Ba1−xKxBiO3 Research Articles

Influence of thickness scaling on the electronic structure and optical properties of oxygen deficient BaBiO3-δ thin films grown on SrTiO3-buffered Si(001) substrate

BaBiO3 has attracted a lot of research attention since it was discovered as the parent compound for the high-Tc superconducting BaPbxBi1–xO3 and Ba1–xKxBiO3. In its pure state, BaBiO3 is an insulator due to the presence of a breathing distortion of the BiO6 octahedra. The distortion is attributed to the valency of Bi in the compound being charge-ordered in the form of Bi3+ and Bi5+ along the lattice, resulting in alternating expanded or contracted BiO6 octahedra. The interaction between the electronic properties and the thickness of the thin film is crucial to study. We conducted a thorough study to investigate the effect of the thickness reduction of BaBiO3-δ grown on SrTiO3-buffered Si substrates on the optical properties as well as the Bi electronic structure of the thin films. We conclude that modifications in the valency of Bi in the ultra-thin film regime result in an optically conducting layer.

Superconductivity and appearance of negative magnetocaloric effect in Ba1–xKxBiO3 perovskites, doped by Y, La and Pr

Recently, substantial attention is given to the bismuth-based perovskites for variety of electronic applications. The perovskites are used for quantum dots displays (QLED), photovoltaic systems and superconducting (HTS) devices. In this paper comprehensive studies of Ba1–xKxBi1‒yREyO3 bismuth perovskites (REBKBO, RE = Y, La or Pr) are reported. Apart from structural anomalous behavior at low temperatures, the electronic properties of the REBKBO can be easily tailored by doping. For instance, critical current densities differ by 3 orders of magnitude from 0.07 kA/cm2 (LaBKBO) to 65 kA/cm2 (BKBO). To explain the above behavior, microstructural, magnetic, calorimetric, spectroscopic (FT-IR) studies and ab-initio electronic structure calculations were carried out. The FT-IR shows that the biggest difference in the spectrum compared to the undoped sample was recorded for PrBKBO, due to magnetic contribution of the Pr3+ multiplet. Electronic structure calculations revealed that the BKBO compound shows the highest DOS(EF) compared to the other samples, however, the electronic structure of PrBKBO is very different. The negative magnetocaloric effect observed below 30 K is a highlight of the peculiar properties of that sample. These findings extend possible usage of bismuth-based perovskites towards new applications.

Mixed Valence and Superconductivity in Perovskite Antimonates

Herein, we present results of the 121Sb Mössbauer spectroscopy in perovskite antimonates Ba1–xKxSbO3, sibling compounds of the well-known high-Tc superconductors Ba1–xKxBiO3. Two Sb valence states SbIII and SbV, forming a charge-density-wave (CDW) order, are unambiguously revealed in un- and under-doped phases at x = 0–0.5. As the CDW order is suppressed at x = 0.65, the compound becomes superconducting and all of the Sb sites become equivalent with the intermediate valence of +4.65, consistent with that anticipated from the potassium content. These results provide the direct evidence of metal s electrons participating in the formation of the CDW order and superconductivity in the material, and thus they have important implications for clarifying the underlying mechanism related to mixed valence and potential valence fluctuations of metal ions. The use of Mössbauer active nuclei offers a deeper insight into the structural properties and superconductivity in mixed-valence main-group oxides.

Phase stability, electronic structures, and superconductivity properties of the BaPb1−xBixO3 and Ba1−xKxBiO3 perovskites

AbstactWith extensive first‐principles calculations, we investigate the phase stability, electronic structures, and superconductivity properties of the BaPb1−xBixO3 (BPBO) and Ba1−xKxBiO3 (BKBO) perovskites with the cubic (C), tetragonal (T), and orthorhombic (O) phases. Our calculations show that the tetragonal superconducting phases of both perovskites are metastable. However, the orthorhombic phase of the BPBO perovskite in the superconductivity region is only slightly more stable than the tetragonal phase. The small energy difference between the T and O phases and the discontinuous T‐to‐O phase transition account for the experimentally observed coexistence of the T and O phases. On the other hand, the BKBO perovskite involves a large energy difference between the T and O phases, which induces a low equilibrium temperature of the discontinuous T‐to‐O phase transition, in agreement with the experimental observation that the tetragonal BKBO is maintained down to low temperatures. Moreover, the electronic structures of both BPBO and BKBO superconductors show a flat band near the Fermi level, which is favorable for superconductivity. Furthermore, we find that the longer the total length of the flat band segment is, the higher the critical temperature of the BPBO or BKBO perovskite is. This key finding could be generalized straightforwardly to other unconventional superconductors and can be used to design and find optimal composition with maximum Tc for new unconventional superconductors.

