Davor Pavuna

Superconductor-insulator Transition in La2-xSrxCuO4 at the Pair Quantum Resistance

High-temperature superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor–insulator transition is a key open question. The cleanest approach is to tune the carrier density using the electric field effect; for example, it was learned in this way that weak electron localization transforms superconducting SrTiO3 into a Fermi-glass insulator. But in the copper oxides this has been a long-standing technical challenge, because perfect ultrathin films and huge local fields (>109 V m−1) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration. Here we report synthesis of epitaxial films of La2− xSrxCuO4 that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor–insulator transition. The observed critical resistance is precisely the quantum resistance for pairs, RQ = h/(2e) = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.

Evidence for enhancement of critical current by intergrain Ag in YBaCuO‐Ag ceramics

We report the evidence for enhancement of critical current density Jc by ∼50%, which occurs when ∼10 wt. % Ag is added to Y1Ba2Cu3O7−δ ceramics. The maximal Jc (∼700 A/cm2 at T=77 K) appears simultaneously with maximum YBaCuO compactness in the samples. The silver fills the intergranular space (holes) without Cu substitution, and the critical temperature Tc is not reduced from the bulk value (∼91 K). Normal‐state resistivity of Ag‐YBaCuO samples is decreased by an order of magnitude, and samples exhibit improved contact resistance and resistance to water. While the critical density is improved by adding ∼10 wt. % Ag, it decreases at higher Ag concentrations.

In situ growth of superconducting YBa2Cu3O7-δ thin films on Si with conducting indium-tin-oxide buffer layers

Superconducting YBa2Cu3O7−δ (YBCO) thin films have been grown in situ on Si with conducting indium‐tin‐oxide (ITO) buffer layers. ITO allows YBCO to be electrically connected to the underlying Si substrate. Both the YBCO film and ITO buffer layer, grown by ion beam sputtering, are textured and polycrystalline with a combined room‐ temperature resistivity of about 2 mΩ cm. Superconducting onsets are 92 K with zero resistance at 68 K.

Quantitative analysis of growth-induced reduction of long range lattice order in ion-beam sputtered YBa2Cu3O6.9 films

We report evidence for the reduction of long range lattice order caused by slight departures from the optimal growth temperature in fully doped (x≊0.9) YBa2Cu3O6+x films deposited by ion‐beam sputtering on 〈001〉 SrTiO3. We estimate the characteristic length of this disorder from the broadening Δϑ of the 〈005〉 x‐ray diffraction rocking curve. The depression of superconductivity and normal conductivity scales as Δϑ and disappears when the in‐plane lattice coherence length rc∼1/Δϑ is larger than ≊10 nm.

Percolation and electronic properties of superconducting (YBa2Cu3O7−δ)1−xAgx ceramics and thick films

We present the percolation and electronic properties of (Y1Ba2Cu3O7−δ)1−xAgx compounds in which silver fills the intergranular space without reducing Tc, which remains at 92 ± 1 K. Normal-state resistivity is decreased by up to two orders of magnitude when adding up to 50 wt.% Ag (Tc=87 K), and samples exhibit improved contact resistance, better mechanical properties, and resistance to water. We analyzed the percolation properties of these compounds and found that the critical indicest, s are in agreement with percolation theory, butpc is higher than expected, probably due to the effect of holes. TheJc estimated from magnetization reaches 5 · 104A/cm2 (atT = 4.2K,H = 0) and shows enhancement of 15–50% by addition of ∼ 10 wt.% Ag, which exists also in samples having a higherJc due to preparation conditions (temperature). We present preliminary results on the 2D percolation problem in (Y1Ba2Cu3O[7−δ)1−xAgx samples, obtained by preparing Y1Ba2Cu3O7−δ thick films using the spin-on technique. Preliminary results show good adhesion but a reduced Tc of Y1Ba2Cu3O7−δ films compared with bulk samples.

Large area superconducting YBa2Cu3O7-x films grown by single target ion beam sputtering

The authors have demonstrated, by using a simple single YBa2Cu3O7-x target ion beam system that, with a sufficiently low power ion beam, preferential sputtering is avoided and high-quality YBa2Cu3O7-x films are deposited over areas larger than a30 cm2 in a reproducible way. As-deposited films on 〈100〉SrTiO3 are 50-100 nm thick, c-oriented and show the following reproducible electrical properties (within the given variations): Tc0=90±0.5 K, transitions widths less than 1 K, jc(77 K)=1.0-1.2×106 A cm-2, ρ(300 K)=300±50 μΩ cm, ρ(300 K)/ρ(100 K)=2.9±0.1. The extrapolated residual resistivity ρres(0 K) is between 0 and 5% of ρ(300 K)

In-situ superconducting YBa2Cu3O7 thin films grown by ion beam co-deposition

The authors present superconducting YBa2C3O7 (YBCO) thin films grown in-situ by three-ion-beam sputtering. Y, Y2O3, Cu, Cu2O, BaF2 and BaCO3 sputter targets have been investigated. The highest quality films were prepared using a BaCo3 target. Auger analysis of films grown using a BaCO3 target show no carbon content. Y2O3 and Cu2O are more suitable than the native metals as sputter targets for YBCO growth as they are much less prone to sputter rate variations with oxygen partial pressure. They also supply oxygen to the growing film. As-deposited YBCO films are metallic (resistivity 240 μΩ cm at 100 K), reflective, and of highly homogeneous composition with Tco transition temperatures of 73 K and transition widths of 15 K. Post-annealing in flowing oxygen improves TCOs to 82 K. Critical currents are in excess of 105 A cm-2 at 77 K. Films are textured with c-axis orientation perpendicular to the (100) SrTiO3 substrate surface. As-deposited superconducting YBCO films have also been prepared on SiO2 and Y2O3 buffer layers on Si wafers

Electric field effect on superconductivity in La2−xSrxCuO4

We demonstrate a method to tune the carrier concentration of a high temperature superconductor over a wide range, using an applied electric field. Thin film devices were made in an electrical double layer transistor configuration utilizing an ionic liquid. In this way, the surface carrier density in La2−xSrxCuO4 films can be varied between 0.01 and 0.14 carriers per Cu atom with a resulting change in critical temperature of 25 K (∼70% of the maximum critical temperature in this compound). This allows one to study a large segment of the cuprate phase diagram without altering the level of disorder. We used this method [A. T. Bollinger et al., Nature 472, 458–460 (2011)] to study the quantum critical point at the superconductor to insulator phase transition on the underdoped side of superconducting dome, and concluded that this transition is driven by quantum phase fluctuations and Cooper pair delocalization.

The preparation of YBCO thin films by a four ion beam co-deposition system

We present preliminary results for thin films grown with a recently commissioned ion beam codeposition system designed for in situ growth and etching of YBCO films. The apparatus consists of a high-vacuum chamber (10-7 Torr) equipped with four Kaufmann ion beam sources. Three of the sources are directed at targets for ion beam sputter deposition of materials; the fourth ion source is directed at the substrate for substrate precleaning, oxygen ion beam assisted deposition and ion beam etching. We have determined individual sputter deposition rates from Y2O3, BaF2 and copper targets as a function of ion beam currents on unheated substrates in argon background, normal background, oxygen background, and oxygen ion assist conditions. Films were studied by X-ray diffraction (XRD), Auger electron spectroscopy (AES), resistivity and Rutherford back-scattering analysis (RBS).

Oxygen Displacement in Cuprates under Ionic Liquid Field-Effect Gating.

We studied structural changes in a 5 unit cell thick La1.96Sr0.04CuO4 film, epitaxially grown on a LaSrAlO4 substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer, and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered, and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms.