Bostonia. Volume 12

Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs

Fundamentals of Focused Ion Beam Nanostructural Processing: Below, At, and Above the Surface

This article considers the fundamentals of what happens in a solid when it is impacted by a medium-energy gallium ion. The study of the ion/sample interaction at the nanometer scale is applicable to most focused ion beam (FIB)–based work even if the FIB/sample interaction is only a step in the process, for example, micromachining or microelectronics device processing.Whereas the objective in other articles in this issue is to use the FIB tool to characterize a material or to machine a device or transmission electron microscopy sample, the goal of the FIB in this article is to have the FIB/sample interaction itself become the product.To that end, the FIB/sample interaction is considered in three categories according to geometry:below, at, and above the surface. First, the FIB ions can penetrate the top atom layer(s) and interact below the surface. Ion implantation and ion damage on flat surfaces have been comprehensively examined; however, FIB applications require the further investigation of high doses in three-dimensional profiles.Second, the ions can interact at the surface, where a morphological instability can lead to ripples and surface self-organization, which can depend on boundary conditions for site-specific and compound FIB processing. Third, the FIB may interact above the surface (and/or produce secondary particles that interact above the surface).Such ion beam–assisted deposition, FIB–CVD (chemical vapor deposition), offers an elaborate complexity in three dimensions with an FIB using a gas injection system. At the nanometer scale, these three regimes—below, at, and above the surface—can require an interdependent understanding to be judiciously controlled by the FIB.Engineering and Applied Science

Loss of PBRM1 rescues VHL dependent replication stress to promote renal carcinogenesis

AbstractInactivation of the VHL (Von Hippel Lindau) tumour suppressor has long been recognised as necessary for the pathogenesis of clear cell renal cancer (ccRCC); however, the molecular mechanisms underlying transformation and the requirement for additional genetic hits remain unclear. Here, we show that loss of VHL alone results in DNA replication stress and damage accumulation, effects that constrain cellular growth and transformation. By contrast, concomitant loss of the chromatin remodelling factor PBRM1 (mutated in 40% of ccRCC) rescues VHL-induced replication stress, maintaining cellular fitness and allowing proliferation. In line with these data we demonstrate that combined deletion of Vhl and Pbrm1 in the mouse kidney is sufficient for the development of fully-penetrant, multifocal carcinomas, closely mimicking human ccRCC. Our results illustrate how VHL and PBRM1 co-operate to drive renal transformation and uncover replication stress as an underlying vulnerability of all VHL mutated renal cancers that could be therapeutically exploited.</jats:p

Enhanced stability of layered phases in parallel hard-spherocylinders due to the addition of hard spheres

There is increasing evidence that entropy can induce microphase separation in binary fluid mixtures interacting through hard particle potentials. One such phase consists of alternating two dimensional liquid-like layers of rods and spheres. We study the transition from a uniform miscible state to this ordered state using computer simulations and compare results to experiments and theory. We conclude that (1) there is stable entropy driven microphase separation in mixtures of parallel rods and spheres, (2) adding spheres smaller then the rod length decreases the total volume fraction needed for the formation of a layered phase, therefore small spheres effectively stabilize the layered phase; the opposite is true for large spheres and (3) the degree of this stabilization increases with increasing rod length.Comment: 11 pages, 9 figures. Submitted to Phys. Rev. E. See related website http://www.elsie.brandeis.ed

Nanoscale anisotropic structural correlations in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5 MnO3

We report x-ray scattering studies of short-range structural correlations and diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5MnO3.Comment: 5 pages, 4 embedded figure

A wide deep infrared look at the Pleiades with UKIDSS: new constraints on the substellar binary fraction and the low mass IMF

We present the results of a deep wide-field near-infrared survey of 12 square degrees of the Pleiades conducted as part of the UKIDSS Deep Infrared Sky Survey (UKIDSS) Galactic Cluster Survey (GCS). We have extracted over 340 high probability proper motion members down to 0.03 solar masses using a combination of UKIDSS photometry and proper motion measurements obtained by cross-correlating the GCS with data from the Two Micron All Sky Survey (2MASS), the Isaac Newton (INT) and the Canada-France-Hawai'i (CFHT) telescopes. Additionally, we have unearthed 73 new candidate brown dwarf members on the basis of five band UKIDSS photometry alone. We have identified 23 substellar multiple system candidates out of 63 candidate brown dwarfs from the (Y-K,Y) and (J-K,J) colour-magnitude diagrams, yielding a binary frequency of 28-44% in the 0.075-0.030 Msun mass range. Our estimate is three times larger than the binary fractions reported from high-resolution imaging surveys of field ultracool dwarfs and Pleiades brown dwarfs. However, it is marginally consistent with our earlier ``peculiar'' photometric binary fraction of 50+/-10% presented in Pinfield et al. (2003), in good agreement with the 32-45% binary fraction derived from the recent Monte-Carlo simulations of Maxted & Jeffries (2005) and compatible with the 26+/-10% frequency recently estimated by Basri & Reiners (2006). A tentative estimate of the mass ratios from photometry alone seems to support the hypothesis that binary brown dwarfs tend to reside in near equal-mass ratio systems. (abridged)Comment: 21 pages, 8 figures, 6 tables, 1 electronic table, 6 appendices with tables, accepted to MNRA