Biaxially textured LuNi2BC thin films with Tc up to 15.8 K and a residual resistivity ratio up to 15 have been prepared. The films were deposited on MgO(110) substrates using pulsed laser deposition from a stoichiometric target. Via the precise control of the deposition rate, a biaxial texture is favored within a broad range of the deposition temperature. The full epitaxial relationship between the borocarbide film, the \luo interface layer and the substrate is (110)[001]MgO II (110)[001]Lu2O3 II (001)[100]LuNi2B2C. A very sharp in--plane alignment of about 1\textdegree and an out--of--plane order of about 2.5\textdegree was achieved in the \luni layer. However, a systematic study using different deposition temperatures shows that the temperature is crucial for phase formation and appealing superconducting properties. Applying optimized deposition conditions, critical temperatures of up to 15.8 K and steep superconducting transitions of about 0.3 K are reproducibly obtained. We further analyzed the upper critical field Hc2 of one film deposited under optimized conditions. It was resistively measured along the [001] direction and fitted with a power law. The value of Hc2(0) = 9.82 T obtained from that fit is in very good agreement with single crystal data and the power exponent describing the positive curvature for small external magnetic fields of about 0.19 indicates a relatively low intraband scattering in the films.
The biaxially textured growth of superconducting Co-doped BaFe2As2 (Ba-122) thin films has been realized on ion beam assisted deposition (IBAD) MgO coated conductor templates by employing an iron buffer architecture. The iron pnictide coated conductor showed a superconducting transition temperature of 21.5 K, which is slightly lower than that of Co-doped Ba-122 films on single crystalline MgO substrates. A self-field critical current density of over 10^5 A/cm2 has already been achieved even at 8 K. The current experiment highlights the potential of possible coated conductor applications of the…
We have investigated the nonlinear microwave properties of electron-beam coevaporated YBaCuO films on MgO, using stripline resonators at 2.3 GHz and temperatures 1.7-80 K. The oxygen content of the films ranged from strongly underdoped to overdoped. Above 20 K, the nonlinear response of the resonators was dominated by the superconductor. We could establish clear correlations between the nonlinear surface resistance, two-tone intermodulation (IMD), and oxygen content of the films, which indicate that the superconducting order parameter is important for the nonlinearities. An exponential rather …
High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d=1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm$^2$ have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.
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.
We have investigated the anomalous nonlinear microwave response of epitaxial electron-beam coevaporated YBa2Cu3O7-x films on MgO. The power and temperature dependent surface impedance and two-tone intermodulation distortion were measured in stripline resonators at several frequencies between 2.3 and 11.2 GHz and at temperatures between 1.7 K and Tc. All of the eight films measured to date show a decrease of the surface resistance Rs of up to one order of magnitude as the microwave current is increased up to approximately 1 mA for T < 20 K. The surface reactance Xs showed only a weak increase i…
Most commercial high-temperature superconducting coated conductors based on ion beam assisted MgO deposited templates use $\mathrm{LaMnO_3}$ (LMO) films as the terminating buffer layer. In contrast, coated conductors based on inclined substrate deposition (ISD)-MgO technology are still produced with homoepitaxial (homoepi)-MgO as the cap layer. In this work we report on the deposition of LMO buffer layers on ISD-MgO/homoepi-MgO by electron beam physical vapor deposition. The growth parameters of textured LMO films were studied systematically and their properties were optimized regarding the cr…
We have grown untwinned YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films on (110) MgO substrates that were pre-annealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to th…
We fabricated superconducting MgB2 thin films on (001) MgO substrates. The samples were prepared by magnetron rf and dc co-sputtering on heated substrates. They were annealed ex-situ for one hour at temperatures between 450°C and 750°C. We will show that the substrate temperature during the sputtering process and the post annealing temperatures play a crucial role in forming MgB2 superconducting thin films. We achieved a critical onset temperature of 27.1K for a film thickness of 30nm. The crystal structures were measured by x-ray diffraction.
