No abstract

We present simulations of flux-gradient-driven superconducting rigid vortices interacting with square and triangular arrays of columnar pinning sites in an increasing external magnetic field. These simulations allow us to quantitatively relate spatio-temporal microscopic information of the vortex lattice with typically measured macroscopic quantities, such as the magnetization M (H). The flux lattice does not become completely commensurate with the pinning sites throughout the sample at the magnetization matching peaks, but forms a commensurate lattice in a region close to the edge of the sample. Matching fields related to unstable vortex configurations do not produce peaks in M (H). We observe a variety of evolving complex flux profiles, including flat terraces or plateaus separated by winding current-carrying strings and, near the peaks in M (H), plateaus only in certain regions, which move through the sample as the field increases.

In this study the occlusion effect was quantified for five types of earmolds with different venting. Nine normal-hearing listeners and ten experienced hearing aid users were provided with conventional earmolds with 1.6 and 2.4 mm circular venting, shell type earmolds with a novel vent design with equivalent cross-sectional vent areas, and nonoccluding soft silicone eartips of a commercial hearing instrument. For all venting systems, the occlusion effect was measured using a probe microphone system and subjectively rated in test and retest sessions. The results for both normal-hearing subjects and hearing aid users showed that the novel vents caused significantly less occlusion than the traditional vents. Occlusion effect associated with the soft silicone eartip was comparable to the nonoccluded ear. Test-retest reproducibility was higher for the subjective occlusion rating than for the objectively measured occlusion. Perceived occlusion revealed a closer relationship to measured occlusion in the ear in which the measured occlusion effect was higher ("high OE" ear) than in the "low OE" ear. As our results suggest that subjective judgment of occlusion is directly related to the acoustic mass of the air column in the vent, the amount of perceived occlusion may be predicted by the vent dimensions.

We present a microscopic derivation, without electrodynamical assumptions, of B(x, y, H(t)), M (H(t)), and Jc(H(t)), in agreement with experiments on strongly pinned superconductors, for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion of these flux-gradient-driven vortices which can be temporarily trapped at pinning centers. The field is increased (decreased) by the addition (removal) of flux lines at the sample boundary, and complete hysteresis loops can be achieved by using flux lines with opposite orientation. The pinning force per unit volume we obtain for strongly-pinned vortices, JcB ∼ npf 1.6 p , interpolates between the following two extreme situations: very strongly-pinned independent vortices, where JcB ∼ npfp, and the 2D Larkin-Ovchinikov collective-pinning theory for weakly-pinned straight vortices, where JcB ∼ npf 2 p . Here, np and fp are the density and maximum force of the pinning sites.PACS numbers: 74.60.Ec, 74.60.Ge, 74.60.Jg I. INTRODUCTIONFlux distributions in type-II superconductors are commonly inferred from magnetization and critical current measurements [1] and interpreted in the context of the Bean model [2] or its variations. The Bean model, which has been widely used for over three decades, postulates that the current density in a hard superconductor (i.e., with strong pinning) can only have three values: −J c , 0, and +J c , where J c is the critical current density, which is independent of the local magnetic flux density B(x, y, t). The Bean model and its many variants make no specific claims with regard to the microscopic mechanism controlling the trapping of vortices. Bean's postulate, J c =constant, was modified several times by Kim et al. [3]:On the other hand, Fietz et al. [4] suggested that J c ∼ exp(−B/b 0 ) ; while Yasukōchi et al.[5] suggested J c ∼ 1/B 1/2 . These, and other proposals made during the 1960s, were followed by several other phenomenological modifications of J c (H) during the following two decades [1,6]. A microscopic description, without assuming any particular B-dependence of J c , of these flux distributions-in terms of interacting vortices and pinning sites-can be very valuable for a better understanding of commonly measured bulk quantities.One of the most effective methods of investigating the microscopic behaviour of flux in a hard superconductor is with computer simulations (see, e.g., [7,8], and references therein). In this paper, we present molecular dynamics (MD) simulations of the evolution of rigid flux lines in a hard superconductor. We first introduce our model for vortex-vortex and vortex-pin interactions as well as

We present simulations, without electrodynamical assumptions, of $B(x,y,H(t)), M(H(t))$, and $J_c(H(t))$, in hard superconductors, for a variety of twin-boundary pinning potential parameters, and for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion of up to 10^4 flux-gradient-driven vortices which can be temporarily trapped at $\sim 10^6$ pinning centers. These simulations relate macroscopic measurements (e.g., M(H), ``flame'' shaped $B(x,y)$ profiles) with the underlying microscopic pinning landscape and the plastic dynamics of individual vortices