temperature indicating a semiconducting-like behavior.

Towards Isotropic Vortex Pinning in YBCO Films with Double-doping BHO-Y2O3 and BZO-Y2O3 Artificial Pining Centers

Strong and isotropic vortex pinning landscape is demanded for high field applications of YaBa2Cu3O7-x (YBCO) epitaxial thin films. Double-doping (DD) of artificial pinning centers (APCs) of mixed morphologies has been identified as a viable approach for this purpose. This work presents a comparative study on the transport critical current density J c (H, θ) of 3.0 vol.%Y2O3+2.0 (or 6.0) vol.% BaZrO3 (BZO DD) and 3.0 vol.%Y2O3+ 2.0 (or 6.0) vol.% BaHfO3 (BHO DD) films. Based on the elastic strain model, BaHfO3 (BHO) nanorods have lower rigidity than their BaZrO3 (BZO) counterparts, which means their c-axis alignment is more susceptible to the local strain generated by the secondary dopant of Y2O3. Considering the increasing strain field with higher BZO (or BHO doping), the higher susceptibility may result in a large portion of the BHO APCs moving away from perfect c-axis alignment and enhancing isotropic pinning with respect to the H orientation. This is confirmed since the BHO DD films illustrate a less pronounced J c peak at H//c-axis and hence more isotropic J c(θ) than their BZO DD counterparts. At 9.0 T, the variation of the J c across the entire θ range (0-90 degree) is less than 18% for the BHO DD film, in contrast to about 100% for the 2.0 vol.% BZO DD counterpart. At the higher BHO concentration of 6.0 vol.%, this higher tunability of the Y2O3 leads to increased ab-plane aligned BHO APCs and hence enhanced J c at H//ab-plane.