High critical current densities (Jc) > 1 MA/cm2 were realized in cobalt-doped BaFe2As2 (BaFe2As2:Co) films on flexible metal substrates with biaxially-textured MgO base-layers fabricated by an ion-beam assisted deposition technique. The BaFe2As2:Co films showed small in-plane crystalline misorientations (delta fai BaFe2As2:Co) of ~3o regardless of doubly larger misorientaions of the MgO base-layers (delta fai MgO = 7.3o), and exhibited high self-field Jc up to 3.5 MA/cm2 at 2 K. These values are comparable to that on MgO single crystals and the highest Jc among iron pnictide superconducting ta…
We have investigated the nonlinear surface impedance and two-tone intermodulation distortion of nine epitaxial YBa2Cu3O7-d films on MgO substrates, using stripline resonators, at frequencies f=2.3-11.2GHz and temperatures T=1.7K-Tc. The power dissipation decreased by up to one order of magnitude as the microwave electric field was increased to about 100V/m for T<20 K, while the reactance Xs showed only a weak increase. The minimum of the losses correlated with a plateau in the intermodulation signal. The same features were observed for a Nb film on MgO. The anomalous response is due to nonline…
In this work, we report a strengthened superheating effect caused by a buffering YBa2Cu3Oy (Y123 or YBCO) layer in the Nd1+xBa2-xCu3O7-y (Nd123 or NdBCO) thin film with MgO substrate (i.e., NdBCO/YBCO/MgO thin film). In the cold-seeding melt-textured (MT) growth, the NdBCO/YBCO/MgO film presented an even higher superheating level, about 20 °C higher than that of non-buffered NdBCO film (i.e., NdBCO/MgO film). Using this NdBCO/YBCO/MgO film as seeds and undergoing a maximum processing temperature (Tmax) up to 1120 °C, we succeeded in growing various RE1+xBa2-xCu3O7-y (REBCO, RE=rare elements) b…
The effects of electron doping and phonon vibrations on the magnetic properties of monolayer and bilayer FeSe epitaxial films on SrTiO$_3$ have been studied, respectively, using first-principles calculations with van der Waals correction. For monolayer FeSe epitaxial film, the combined effect of electron doping and phonon vibrations readily leads to magnetic frustration between the collinear antiferromagnetic state and the checkerboard antiferromagnetic Néel state. For bilayer FeSe epitaxial film, such magnetic frustration is much more easily induced by electron doping in its bottom layer than…
We report the successful fabrication of superconducting tantalum nitride (TaN) thin films using a pulsed laser deposition technique with 1064 nm radiation. Films with thickness $ \sim $ 100 nm deposited on MgO (100) single crystals and on oxidized silicon (SiO$_{2} $) substrates exhibited a superconducting transition temperature of $\sim $ 8 K and 6 K, respectively. The topography of these films were investigated using atomic force and scanning electron microscopy, revealing fairly large area particulate free and smooth surfaces, while the structure of the films were investigated using standar…
Ba(Fe1-xCox)2As2 superconducting thin films have been grown on SrTiO3, (La,Sr)(Al,Ta)O3, LaAlO3 and YAlO3 (YAO) single crystal substrates by pulsed laser deposition. All the films, except on YAO, have been grown epitaxially without buffer layers. The films deposited on YAO contained 45 degree in-plane rotated grains and showed a broad superconducting transition. The onset Tc of the films is observed to increase from 16.2 K to 24.5 K with increasing c/a, mainly due to a slight distortion of the AsFe4 tetrahedron. From this correlation, we expect that higher superconducting transition temperatur…
Superconducting thick films (200$μ$m-300$μ$m) of YBCO have been fabricated successfully on MgO substrates by a new approach. The precursor powders (YBa2Cu3O7-$δ$(Y123)+0.1molY2BaCuO5(Y211)) are placed between two MgO (10mm x 10mm) slices to form sandwich structure. The YBCO thick films have been obtained from the precursors by modified melt growth process. Resistance measurements of YBCO thick films show Tcon of 87.3K and $Δ$T of 4.5K. Estimates using hysteresis loops and the Bean model give a value of 2.78 x 10$^{3}$ A/cm2 (77K, 0T) for the critical current density. The observations of scanni…
We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of High critical Temperature Superconductor thin films. We present measurement of niobium superconducting $λ/2$ coplanar waveguide resonators, fabricated on $La_{0.3}Sr_{0.7}Al_{0.65}Ta_{0.35}O_3$ (LSAT), $MgO$ (MgO) and $LaAlO_3$ (LAO), at the millikelvin temperature range and at low input power. By comparing our results with the Two-Level System (TLS) model, we have discriminated among different dominant loss mechanisms. LSAT has shown the …
Co-doped BaFe2As2 (Ba-122) can be realized on both MgO and SrTiO3 bicrystal substrates with [001]-tilt grain boundary by employing Fe buffer layers. However, an additional spinel (i.e. MgAl2O4) buffer between Fe and SrTiO3 is necessary since an epitaxial, smooth surface of Fe layer can not be grown on bare SrTiO3. Both types of bicrystal films show good crystalline quality.
