Raghuram Gandikota

Effect of damage by 2MeV He ions on the normal and superconducting properties of magnesium diboride

We have studied the effect of damage induced by 2MeV alpha particles on the critical temperature, Tc, and resistivity, ρ, of MgB2 thin films. This technique allows defects to be controllably introduced into MgB2 in small successive steps. Tc decreases linearly as the intragrain resistivity at 40K increases, while the intergrain connectivity is not changed. Tc is ultimately reduced to less than 7K and we see no evidence for a saturation of Tc at about 20K, contrary to the predictions of the Tc of MgB2 in the dirty limit of interband scattering.

MgB2 tunnel junctions with native or thermal oxide barriers

MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers therefore survive the deposition of the second electrode at 300°C, even over junction areas of ∼1mm2. Studies of such junctions and those of the type MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers grown on MgB2 exhibit a wide range of barrier heights and widths.

Internally shunted Josephson junctions with barriers tuned near the metal–insulator transition for RSFQ logic applications

The fabrication of self-shunted SNS (superconductor/normal conductor/superconductor) Josephson junctions for rapid single flux quantum (RSFQ) logic could potentially facilitate increased circuit density, as well as reduced parasitic capacitance and inductance over the currently used externally shunted SIS (superconductor/insulator/superconductor) trilayer junction process. We report the deposition, fabrication, and device characterization of Josephson junctions prepared with Nb1?yTiyN electrodes and TaxN barriers tuned near the metal?insulator transition, deposited on practical large-area oxide-buffered silicon wafers. When scaled to practical device dimensions, this type of junction is found to have an IcRn product of over 0.5?mV and a critical current (Ic) and normal resistance (Rn) of magnitudes suitable for single flux quantum digital circuits. A longer than expected normal-metal coherence length (?n) of 5.8?nm is inferred from the thickness dependence of Jc at 4.2?K for junctions fabricated using a barrier resistivity of 13?m??cm. Although not well understood and not quantitatively predicted by conventional theories, this results in a sufficiently high Ic and IcRn to make the junctions suitable for practical applications. Similar observations of unexpectedly large Josephson coupling currents in SNS junctions have been documented in other systems, particularly in cases when the barrier is near the M?I transition, and have become known as the giant proximity effect. The temperature dependence of ?n, IcRn, and Jc are also reported. For this technology to be used in practical applications, significant improvements in our fabrication process are needed as we observe large variations in Ic and Rn values across a 100?mm wafer, presumably as a result of variations in the Ta:N stoichiometry and the resulting changes in the TaxN barrier resistivity.

Effect of stoichiometry on oxygen incorporation in MgB2 thin films

The amount of oxygen incorporated into MgB2 thin films upon exposure to atmospheric gasses is found to depend strongly on the materials stoichiometry. Rutherford backscattering spectrometry was used to monitor changes in oxygen incorporation resulting from exposure to: (a) ambient atmosphere, (b) humid atmospheres, (c) anneals in air and (d) anneals in oxygen. The study investigated thin-film samples with compositions that were systematically varied from Mg0.9B2 to Mg1.1B2. A significant surface oxygen contamination was observed in all of these films. The oxygen content in the bulk of the film, on the other hand, increased significantly only in Mg-rich films and in films exposed to humid atmospheres.

Effect of oxygen incorporation on normal and superconducting properties of MgB2 films

Oxygen was systematically incorporated in MgB2 films using in situ postgrowth anneals in an oxygen environment. Connectivity analysis in combination with measurements of the critical temperature (Tc) and resistivity (ρ) indicate that oxygen is distributed both within and between the grains. High values of critical current densities (Jc) in field (∼4×105A∕cm2 at 8T and 4.2K), extrapolated critical fields [Hc2(0)] (>45T), and slopes of critical field versus temperature (1.4T∕K) are observed. Our results suggest that low growth temperatures (300°C) and oxygen doping (⩾0.65%) can produce MgB2 with high Jc values in field and Hc2 for high-field magnet applications.

A novel technique for synthesizing MgB2 thin films with high upper critical fields

We report a new method for synthesizing magnesium diboride (MgB2) thin films which exhibit a steep increase in the slope of the upper critical field Hc2 versus decreasing temperature (i.e. |dHc2/dT|Tc|≈2.5 T K−1). An Hc2 of 38.4 T at 4.2 K and an extrapolated value of Hc2(0) above 43 T at 0 K are reported. Films of magnesium plus boron were deposited on unheated c-plane sapphire substrates by co-evaporation, and were subsequently annealed in a reducing atmosphere at temperatures below 600 °C. The use of a combination of a magnesium-rich stoichiometry (Mg/B>1/2) for the as-deposited film and a two-step annealing process was found to be critical in obtaining these high values of Hc2 and |dHc2/dT|Tc|.

Effect of Rb and Cs doping on superconducting properties of MgB2 thin films

A Rutherford backscattering spectrometry (RBS) study has found that concentrations up to 7 at.% of Rb and Cs can be introduced to a depth of ~700 A in MgB2 thin films by annealing in quartz ampoules containing the elemental alkali metals at 50 K) from susceptibility measurements on MgB2 powders. The lack of a significant change in Tc and intra-granular carrier scattering suggests that Rb and Cs diffuse into the film, but do not enter the grains. Instead, the observed changes in the electrical properties, including a significant drop in Jc and an increase in Δρ(ρ300−ρ40), arise from a decrease in inter-granular connectivity due to segregation of the heavy alkaline metals and other impurities (i.e. C and O) introduced into the grain boundary regions during the anneals.

Saturation and intrinsic dynamics of fluxons in NbTi and MgB2

The authors measured the remanent magnetization of superconducting NbTi and MgB2 as a function of time after removing an applied field. At similar reduced temperatures, the density of magnetic flux quanta (fluxons) is found to relax faster in NbTi than in MgB2. Nearer the transition, the relaxation rate in both materials exhibits a saturation that is independent of the initially applied field. This saturation occurs when the distance between fluxons is comparable to the London penetration depth. The temperature dependence of the saturation and time dependence of the relaxation can be characterized by the Anderson-Kim model [Rev. Mod. Phys. 36, 39 (1964)] for fluxon dynamics.