high-Tc Films Research Articles

Radiatively cooled quantum microwave amplifiers

Superconducting microwave amplifiers are essential for sensitive signal readout in superconducting quantum processors. Typically based on Josephson junctions, these amplifiers require operation at milli-Kelvin temperatures to achieve quantum-limited performance. Here, we demonstrate a quantum microwave amplifier that employs radiative cooling to operate at elevated temperatures and maintain near quantum-limited added noise. This kinetic-inductance-based parametric amplifier, patterned from a single layer of relatively high-Tc NbN thin film, maintains a high gain and meanwhile enables low added noise of 1.3 quanta when operated at 1.5 K. Remarkably, this represents only a 0.2 quanta increase compared to the performance at a base temperature of 0.1 K. Based on our findings, we also discuss the practicality of such an operating scheme for various quantum applications. By uplifting the parametric amplifiers from the mixing chamber without compromising readout efficiency, this work represents an important step toward more scalable microwave quantum technologies.

Microwave surface resistance in nanostructured high-Tc superconductor films

The impact of artificially created defects nanostructure, formed by implanted dielectric nanoparticles or irradiation defects, on microwave properties of high-Tc superconductor films is analyzed in the framework of phenomenological theory for microwave response of type-II superconductors. We have calculated the surface resistance for such a kind of nanostructured type-II superconductor film and investigated conditions for the emergence of nonlinear response caused by the entrance of microwave-induced vortices in the film's interior through its edges. The obtained results indicate that artificial defect nanostructure in the film's interior formed by point-like or columnar structural defects can significantly improve its microwave characteristics in both the Meissner and mixed states and also increase the threshold for the onset of nonlinear response.

Increasing the Transition Temperature of High-TC Superconductor Thin Films by Organic Linking of Gold Nanoparticles

The transition temperature, TC, of two types of optimally doped cuprate high-TC superconductor films, YBa2Cu3O7-δ and La1.85Sr0.15CuO4, is found to increase upon linking Au nanoparticles to their surface via organic molecules. At the same time, and quite surprisingly, the critical current is reduced. We attribute these results to screening of the Coulomb interactions and smearing the pinning potential landscape. The transition temperature was increased also when the Au NPs were not chemically linked to the surface, consistent with the screening scenario. The linking molecules, however, enable selective adsorption of the gold nanoparticles and their separation from the surface, thus eliminating the conventional proximity effect.

Microwave Response of Nanostructured High-Tc Superconductor Thin Films

A model for the microwave response of a nanostructured high-Tc superconductor (HTS) film, with implanted nanoparticles and nanorods of a dielectric material or point-like and columnar irradiation defects with a nano-sized cross-section is developed. In this case, the microwave surface resistance Rs(T,H,ω) is calculated both for the Meissner and mixed states of a superconductor film in an applied dc magnetic field. The obtained results indicate that the implantation of dielectric nanoparticles or point-like radiation defects can significantly improve superconductor characteristics at microwave frequencies. Namely, these nano-sized structural defects can decrease the surface resistance in the Meissner state and eliminate the oscillations of Abrikosov vortices and the related microwave energy losses, thus decreasing the contribution of Abrikosov vortices to the Rs value in the mixed state of a HTS film.

On the growth of Co-doped BaFe2As2 thin films on CaF2

The competition between phase formation of BaF2 and Ba(Fe1−xCox)2As2 on CaF2 single crystals has been analysed. Ba(Fe0.92Co0.08)2As2 thin films have been deposited by pulsed laser deposition. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies have revealed that the formation of secondary phases and misorientations as well as the growth modes of the Ba(Fe0.92Co0.08)2As2 thin films strongly depend on the growth rate. At high growth rates, formation of BaF2 is suppressed. The dependency of the Ba(Fe0.92Co0.08)2As2 lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF2 formation similar as was proposed for FeSe1−xTex thin films on CaF2. Furthermore, a growth mode transition from a layer growth mechanism to a three-dimensional growth mode at high supersaturation has been found, suggesting similarities between the growth mechanism of iron-based superconductors and high-Tc cuprate thin films.

