A detailed numerical study of the real space configuration of vortices in disordered superconductors using 2D London-Langevin model is presented. The magnetic field $B$ is varied between 0 and $B_{c2}$ for various pinning strengths $\Delta$. For weak pinning, an inhomogeneous disordered vortex matter is observed, in which the topologically ordered vortex lattice survives in large domains. The majority of the dislocations in this state are confined to the grain boundaries/domain walls. Such quasi-ordered configurations are observed in the intermediate fields, and we refer it as the domain regime (DR). The DR is distinct from the low-field and the high-fields amorphous regimes which are characterized by a homogeneous distribution of defects over the entire system. Analysis of the real space configuration suggests domain wall roughening as a possible mechanism for the crossover from the DR to the high-field amorphous regime. The DR also shows a sharp crossover to the high temperature vortex liquid phase. The domain size distribution and the roughness exponent of the lattice in the DR are also calculated. The results are compared with some of the recent Bitter decoration experiments.Comment: 9 pages, 9 figure

The (111) intrinsic stacking fault energy γISF in Ni and Ni-Co alloy was calculated and compared using two different ab initio methods, viz., the supercell approach and the axial interaction model (AIM), based on density functional theory. The supercell approach uses energies of crystal structure in slab geometry with and without the stacking fault. In the AIM approach, the problem is mapped to a 1D spin-model and the interaction parameters are obtained using energies for ordered structures, thus obviating the need to handle faulted structure. For elemental Ni, the calculated values of γISF from AIM and supercell approaches differ by not more than by 2%, and compares well with experimental value. For Ni-Co alloy, AIM predicts a slightly faster decrease in γISF with increasing Co concentration compared to supercell approach and experimental data. Overall, there is good agreement between the two approaches.

An ab initio method based on density functional theory has been employed to compute the zero-temperature anti-phase boundary (APB) energies for Ni3Al1−xRx (R = Nb, Ta, Ti) system over a range of compositions. The computation is limited to the APB on the (1 1 1) plane for L12 crystal structure, allowing only the volume relaxation, appropriate for the γ′ precipitate in Ni-based superalloy. For the limiting case of the binary system Ni3Al, the APB energy has also been calculated for the (1 0 0) plane. We find that the APB energy for the (1 1 1) plane in Ni3Al is 181 mJ m−2, and substitution of Nb, Ta or Ti at the Al site increases the APB energy to over 600 mJ m−2, leading to higher strengths. While the peak APB energy values for all the ternary systems are quite similar, they are achieved over very different compositional ranges. Nb and Ta are found to have almost identical strengthening effect on Ni3Al. The selected compositional space is of direct relevance to the commercially important family of Ni-based superalloys, and our results provide important guidelines for alloy design strategies.

We study the zero-temperature dynamic transition from the disordered flow to an ordered flow state in driven vortices in type-II superconductors. The transition current Ip is marked by a sharp kink in the V (I) characteristic with a concomitant large increase in the defect concentration. On increasing magnetic field B, the Ip(B) follows the behaviour of the critical current Ic(B). Specifically, in the peak effect regime Ip(B) increases rapidly along with Ic. We also discuss the effect of varying disorder strength on Ip.

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