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Register a new userThin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field
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The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. With twice the critical temperature of Nb_3Sn and four times that of Nb-Ti alloy, MgB_2 has the potential to reach much higher fields and current densities than either of these technological superconductors. A vital prerequisite, strongly linked current flow, has already been demonstrated even at this early stage. One possible drawback is the observation that the field at which superconductivity is destroyed is modest. Further, the field which limits the range of practical applications, the irreversibility field H*(T), is ~7 T at liquid helium temperature (4.2 K), significantly lower than ~10 T for Nb-Ti and ~20 T for Nb_3Sn. Here we show that MgB_2 thin films can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding H*(4.2 K) above 14 T. In addition, very high critical current densities at 4.2 K, 1 MA/cm_2 at 1 T and 10_5 A/cm_2 at 10 T, are possible. These data demonstrate that MgB_2 has credible potential for high-field superconducting applications.
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We present a detailed study of the electrical transport properties of YBa2Cu3O7-{delta} thin film. The irreversibility fields ({mu}_0 H_irr), upper critical fields ({mu}_0 H_C2), penetration depths ({lambda}) and coherence lengths ({xi} ) of the YBa2Cu3O7-{delta} materials are deduced from the resistivity curves. Itis observed that {mu}_0 H_irr, {mu}_0 H_C2 and {Delta}Tc of the film strongly depend on the direction and strength of the field. The coherence length {xi} (0) and penetration depth {lambda} (0) values at T = 0 K has been calculated from the irreversibility fields ({mu}_0 H_irr) and upper critical fields ({mu}_0 H_C2) respectively. Based on all the results, the change of the superconducting properties as a function of the magnetic field direction presents the anisotropy of the sample produced.
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes. Full C incorporation required only 30-60 min of milling. Grain size of sintered samples decreased with increased milling time to less than 30 nm for 20-50 hrs of milling. Milling had a weak detrimental effect on connectivity. Strong irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, high field Jc benefited greatly from lengthy powder milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nano-dispersions.
For any practical superconductor the magnitude of the critical current density, $J_textrm{c}$, is crucially important. It sets the upper limit for current in the conductor. Usually $J_textrm{c}$ falls rapidly with increasing external magnetic field but even in zero external field the current flowing in the conductor generates a self-field which limits $J_textrm{c}$. Here we show for thin films of thickness less than the London penetration depth, $lambda$, this limiting $J_textrm{c}$ adopts a universal value for all superconductors - metals, oxides, cuprates, pnictides, borocarbides and heavy Fermions. For type I superconductors, it is $H_{textrm{c}}/lambda$ where $H_textrm{c}$ is the thermodynamic critical field. But surprisingly for type II superconductors we find the self-field $J_textrm{c}$ is $H_{textrm{c}1}/lambda$ where $H_{textrm{c}1}$ is the lower critical field. $J_textrm{c}$ is thus fundamentally determined and this provides a simple means to extract absolute values of $lambda(T)$ and, from its temperature dependence, the symmetry and magnitude of the superconducting gap.
We show how to calculate the magnetic-field and sheet-current distributions for a thin-film superconducting annular ring (inner radius a, outer radius b, and thickness d<<a) when either the penetration depth obeys lambda < d/2 or, if lambda > d/2, the two-dimensional screening length obeys Lambda = 2 lambda^2/d << a for the following cases: (a) magnetic flux trapped in the hole in the absence of an applied magnetic field, (b) zero magnetic flux in the hole when the ring is subjected to an applied magnetic field, and (c) focusing of magnetic flux into the hole when a magnetic field is applied but no net current flows around the ring. We use a similar method to calculate the magnetic-field and sheet-current distributions and magnetization loops for a thin, bulk-pinning-free superconducting disk (radius b) containing a dome of magnetic flux of radius a when flux entry is impeded by a geometrical barrier.
We study long Josephson junctions with the critical current density alternating along the junction. New equilibrium states, which we call the field synchronized or FS states, are shown to exist if the applied field is from narrow intervals centered around equidistant series of resonant fields, $H_m$. The values of $H_m$ are much higher than the flux penetration field, $H_s$. The flux per period of the alternating critical current density, $phi_i$, is fixed for each of the FS states. In the $m$-th FS state the value of $phi_i$ is equal to an integer amount of flux quanta, $phi_i =mphi_0$. Two types of single Josephson vortices carrying fluxes $phi_0$ or/and $phi_0/2$ can exist in the FS states. Specific stepwise resonances in the current-voltage characteristics are caused by periodic motion of these vortices between the edges of the junction.
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