[en] Over the course of three decades of intense study, apart from the exceptionally high critical temperature, many unusual properties of cuprates have been discovered, notably including resistivity linear in temperature, electronic Raman continuum and optical absorption extending throughout the infrared region, pseudogap, hour-glass spin excitation spectrum, etc. However, each of these features have been also observed in other materials, including some that are not even superconducting at all. Here, we describe an extensive experiment in which over 2,000 films of the La_2-xSr_xCuO₄ have been synthesized and studied in detail over the course of the last twelve years. We argue here that, uniquely, in the cuprates an unusual superconducting state, that defies the standard BCS description, develops from an unusual metallic state, in which the rotational symmetry of the electron fluid is spontaneously broken.

[en] A conformal pinning array can be created by conformally transforming a uniform triangular pinning lattice to produce a new structure in which the six-fold ordering of the original lattice is conserved but where there is a spatial gradient in the density of pinning sites. Here we examine several aspects of vortices interacting with conformal pinning arrays and how they can be used to create a flux flow diode effect for driving vortices in different directions across the arrays. Under the application of an ac drive, a pronounced vortex ratchet effect occurs where the vortices flow in the easy direction of the array asymmetry. When the ac drive is applied perpendicular to the asymmetry direction of the array, it is possible to realize a transverse vortex ratchet effect where there is a generation of a dc flow of vortices perpendicular to the ac drive due to the creation of a noise correlation ratchet by the plastic motion of the vortices. We also examine vortex transport in experiments and compare the pinning effectiveness of conformal arrays to uniform triangular pinning arrays. In conclusion, we find that a triangular array generally pins the vortices more effectively at the first matching field and below, while the conformal array is more effective at higher fields where interstitial vortex flow occurs.

[en] Highlights: • IV characteristic of electron doped superconductors ECBCO has been studied below T_c. • Role of angle between two grains in controlling J_c has been found to be dominating over electron doping. • Variation of the low critical current density with temperature has been explained by using an exponential dependence on T • Vortex solid to vortex fluid state near T_c has been attributed to the exponential variation of J_c(T) We have studied how the current-voltage (IV) characteristic below the critical temperature gets affected by electron doping in Eu_1-xCe_xBa₂Cu₃O_7-δ (ECBCO) superconducting systems. The critical current density as a function of T has been extracted in the vicinity but below the critical temperature. Variation of the low critical current density with temperature in the limited range of T has been explained by using an exponential dependence on T. Possibility of the Ambegaokar-Baratoff (AB) to Ginzburg-Landau (GL) crossover has been explored. An exponential nature of the variation in all electron doped superconductors with large angular separation between grain boundaries is attributed to the existence of the vortex fluid state in a narrow range of T.

[en] Highlights: • Nb₃Sn superconducting films were fabricated by magnetron sputtering from a single target. • Effects of annealed pressure on the thin films' superconductivity and microstructural properties is examined. • argon atmosphere protection can significantly alleviate the tin loss problem. • the tc of the film on the sapphire substrate was 17 K, generally about 3.5 K higher than that on the copper substrate. Nb₃Sn is an alternative material for future superconducting accelerators. With its higher critical temperature and superheating field than that of niobium, its application in the field of SRF is a great advantage. In this paper, the magnetron sputtering method is applied to prepare the Nb₃Sn thin film on the oxygen-free copper substrate by the stoichiometric target—the ratio of 3:1 of niobium to tin, so as to explore the effect of different annealing air pressures on the properties of the film in detail. The film crystal structure, morphology, composition, and superconducting critical temperature were characterized by XRD, SEM, EDS, and MPMS. After in-situ annealing, XRD and MPMS reveal the existence of Nb₃Sn crystal. The compact film with good surface properties is free of cracks and tin islands. The superconducting critical temperature reaches 13.6 K. The results show that if annealing was conducted in Ar atmosphere at 1 Pa, the obtained film exhibits better performance with a sharp transition from the superconducting state to the normal state.

