BKBO Films Research Articles

Ablation laser fluence as an effective parameter to control superconductivity in Ba1−xKxBiO3 films

Potassium doping in insulating BaBiO3 induces superconductivity, with high superconducting transition temperatures, Tc, of up to 31 K in bulk. We investigated growth control of superconducting properties of BKBO films, by varying laser fluence using pulsed laser deposition technique. As cation stoichiometry, especially potassium concentration in BKBO films, was sensitively changed with laser fluence, we were able to precisely control Tc of BKBO films. Following the trend of the bulk phase diagram, Tc showed the highest value of 24.5 ± 0.5 K at the optimal stoichiometry. This result can provide optimal guidance for the synthesis of high-quality BKBO films, and demonstrates the effectiveness of laser fluence to study emerging superconducting phenomena in PLD-grown complex oxide thin films.

A layer by layer growth of Ba0.6K0.4BiO3superconductor: liquid phase epitaxy with a new substrate

With a view to find a new substrate for the liquid phase epitaxy of superconducting Ba 1-x K x BiO 3-δ (BKBO) films single crystals of the general formula Ba 0.96 Ca 0.19 Bi 0.85 O 2.97±0.03 (BOBO) were grown first. The BCBO crystals belong to a cubic structure of distorted perovskite with the unit cell parameter a = 8.6425 A. A set of superconducting BKBO films onto BCBO substrates was successfully prepared to reveal the main features of a layer by layer epitaxial growth, comparable to single crystals parameters of superconductivity and unusually flat the surface morphology. The data of polarization-dependent O 1s NEXAFS studies provide first evidence for Bi(III)-Bi(V) charge disproportion in BKBO system.

Temperature dependences of the surface resistance and the diamagnetic shielding susceptibility at T/sub c/-T≪T/sub c/ for high-T/sub c/ superconductors

At T_c-T<<T_c(i.e., near T_c), in order to demonstrate the condudction mechanism and temperature dependencies of the diamagnetic-shielding susceptibility and the penetration depth, we fabricated Ba_1_xBiO_3(BKBO) thin films and measured the energy gap by tunnel effect and shielding susceptibilities which are compared with those measureed for BKBO and YBCO single crystals. The shielding susceptibilities for BKBO and YBCO single crystals well-fit \chi(T)/\chi(0)=1-exp(-2\triangle(T)/k_BT), while that for the BKBO film follows \chi(T)/\chi(0)=1-T/T_c) which may not be intrinsic. The exponential decrease of the susceptibilities near T_c indicates that the conduction mechanism is hopping. The energy gaps are observed as 2\triangle(0)=(3.5+-0.1)k_BT_c for the BKBO film by the tunnel effect, 2\triangle(0)=(3.9+-0.1)k_BT_c for the BKBO single crystal, and 2\triangle(0)=(8+-0.2)k_BT_c for the YBCO single crystal. Furthermore, for microwave device applications of superconductors, at T_c-T<<T_c, the surface resistance R_s(T) is derived from the surface impedance at \omega\tau_(tr)<<1, where \sigma_s(0) and \sigma_n are the conductivities of the superconducting state and the normal state, respectively, and f(T)=\chi(T)/\chi(0)=1-exp(-2\tringle(T)/k_BT).

Ba/sub 1-x/K/sub x/BiO/sub 3/ epitaxy on various substrate materials

We have fabricated Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) films by thermal co-evaporation of the metals K, Ba and Bi. To improve the epitaxy a high temperature BaBiO/sub 3/ seed layer is deposited prior to the BKBO film which results in T/sub c/ values up to 24 K on standard substrates. As BKBO grows well on perovskites we deposited thin PrBa/sub 2/Cu/sub 3/O/sub 7/ (PBCO) layers by thermal co-evaporation on MgO, CeO/sub 2/ buffered sapphire, YSZ buffered Si and YSZ substrates. BKBO films on these systems are epitaxial and show the same T/sub c/ values as on standard substrates. Thus, by using a PBCO buffer layer technical substrates are available for BKBO thin films. >

Property Control for High-Quality Ba1-xKxBiO3 Epitaxial Thin Films Prepared by High-Pressure Reactive rf-Magnetron Sputtering

The electrical properties of Ba1-xKxBiO3(BKBO) epitaxial thin films on SrTiO3(110) substrates have been successfully controlled using high-pressure reactive rf-magnetron sputtering under an 80-Pa discharge gas. We investigated the relationship between the target composition and the electrical properties of BKBO films. The zero resistance temperature (Tce) and the temperature dependence of resistivity above the superconducting onset temperature (Tco) were found to be primarily influenced by the ratio of Ba/K and the Bi content of the target composition, respectively. Electrical measurements showed that the highest Tce of as-grown BKBO epitaxial film was 28 K.

Electrical Responses of Ln-Ba-Cu-O (Ln=Eu, Y), Ba-K-Bi-O and Ba-Pb-Bi-O Superconducting Thin Films to Pulsed Infrared Radiation

The electrical responses to pulsed infrared radiation (λ=1.31 µm and a typical duration=1-4 ns) of superconducting LnBa2Cu3Oy (Ln=Eu, Y) (LnBCO), Ba1-xKxBiO3 (BKBO) and BaPb1-xBixO3 (BPBO) thin films are examined. The fall time of the voltage response for BKBO and BPBO thin films shows a significant increase at temperatures close to Tc or when the irradiation power is high enough to generate an excess quasiparticle density comparable to the pair density of these materials. The fall time for these films at lower temperatures and lower irradiation powers is 0.6-2 ns. This magnitude as well as the dependences of the fall time can be well explained on the basis of the nonequilibrium gap suppression model. LnBCO thin films, in contrast, exhibit a fall time which is almost temperature and power invariant. It is approximately proportional to the film thickness and a typical value for a 63-nm-thick film is 4 ns. This fall time is probably attributable to a bolometric response dominated by a large thermal boundary resistance at the film-substrate interface, although the possibility of the existence of a nonbolometric component in the observed responses cannot be ruled out.

