Search Results (51)

It is known that noble metals such as gold, silver and copper are not superconductors; this is also true for magnesium. This is due to the weakness of the electron–phonon interaction, which makes them excellent conductors but not superconductors. As has recently been shown for gold, silver and copper, and even for magnesium, it is possible that in very particular situations, superconductivity may occur. Quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors. It is therefore an important fundamental question whether ultra-thin film confinement may induce observable superconductivity in non-superconducting metals such as magnesium. We study this problem using a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thin-film confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness, L. This parameter-free theory predicts superconductivity in very thin magnesium films. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer. Full article

The process of normal (N) zone propagation in three superconducting YBaCuO thin films with different Pearl length values was theoretically studied. The point appearance of the N zone was found to result from powerful energy release caused by micro-sized magnetic cumulation. Solutions of the heat equation for hot electrons, diffusing to ~15 nm depth into the edge of the Pearl length, were obtained for the two length cases. The hot electron thermalization induced a transition to N state at the aforementioned depth due to fast exceeding of T_c, followed by flash high temperature growth. In the third case, we considered a process of crack branching when the superconducting current concentrated at the tips, followed by the transition to N state caused by exceeding j_c. The superfast reaction of the superconductor allowed it to restore the energy loss at the Pearl length in all cases. This explains the step propagation process of the N zone with velocities up to 2.7 × 10³ and 1.1 × 10³ m/s in the first and second cases. In the third, the propagation can reach the detonation wave velocity of about 1 × 10⁴ m/s. It is concluded that the process of the N zone propagation has the character of a combustion wave. Full article

We investigated the effect of 0.6 MeV proton irradiation on the superconducting and normal-state properties of thin-film ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ superconductors. A thin-film YBCO superconductor (≈567 nm thick) was subject to a series of proton irradiations with a total fluence of $7.6\times {10}^{16}$ ${\mathrm{p}/\mathrm{cm}}^2$. Upon irradiation, $T_c$ was drastically decreased from 89.3 K towards zero with a corresponding increase in the normal-state resistivity above $T_c$. This increase in resistivity, which indicates an increase in defects inside the thin-film sample, can be converted to the dimensionless scattering rate. We found that the relation between $T_c$ and the dimensionless scattering rate obtained during proton irradiation approximates the generalized d-wave Abrikosov–Gor’kov theory better than the previous results obtained from electron irradiations. This is an unexpected result, since the electron irradiation is known to be most effective to suppress superconductivity over other heavier ion irradiations such as proton irradiation. In comparison with the previous irradiation studies, we found that the result can be explained by two facts. First, the dominant defects created by 0.6 MeV protons can be point-like when the implantation depth is much longer than the sample thickness. Second, the presence of defects on all element sites is important to effectively suppress $T_c$. Full article

Thin films of the superconductor YBa₂Cu₃O_7−δ (YBCO) were modified by low-energy light-ion irradiation employing collimated or focused He⁺ beams, and the long-term stability of irradiation-induced defects was investigated. For films irradiated with collimated beams, the resistance was measured in situ during and after irradiation and analyzed using a phenomenological model. The formation and stability of irradiation-induced defects are highly influenced by temperature. Thermal annealing experiments conducted in an Ar atmosphere at various temperatures demonstrated a decrease in resistivity and allowed us to determine diffusion coefficients and the activation energy $\Delta E=(0.31\pm 0.03)$ eV for diffusive oxygen rearrangement within the YBCO unit cell basal plane. Additionally, thin YBCO films, nanostructured by focused He⁺-beam irradiation into vortex pinning arrays, displayed significant commensurability effects in magnetic fields. Despite the strong modulation of defect densities in these pinning arrays, oxygen diffusion during room-temperature annealing over almost six years did not compromise the signatures of vortex matching, which remained precisely at their magnetic fields predicted by the pattern geometry. Moreover, the critical current increased substantially within the entire magnetic field range after long-term storage in dry air. These findings underscore the potential of ion irradiation in tailoring the superconducting properties of thin YBCO films. Full article

Iron-based superconductors are under study for their potential for high-field applications due to their excellent superconducting properties such as low structural anisotropy, large upper critical fields and low field dependence of the critical current density. Between them, Fe(Se,Te) is simple to be synthesized and can be fabricated as a coated conductor through laser ablation on simple metallic templates. In order to make all the steps simple and fast, we have applied the spark plasma sintering technique to synthesize bulk Fe(Se,Te) to obtain quite dense polycrystals in a very short time. The resulting polycrystals are very well connected and show excellent superconducting properties, with a critical temperature onset of about 16 K. In addition, when used as targets for pulsed laser ablation, good thin films are obtained with a critical current density above 10⁵ A cm⁻² up to 16 T. Full article

