Superconductor Thin Films Research Articles

Control of the nanosized defect network in superconducting thin films by target grain size

A nanograined YBCO target, where a great number of grain boundaries, pores etc. exist, is shown to hold an alternative approach to future pulsed laser deposition based high-temperature superconductor thin film and coated conductor technologies. Although the nanograined material is introduced earlier, in this work, we comprehensively demonstrate the modified ablation process, together with unconventional nucleation and growth mechanisms that produces dramatically enhanced flux pinning properties. The results can be generalized to other complex magnetic oxides, where an increased number of defects are needed for modifying their magnetic and electrical properties, thus improving their usability in the future technological challenges.

Synthesis of Thin Niobium Films on Silicon and Study of Their Superconducting Properties in the Dimensional Crossover Region

Niobium films with a thickness of 4–100 nm are synthesized on a silicon substrate under ultrahigh vacuum conditions. Measurements of the electrical resistance show a high superconducting transition temperature Tc in the range of 4.7–9.1 K and record-breaking small transition widths ΔTc in the range of 260–11 m. The dependences of Tc and ΔTc on the magnetic field are investigated, and the superconducting coherence lengths and mean free paths of conduction electrons for different thicknesses of the synthesized films are determined. A significant influence of the magnetic field on ΔTc is found, which reveals the transition from three- to two-dimensional superconductivity at thicknesses below 10 nm. The dependences of Tc and ΔTc on the thickness of the films and the magnitude of the magnetic field are discussed within the framework of existing theories of superconductivity in thin films of superconducting metals.

Moving Pearl Vortices in Thin-Film Superconductors

The magnetic field hz of a moving Pearl vortex in a superconducting thin-film in (x,y) plane is studied with the help of the time-dependent London equation. It is found that for a vortex at the origin moving in +x direction, hz(x,y) is suppressed in front of the vortex, x>0, and enhanced behind (x<0). The distribution asymmetry is proportional to the velocity and to the conductivity of normal quasiparticles. The vortex self-energy and the interaction of two moving vortices are evaluated.

Spin pumping between noncollinear ferromagnetic insulators through thin superconductors

Dynamical magnets can pump spin currents into superconductors. To understand such a phenomenon, we develop a method utilizing the generalized Usadel equation to describe time-dependent situations in superconductors in contact with dynamical ferromagnets. Our proof-of-concept theory is valid when there is sufficient dephasing at finite temperatures, and when the ferromagnetic insulators are weakly polarized. We derive the effective equation of motion for the Keldysh Green's function focusing on a thin film superconductor sandwiched between two noncollinear ferromagnetic insulators of which one is dynamical. In turn, we compute the spin currents in the system as a function of the temperature and the magnetizations' relative orientations. When the induced Zeeman splitting is weak, we find that the spin accumulation in the superconducting state is smaller than in the normal states due to the lack of quasiparticle states inside the gap. This feature gives a lower backflow spin current from the superconductor as compared to a normal metal. Furthermore, in superconductors, we find that the ratio between the backflow spin current in the parallel and anti-parallel magnetization configuration depends strongly on temperature, in contrast to the constant ratio in normal metals.

All-optical and microwave-free detection of Meissner screening using nitrogen-vacancy centers in diamond

Microscopic studies on thin film superconductors play an important role for probing non-equilibrium phase transitions and revealing dynamics at the nanoscale. However, magnetic sensors with nanometer scale spatial and picosecond temporal resolution are essential for exploring these. Here, we present an all-optical, microwave-free method that utilizes the negatively charged nitrogen-vacancy (NV) center in diamond as a non-invasive quantum sensor and enables the spatial detection of the Meissner state in a superconducting thin film. We place an NV implanted diamond membrane on a 20nm thick superconducting La2−xSrxCuO4 (LSCO) thin film with Tc of 34K. The strong B-field dependence of the NV photoluminescence allows us to investigate the Meissner screening in LSCO under an externally applied magnetic field of 4.2mT in a non-resonant manner. The magnetic field profile along the LSCO thin film can be reproduced using Brandt’s analytical model, revealing a critical current density jc of 1.4×108A/cm2. Our work can be potentially extended further with a combination of optical pump probe spectroscopy for the local detection of time-resolved dynamical phenomena in nanomagnetic materials.

Extreme high-field superconductivity in thin Re films

We report the high-field superconducting properties of thin, disordered Re films via magneto-transport and tunneling density of states measurements. Films with thicknesses in the range of 9 nm to 3 nm had normal state sheet resistances of $\sim$0.2 k$\Omega$ to $\sim$1 k$\Omega$ and corresponding transition temperatures in the range of 6 K to 3 K. Tunneling spectra were consistent with those of a moderate coupling BCS superconductor. Notwithstanding these unremarkable superconducting properties, the films exhibited an extraordinarily high upper critical field. We estimate their zero-temperature $H_{c2}$ to be more than twice the Pauli limit. Indeed, in 6 nm samples the estimated reduced critical field $H_{c2}/T_c\sim$ 5.6 T/K is among the highest reported for any elemental superconductor. Although the sheet resistances of the films were well below the quantum resistance $R_Q=h/4e^2$, their $H_{c2}$'s approached the theoretical upper limit of a strongly disordered superconductor for which $k_F\ell\sim1$.