SnAs with the NaCl-type Structure: Type-I Superconductivity and Single Valence State of Sn

We examined the superconductivity of SnAs and the valence state of Sn in its lattice with an expectation that SnAs and related compounds with the NaCl-type structure might exhibit superconductivity at a high critical temperature (Tc), as (Ba1–xKx)BiO3 (BKBO) does, owing to the similar crystallographic and chemical environments of Sn in SnAs and Bi in BKBO. Although SnAs had low Tc, 3.58 K, we clarified two important characteristics: First, SnAs exhibits weakly coupled type-I superconductivity. Second, Sn has a single valence state, like Sn3+(5s1), originating from there being only one crystallographically independent site in its NaCl-type structure. This unconventional single chemical state of Sn in SnAs would explain the superconductivity of SnAs with a three-dimensional NaCl-type structure, rather than a two-dimensional layered structure.

Correlation of the local and the macroscopic properties of high-temperature superconductors

Temperature dependent comparative EXAFS studies of Ba1–xKxBiO3 superconducting oxides and copper based superconductors with hole (La2–xSrxCuO4) and electron (Nd2–xCexCuO4–δ) doping showed the common dynamical local structure nonuniformity observed as the double-well potential of oxygen ion oscillations. Obtained data evidence that these local structure nonuniformity arise from the local dynamic charge ordering due to different electronic structure of the neighbouring BiO6 or CuOn (n = 4, 6) complexes in perovskite-like lattice. Based on the experimental results, the phenomenological description was proposed which allowed us to establish the correlations between the local and the macroscopic properties and to explain the insulator-metal phase transition and the appearance of the superconductive state with both hole and electron doping of parent BaBiO3, La2CuO4 and Nd2CuO4 insulators. The presented results makes clear the role of electron-phonon coupling in the mechanism of high temperature superconductivity.

Self-consistent spatially inhomogeneous state in HTSC Ba1-x KxBiO3 single crystals

A spatially inhomogeneous state in HTSC Ba1-xKxBiO3 single crystals (Tc ≈ 30 K, x = 0.4) has been studied at T < Ť ≈ 17 K, where Ť is the temperature of separation into superconducting and dielectric phases. This separation has earlier been detected in Ba1-xKxBiO3 polycrystals. In the single crystals, the main results are obtained from hysteretic magnetization reversal curves, whereas the character of separation in the polycrystals is clearly visible from I–V curves. The results obtained support the theoretical model of HTSC separation proposed in [1].

Superconductivity of Ba1 − xKxBiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba1 − x K x BiO3 (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.

Superconducting thick films prepared via electrodeposition

M. S. Martín-González, J. García-Jaca, E. Morán and M. Á. Alario-Franco, J. Mater. Chem., 1999, 9, 137 DOI: 10.1039/A805310C

Electrodeposition of large Ba1−xKxBiO3 crystals from molten KOH/KNO3 solution

Large Ba1−xKxBiO3 (BKBO) single crystals were electrosyntheiszed in air by using KOH/KNO3 flux. The effect of KNO3 concentration (CKNO3) in the flux on the K content (x) and superconductivity of BKBO was also investigated. The results show that addition of KNO3 into KOH not only makes the electrolyte stable in air, but also promotes the growth of BKBO crystals. The results also show that the electrodeposited BKBO crystals have a superconducting transition at 31K when CKNO3 (wt.%)≤17%. However, the superconductivity is suppressed when CKNO3>25%. EDX analyses reveal that CKNO3 affects the K content, for example, x=0.40 for CKNO3≤17%, however, x decreases with increasing CKNO3 when CKNO3≥25%.

