Fabrication of high quality ferromagnetic Josephson junctions

We present ferromagnetic Nb/Al2O3/Ni60Cu40/Nb Josephson junctions (SIFS) with an ultrathin Al2O3 tunnel barrier. The junction fabrication was optimized regarding junction insulation and homogeneity of current transport. Using ion-beam-etching and anodic oxidation we defined and insulated the junction mesas. The additional 2 nm thin Cu layer below the ferromagnetic NiCu (SINFS) lowered interface roughness and ensured very homogeneous current transport. A high yield of junctional devices with jc spreads less than 2% was obtained.Comment: 5 pages, 6 figures; VORTEX IV conference contribution; Submitted to Physica

The Computational Complexity of the Restricted Isometry Property, the Nullspace Property, and Related Concepts in Compressed Sensing

This paper deals with the computational complexity of conditions which guarantee that the NP-hard problem of finding the sparsest solution to an underdetermined linear system can be solved by efficient algorithms. In the literature, several such conditions have been introduced. The most well-known ones are the mutual coherence, the restricted isometry property (RIP), and the nullspace property (NSP). While evaluating the mutual coherence of a given matrix is easy, it has been suspected for some time that evaluating RIP and NSP is computationally intractable in general. We confirm these conjectures by showing that for a given matrix A and positive integer k, computing the best constants for which the RIP or NSP hold is, in general, NP-hard. These results are based on the fact that determining the spark of a matrix is NP-hard, which is also established in this paper. Furthermore, we also give several complexity statements about problems related to the above concepts.Comment: 13 pages; accepted for publication in IEEE Trans. Inf. Theor

DOLPHIn - Dictionary Learning for Phase Retrieval

We propose a new algorithm to learn a dictionary for reconstructing and sparsely encoding signals from measurements without phase. Specifically, we consider the task of estimating a two-dimensional image from squared-magnitude measurements of a complex-valued linear transformation of the original image. Several recent phase retrieval algorithms exploit underlying sparsity of the unknown signal in order to improve recovery performance. In this work, we consider such a sparse signal prior in the context of phase retrieval, when the sparsifying dictionary is not known in advance. Our algorithm jointly reconstructs the unknown signal - possibly corrupted by noise - and learns a dictionary such that each patch of the estimated image can be sparsely represented. Numerical experiments demonstrate that our approach can obtain significantly better reconstructions for phase retrieval problems with noise than methods that cannot exploit such "hidden" sparsity. Moreover, on the theoretical side, we provide a convergence result for our method

An Infeasible-Point Subgradient Method Using Adaptive Approximate Projections

We propose a new subgradient method for the minimization of nonsmooth convex functions over a convex set. To speed up computations we use adaptive approximate projections only requiring to move within a certain distance of the exact projections (which decreases in the course of the algorithm). In particular, the iterates in our method can be infeasible throughout the whole procedure. Nevertheless, we provide conditions which ensure convergence to an optimal feasible point under suitable assumptions. One convergence result deals with step size sequences that are fixed a priori. Two other results handle dynamic Polyak-type step sizes depending on a lower or upper estimate of the optimal objective function value, respectively. Additionally, we briefly sketch two applications: Optimization with convex chance constraints, and finding the minimum l1-norm solution to an underdetermined linear system, an important problem in Compressed Sensing.Comment: 36 pages, 3 figure

Degenerations of ideal hyperbolic triangulations

Let M be a cusped 3-manifold, and let T be an ideal triangulation of M. The deformation variety D(T), a subset of which parameterises (incomplete) hyperbolic structures obtained on M using T, is defined and compactified by adding certain projective classes of transversely measured singular codimension-one foliations of M. This leads to a combinatorial and geometric variant of well-known constructions by Culler, Morgan and Shalen concerning the character variety of a 3-manifold.Comment: 31 pages, 11 figures; minor changes; to appear in Mathematische Zeitschrif

Between Gay and Straight-1 Before

In chapter 1, “Before,” of the book Between Gay and Straight: Understanding Friendship Across Sexual Orientation (AltaMira Press, 2001), I story the absences, silences, and stereotypes surrounding same-sex orientation when I came of age in the 1980s and early 90s

Wedding Album: An Antiheterosexist Performance Text

Historical and personal snapshots of weddings become poetic stanzas that advocate for marriage equality and for a social safety net strong enough to protect the human rights and meet the human needs of everyone, regardless of relational—or any other—statu

Labor Pains in the Academy

This piece offers autoethnographic reflections on crossroads to which many academics come: whether to seek (or postpone or avoid) parenthood and when. The author deeply explores the personal (her own trajectories from daughter and sister to potential mother and from graduate student to full professor) in order to reflect on structural constraints associated with graduate education, the academic job market, and institutional policies and politics

Body and Bulimia Revisited: Reflections on A Secret Life

In 1996, the author published “A Secret Life in a Culture of Thinness: Reflections on Body, Food, and Bulimia” (Tillmann-Healy, 1996), an account of her struggle with binging and purging from ages 15 to 25. She came to understand bulimia as a communicative act, expressing fear, anxiety, and grief. From 25 to 35, her recovery from bulimia involved learning to “purge” emotion through other forms of communication (e.g., dialogue, writing, and teaching). At 35, separation and divorce pose the greatest challenge to the author’s 10-year recovery, yet she does not return to bulimic expression. This article invites readers to sense and feel pathways in, through, and out of unhealthy relationships with our bodies and ourselves