The study of granularity in superconducting films by using AC susceptibility has a crucial role in the development of and improvement in the ReBCO-coated conductors, which are a constantly evolving reality in the modern power applications of superconductivity. Specifically, the study of the granularity is essential because the ReBCO superconducting wires and tapes are far from the regularity of a single crystal while they often present an inter- and intragranular contribution to the critical current density. On the other hand, the AC susceptibility is a key part of the characterization of a granular sample because this technique is very sensitive to the presence of granularity in the superconductors and, moreover, the study of its first harmonic allows for determining pivotal properties such as the pinning energy as well as the dissipation processes acting in the sample. The pinning energy values and the granularity of an YBCO thin film have been studied by means of AC susceptibility measurements as a function of the AC amplitude, temperature, and DC field. In particular, the first harmonic imaginary component of the AC susceptibility ${\chi }_1^{″}$ related to the dissipation processes of the sample has been studied. First, starting from the Brandt approach, the critical current density J_c and the pinning energy U of the sample have been extracted at 77 K by using the ${\chi }_1^{″}$ measurements as a function of the AC amplitude at different AC frequencies and DC fields. From these measurements, a first signal of granularity appears. In order to confirm it, the temperature dependence of the ${\chi }_1^{″}$ at different DC fields has been studied and a contribution deriving from the inter- and intragranular part of the sample has emerged. By taking the temperature corresponding to the crossover between the two contributions at the different DC fields, the intergranular and intragranular response has been separated. Successively, the temperature has been fixed to 77 K, together with an AC frequency equal to 1597.9 Hz, and the ${\chi }_1^{″}$ as a function of the DC field at different AC amplitudes has been analyzed showing a clear presence of granularity in all the curves. By drawing the contour plot of the ${\chi }_1^{″}$ with the DC and AC values, it was possible to determine the best parameters to put at 77 K in order to exploit the material for applications. Full article

In planar superconductor thin films, the places of nucleation and arrangements of moving vortices are determined by structural defects. However, various applications of superconductors require reconfigurable steering of fluxons, which is hard to realize with geometrically predefined vortex pinning landscapes. Here, on the basis of the time-dependent Ginzburg–Landau equation, we present an approach for the steering of vortex chains and vortex jets in superconductor nanotubes containing a slit. The idea is based on the tilting of the magnetic field $\mathbf{B}$ at an angle $\alpha$ in the plane perpendicular to the axis of a nanotube carrying an azimuthal transport current. Namely, while at $\alpha =0^{\circ }$, vortices move paraxially in opposite directions within each half-tube; an increase in $\alpha$ displaces the areas with the close-to-maximum normal component $|B_{\mathrm{n}}|$ to the close(opposite)-to-slit regions, giving rise to descending (ascending) branches in the induced-voltage frequency spectrum $f_{\mathrm{U}}\left(\alpha \right)$. At lower B values, upon reaching the critical angle ${\alpha }_{\mathrm{c}}$, the close-to-slit vortex chains disappear, yielding $f_{\mathrm{U}}$ of the $n{f}_1$ type ($n\ge 1$: an integer; $f_1$: the vortex nucleation frequency). At higher B values, $f_{\mathrm{U}}$ is largely blurry because of multifurcations of vortex trajectories, leading to the coexistence of a vortex jet with two vortex chains at $\alpha ={90}^{\circ }$. In addition to prospects for the tuning of GHz-frequency spectra and the steering of vortices as information bits, our findings lay the foundation for on-demand tuning of vortex arrangements in 3D superconductor membranes in tilted magnetic fields. Full article

Hafnium is a superconductor with a transition temperature slightly above 100 mK. This makes it attractive for such applications as microcalorimeters with high energy resolution. We report the superconducting properties of Hf films of thicknesses ranging from 60 to 115 nm, deposited on Si and Al₂O₃ substrates by electron beam evaporation. Besides that, we fabricated and measured combinations of hafnium with thin layers of normal metals, decreasing the critical temperature by the proximity effect. The critical temperature of the studied films varied from 56 to 302 mK. We have observed a significant change in the critical temperature of some films over time, which we propose to prevent by covering hafnium films with a thin layer of titanium. Full article