Design of sextuple-mode triple-ring HTS UWB filter using two-round interpolation* *Project supported by the National Natural Science Foundation of China (Grant No. 61471094).

A single-stage ring resonator capable of introducing six modes within the ultra-wideband (UWB) passband is presented. The sextuple-mode resonator consists of three rings and three sets of stepped-impedance open stubs. Based on this sextuple-mode resonator, a UWB filter fed by the interdigital-coupling line (ICL) is designed. And we propose a two-round interpolation method to obtain the filter’s initial dimensions. The designed filter is fabricated on a double-sided YBCO/MgO/YBCO high-temperature superconducting (HTS) thin film for demonstration. The experimental results show that this UWB filter produces eight resonances in the passband eventually, which effectively improves the in-band reflection and the band-edge steepness. Moreover, the upper stopband performance is enhanced due to the transmission zeros (TZs) generated by the stepped-impedance open stubs and the ICL structure. The measured good performance verifies the practicability of the two-round interpolation approach, which can also be extended to other odd–even-mode filter designs.

Effects of Pb assisted cation chemistry on the superconductivity of BSCCO thin films.

Thin films of Bi-based superconductors, highly c-axis oriented, were deposited on single crystalline substrates of SrTiO3, LaAlO3, and MgO using a pulsed laser deposition technique with a Bi-2223 target of nominal composition Bi1.75Pb0.25Sr2Ca2Cu3O10±δ prepared by the solid state reaction method. The effect of different deposition parameters on the evolution of the requisite properties in the thin films has been studied. These films have been characterized by X-ray diffraction to investigate their structural properties, scanning electron microscopy to understand the effect of ex situ annealing on the grain growth, and DC resistivity measurements to quantify their superconducting critical temperature. Furthermore, the chemical states of the constituent elements Bi, Pb, Sr, Ca, Cu and O were confirmed using X-ray photoelectron spectroscopy. This information has helped in deciphering the empirical stoichiometry of the films on each of the chosen substrates. We have also been able to comment on the influence made by the choice of the substrates on the mechanism of evolution of superconductivity based on the interplay of the cation chemistry between the substituent and the constituent elements. Thin films with superior superconducting properties were obtained on SrTiO3 substrates with 58% of Bi-2223 phase fraction yielding a superconducting transition temperature (TC,offset) of 107 K. Magnetotransport studies were performed on these films to quantify their superconducting upper critical field and to comprehend the pinning mechanism.

Температурные зависимости критических параметров неоднородных сверхпроводящих пленок

The results of calculating the temperature dependences of the critical current density and critical magnetic field of thin inhomogeneous superconducting films are presented. Comparison of the results obtained for inhomogeneous films with the results of calculations for homogeneous ones showed that in both cases, the decrease in the critical magnetic field occurs according to the root law, and the critical current density changes according to a power law with a degree of 3/2 when approaching the critical temperature. Quantitatively, the critical current density for inhomogeneous films in the absence of an external magnetic field is lower than for homogeneous ones. In turn, the critical magnetic field of inhomogeneous films is much larger than the critical field of homogeneous films.

Synthesis, Characterization, and Magnetoresistive Properties of the Epitaxial Pd0.96Fe0.04/VN/Pd0.92Fe0.08 Superconducting Spin-Valve Heterostructure.

A thin-film superconductor(S)/ferromagnet(F) F1/S/F2-type Pd0.96Fe0.04(20 nm)/VN(30 nm)/Pd0.92Fe0.08(12 nm) heteroepitaxial structure was synthesized on (001)-oriented single-crystal MgO substrate utilizing a combination of the reactive magnetron sputtering and the molecular-beam epitaxy techniques in ultrahigh vacuum conditions. The reference VN film, Pd0.96Fe0.04/VN, and VN/Pd0.92Fe0.08 bilayers were grown in one run with the target sample. In-situ low-energy electron diffraction and ex-situ X-ray diffraction investigations approved that all the Pd1−xFex and VN layers in the series grew epitaxial in a cube-on-cube mode. Electric resistance measurements demonstrated sharp transitions to the superconducting state with the critical temperature reducing gradually from 7.7 to 5.4 K in the sequence of the VN film, Pd0.96Fe0.04/VN, VN/Pd0.92Fe0.08, and Pd0.96Fe0.04/VN/Pd0.92Fe0.08 heterostructures due to the superconductor/ferromagnet proximity effect. Transition width increased in the same sequence from 21 to 40 mK. Magnetoresistance studies of the trilayer Pd0.96Fe0.04/VN/Pd0.92Fe0.08 sample revealed a superconducting spin-valve effect upon switching between the parallel and antiparallel magnetic configurations, and anomalies associated with the magnetic moment reversals of the ferromagnetic Pd0.92Fe0.08 and Pd0.96Fe0.04 alloy layers. The moderate critical temperature suppression and manifestations of superconducting spin-valve properties make this kind of material promising for superconducting spintronics applications.