Magnetoresistance due to the Plane Current Effect through the Gap Layer Between the High-Tc Superconductor Film and the Giant Magnetoresistance-Spin Valve Multilayer

Magnetoresistance due to the Plane Current Effect through the Gap Layer Between the High-Tc Superconductor Film and the Giant Magnetoresistance-Spin Valve Multilayer

Peculiarities in magnetron sputtering of YBCO epitaxial films for applications in superconductor electronics devices

We consider the main factors determining the growth of YBa2Cu3O7–δ high-Tc superconductor films during magnetron sputtering in the planar axial geometry. Special attention is paid to the increase of the growth rate of the films suitable for superconductor electronics devices. Magnetron sputtering is used for obtaining YBa2Cu3O7–δ films with high structural and electrophysical characteristics for a growth rate up to 200 nm/h, which were used in constructing microwave disk resonators and long Josephson junctions on bicrystal substrates. The unloaded Q factor of cavities exceeds 80000 at a frequency of 7.1 GHz at a temperature of 77 K, which corresponds to the best results in this field. Josephson junction of length 50–350 μm are characterized by critical current density jc = 12–33 kA/cm2 at T = 77 K and jc = 93–230 kA/cm2 at T = 6 K in zero magnetic field. The characteristic voltage IcRn is 0.8–1.96 mV.

Structural and magnetic characterization of ZnCo2O4 thin film prepared by pulsed laser deposition

We report the structural and magnetic characterization of ZnCo2O4 (ZCO) thin films fabricated by pulsed laser deposition on sapphire substrates. Structural analysis measured by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and atomic force microscope (AFM) shows that the ZCO thin film grew in a layer-by-layer mode, and forms a cubic spinel structure with Fd-3m space group. Electronic states detected by X-ray photoelectron spectroscopy (XPS) prove that Co2+ ions occupied on the Zn sites in the ZCO film and oxygen vacancies exist. Magnetic measurements show a strong ferromagnetic behavior. The possible mechanisms for the ferromagnetic property are extensively discussed. Our work is believed to contribute a good understanding of the microscopic ferromagnetism origin of the high-Tc ZCO film, which is significant for expanding their applications into high-effective spin-electronic devices.

Microwave losses of undoped n-type silicon and undoped 4H-SiC single crystals at cryogenic temperatures

We investigated microwave losses of single-crystalline Si and 4H-SiC at cryogenic temperatures at 8.6–24 GHz using a method involving a dielectric resonator with high-Tc superconductor YBa2Cu3O7-δ films used to improve the measurement sensitivity. The loss tangent of our undoped n-type Si appeared to be extremely low at temperatures below 20 K with a value of 1 × 10−6 at 24 GHz at 10 K, which is more than 100 times lower than the value of 2 × 10−4 at 6.8 GHz at 10 K reported by Krupka et al. [IEEE Trans. Microw. Theory Tech. 54, 3995 (2006)] for undoped p-type Si. Meanwhile, the loss tangent of pristine 4H-SiC appeared to be very high with a value of 0.01 at 10 K at 8.6 GHz, which is 4000 times higher than that of our undoped Si. When the pristine 4H-SiC was irradiated with thermal neutrons, the loss tangent was enhanced by seven times due to the significantly reduced electrical resistivity. Our results show that, at temperatures below 20 K, the loss tangent of undoped n-type Si is low enough for various cryogenic applications and that thermal neutron irradiation could provide a useful means of reducing the electrical resistivity of SiC possibly by means of neutron transmutation doping.

Pulsed laser deposition of BaFe2(As,P)2 superconducting thin films with high critical current density

Superconducting BaFe2(As0.6P0.4)2 (Ba122:P) thin films were fabricated on MgO(100) substrates by a pulsed laser deposition (PLD) method using a second-harmonic Nd:YAG (yttrium aluminum garnet) laser. Structural investigation by means of x-ray diffraction confirmed both c-axis orientation and in-plane alignment, or epitaxial growth on the substrate. The film exhibited Tc(onset) = 26.5 K and Tc(zero) = 24.0 K. High Jc values of 3.5 MA cm−2 at 4.2 K in the self-field and over 1 MA cm−2 at 10 K under 1 T were also obtained. Fabrication of Ba122:P film by Nd:YAG PLD seems to be a promising approach for preparing superconducting tapes, since a high-Tc and high-Jc film can be relatively easily obtained.