In recent years, metal selenide materials have attracted attention due to their wide application prospects. In this family of materials, FeSe is particularly studied since it is both a semiconductor used in solar cells and a superconductor with a critical transition temperature, T_c, of 8 K. For any envisaged application, the possibility of preparing large-area FeSe thin films at low cost is extremely appealing, and one possible technique suitable for this purpose is electrodeposition. Several groups have reported successful electrodeposition of FeSe, but the investigated systems are different in many aspects, and the results are difficult to compare. The aim of this review is to collect the available information on FeSe electrodeposition and the thermodynamic laws controlling this process; to catalog the literature pointing out the differences in the experimental procedure and how they influence the results; and to draw general conclusions, if any, on this topic. Full article

We review the topological gauge theory of Josephson junction arrays and thin film superconductors, stressing the role of the usually forgotten quantum phase slips, and we derive their quantum phase structure. A quantum phase transition from a superconducting to the dual, superinsulating phase with infinite resistance (even at finite temperatures) is either direct or goes through an intermediate bosonic topological insulator phase, which is typically also called Bose metal. We show how, contrary to a widely held opinion, disorder is not relevant for the electric response in these quantum phases because excitations in the spectrum are either symmetry-protected or neutral due to confinement. The quantum phase transitions are driven only by the electric interaction growing ever stronger. First, this prevents Bose condensation, upon which out-of-condensate charges and vortices form a topological quantum state owing to mutual statistics interactions. Then, at even stronger couplings, an electric flux tube dual to Abrikosov vortices induces a linearly confining potential between charges, giving rise to superinsulation. Full article

The dc Josephson current is generated from phase difference between two superconductors separated by a mesoscopic thin film (Josephson junction) without external bias voltage. In the presence of a temperature gradient across the superconductors, a thermal phase is induced under the condition of open circuit. This is very similar to the Seebeck effect in the usual thermoelectric effect, and the thermal phase is thus named as thermophase Seebeck coefficient (TPSC). Here we find obvious enhancement and sign change of the TPSC unique to the Josephson junction composing of two superconductors connected to a semiconductor quantum dot (QD), which is additionally side-coupled to a nanowire hosting Majorana bound states (MBSs), the system denoted by S-MQD-S. These result arise from the newly developed states near the Fermi level of the superconductors due to the QD-MBS hybridization when the dot level is within the superconducting gap. The sign change of the TPSC provides a strong evidence of the existence of MBSs, and is absent if the QD is coupled to regular fermion, such as another QD (system denoted by S-DQD-S). We show that the magnitude and sign of the TPSC are sensitive to the physical quantities including interaction strength between the QD and MBSs, direct overlap between the MBSs, system equilibrium temperature, as well as hopping amplitude between the QD and the superconductors. The obtained results are explained with the help of the current-carrying density of the states (CCDOS), and may be useful in interdisciplinary research areas of Josephson and Majorana physics. Full article

We have theoretically studied the transport properties of the SIsN${}_{SO}$F structure consisting of thick (S) and thin (s) films of superconductor, an insulator layer (I), a thin film of normal metal with spin–orbit interaction (SOI) (N${}_{SO}$), and a monodomain ferromagnetic layer (F). The interplay between superconductivity, ferromagnetism, and spin–orbit interaction allows the critical current of this Josephson junction to be smoothly varied over a wide range by rotating the magnetization direction in the single F-layer. We have studied the amplitude of the spin valve effect and found the optimal ranges of parameters. Full article

The experimental data in the literature concerning the Paramagnetic Meissner Effect (PME) or also called Wohlleben effect are reviewed with the emphasis on the PME exhibited by metallic, s-wave superconductors. The PME was observed in field-cool cooling (FC-C) and field-cool warming (FC-W) $m\left(T\right)$-measurements on Al, Nb, Pb, Ta, in compounds such as, e.g., NbSe${}_2$, In-Sn, ZrB${}_{12}$, and others, and also in MgB${}_2$, the metallic superconductor with the highest transition temperature. Furthermore, samples with different shapes such as crystals, polycrystals, thin films, bi- and multilayers, nanocomposites, nanowires, mesoscopic objects, and porous materials exhibited the PME. The characteristic features of the PME, found mainly in Nb disks, such as the characteristic temperatures $T_1$ and $T_p$ and the apparative details of the various magnetic measurement techniques applied to observe the PME, are discussed. We also show that PME can be observed with the magnetic field applied parallel and perpendicular to the sample surface, that PME can be removed by abrading the sample surface, and that PME can be introduced or enhanced by irradiation processes. The PME can be observed as well in magnetization loops (MHLs, $m\left(H\right)$) in a narrow temperature window $T_p<T_c$, which enables the construction of a phase diagram for a superconducting sample exhibiting the PME. We found that the Nb disks still exhibit the PME after more than 20 years, and we present the efforts of magnetic imaging techniques (scanning SQUID microscopy, magneto-optics, diamond nitrogen-vacancy (NV)-center magnetometry, and low-energy muon spin spectroscopy, (LE-$\mu$SR)). Various attempts to explain PME behavior are discussed in detail. In particular, magnetic measurements of mesoscopic Al disks brought out important details employing the models of a giant vortex state and flux compression. Thus, we consider these approaches and demagnetization effects as the base to understand the formation of the paramagnetic signals in most of the materials investigated. New developments and novel directions for further experimental and theoretical analysis are also outlined. Full article