Reconfigurable Pinwheel Artificial-Spin-Ice and Superconductor Hybrid Device.

The ability to control the potential landscape in a medium of interacting particles could lead to intriguing collective behavior and innovative functionalities. Here, we utilize spatially reconfigurable magnetic potentials of a pinwheel artificial-spin-ice (ASI) structure to tailor the motion of superconducting vortices. The reconstituted chain structures of the magnetic charges in the pinwheel ASI and the strong interaction between magnetic charges and superconducting vortices allow significant modification of the transport properties of the underlying superconducting thin film, resulting in a reprogrammable resistance state that enables a reversible and switchable vortex Hall effect. Our results highlight an effective and simple method of using ASI as an in situ reconfigurable nanoscale energy landscape to design reprogrammable superconducting electronics, which could also be applied to the in situ control of properties and functionalities in other magnetic particle systems, such as magnetic skyrmions.

Multilayered Heater Nanocryotron: A Superconducting-Nanowire-Based Thermal Switch

We demonstrate a multilayer nanoscale cryogenic heater-based switch (M-hTron) that uses a normal-metal heater overlapping a thin-film superconductor separated by a thin insulating layer. The M-hTron eliminates leakage current found in three-terminal superconducting switches and applies heat locally to the superconductor, reducing the energy required to switch the device. Modeling using the energy-balance equations and the acoustic mismatch model demonstrates reasonable agreement with experiment. The M-hTron is a promising device for digital superconducting electronics that require high fan-out and offers the possibility of enhancing readout for superconducting-nanowire single-photon detectors.

Optical and hidden transport properties of BaFe1.91Ni0.09As2 film

Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe1.91Ni0.09As2 thin superconducting (SC) film. We analyzed the normal state data using a Drude–Lorentz model with two Drude components: one narrow (D1) and another broad one (D2). In the SC state, two gaps with –2.0 and –4.3 are formed from the narrow component D1 while the broad component D2 remains ungapped. The calculated total DC resistivity of the film and the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. The change of total electron–boson coupling (λ tot) and representative energy (Ω0) in the normal state with respect to the SC state is typical of other iron-based materials as well as high-temperature superconducting (HTSC) cuprates.

22 K superconductivity in BaFe2As2 exposed to F2

Several previous reports on undoped $AE\mathrm{}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($AE=$ alkaline earth metal) point to drops in resistivity indicative of possible superconductivity and some degree of Meissner effect in the magnetic susceptibility. Also, based on both resistivity and magnetic susceptibility measurements, controlled exposure to water vapor has been shown to induce superconductivity in $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. In our study, $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals grown using the self-flux method showed a full resistive drop around 22 K and magnetic shielding, when exposed to fluorine gas postgrowth. Our measurements indicate electron (donor) doping via atomic substitution of F for As is concurrent with the observed superconductivity, which sheds light on the likely effect of exposure to water vapor in previous work. Like doping experiments (such as substitution of Co for Fe, P for As, or K for Ba) in $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ to date, the present work is consistent with suppression of the spin density wave transition coincident with the appearance of ${T}_{c}$. In addition to answering the puzzle of superconductivity in undoped or water vapor exposed $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, our results also represent a fast (20 min exposure to 5% F in He) and reliable method suitable for inducing superconductivity in thin films of $AE{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Some supportive work on $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ exposed to ${\mathrm{Cl}}_{2}$ is also presented.

Синтез тонких пленок ниобия на кремнии и исследование их сверхпроводящих свойств в области размерного кроссовера

Niobium films of 4–100 nm thickness were synthesized on a silicon substrate under ultrahigh vacuum conditions. Measurements of electrical resistance showed a high temperature of the superconducting transition Tc, in the range of 4.7–9.1 K, and extremely small transition widths ΔTc in the range of 260–11 mK. The dependences of Tc and ΔTc on the magnetic field were studied, and superconducting coherence lengths and mean free paths of the conduction electrons were determined for different thicknesses of the synthesized films. A specific effect of the magnetic field on ΔTc was found, which reveals a transition from three-dimensional to two-dimensional superconductivity at thicknesses below 10 nm. The dependences of Tc and ΔTc on the films thickness and the magnitude of the magnetic field are discussed in the framework of existing concepts of superconductivity in thin films of superconducting metals.

