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The most effective way to enhance the dissipation-free supercurrent in presence of magnetic field for type II superconductors is the introduction of defects that acts as artificial pinning centres (APCs) for the vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last decade by APCs created by ion-irradiation. The technique of ion-irradiation has been commonly implemented to determine the ultimate superconducting properties. However, this method is rather complicated and expensive. Here, we report on a surprisingly high critical current density and strong pinning efficiency close to the crystallographic c-axis for a K-doped Ba122 epitaxial thin film without APCs, achieving performance comparable to ion-irradiated K-doped Ba122 single crystals. Microstructural analysis reveals that the film is composed of columnar grains having width around 30-60 nm. The grains are rotated around the b- (or a-) axis by 1.5 degree and around the c-axis by -1 degree, resulting in the formation of low-angle grain boundary networks. This study demonstrates that the upper limit of in-field properties reached in ion-irradiated K-doped Ba122 is achievable by grain boundary engineering, which is a simple and industrially scalable manner., Main text with Supplement Material

Molecular beam epitaxy of K-doped Ba122 (Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$) superconductor was realized on a MgO substrate. Microstructural observation revealed that the undoped Ba122 served as a perfect buffer layer for epitaxial growth of the K-doped Ba122. The film exhibited a high critical temperature of 39.8 K and a high critical current density of 3.9 MA/cm$^\text{2}$ at 4 K. The successful growth of epitaxial thin film will enable artificial single grain boundary on oxide bicrystal substrates and reveal the grain boundary transport nature of K-doped Ba122., 5 pages, 4 figures, accepted manuscript Supercond. Sci. Technol 2022

The iron-based superconductor ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates $({\mathrm{CaF}}_{2},$ ${\mathrm{SrF}}_{2}, \mathrm{and} {\mathrm{BaF}}_{2})$ and a low growth temperature (350--420 \ifmmode^\circ\else\textdegree\fi{}C). Our epitaxial thin film grown on ${\mathrm{CaF}}_{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by \ensuremath{\sim}2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density $({J}_{\mathrm{c}})$ determined by the magnetization hysteresis loop is as high as $2.2\phantom{\rule{0.28em}{0ex}}\mathrm{MA}/\mathrm{c}{\mathrm{m}}^{2}$ at 4 K under self-field. In-field ${J}_{\mathrm{c}}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$.

La 2- x Sr x CuO 4 (LSCO) and La 2- x Ba x CuO 4 (LBCO) have been recognized as the archetype materials of “hole-doped” high- T c superconductors. Their crystal structures are relatively simple with a small number of constituent cation elements. In addition, the doping level can be varied by the chemical substitution over a wide range enough to obtain the full spectrum of doping-dependent electronic and magnetic properties. These attractive features have dedicated many researchers to thin-film growth of LSCO and LBCO. The critical temperature ( T c ) of LSCO and LBCO is sensitive to strain as manifested by a positive pressure coefficient of T c in bulk samples. In general, films are strained if they are grown on lattice-mismatched substrates (epitaxial strain). Early attempts (before 1997) at the growth of LSCO and LBCO films resulted in depressed T c below 30 K as they were grown on a commonly used SrTiO 3 substrate (in-plane lattice parameter a sub = 3.905 A): the in-plane lattice parameters of LSCO and LBCO are ≤3.80 A, and hence tensile epitaxial strain is introduced. The situation was changed by the use of LaSrAlO 4 substrates with a slightly shorter in-plane lattice constant ( a sub = 3.756 A). On LaSrAlO 4 substrates, the T c reaches 45 K in La 1.85 Sr 0.15 CuO 4 , 47 K in La 1.85 Ba 0.15 CuO 4 , and 56 K in ozone-oxidized La 2 CuO 4+ δ films, substantially higher than the T c ’s of the bulk compounds. The T c increase in La 1.85 Sr 0.15 CuO 4 films on LaSrAlO 4 and decrease on SrTiO 3 are semi-quantitatively in accord with the phenomenological estimations based on the anisotropic strain coefficients of T c (d T c /d e i ). In this review article, we describe the growth and properties of films of cuprates having the K 2 NiF 4 structure, mainly focusing on the increase/decrease of T c by epitaxial strain and quasi-stable phase formation by epitaxial stabilization. We further extract the structural and/or physical parameters controlling T c toward microscopic understanding of the variation of T c by epitaxial strain.

