Kookrin Char

Physical properties of transparent perovskite oxides (Ba,La)SnO3with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.

High Tc superconductor‐normal‐superconductor Josephson junctions using CaRuO3 as the metallic barrier

Josephson junctions of the SNS type have been fabricated in the edge and trilayer junction geometries using YBa2Cu3O7−δ/CaRuO3/YBa2Cu3O7−δ. The conducting oxide CaRuO3 was chosen because it grows epitaxially on YBa2Cu3O7−δ and its resistivity shows metallic behavior. Current‐voltage characteristics display behavior with significant deviations from the resistivity shunted junction model. The dependence of critical currents and junction resistances on the CaRuO3 thickness suggest that the interface resistance between YBa2Cu3O7−δ and CaRuO3 plays an important role in determining the critical parameters of the junctions. A dc SQUID functioning at 77 K has been demonstrated with this type of junction.

Study of interface resistances in epitaxial YBa2Cu3O7−x/barrier/YBa2Cu3O7−x junctions

We have measured interface resistances in YBa2Cu3O7−x/barrier/YBa2Cu3O7−x junctions with different barrier materials in an edge junction geometry. CaRuO3, La0.5Sr0.5CoO3, Y0.7Ca0.3Ba2Cu3O7−x, YBa2Cu2.79Co0.21O7−x, and La1.4Sr0.6CuO4 have been employed as the epitaxial barrier materials. We observe interface resistances of the order of 1×10−8 Ω cm2 when we use CaRuO3 and La0.5Sr0.5CoO3 barriers. These two barrier materials are cubic perovskites. However, in the case of the layered barrier materials, the measured interface resistances are smaller than 1×10−10 Ω cm2. Our study suggests that the oxygen disorder at the YBa2Cu3O7−x surface, due to stress created by the thermal expansion mismatch between YBa2Cu3O7−x and the barrier, may be the origin of the interface resistances, and that the magnitude of this stress can change the resistance by orders of magnitude.

Reactive magnetron sputtering of thin‐film superconductor YBa2Cu3O7−x

Superconducting thin films of YBa2Cu3O7−x have been successfully made by co‐sputtering from the three targets in an Ar and O2 mixture atmosphere. After high‐temperature annealfilms are superconducting with an onset temperature at 90 K, a full transitiontemperature as high as 88 K, and a critical current density in excess of 1×105 A/cm2 at 78 K and 2×106 A/cm2 at 4.2 K. Highly oriented thin films have been obtained on SrTiO3 {100} substrates.

Growth of high Tc superconducting thin films using molecular beam epitaxy techniques

Thin films of the high‐temperature superconductor DyBa2Cu3O7−x have been grown on SrTiO3 substrates using molecular beam epitaxy techniques. Reflection high‐energy electron diffraction patterns observed during deposition indicate incomplete oxidation of copper and growth of oriented metallic copper microcrystals in a matrix of amorphous barium and dysprosium oxides. After post‐growth anneal the films exhibited sharp superconducting transitions with zero resistance observed as high as 89 K and critical current densities of 4.8×105 A/cm2 at 4.2 K and 3.9×104 A/cm2 at 77 K.

Ferroelectric field effect in ultrathin SrRuO3 films

We report the observation of a ferroelectric field effect in the conducting oxide SrRuO3 using Pb(Zr0.52Ti0.48)O3/SrRuO3 epitaxial heterostructures. Upon reversing the polarization of the ferroelectric Pb(Zr0.52Ti0.48)O3 layer, we measured a 9% change in the resistance of a nominally 30 A SrRuO3 film at room temperature. This change was nonvolatile for a period of several days. Conductivity measurements taken between 4.2 and 300 K are consistent with n-type conduction throughout this temperature range. Hall effect measurements also yield n-type conduction, with n≈2×1022 electrons/cm3, and furthermore allow us to understand quantitatively the magnitude of the observed resistivity change.

Josephson coupling of YBa2Cu3O7−x through a ferromagnetic barrier SrRuO3

Epitaxial barriers of ferromagnetic SrRuO3 have been used to fabricate high Tc superconductor‐normal‐superconductor Josephson junctions in the edge junction geometry. At small barrier thicknesses the SrRuO3 junctions follow the behavior of nonferromagnetic but otherwise closely related CaRuO3 junctions. A rapid disappearance of critical current is observed when the barrier thickness is increased to 250 A. Possible origins of such a large critical thickness are discussed.

Perpendicular magnetic anisotropy and strong magneto‐optic properties of SrRuO3 epitaxial films

Epitaxial films of the ferromagnetic perovskite SrRuO3 were measured with a bulk magnetometer and with a local magneto‐optic Sagnac interferometer in transmission and in reflection. We find a magnetic easy axis perpendicular to the films, and for saturated remanent magnetization along this direction the Faraday rotation and the Kerr rotation at λ=840 nm are about 0.75×105 deg/cm and 0.85°, respectively. The temperature dependence of the remanent magnetization in the low temperature limit is dominated by spin‐wave excitations, yielding a notable decrease with T3/2. Using Sagnac–Kerr scanning and transmission electron microscopy imaging we correlate the coercivity with the grain size.

Magnetic flux motion and its influence on transport properties of the high Tc oxide superconductors

High Tc superconductors exhibit significant flux motion due to a thermally activated creep process. The motion of the fluxoids dissipates energy. This dissipation causes a broadening of the resistive transition below Tc in a magnetic field as well as a logarithmic decay of the diamagnetic shielding moment in time. From the detailed structure of the broadened transition, empirical parameters are derived. The pinning strength is highly anisotropic, reflecting the anisotropy of the coherence length. Implications of an extraordinarily low activation energy are also addressed.

A multilayer YBa2Cu3Ox Josephson junction process for digital circuit applications

We have demonstrated a multilayer high‐Tc junction process capable of fabricating small‐scale digital circuits. The process uses superconductor/normal–metal/superconductor YBa2Cu3Ox (YBCO) edge junctions with a cobalt‐doped YBCO barrier and an integrated YBCO ground plane. We have measured spreads in the junctions’ critical currents as low as 12% (1σ) both on and off the ground plane. The proper functioning of the ground plane was verified by measuring the reduced inductance of superconducting quantum interference devices (SQUIDs) on the ground plane compared to identical SQUIDs off the ground plane. At a temperature of 70 K, the inductance on the ground plane is as low as 1.2 pH/⧠. The inferred YBCO penetration depth is 250 nm at 70 K and 280 nm at 77 K.