Artificial barriers for Ba-K-Bi-O tunnel junctions

The authors have grown epitaxial bilayers and trilayers which utilize Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) base electrodes, native or artificial (MgO or SrTiO/sub 3/) insulating barriers, and Ag or BKBO counterelectrodes, respectively. The layers were deposited in situ by RF magnetron sputtering and characterized in situ. Electron diffraction showed the layers to be epitaxial with the symmetry of the desired structure maintained even in the first monolayer at the surface. X-ray photoelectron spectroscopy (XPS) showed no evidence of a chemical reaction at the BKBO/barrier interfaces. Tunneling measurements through a native insulating layer grown by exposing BKBO surfaces to air showed a gap of approximately 25 mV to be present. In the range of thicknesses used for artificial barriers, 3-6 nm, the barriers apparently contained pin holes which caused the junctions to short. The BKBO films were K-rich with x approximately=0.5. Critical temperatures and lattice constants were consistent with reports for bulk superconductors with this composition. Normal-state resistivities of 70-100 mu Omega -cm at 30 K were the lowest ever reported in this system. >

Hysteretic Josephson junctions from high T/sub c/ superconducting thin films

Hysteretic superconductor-insulator-superconductor (SIS) and superconductor-normal metal-superconductor (SNS) Josephson junction devices have been fabricated from epitaxial thin-film structures. The SIS tunnel junctions were made of BKBO (Ba/sub 1-x/K/sub x/BiO/sub 3/) thin-film electrodes and a KNbO/sub 3/ barrier. Tunnel structures with gap voltages as high as 8 mV and switching voltages as high as 5 mV have been demonstrated. Nearly ideal SIS tunnel characteristics for high-temperature superconductive thin-film trilayer structures are reported. Zero bias currents are modulated by small magnetic fields and Shapiro steps are observed under RF-irradiation. SNS devices made from a YBCO (YBa/sub 2/Cu/sub 3/O/sub 7-y/) bottom thin-film electrode, a SrTiO/sub 3/ barrier, and a top BKBO film possessed McCumber parameters in excess of 100 and displayed I/sub c/R (critical-current resistance) products of up to 1 mV at 4.2 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pauli-limited critical field of Ba/sub 1-x/K/sub x/BiO/sub 3/ determined from quasiparticle tunneling

A planar tunnel diode with Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) and Ag electrodes were fabricated. The dynamic conductance of the junction was similar to that found with conventional BCS superconductors. Using weak coupling theory, the undepaired transition temperature, spin-orbit scattering rate, and diffusion constant of BKBO were determined. The Pauli-limited critical field was calculated and compared to the measured resistive transitions of the BKBO film in magnetic fields. The Pauli-limited field is observed to occur generally at the field where the normal-state resistance is restored.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electronic noise properties of barium potassium bismuth oxide (BKBO) thin films

The low-frequency electrical noise fluctuations in superconducting barium potassium bismuth oxide (BKBO) thin films deposited by magnetron sputtering and by laser ablation have been measured. The excess noise in the presence of a bias current was 1/f noise. The magnitudes of the room temperature 1/f noise were somewhat higher than observed in the best cuprate high-temperature superconducting thin films. The noise magnitude did not scale as expected with film geometry, and the noisier BKBO film also had the highest superconducting transition temperature. It is suggested that etch pits observed on the patterned films contributed to the total 1/f noise measured. Both films exhibit a peak in the noise voltage at the foot of the superconducting transition which cannot be explained from a model of 1/f noise resulting from temperature fluctuations. >

Oxygen Partial Pressure Dependence of the In Situ Growth of Ba1-xKxBiO3 Films by Laser Ablation

The in-situ growth of the superconducting Ba1-xKxBiO3 (BKBO) films as a function of oxygen partial pressure and substrate temperature by laser ablation has been studied. The superconducting BKBO films can be grown at a temperature of 410-500°C within two pressure ranges of 15-60 mTorr and 200-400 mTorr, respectively. Oxygen partial pressure was found to be important to control the growth of the superconducting BKBO films. The optimum pressure for growing the superconducting BKBO films is 40 mTorr, under which potassium is readily doped into the BKBO phase by oxygen. In the present study the best film with Tc(onset) of 28 K and Tc(zero) of 25 K is grown at 450°C in 40 mTorr of O2.

Characterization of a Natural Barrier in an Au/Ba1-XKXBiO3 Junction

X-ray photoelectron spectroscopy (XPS) has revealed that the “natural barrier” on a Ba1-XKXBiO3 (BKBO) thin-film surface is composed of a K-compound layer and a nonstoichiometric layer resulting from Ba depletion. An Au/BKBO junction was fabricated at various temperatures, and its electrical properties were investigated. Junction resistivity could be reproducibly controlled between 10-1 and 10-5 Ω·cm2 by varying the temperature of Au deposition between room temperature (RT) and 300°C. The decrease in junction resistivity with increasing Au-deposition temperature was found to be a result of decomposition of the K-compound layer and rearrangement of the nonstoichiometric layer by annealing the BKBO film.

Epitaxial growth of Ba 1−xK xBiO 3 thin films by rf-magnetron sputtering

As-grown Ba 1−xK xBiO 3 (BKBO) epitaxial thin films with a critical temperature of 16.3 K have been successfully prepared at 300°C on SrTiO 3(110) substrates by a high-pressure rf-magnetron sputtering. A preferred orientation of (110) and epitaxial growth of the BKBO film on the substrates were confirmed by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED), respectively.