Flux transport in nanostructured high-Tc films at microwave frequencies

The understanding of flux transport in micro- and nanostructured superconducting systems that are exposed to an electromagnetic field at microwave frequencies is of interest for basic aspects of vortex matter and for potential application of superconductivity in fluxonic devices. We report on the combination of dc and microwave electronic measurements on submicron-patterned high-Tc films. The frequency dependence of the forward transmission coefficient S21 indicates that the mechanism of flux transport depends on the velocity of vortices. At low frequencies, flux transport via Abrikosov vortices is present leading to additional microwave losses. Above a geometrically defined frequency, a different, loss-free mechanism seems to be responsible for flux transport. This mechanism most likely represents a phase-slip type of mechanism. The limiting vortex velocity obtained from the frequencies dependence of the microwave properties agrees with the Larkin–Ovchinnikov critical vortex velocity determined via dc pulse measurements. In spite of the change of mechanism, guidance of flux persists in these nano-patterns up to high frequencies.

Ferroelectric Polarization Effects in the Surface Superconductivity

Ferroelectric polarization effects of the heterostructures normal metal-ferroelectric-superconducting high-Tc film are studied in the transverse electrostatic field. A boundary condition for the Ginzburg-Landau wave function of a one-band superconductor at surfaces biased by a strong electrostatic transverse electric field is derived within the de Gennes approach. This boundary condition is applied to the metal-ferroelectric-superconductor structures. The coherence length is critical and it supports the existence of thin superconducting films, such as high-Tc cuprates and other compounds with high transition temperature. We have found various electric field effects in these systems.

Characterization of a Transition-Edge Bolometer Made of YBCO Thin Films Prepared by Nonfluorine Metal–Organic Deposition

We present the results of a bolometric transition-edge sensor made of a high- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$T_{c}$</tex></formula> superconductor YBCO thin film prepared by fluorine-free metal–organic deposition. The structure of the films was characterized by X-ray diffraction and scanning electron microscopy, and the superconducting properties were determined by <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$R{-}T$</tex></formula> measurements. The applicability of the resulting film as an infrared sensor is reported here. The optical response in the range of near infrared and the noise characteristics of the patterned bolometer are measured and analyzed. The dependence of device sensitivity on the bias current and modulation frequency is also investigated. As it is presented in this paper, the results of the fabricated device are comparable to the devices made of high-quality pulsed laser deposition YBCO thin films.

Pattern induced phase transition of vortex motion in high-Tc films

A micropattern induced transition in the mechanism of vortex motion and vortex mobility is demonstrated for high-Tc films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) leads to a sudden change in the direction of vortex motion that is accompanied by a change of the critical current density and microwave losses. The latter demonstrates the difference in vortex mobility in the different phases of vortex motion in between and within the rows of antidots.

Influence of thermal, morphological and measuring variables on the thermal runaway of superconducting films under high current densities

We study the abrupt jump in voltage seen in the current-voltage characteristics of high-Tc films at current densities, J*, around 2–3 times the critical one, and for perturbations with characteristic times in the millisecond range. Customarily, even for these long characteristic times, the voltage jump has been ascribed to an electrodynamic vortex instability. Recent results based on finite-element analyses show quantitatively that smooth, i.e., jumpfree isothermal curves may show up themselves as the jumping, i.e., discontinuous curves experimentalists observe, once the nonlinearity, thermal feedback and measurement rate are properly accounted for. The thermal nature of the transition to a highly dissipative state in high-Tc superconductors at zero magnetic field makes the control of the instability transition easier in that there are more parameters available to experimentalists that may be tuned to tailor the phenomenon. This is specially relevant for devices working on their superconducting borderline as current-limiters or even magnets. Here we explore through finite-element analysis the role of film geometry, measurement rate and substrate thermal conductivity in the triggering of the thermal runaway.