Among the recently discovered iron-based superconductors, ultrathin films of FeSe grown on SrTiO3 substrates have uniquely evolved into a high superconducting-transition-temperature (TC) material. The mechanisms for the high-TC superconductivity are ongoing debate mainly with the superconducting gap characterized with in-situ analysis for FeSe films grown by bottom-up molecular-beam epitaxy. Here, we demonstrate the alternative access to investigate the high-TC superconductivity in ultrathin FeSe with top-down electrochemical etching technique in three-terminal transistor configuration. In add…
This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain boundaries perpendicular to the $ab$-plane, good superconducting properties are measured: a critical temperature, $T _{\rm c}$, of 46 K and a self-field critical current density, $J_{\rm c}$, of $2 \times 1…
We used STM to study the tunneling spectra of FeSe0.3Te0.7 films with two orientations of ab-planes and the connection ramp between them. We have discovered that, using pulse laser deposition (PLD) method, the a- and b-axis of the FeSe0.3Te0.7 film deposited on Ar-ion-milled Magnesium Oxide (MgO) substrate are rotated 45 degree with respect to those of MgO, while the a- and b-axis of the film grown on pristine MgO substrate are parallel to those of MgO. With photolithography and this technique, we can prepare FeSe0.3Te0.7 film with two kinds of orientations on the same MgO substrate, and the c…
Thin film growth of iron chalcogenides by pulsed laser deposition (PLD) is still a delicate issue in terms of simultaneous control of stoichiometry, texture, substrate/film interface properties, and superconducting properties. The high volatility of the constituents sharply limits optimal deposition temperatures to a narrow window and mainly challenges reproducibility for vacuum based methods. In this work we demonstrate the beneficial introduction of a semiconducting FeSe$_{1\text{--}x}$Te$_x$ seed layer for subsequent homoepitaxial growth of superconducting FeSe$_{1\text{--}x}$Te$_x$ thin fi…
Superconducting epitaxial films of Fe-based layered arsenide, Co-doped SrFe2As2, were grown at 700oC on mixed perovskite (La, Sr)(Al, Ta)O3 (001) single-crystal substrates by pulsed-laser deposition. Both the epitaxial film and an (001)-oriented film grown at 600oC exhibited superconducting transitions at ~ 20 K. The zero-resistance states of the epitaxial film were sustained under a magnetic field (H) of 9 T at 9 K when H was parallel to the c-axis, while they were sustained at higher temperatures up to 10 K for H parallel to the a-axis. This is the first demonstration of superconducting thin…
High-critical-temperature (Tc) superconductivity at 48 K is reported for hydrogen-doped SmFeAsO epitaxial films on MgO single-crystal substrates. The key processes are pulsed laser deposition to grow undoped SmFeAsO epitaxial films and subsequent topotactic chemical reaction using CaH2 powders under evacuated silica-glass ampule atmosphere. Based on this post-deposition thermal annealing treatment that we have developed, a maximum hydrogen concentration x = ~0.35 was realized in SmFeAs(O1-xHx). Disordered hydrogen-substitution at O sites is experimentally confirmed directly by atomic-scale mic…
We report Nernst effect measurements for some crystalline films of Ca and Zn-doped yttrium barium copper oxide grown by pulsed laser deposition. We argue that our results and most of the published data for LSCO are consistent with the theory of Gaussian superconducting fluctuations.
The nonlinear surface resistance Rs of the YBa2Cu3O7-x superconducting thin films on the MgO substrates was researched in the Hakki-Coleman dielectric resonator at the microwave signal powers from -18 dBm to +30 dBm at the ultra high frequency of 25 GHz in the range of temperatures from 12 K to 85 K. The dependences of the surface resistance on the temperature RS(T) in the YBa2Cu3O7-x superconducting thin films at the microwaves were measured. The dependence of the surface resistance on the microwave power RS(P) in the YBa2Cu3O7-x superconducting thin films at the microwaves were found at the …
While the electrodynamic treatment of a superconducting thin film by Yeh et al. [Phys. Rev. B 48, 9861 (1993)] ostensibly has application for the "thin-film limit," their work is shown to employ bulk-like instead of thin-film electrodynamics, and as such obtains an ostensible violation of the first law of thermodynamics and a surface resistance independent of the resistivity. Other theoretical and experimental difficulties are briefly noted.
We investigate the epitaxial growth of bilayer cuprate La2CaCu2O6+δusing pure ozone as an oxidant, and find that even the crystal with parent composition without cation substitution can show metallic behavior with the aid of epitaxial strain effect. The hole concentration is controlled simply by excess-oxygen doping, and the films grown under the optimum conditions exhibit superconductivity below 30 K. This is the first result on the superconductivity of bilayer La2CaCu2O6+δinduced purely by the excess oxygen.
We have studied structural and transport properties of epitaxial NaxCoO2 thin films on (0001) sapphire substrate prepared by topotaxially converting an epitaxial Co3O4 film to NaxCoO2 with annealing in Na vapor. The films are c axis oriented and in-plane aligned with [10 1 0] NaxCoO2 rotated by 30 degrees from [10 1 0] sapphire. Different Na vapor pressures during the annealing resulted in films with different Na concentrations, which showed distinct transport properties.