[en] Highlights: • Electron-phonon interaction and superconducting properties have been studied for HfOs and HfRu compounds in B₂ CsCl phase. • Phonon dispersion curves show strong bonding forces between Hf and Os(Ru) atoms and HfOs and HfRu compounds are dynamically stable in cubic B₂ CsCl phase at ambient pressure. • Calculated low values of λ are 0.41 and 0.33 for HfOs and HfRu give the low value of superconducting transition temperature T_C is 1.16 K for HfOs and 1.163 K for HfRu which is agree well with experimental results. From the first principles calculation based on the plane-wave pseudopotential method of density functional theory, and the linear response technique, we have performed electronic structure, phonon and electron-phonon interaction of superconducting compounds HfOs and HfRu in B₂ phase. The distribution of energy with different wave vector in the first Brillouin zone and conductive charge carriers (electrons and holes) at Fermi level are described by the electronic band structure and Fermi surfaces (FS), which reveal that both the compounds are metallic in nature. Phonon dispersion curves along with phonon density of states for both the compounds are calculated. Moreover, for the convincement of phonon calculation, we have described the eigen vector displacement of optical phonon modes at the zone centre which reveals that the computed phonon spectra are real. The interaction of electron with acoustic phonon modes and low-frequency optical phonon modes is responsible for superconductivity for both the compounds. The detailed discussion of superconductivity is given in terms of Eliashberg spectral function (α²F(ω)), electron phonon coupling constants (λ) and superconducting transition temperature (T_C). The calculated value of λ is 0.41 and 0.33 for HfOs and HfRu. Using Allen Dynes formula the calculated superconducting transition temperature is 1.16 K and 1.163 K, with good agreement of experimental values.

[en] Highlights: • A modified transverse tensile testing method by using epoxy adhesive was developed. • Decreasing operation temperature can improve the delamination strength of CCs. • At 77 K, the critical stress is almost double compared to the result at RT. A setup of transverse tension measurements of high temperature superconductor (HTS) RE-Ba-Cu-O coated conductors are modified for simplicity. Modification includes using high strength epoxy films instead of solder as adhesive agents and changing the shape of contact surface between test samples and anvils. This method was designed to measure delamination strength of stabilizer free REBCO coated conductors with a width more than 10 mm. It is also applicable to REBCO coated conductors with copper or stainless-steel stabilizers. Samples made by the IBAD/PLD process were tested by this test. Effect of operation temperature on the delamination strength of coated conductors was investigated. In addition, fracture surfaces of delaminated samples were studied.

[en] Highlights: • The mathematical modelling of winding process of CORC cable/ wire is performed, right from the production stage of REBCO tape. • Induced residual strain developed due to thermal loading on the REBCO tape was computed. • Investigated the influence of cable parameters on the degradation of superconducting CORC cable/ wire. • Parametric studies were conducted by varying core diameter, hastelloy thickness, copper thickness, tape width and winding angle, and proposed the limiting values. The design and testing of high current superconducting cables made of ReBa₂Cu₃O_7-z (REBCO) are still under development. It seems to have huge potential to overcome some of the difficulties associated Low Temperature Superconductors. One of the attractive features of CORC® cable compared to other cable types is that a former with a small diameter can be used. This allows a flexible and round wire shape of the cable showing symmetry in the radial direction. This paper aims at analyzing limitations of a simple superconducting CORC® cable design by varying parameters such as substrate thickness, copper stabilizer layer thickness, winding angle, and central former diameter. The CORC® cable modeling is started from the design to the production stage of the tape, then the winding of the tape over the central former is performed at room temperature. The winding is critical as in the process, compressive strain is induced in the tape. This paper also investigates the influence of different parameters on the performance and possible performance degradation of superconducting CORC® wire and cable. The result showed that Hastelloy thickness decreases from 0.05 to 0.02 mm, at 1 mm core diameter, the variation of maximum strain is 47 % and 52%, at 0.01 and 0.02 copper thickness respectively. The compressive strain is increased by 43 % when the core diameter decreases from 3 to 1 mm, at 0.02 mm Hastelloy thickness. Similarly, at 0.01 mm copper thickness, and 0.05 mm Hastelloy thickness, 50% increase of compressive strain is noted when the core diameter decreases from 3 to 1 mm.

