三浦 正志

Abstract The addition of artificial pinning centers has led to an impressive increase in the critical current density (J_c) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The J_c of superconductors has reached ~0.2–0.3 J_d, where J_d is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase J_d. For (Y_0.77Gd_0.23)Ba₂Cu₃O_y ((Y,Gd)123), we can achieve this by controlling the carrier density, which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe₂(As_1−xP_x)₂ films. The variation in λ and ξ leads to an intrinsic improvement in J_c via J_d, allowing extremely high values of J_c of 130 MA/cm² and 8.0 MA/cm² at 4.2 K, consistent with an enhancement in J_d of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe₂(As_1−xP_x)₂ films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m³ at 4.2 K and 18 T (H||c), the highest values ever reported for any superconductor.

Because of pressing global environmental challenges, focus has been placed on materials for efficient energy use, and this has triggered the search for nanostructural modification methods to improve performance. Achieving a high density of tunable-sized second-phase nanoparticles while ensuring the matrix remains intact is a long-sought goal. In this paper, we present an effective, scalable method to achieve this goal using metal organic deposition in a perovskite system REBa2Cu3O7 (rare earth (RE)) that enhances the superconducting properties to surpass that of previous achievements. We present two industrially compatible routes to tune the nanoparticle size by controlling diffusion during the nanoparticle formation stage by selecting the second-phase material and modulating the precursor composition spatially. Combining these routes leads to an extremely high density (8 x 10(22) m(-3)) of small nanoparticles (7 nm) that increase critical currents and reduce detrimental effects of thermal fluctuations at all magnetic field strengths and temperatures. This method can be directly applied to other perovskite materials where nanoparticle addition is beneficial.

Superconductors are excellent testbeds for studying vortices, topological excitations that also appear in superfluids, liquid crystals and Bose-Einstein condensates. Vortex motion can be disruptive; it can cause phase transitions(1), glitches in pulsars(2), and losses in superconducting microwave circuits(3), and it limits the current-carrying capacity of superconductors(4). Understanding vortex dynamics is fundamentally and technologically important, and the competition between thermal energy and energy barriers defined by material disorder is not completely understood. Specifically, early measurements of thermally activated vortex motion (creep) in iron-based superconductors unveiled fast rates (S) comparable to measurements of YBa2Cu3O7-delta (refs 5-10). This was puzzling because S is thought to somehow correlate with the Ginzburg number (Gi), and Gi is significantly lower in most iron-based superconductors than in YBa2Cu3O7-delta. Here, we report very slow creep in BaFe2(As0.67P0.33)(2) films, and propose the existence of a universal minimum realizable S similar to Gi(1/2)(T/T-c) (T-c is the superconducting transition temperature) that has been achieved in our films and few other materials, and is violated by none. This limitation provides new clues about designing materials with slow creep and the interplay between material parameters and vortex dynamics.

We show a simple and effective way to improve the vortex irreversibility line up to very high magnetic fields (60T) by increasing the density of second phase BaZrO3 nanoparticles. (Y-0.77 Gd-0.23)Ba2Cu3Oy films were grown on metal substrates with different concentration of BaZrO3 nanoparticles by the metal organic deposition method. We find that upon increase of the BaZrO3 concentration, the nanoparticle size remains constant but the twin -boundary density increases. Up to the highest nanoparticle concentration (n similar to 1.3 x 10(22)/m(3)), the irreversibility field (H-irr) continues to increase with no sign of saturation up to 60T, although the vortices vastly outnumber pinning centers. We find extremely high H-irr namely H-irr = 30T (H parallel to 45 degrees) and 24T (H parallel to c) at 65 K and 58T (H parallel to 45 degrees) and 45T (H parallel to c) at 50K. The difference in pinning landscape shifts the vortex solid-liquid transition upwards, increasing the vortex region useful for power applications, while keeping the upper critical field, critical temperature and electronic mass anisotropy unchanged.

We study the field (H) and temperature (T) dependence of the critical current density (J(c)) and irreversibility field (H-irr) at different field orientations in Y0.77Gd0.23Ba2Cu3Oy with randomly distributed BaZrO3 nanoparticles (YGdBCO + BZO) and YBa2Cu3Oy (YBCO) films. Both MOD films have large RE2Cu2O5 (225) nanoparticles (similar to 80 nm in diameter) and a high density of twin boundaries (TB). In addition, YGdBCO + BZO films have a high density of BZO nanoparticles (similar to 25 nm in diameter). At high temperatures (T > 40 K), the superconducting properties, such as J(c), H-irr, and flux creep rates, are greatly affected by the BZO nanoparticles, while at low temperatures the superconducting properties of both the YBCO and YGdBCO + BZO films show similar field and temperature dependencies. In particular, while the J(c) of YBCO films follow a power-law dependence (proportional to H-alpha) at all measured T, this dependence is only followed at low T for YGdBCO + BZO films. As a function of T, the YGdBCO + BZO film shows J(c) (T, 0.01T) similar to [1 -(T/T-c)(2)](n) with n similar to 1.24 +/- 0.05, which points to "delta T-c pinning." We analyze the role of different types of defects in the different temperature regimes and find that the strong pinning of the BZO nanoparticles yields a higher H-irr and improved J(c) along the c axis and at intermediate orientations at high T. The mixed pinning landscapes due to the presence of disorder of various dimensionalities have an important role in the improvement of in-field properties.