Properties of Y1Ba2Cu3O7−δ and Ba1−XKXBiO3 step-edge grain boundary junctions

A technique for the fabrication of sharp and straight step-edges on LaAlO3 (LAO) substrates by Ar-ion milling is reported here. Diamond-like carbon (DLC) thin films, deposited by pulsed laser ablation, were used as masks. Clear Shapiro steps were observed in the current-voltage (IV) characteristics of Y1Ba2Cu3O7−δ (YBCO) step-edge junctions with millimeter-wave irradiation. Also, the transport properties of epitaxial Ba1−XKXBiO3 (BKBO) films deposited on LaAlO3 step-edge substrates were studied.

Characterization of Ba1-xKxBiO3 Superconducting Thin Films for Three-Terminal Devices

The physical parameters of a Ba1-x Kx BiO3 (BKBO) superconductor have been successfully estimated by measuring the Hall effect and optical reflectivity spectra for epitaxial thin films. The results of these measurements at room temperature indicated that the optimized BKBO thin film has a carrier density of 5.0×1021 cm-3 and an effective mass of 0.9m0. Supposing the free-electron model, Fermi energy of BKBO could be estimated to be 1.2 eV . Using these parameters, the quasi-particle transmission probability was also calculated for the BKBO/Nb-doped SrTiO3 junction.

Barium potassium bismuth oxide: A review

Ba1−x K x BiO3(BKBO) has aT c (onset) of 34 K. It is the highest-temperature oxide superconductor which is cubic, with a coherence length of 30–60 A. The basic properties of this compound are reviewed.

Tunneling measurement of the quasiparticle lifetime in Ba1−xKxBiO3

The temperature dependence of the energy gap of bulk polycrystalline Ba1−xKxBiO3 (BKBO) was measured with a low-temperature scanning tunneling microscope in the point contact mode. At 4.2 K, the tunneling conductance shows nearly ideal BCS behavior, however at higher temperatures, the lifetime broadening term Γ was required to fit the data. Γ(T) displays a surprising T3 temperature dependence and the magnitude is significantly larger than the calculated theoretical recombination times. Comparisons with other conductance data obtained from grain boundary and sandwich-type BKBO junctions show similar results. The large scattering rates for BKBO may be important for the utility of this material in SIS mixers.

Structural comparison of Ba1−xKxBiO3 superconducting thin films

The microstructure of Ba1−xKxBiO3 (BKBO) thin films from three different sources has been extensively compared by transmission electron microscopy studies. The three films were prepared independently in three different laboratories on three different substrates of (100) orientation and displayed excellent superconducting properties. The observed microstructure is remarkably similar in the three films. They are epitaxial with (100) orientation through all their extension and no cracks have been observed. Their defect density is similar and the resulting extension of defect-free regions is of the order of 50–80 nm.

Parametric study of the in situ growth of BaKBiO thin films by laser ablation

In situ growth of Ba1−xKxBiO3 (BKBO) films by laser ablation has been studied as a function of gas pressure and substrate temperature in O2, N2O, and N2O/O2 (1:1), respectively. The superconducting BKBO films can be grown at 410–500 °C in 15–60 mTorr of O2, at 390–510 °C in 20–2000 mTorr of N2O, and at 390–500 °C in 15–700 mTorr of N2O/O2 (1:1), respectively. Oxygen partial pressure plays an important role in controlling the doping of potassium into the BKBO phase during in situ deposition and hence the growth of the superconducting films. The optimum deposition pressures are 40 mTorr of O2, 30–50 mTorr of N2O, and 20–50 mTorr of N2O/O2 (1:1), under which not only the temperature range available for growing the superconducting films is wider but also the Tc’s of the as-grown films are higher. N2O used as an oxidation gas during growth is efficient in suppressing the crack formation in the BKBO films on cooling. The target composition also plays an important role in film growth. The optimal target composition studied is Ba:K:Bi=0.6:0.8:1.0.