Nickelate superconductors, R_1−xA_xNiO₂ (where R is a rare earth metal and A = Sr, Ca), experimentally discovered in 2019, exhibit many unexplained mysteries, such as the existence of a superconducting state with T_c (up to 18 K) in thin films and yet absent in bulk materials. Another unexplained mystery of nickelates is their temperature-dependent upper critical field, $B_{c2}\left(T\right)$, which can be nicely fitted to two-dimensional (2D) models; however, the deduced film thickness, $d_{sc,GL}$, exceeds the physical film thickness,$d_{sc}$, by a manifold. To address the latter, it should be noted that 2D models assume that $d_{sc}$ is less than the in-plane and out-of-plane ground-state coherence lengths, $d_{sc}<{\xi }_{ab}\left(0\right)$ and $d_{sc}<{\xi }_c\left(0\right)$, respectively, and, in addition, that the inequality ${\xi }_c\left(0\right)<{\xi }_{ab}\left(0\right)$ satisfies. Analysis of the reported experimental $B_{c2}\left(T\right)$ data showed that at least one of these conditions does not satisfy for R_1-xA_xNiO₂ films. This implies that nickelate films are not 2D superconductors, despite the superconducting state being observed only in thin films. Based on this, here we propose an analytical three-dimensional (3D) model for a global data fit of in-plane and out-of-plane $B_{c2}\left(T\right)$ in nickelates. The model is based on a heuristic expression for temperature-dependent coherence length anisotropy: ${\gamma }_{\xi }\left(T\right)=\frac{{\gamma }_{\xi }\left(0\right)}{1-\frac{1}{a}\times \frac{T}{T_c}}$, where $a>1$ is a unitless free-fitting parameter. The proposed expression for ${\gamma }_{\xi }\left(T\right)$, perhaps, has a much broader application because it has been successfully applied to bulk pnictide and chalcogenide superconductors. Full article

Ensuring the energy transition in order to decrease CO₂ and volatile organic compounds emissions and improve the efficiency of energy processes requires the development of advanced materials and technologies for the electrical energy sector. The article reviews superconducting materials, functional nanomaterials used in the power industry mainly due to their magnetic, electrical, optical, and dielectric properties and the thin layers of amorphous carbon nitride, which properties make them an important material from the point of view of environmental protection, optoelectronic, photovoltaic and energy storage. The superconductivity-based technologies, material processing, and thermal and nonthermal plasma generation have been reviewed as technologies that can be a solution to chosen problems in the electrical energy sector and environment. The study explains directly both—the basics and application potential of low and high-temperature superconductors as well as peculiarities of the related manufacturing technologies for Roebel cables, 1G and 2G HTS tapes, and superconductor coil systems. Among the superconducting materials, particular attention was paid to the magnesium di-boride MgB₂ and its potential applications in the power industry. The benefits of the use of carbon films with amorphous structures in electronics, sensing technologies, solar cells, FETs, and memory devices were discussed. The article provides the information about most interesting, from the R&D point of view, groups of materials for PV applications. It summarises the advantages and disadvantages of their use regarding commercial requirements such as efficiency, lifetime, light absorption, impact on the environment, costs of production, and weather dependency. Silicon processing, inkjet printing, vacuum deposition, and evaporation technologies that allow obtaining improved and strengthened materials for solar cell manufacturing are also described. In the case of the widely developed plasma generation field, waste-to-hydrogen technology including both thermal and non-thermal plasma techniques has been discussed. The review aims to draw attention to the problems faced by the modern power industry and to encourage research in this area because many of these problems can only be solved within the framework of interdisciplinary and international cooperation. Full article