Nd2CuO4 has a crystal structure called the Nd2CuO4 (T’) structure in which fluorite-like Nd2O2 slabs and CuO2 planes stack alternately. Nd2CuO4 is known to show superconductivity by carrier doping via anion/cation substitution, or making the oxygen sublattice highly ordered via a stringent control of thermodynamic conditions during crystal growth. In this study, CaF2 is used for growing Nd2CuO4 films, as a substrate material which contains fluorine atoms. The films show superconducting onset ($T_{\mathrm {c}}^{\text {on}}$) beyond 30 K. Furthermore, in contrast to reported superconductivity in this system, the emergence of superconductivity is found to be insensitive to post annealing procedures.

We report the fabrication of superconducting tunnel junctions, both of superconductor–insulator-normal metal (SIN) and superconductor–insulator-superconductor (SIS), on MgB 2 using MgO and CaF 2 as a barrier. The SIN junctions fabricated using an MgO barrier showed excellent quasi-particle characteristics, including a large superconducting gap ( Δ ) of 2.5–3 meV and a low zero-bias conductance. We have also fabricated SIS junctions with an MgO barrier, but the quasi-particle characteristics of the SIS junctions are not as good as those of the SIN junctions, namely a reduced superconducting gap and a high zero-bias conductance. It appears that top MgB 2 electrodes do not grow well on an MgO barrier, which is also suggested from in-situ RHEED observation. The SIN junctions fabricated using a CaF 2 barrier showed less sharp quasi-particle characteristics than using an MgO barrier. However, the SIS junctions using a CaF 2 barrier showed a fairly large I c R N value at 4.2 K over 1 mV and also exhibited finite Josephson current up to almost the film's T c (∼30 K). The RHEED observation revealed that top MgB 2 electrodes grow well on a CaF 2 barrier.

The electronic phase diagram of the cuprates remains enigmatic and is still a key ingredient to understand the mechanism of high- T c superconductivity. It has been believed for a long time that parent compounds of cuprates were universally antiferromagnetic Mott insulators (charge-transfer insulators) and that high- T c superconductivity would develop upon doping holes or electrons in a Mott–Hubbard insulator (“doped Mott-insulator scenario”). However, our recent discovery of superconductivity in the parent compounds of square-planar cuprates with the Nd 2 CuO 4 (T’) structure and the revised electronic phase diagram in T’ cuprates urged a serious reassessment to the above scenario. In this review, we present the main results derived from our synthesis and experiments on T’ cuprates in the undoped or heavily underdoped regime over 20 years, including material issues and basic physics. The key material issue is how to remove excess oxygen ions at the apical site without introducing oxygen vacancies in the CuO 2 planes. In order to put this into practice, the basic knowledge of complex solid-state chemistry in T’ cuprates is required, which is also included in this review.

We synthesized high-quality, single-crystalline thin films of layered rare-earth (R) palladates R${}_{2}{\mathrm{PdO}}_{4}$ ($R$ = La, Nd, and Sm) and Ce substituted ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ by reactive molecular beam epitaxy. For ${\mathrm{La}}_{2}{\mathrm{PdO}}_{4}$, ${\mathrm{Nd}}_{2}{\mathrm{PdO}}_{4}$, and ${\mathrm{Sm}}_{2}{\mathrm{PdO}}_{4}$, we find that the electronic conduction is independent of R elements. Doping charge carriers into the ${\mathrm{PdO}}_{2}$ planes of ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ increases the electronic conduction and this effect is enhanced by vacuum annealing. The enhanced electronic conduction originates not solely from the doped charge carriers but is superimposed by Pd vacancies. X-ray photoelectron spectroscopy combined with inductively coupled plasma mass spectrometry revealed Pd deficiencies in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ and these defects play a crucial role for the electronic conduction. We observe a monotonic enhancement of the electronic conduction in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ thin films induced by Ce substitution and vacuum annealing. The estimated charge carrier concentration necessary for metallic conduction ($x\ensuremath{\approx}0.45$) at $T=0$ K is far beyond the experimentally accessible solubility limit.