Superconducting Ba1−xKxBiO3 thin films and junctions

Superconducting Ba1−xKxBiO3 (BKBO) thin films and heteroepitaxial multilayers were deposited by in situ off-axis sputtering on various substrate at 400 °C. A typical superconducting BKBO thin film on the SrTiO3 and MgO substrates had a very sharp zero resistance transition at a temperature of 22–24 K as measured by transport four point and mutual inductance probes. The normal state resistivity exhibited metallic to semiconductive characteristics depending on film composition with the lowest room temperature resistivity of about 0.25 mΩ cm. The film orientation was highly epitaxial to both SrTiO3 and MgO substrates. Normal metal-insulator-superconductor (NIS) type tunnel functions with native insulator layer and deposited MgO layer were reproducibly fabricated. The superconducting energy gap of about 3.6 meV was obtained from those NIS junctions. Heteroepitaxial superconductor- insulator-superconductor (SIS) trilayer junctions were also fabricated using MgO insulator layers. For a 5 nm thick insulator layer, quasiparticle SIS tunnel junctions with nonhysteric I-V characteristics were obtained. By reducing MgO insulator layer thickness down to 3 nm, SIS tunnel junctions with Josephson supercurrent were obtained with hysteric I-V characteristics.

Electrical and magnetic transport properties of laser-deposited Ba1−xKxBiO3 thin films

We report measurements of the electrical and magnetic transport properties of high-quality superconducting Ba1−xKxBiO3 (BKBO) thin films grown by in situ pulsed laser deposition. The best films exhibit sharp resistance and magnetic transitions with zero resistance above 28 K and widths of 0.3 K, and nearly linear normal state resistance extending from the transition to above room temperature. Critical current densities obtained by transport in patterned BKBO lines are greater than 3×106 A/cm2 at 4 K. The transverse magnetoresistance exhibits predominantly BCS-like behavior with an upper critical field slope of −0.78 T/K near Tc, giving Hc2(0)=15.2 T and a BCS coherence length ξ=46 Å. We also observe substantial field broadening suggestive of flux flow dissipation.

Pulsed Laser Deposition and Characterization of Superconducting Ba1−xKxBiO3 Thin Films

ABSTRACTWe report on high quality Ba1−xKxBiO3 (BKBO) thin films grown by pulsed laser deposition. The best films exhibit resistive and magnetic transition temperatures of 28 K, with a transition width of 0.4 K and critical current densities above 106 A/cm2 measured at 5 K. Surface impedance is measured to be 1 mΩ. Films are single phase and highly oriented in the (100) direction and electron diffraction shows no other phases. Analysis using Moire' fringes shows films to consist of 100 nm size dislocation-free regions which show a high degree of in-plane epitaxy.

Tunnelling evidence for predominantly electron–phonon coupling in superconducting Ba1−xKxBiO3 and Nd2−xCexCuO4−y

AMONG the superconducting oxides, the cubic, copper-free Ba1−xKxBiO3 (BKBO) and the electron-doped compound Nd2−xCexCuO4−y, (NCCO) stand out as being somewhat different from the rest. Nevertheless, an understanding of the pairing mechanism in BKBO (transition temperature, Tc ≈ 30K) and NCCO (Tc ≈ 23 K) may give important insights into the mechanisms of the higher- Tc superconductors. Here we report tunnelling spectroscopy measurements on BKBO and NCCO, using point-contact junctions that exhibit low leakage currents and sharp conductance peaks at the gap voltages V = ±Δ/e. Reasonably symmetric and reproducible structures are observed in the high-bias tunnelling conductances which are characteristic of phonon effects as seen in conventional superconductors. We have inverted the tunnelling data and obtained the Eliashberg functions, α2F(ω), where F(ω) is the phonon density of states at energy hω. For BKBO, α2F(ω) bears a close resemblance to the available phonon density of state determined by inelastic neutron scattering, most importantly consistently reproducing the minima. The fact that the α2F(ω) are not identical for different junctions leads to some uncertainty, but the calculated values of Tc are in good agreement with experiment for both BKBO and NCCO. Also, there is a good match between the calculated values of the total electron–phonon coupling constant λ and the measured 2Δ/κTc, consistent with predominantly phonon-mediated pairing mechanisms in these compounds.