[en] Highlights: • MgTa${}_2$S${}_5$ is revealed to be a BCS superconductor with T${}_c$ = 1 K, and with a superconducting energy gap of 0.25 meV. • It is the newest member of the AM${}_2$S${}_5$ compounds, A = alkaline earth metal, M = transition metal. • The electronic specific heat coefficient 11.1 mJ/mole K${}^2$ is reported. Superconductivity in the new compound, MgTa${}_2$S${}_5$, is reported. Its crystal structure was indexed to space group $C2/m$, which is monoclinic with lattice parameters $a$ = 5.784(5) Å, $b$ = 12.680(3) Å, and $c$ = 6.106(5) Å, with $\beta$ = 112.64${}^{\circ }$. Electrical resistivity reveals metallic behavior with $\rho$(296 K) = 5.2 m$\Omega$ cm and bulk superconductivity below $T_{\mathrm{c}}$ = 1 K. Measurements of the specific heat $C$ reveal the electronic specific heat coefficient $\gamma$ = 11.1(5) mJ/mol K${}^2$, a jump in $C$ at $T_{\mathrm{c}}$ illustrating bulk superconductivity, and an energy gap associated with the superconducting state of $E_g$ = 0.25(3) meV. MgTa${}_2$S${}_5$ can be classified as a BCS superconductor. An unidentified minority phase present in the sample ($\sim$ 4%) exhibits a superconducting phase transition near 2.6 K.

[en] Highlights: • $\beta$-FeSe was synthesized by solvothermal method. • Magnetic susceptibility and phase composition were studied. • Superconductivity at 8 K was observed. • The superconducting phase is instable under normal conditions. Using elemental iron and selenium as well as NH${}_4$Cl and 2-propanol as starting materials, a low temperature (473 K) method of solvothermal synthesis of $\beta$–FeSe superconducting at $T_c\simeq 8$K has been successfully applied. This method may result in new possibilities for soft chemistry approaches to the synthesis and modification of iron-based superconductors.

[en] Highlights: • Nonlinear temperature distribution is observed along the length of HTS cable. • Lower heat loads and higher volumetric flow rates reduce the maximum temperature in the cable. • The maximum possible length of 2.5 km can be achieved at the flow rate of 30 L/min and with heat fluxes of 2.3–2.6 W/m within the defined operating range. -- Abstract: The technical advantages of using High Temperature Superconducting (HTS) cables have drawn the attention of most of the researchers around the world. However, the cooling design of these cables nevertheless remains a challenge for the researchers and manufacturers. Therefore, in the present work, an attempt has been made to ease the challenge of cooling of long length High Temperature Superconducting (HTS) cables. Counter flow cooling arrangements of liquid nitrogen (LN2) have been proposed and analysed the longitudinal temperature distribution along the superconducting cable using heat balance equations. Also, in the present work the solution for the temperature distribution profiles of LN2 in counter cooled HTS cable are also obtained assuming one dimensional (1-D) model. In particular, nonlinear axial temperature profiles of LN₂ in the counter cooled HTS cable are found. The single phase cold dielectric HTS cable with stainless steel corrugated pipes as flow paths are used for the analysis. Liquid nitrogen with varying volumetric flow rates of 20–35 L/min and total heat loss (A.C. loss and exterior heat load) of 2.3–2.6 W/m reveals interesting facts about cooling of cables with longer lengths. The defined cable length range at constant diameter (inner and annulus pipe) for the analysis was considered as 1.5–3 km. As a result, the maximum possible HTS cable length of 2.5 km can be achieved at the flow rate of 30 L/min and heat fluxes of 2.3–2.6 W/m within the defined operating range.