Stress Distributions of Rectangular Thin-Film Superconductor with a Centric Hole

In this paper, the stress distributions of rectangular thin-film superconductors with a centric rectangular or elliptical hole are investigated. The bean critical current density model is adopted. In the present work, the influence of the existing hole on both the current density and magnetic field in the rectangular thin film is considered. By virtue of the minimum magnetic energy method, the principal stresses are calculated. Finally, the numerical examples about stress analysis along hole edge are examined and discussed in detail. Some important phenomena are found, which should be helpful for the analysis and design of superconducting film structure.

Electrically Tunable Superconductivity Through Surface Orbital Polarization

We investigate the physical mechanisms for achieving an electrical control of conventional spin-singlet superconductivity in thin films by focusing on the role of surface orbital polarization. Assuming a multi-orbital description of the metallic state, due to screening effects the electric field acts by modifying the strength of the surface potential and, in turn, yields non-trivial orbital-Rashba couplings. The resulting orbital polarization at the surface and in its close proximity is shown to have a dramatic impact on superconductivity. We demonstrate that, by varying the strength of the electric field, the superconducting phase can be either suppressed, i.e. turned into normal metal, or undergo a $0-\pi$ transition with the $\pi$ phase being marked by non-trivial sign change of the superconducting order parameter between different bands. These findings unveil a rich scenario to design heterostructures with superconducting orbitronics effects.

Coexistence of magnetism and superconductivity in thin films of the Fe-based superconductor Ba1−xLaxFe2As2

Magnetization measurements have been performed to understand the role of the magnetic structure on the superconducting properties of epitaxial thin films of Ba1−xLaxFe2As2 (x = 0.08, 0.13, and 0.18) deposited on single crystal (001)-oriented MgO substrates by pulsed laser deposition. All samples exhibit a reentrant-spinglass like behavior at normal state. At lower temperatures, we observe the same magnetic state coexisting with superconductivity and it is also observed a prominent non-linear giant diamagnetism in an intermediate temperature range just above the superconducting phase transition temperature. Furthermore, no significant change in the magnetic domain structure was detected by the onset of superconductivity. Based on their magnetic states, we claim that each domain (as a disconnected superconducting island) has its own bulk superconducting properties. Finally, we discussed the dual character played by the La atoms in the superconducting properties. That duality character has been also confirmed by analyzing resistivity data.

Controlling the proximity effect in a Co/Nb multilayer: the properties of electronic transport.

We present both theoretical and experimental investigations of the proximity effect in a stack-like superconductor/ferromagnetic (S/F) superlattice, where ferromagnetic layers with different thicknesses and coercive fields are made of Co. Calculations based on the Usadel equations allow us to find the conditions at which switching from the parallel to the antiparallel alignment of the neighboring F-layers leads to a significant change of the superconducting order parameter in superconductive thin films. We experimentally study the transport properties of a lithographically patterned Nb/Co multilayer. We observe that the resistive transition of the multilayer structure has multiple steps, which we attribute to the transition of individual superconductive layers with the critical temperature, Tc, depending on the local magnetization orientation of the neighboring F-layers. We argue that such superlattices can be used as tunable kinetic inductors designed for artificial neural networks representing the information in a “current domain”.

N concentration effects on structure and superconductivity of NbN thin films

In this work, we present a study on niobium nitride (NbN) thin films deposited using a reactive magnetron sputtering of Nb at different partial pressures of N2 (RN2). The effect of N concentration on the structural and superconducting properties of niobium mononitride (NbN) thin films was investigated using x-ray diffraction and resistivity measurements. During the initial nitridation process (RN2 = 0.5 to 2%) N atoms occupy interstitial sites within Nb, leading to an expansion and distortion in Nb lattice. At an intermediate RN2 range (RN2 = 8 to 16%), nearly stoichiometric NbN phase emerges and further nitridation (RN2 = 20 to 100%) results in expansion and distortion of NbN, similar to the effect observed in Nb. Low temperature and high magnetic field resistivity measurements reveal that the superconducting transition temperature (TC) and the upper critical field (HC2(0)) maximizes to 12.8 K and 30 T at RN2 = 16%. Both deficiency or excess of N from equiatomic NbN composition leads to a reduction in TC and HC2(0). The effect of an applied radio frequency (rf) bias during the growth of NbN thin films was also studied and it was found that under optimized rf biasing conditions, the adatom mobility can be enhanced leading to superior long-range crystalline ordering with reduced disorder/defects. As a result, higher value of TC = 13.2 K was obtained. In essence through this work, we demonstrate possibilities to attain a stoichiometric NbN phase without the need of high growth temperatures during the deposition process.