Iron nitride films were grown by atomic-nitrogen-assisted molecular beam epitaxy. ZnS-type FeN (γ″-FeN) was synthesized with low Fe evaporation rates (≤ 0.2 A/s) at low growth temperatures below 210 °C whereas γ′-Fe 4 N was synthesized with high Fe evaporation rates (≥1.0 A/s) at 130–415 °C. Our results indicate that the stabilization of ZnS-type FeN requires not only the fulfillment of thermodynamic constraints but also the control of a delicate balance of kinetically driven competition. The use of lattice-matched GaN(0001) substrates enables the growth of epitaxial films of ZnS-type FeN.

We report the first determination of the in-plane complex optical conductivity of 1111 high-${T}_{c}$ superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at ${T}_{c}$ and experiences an anomalous $T$-linear softening and narrowing below ${T}^{*}\ensuremath{\approx}110\text{ }\text{ }\mathrm{K}\ensuremath{\gg}{T}_{c}$. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\approx}7$ in both materials, indicating strong pairing.

Reduction agents, such as activated carbon, TiH 2 , and CaH 2 , were compared from a viewpoint of the facility for the topotactic reduction of LaNiO 3 to LaNiO 2 films. Activated carbon did not yield infinite-layer LaNiO 2 whereas both of TiH 2 and CaH 2 yielded infinite-layer LaNiO 2 with low resistivity (∼1 mΩ cm at 300 K) as well as metallic behavior down to 70 K. Thermal desorption spectroscopy indicated that H 2 released from metal hydrides plays a dominant role in the topotactic reduction.

We report the synthesis of perovskite RE NiO 3 films ( RE = La, Pr, Nd, Sm, and Eu) by metal organic decomposition (MOD). The RE NiO 3 family is an ideal system for studying the metal–insulator transition due to the simplicity of the materials. One of the drawbacks is that the bulk synthesis of the RE NiO 3 requires processing at high oxygen pressures to stabilize Ni 3+ . Fundamentally, MOD is similar to solid-state reaction, but it turned out that the MOD synthesis tends to stabilize RE NiO 3 without the need for high oxygen pressure. The films prepared by MOD show high crystallinity and low resistivity. Furthermore, we have investigated the epitaxial strain effect and observed a dramatic effect in PrNiO 3 and NdNiO 3 films on LaAlO 3 substrates. The metal–insulator transition in the PrNiO 3 films on LaAlO 3 is fully suppressed, whereas the metal–insulator transition temperature is considerably lowered in the NdNiO 3 films on LaAlO 3 .

Infinite-layer LaNiO2 thin films were synthesized by metal organic decomposition and subsequent topotactic reduction in hydrogen, and their transport properties were investigated. LaNiO2 is isostructural to SrCuO2, the parent compound of high-Tc Sr0.9La0.1CuO2 with Tc = 44 K, and has 3d9 configuration, which is very rare in oxides but common to high-Tc copper oxides. The bulk synthesis of LaNiO2 is not easy, but we demonstrate in this article that the thin-film synthesis of LaNiO2 is rather easy, thanks to a large-surface-to-volume ratio, which makes oxygen diffusion prompt. Our refined synthesis conditions produced highly conducting films of LaNiO2. The resistivity of the best film is as low as 640 μΩ cm at 295 K and decreases with temperature down to 230 K but it shows a gradual upturn at lower temperatures.

Recently we reported the growth of MgB 2 films by pyrolysis of decaborane (B 10 H 14 ) in Mg vapor. In this article, we discuss the intrinsic problem at the MgB 2 /Al 2 O 3 interface, which was encountered during our optimization of the growth temperature. Mg is a stronger reductant than Al, and takes out oxygen from Al 2 O 3 . Increasing the growth temperature, this reaction becomes noticeable and leads to the formation of MgO and Mg 1− x Al x B 2 , which deteriorates the properties of resultant MgB 2 films. Our observations present a general issue in the choice of substrate and barrier materials in thin film growth and/or junction fabrication of compounds containing reactive cations such as Mg.

Molecular beam epitaxy (MBE) has been an excellent tool to design artificial heterostructures in the research field of semiconductors by means of an alternate stacking of existing lattices. This article describes further capabilities of MBE as a synthesis tool sui generis, especially for multi-cation oxides, due to low-temperature reaction and the pseudomorphic stabilization effect by epitaxy. Single-crystalline Sr0.9La0.1CuO2 films exhibiting metallic conduction and superconductivity are successfully prepared by MBE. A new phase, T⁎-La2CuO4, is also stabilized on a DyScO3 substrate. Methods of high-precision rate control of each constituent element, which is prerequisite for a reproducible growth of the multi-cation oxide films, are also discussed.