Imaging of vortex flow in microstructured high-Tc films by laser scanning microscope

We studied the YBa2Cu3O7 thin films with arrays of antidots by imaging the resistive photoresponse with a laser scanning microscope. The films were initially covered by a thin Au layer, which we removed in order to improve the spatial resolution and imaging contrast. This procedure did not change the superconducting properties of the samples. The improved spatial resolution allowed for visualizing the individual flow channels of vortices. The reported measurements with the laser scanning microscope feature direct measurements of the local critical currents of the samples. We show that minor variations of the bias current leave the spatial distribution of vortex flow channels unchanged.

Direct angle resolved photoelectron spectroscopy (DARPES) on high-Tc films: doping, strains, Fermi surface topology and superconductivity

Since 1997 we systematically perform Direct ARPES ( = DARPES) on in-situ grown, non-cleaved, ultra-thin (<25nm) cuprate films. Specifically, we probe low energy electronic structure and properties of high-Tc films under different degree of epitaxial (compressive vs tensile) strain. In overdoped in-plane compressed La2-xSrxCuO4 (LSCO) thin films we double Tc from 20K to 40K, yet the Fermi surface (FS) remains essentially 2-dimensional (2D). In contrast, tensile strained films show 3-dimensional (3D) dispersion, while Tc is drastically reduced. It seems that the in-plane compressive strain tends to push the apical oxygen far away from the CuO2 plane, enhances the 2D character of the dispersion and increases Tc, while the tensile strain seems to act exactly in the opposite direction and the resulting dispersion is 3D. We have the FS topology for both cases. As the actual lattice of cuprates is ‘Napoleon-cake’ -like i.e. rigid CuO2 planes alternate with softer ‘reservoir’ (that strains distort differently) our results tend to rule out 2D rigid lattice mean field models. Finally, we briefly discuss recent successful determination of the FS topology from the observed wavevector quantization by DARPES in cuprate films thinner than 18 units cells (<24nm). Such an approach is of broader interest as it can be extended to other similar confined (ultra-thin) functional oxide systems.

Epitaxial thick film high-Tc SQUIDs

Low-noise operation of superconducting quantum interference devices (SQUIDs) in magnetic fields requires high critical current and strong pinning of vortices in the superconducting electrodes and in the flux transformer. Crack-free epitaxial high-Tc dc-SQUID structures with a total thickness ?5 μm and a surface roughness determined by 30 nm high growth spirals were prepared with YBa2Cu3O7-x (YBCO) films on MgO substrates buffered by a SrTiO3/BaZrO3-bilayer. HRTEM demonstrated a high quality epitaxial growth of the films. The YBCO films and SQUID structures deposited on the buffered MgO substrates had a superconducting transition temperature Tc exceeding 91 K and critical current densities Jc > 3 MA/cm2 at 77 K up to a thickness ~5 μm. The application of thicker superconducting and insulator films helped us to increase the critical current and dynamic range of the multilayer high-Tc flux transformer and improve the insulation between the superconducting layers. An optimization of SQUID inductance allowed us to fabricate 8 mm SQUID magnetometers with SQUID voltage swings of ~60 μV and a field resolution of ~30 fT/√Hz at 77 K.

Using ion irradiation to make high-Tc Josephson junctions

In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.

Vortex Interaction Energy in Planar Josephson Junction Arrays at High Density

We study the vortex-vortex and vortex-antivortex interaction in arrays of Josephson junction particularly addressing the effect of mutual induction in the intermediate screening ranges (betaa = 0.5). The modifications of the barrier energy for the displacement of a vortex in presence of a second vortex (antivortex), or more vortices (antivortices), are derived in an adiabatic approach. To this end, we have developed a fast explicit algorithm, which permits to retrieve the phases and the magnetization of the array in a reasonable time and with little memory occupation for relatively large arrays [e.g. o(100 times 100)]. This new model is able to catch the essential characters of the more complex fully mutual coupling model. We provide also some indications on the relation between energy barriers and the observation of vortex quantum tunneling phenomena in Josephson arrays and related systems as high-TC films.