PurposeThis study seeks to investigate doctors' desire to change the hospital where they work to sustain higher quality care.Design/methodology/approachSelf‐administered questionnaires were sent to doctors in Aichi Prefecture, Japan. Data were analyzed using univariate and logistic regression analysis and recursive partitioning.FindingsFactors related to doctors' desire to change hospitals, according to logistic regression, were interaction between working hours and satisfaction with the hospital, evaluation, local government hospitals versus private ones, small vs large hospitals, ophthalmology versus internal medicine, desire to continue working as a hospital doctor and age. Additionally, working hours were also found to be related, based on recursive partitioning.Research limitations/implicationsThe response rate was low and sampling bias was observed – therefore results need careful interpretation. Also, because this was a cross‐sectional study, causal relationships could not be identified. Desire to change hospitals, but not actual behavior, was measured.Practical implicationsEfforts to prevent doctors from changing hospitals should include considering job satisfaction and workload, doctor evaluation methods, support for career progression and organizational management.Originality/valueAs the hospital doctor shortage in rural areas becomes more serious, exploring doctors' desire to leave their current hospital is meaningful for Japanese hospital managers and hospitals worldwide aiming to provide sustainable and higher quality care.

Single-crystalline films of superconducting Ba1−xKxFe2As2 and Sr1−xKxFe2As2 were grown at temperatures below 350 °C under reduced As flux by molecular beam epitaxy. We performed a systematic study of the doping dependence of Tc in Ba1−xKxFe2As2 for x = 0.0 to 1.0 and Sr1−xKxFe2As2 for x = 0.0 to 0.5. The highest T c on ( T c end ) so far attained is 38.3 K (35.5 K) at x ∼ 0.3 for Ba1−xKxFe2As2, and 33.4 K (31.0 K) at x ∼ 0.4 for Sr1−xKxFe2As2. The optimum K-doping level inducing the highest Tc is different in the two systems. The implication of the results is briefly discussed.

We have synthesized La 2− x Sm x CuO 4 (0 ⩽ x ⩽ 2.0) with the Nd 2 CuO 4 structure via a molten alkaline hydroxide route at temperatures as low as 400–480 °C. After reduction heat treatment in vacuum at 600–750 °C for removal of excess oxygen atoms at the interstitial apical site, superconductivity with T c = 20–24 K was observed in the samples with x = 0.05–1.0. The superconducting volume fraction is nearly 100% for x = 0.3–0.7. Our results demonstrate that La 2− x Sm x CuO 4 with no nominal carrier doping is a bulk superconductor.

We report the growth of high-quality MgB 2 films by thermally decomposing decaborane (B 10 H 14 ) in Mg vapor. B 10 H 14 is solid at ambient temperatures and stable in air, and hence it is much more convenient to use in thin film growth than highly explosive diborane that has been commonly used in hybrid physical–chemical vapor deposition of MgB 2 films. We grew MgB 2 films on c -cut sapphire substrates held at 400–450 °C by supplying vaporized B 10 H 14 into a pocket heater with high Mg vapor pressure. The resultant films as thin as 100 nm showed strong and sharp c -axis lattice peaks in X-ray diffraction, indicating nearly ideal epitaxial growth. The films showed a superconducting transition at around 39 K.

We present the results of our systematic investigation of the RE dependency of superconductivity in the parent compounds T′- RE 2 CuO 4 ( RE = Pr, Nd, Sm, Eu, Gd, and Tb). Superconductivity with a transition temperature ( T c ) ≥ 30 K is achieved for RE = Pr and Nd by the metal organic decomposition (MOD) method. By contrast, T c onset is at highest 20 K for RE = Gd. A stringent control of synthesis- and post-annealing-conditions is required to obtain superconducting films. Our results indicate that the induction of superconductivity into T′- RE 2 CuO 4 cuprates strongly depends on the RE 3+ ionic size. This trend is discussed from the viewpoint of RE -dependent thermodynamic stability of T′- RE 2 CuO 4 . For smaller RE 3+ ions, the thermodynamic boundary conditions become tighter.

Recent reports on superconductivity in parent compounds T ′ - RE 2 CuO 4 [RE = Pr, Nd, Sm, Eu, Gd] prepared by metal-organic decomposition (MOD) shed new light on the electronic and magnetic phase diagram of electron-doped cuprates. A thorough reduction process is the key to inducing superconductivity in square-planar coordinated cuprates. In this work, Pr2CuO4 films were grown epitaxially by molecular beam epitaxy (MBE) on (001)SrTiO3, (110)GdScO3 and (110)DyScO3 substrates. A modified two-step reduction process is required in order to obtain single-phase superconducting samples. The optimized superconducting properties are T c zero = 26 K , while the room-temperature resistivity is about 400 μ Ω cm . Hence, we show that even amorphous Pr2CuOy films deposited on (110)DyScO3 become superconducting after the application of the two-step annealing process. These results indicate that superconductivity is induced to Pr2CuO4 by systematic optimization of the reduction conditions, irrespective of the synthesis route.

La 2− x Sm x CuO 4+ δ with the Nd 2 CuO 4 structure was synthesized by precipitation from fused alkaline hydroxide, CsOH, at 400 °C. The as-sintered polycrystalline samples showed no diamagnetic signal, but after post-annealing in vacuum of ≈10 −8 atm at 650–700 °C, the samples with x = 0.1 and 0.3 exhibited superconductivity at T c on = 25 K. The volume fraction of the superconducting phase estimated from the slope of the Zero-Field-Cooled magnetization data was over 30% for x = 0.3, which confirmed that the La 1.7 Sm 0.3 CuO 4+ δ is a bulk superconductor.

We report molecular beam epitaxy growth of FeSe and Sr1−xKxFe2As2. In FeSe growth, the compositional window to obtain the superconducting phase (β-FeSe) is very limited. The use of Al2O3–R substrates instead lattice-matched LaAlO3 substrates slightly expands the compositional window, and we observed a substantial drop of resistivity but with no zero resistivity attained. In Sr1−xKxFe2As2 growth, the largest problem is the volatility of K. Films grown at substrate temperatures higher than 300 °C contain no K. Superconducting films of Sr1−xKxFe2As2 can be obtained at the growth temperature of 250 °C with very low As flux. The best film has T c on = 30.3 K and T c end = 22.9 K .

We prepared thin films of T′-La2CuO4, which usually crystallizes in the T structure, by MBE, and investigated their properties while systematically changing the post-reduction conditions with a view towards obtaining superconductivity along the lines of the parent compound superconductors we have recently reported (O. Matsumoto et al., Phys. Rev. B 79 (2009) 100508(R)). The results indicate that the optimal reduction window is very narrow, near which metallic conductivity is obtained down to 50 K. The resistivity of the T′-La2CuO4 films is in the range of 10−2–10−3 Ω cm, which is several orders of magnitude lower than that of the counterpart T-La2CuO4, the implication of which is briefly discussed from the viewpoint of the difference in electronic structure induced by different oxygen coordination.

Recently we have achieved superconductivity in T ′- RE 2 CuO 4 ( RE = Pr, Nd, Sm, Eu, and Gd), films by metal organic decomposition (MOD). In this article, we first report the cation off-stoichiometry effect, which aims at screening out the possibility of hole doping by cation deficiencies. We also investigated the structure and microstructure of superconducting T ′-Nd 2 CuO 4 films synthesized by MOD. This investigation aims at elucidating why MOD-grown Nd 2 CuO 4 films become superconducting whereas our previous Nd 2 CuO 4 films grown by molecular beam epitaxy were not superconducting.

We investigated the generic phase diagram of the electron doped superconductor, Nd2-xCexCuO4, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial Oap atoms, we found that the Tc increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. The implication of our results is discussed on the basis of tremendous influence of Oap "impurities" on superconductivity and also magnetism in T' cuprates. Then we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd2-xCexCuO4., Comment: 12 pages, 4 figures; International Symposium on Superconductivity (ISS) 2008

We have recently achieved superconductivity in T'-RE 2 CuO 4 (RE: Pr, Nd, Sm, Eu, and Gd), using epitaxial thin films by metal organic decomposition. The key recipes to achieve superconductivity are low-P O2 firing and subsequent vacuum reduction to minimize the amount of impurity oxygen atoms, which are very harmful to high-T c superconductivity. In this article, we report our investigation on the reduction dependence of superconductivity of T'-RE 2 CuO 4 . For thin films, the amount of remnant O ap atoms is difficult to evaluate but we propose that one good measure for this may be the c-axis lattice constant, which tells us whether the reduction is insufficient or excessive.

Lattice instability in the planar Cu–O bond in high-temperature superconducting cuprates is probed by the Cu K-edge extended X-ray absorption fine structure (EXAFS). Refined temperature-dependent polarized EXAFS data for high-quality La1.85Sr0.15CuO4 (LSCO) single crystals grown by MBE and TSFZ methods are analyzed and compared with the transport properties. Temperature-dependent oxygen displacement shows a signature of bond splitting at T * and a sharp drop due to superconductivity coherence at T c. Detailed analysis of the anomalous oxygen displacement demonstrates the Cu–O bond splitting, i.e., long and short bonds (ΔR≈0.12 A), which is in good agreement with the recent cluster calculation. The results indicate that the origin of pseudogap is related to the formation of charged dynamical lattice distortions (polarons) that are coexistent with metallic (superconducting) domains.

Undoped T ′-type cuprates are believed to be charge transfer (Mott–Hubbard) insulators. Recent observation of undoped superconducting T ′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T ′-(La 3+ ,Y 3+ ) 2 CuO 4 and doped T ′-(La 3+ ,Ce 4+ ) 2 CuO 4 thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane ( E // ab ) polarized Cu K-EXAFS data shows a remarkable Cu–O bond distance displacement. The broadened Cu–O distribution suggests a large local lattice distortion in T ′-type structures, which might influence the electronic structure.

The possibility of preparing bicrystalline Josephson junctions and bolometers based on superconducting MgB2 on specially prepared bicrystalline MgO substrates is investigated. Microbridges 0.85–6.00 μm in width, intersecting the bicrystalline interface, are formed in epitaxial bicrystalline MgB2 films grown on these substrates. It is found that annealing of bicrystalline samples in oxygen leads to a systematic decrease in the critical current, an increase in the temperature width of the superconducting transition region, and to an improvement of the current-voltage (IV) characteristic, which becomes close in shape to the IV characteristic of a Josephson junction. The response of such a junction to radiation at a frequency of 110 GHz with an amplitude attaining 0.5 mV is measured.

Local structure of optimally doped T -(La,Sr) 2 CuO 4 (LSCO) single-crystal thin films grown by molecular-beam epitaxy on LaSrCuO 4 and SrTiO 3 substrates was studied by the in-plane polarized Cu K-edge extended X-ray absorption fine structure (EXAFS). The results of data analysis show that the local structure (CuO 6 octahedra) of LSCO thin films is strained by ca. 5×10 −3 in accordance with the crystallographic data. Although the planar and apical oxygen displacements Δ R CuO are related to the critical temperature in a simple relation, the magnitude of strain-induced oxygen displacements (leading to anti-Jahn–Tell or Jahn–Teller distortion) is less than 0.1% of the equilibrium bond length. This suggests that the strain dependence of electronic states involves a different mechanism than a purely electronic model in a homogeneous conducting plane. The fact that LSCO thin films under tensile strain have a large local distortion of planar oxygens indicates that inhomogeneity may affect the critical temperature rather than the apical oxygen displacement.

A Ge pixel array detector with 100 segments was applied to fluorescence X-ray absorption spectroscopy, probing the local structure of high-temperature superconducting thin-film single crystals (100 nm in thickness). Independent monitoring of pixel signals allows real-time inspection of artifacts owing to substrate diffractions. By optimizing the grazing-incidence angle theta and adjusting the azimuthal angle phi, smooth extended X-ray absorption fine structure (EXAFS) oscillations were obtained for strained (La,Sr)2CuO4 thin-film single crystals grown by molecular beam epitaxy. The results of EXAFS data analysis show that the local structure (CuO6 octahedron) in (La,Sr)2CuO4 thin films grown on LaSrAlO4 and SrTiO3 substrates is uniaxially distorted changing the tetragonality by approximately 5 x 10(-3) in accordance with the crystallographic lattice mismatch. It is demonstrated that the local structure of thin-film single crystals can be probed with high accuracy at low temperature without interference from substrates.

Local lattice anomalies in optimally doped T-(La,Sr)2CuO4 single crystal like thin films (T c = 43.4 K) grown by molecular-beam epitaxy have been studied by the in-plane polarized Cu K-edge extended X-ray absorption fine structure (EXAFS). The results indicate temperature-dependent local atomic displacements which are anomalous at the T c and below a higher temperature T s as demonstrated by a change in the mean square relative displacement of the Cu–O bond $$\sigma _{{\rm Cu}-{\rm O}}^2$$ , i.e., a sharp drop at the T c and a gradual deviation from a noncorrelated Debye-like behavior below T s where the spatial inhomogeneity appears. We find that the magnitude of the Cu–O displacement changes at the T c, $$\Delta \sigma _{{\rm Cu}-{\rm O}}^2$$ is enhanced by compressive strain while the tendency of charge segregation is suppressed. The results suggest that the uniaxial pressure effects stabilize the system by decreasing the onset temperature and magnitude of spatial heterogeneity.

The angular dependence of the critical current density (Jc(?)) of La1.87Y0.13CuO4 was measured at different fields and temperatures. Jc(?) showed a strong angular variation, which is typical for uniaxial anisotropic superconductors. Although La1.87Y0.13CuO4 has a quasi-two-dimensional (2D) crystal structure, the angular variation could be consistently described using the usual anisotropic three-dimensional (3D) Ginzburg?Landau theory with a small anisotropy ratio, but not by Tinkham's multi-layer model, which is known to be suitable for describing quasi-2D superconductors. These results indicate that the 2D crystallographic structure plays a minor role in La1.87Y0.13CuO4 because of the elongated c axis coherence length. From our analysis, we were also able to estimate the anisotropy ratio, the ab plane, and the c axis coherence length.

High-temperature superconductivity has been discovered in La2−xBaxCuO4 [J.G. Bednorz, K.A. Muller, Z. Phys. B 64 (1986) 189. [1] ], a compound that derives from the undoped La2CuO4 crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity [V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. [2] , [3] , [4] ]. The undoped material La2CuO4 is also the basis of the electron-doped cuprate superconductors [Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [5] ] of the form La2−xCexCuO4+y [M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. [6] , [7] ] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO2-plane. It is well known that for La2−xCexCuO4+y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La2−xRExCuO4 can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La2−xRExCuO4, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place.

We report the growth of NdBa2Cu3O7-d films on (100) MgO substrate by Molecular Beam Epitaxy (MBE). Large area NdBa2Cu3O7-d films with homogeneous superconducting properties were grown by precise control of stoichiometry and the optimisation of growth parameters. The stoichiometric ratio of Nd:Ba:Cu close to 1:2:3 yields films with TC of 94 K and JC values above 3.5 MA/cm2 at 77 K on bare MgO substrate. The NdBa2Cu3O7-d films grown under optimised conditions had excellent in-plane texture and good metallicity. The most significant characteristic of our MBE grown NdBa2Cu3O7-d films is the very low microwave surface resistance values at all temperature range compared to its YBa2Cu3O7-d counterpart with typical value of ~870 micro-ohm at 77 K & 22 GHz. Our results on the MBE grown NdBa2Cu3O7-d films suggests that NdBa2Cu3O7-d is a superior choice for the realisation of commercial microwave applications., Submitted to Jpn. J. Appl. Phys

Single-crystalline Ba1 − xKxBiO3 (BKBO) films with x between 0 and 0.54 are prepared on SrTiO3(1 0 0) substrates by molecular beam epitaxy (MBE). The key to the MBE growth of BKBO is the reproducible deposition of K, which is achieved by using an alkaline metal dispenser (K2CrO4 with a reducing agent) to make the K source easy to handle, and also by using atomic absorption spectrometry to provide reliable K flux rate control. The films are superconducting for ∼0.31⩽x⩽∼0.54. The best film (x=0.46) shows Tconset=24.5 K, Tcend=24 K, ρ(300 K) ∼ 220 μΩ cm, and ρ(30 K) ∼ 150 μΩ cm.