Transition disks

Figure 9.1 Examples of spectral energy distributions from young Sun-like stars with circumstellar dust disks. Optically thick dust disks (solid line) have excess emission relative to the stellar photosphere over a broad wavelength range, from near-infrared to millimeter wavelengths. Transition disks (dashed line) lack near-infrared excess emission, but have large mid- and far-infrared emission. Debris disks (dotted line) have small excess emission starting at wavelengths typically longer than 10 pm. Primordial and transition disks often show a prominent 10 pm silicate emission feature from warm dust grains in the disk atmosphere.

The Infrared Spectrograph (IRS) provided crucial data to characterize the mid-infrared rise in the spectral energy distribution of transition disks, which bear... 

Figure Bl.19.22. Magnetic force microscopy image of an 8 pm wide track on a magnetic disk. The bit transitions are spaced every 2 pm along the track. Arrows point to the edges of the DC-erased region. (Taken from [109], figure 7.)...

Alder B J and Wainwright T E 1957 Phase transition for a hard sphere system J. Chem. Phys. 27 1208-9 Alder B J and Wainwright T E 1962 Phase transition in elastic disks Phys. Rev. 127 359-61... 

Note that large density fluctuations are suppressed by construction in a random lattice model. In order to include them, one could simply simulate a mixture of hard disks with internal conformational degrees of freedom. Very simple models of this kind, where the conformational degrees of freedom affect only the size or the shape of the disks, have been studied by Fraser et al. [206]. They are found to exhibit a broad spectrum of possible phase transitions. 

FIG. 16 Phase diagram of fluid vesicles as a function of pressure increment p and bending rigidity A. Solid lines denote first-order transitions, dotted lines compressibility maxima. The transition between the prolate vesicles and the stomatocytes shows strong hysteresis efifects, as indicated by the error bars. Dashed line (squares) indicates a transition from metastable prolate to metastable disk-shaped vesicles. (From Gompper and KroU 1995 [243]. Copyright 1995 APS.)... 

One prominent example of rods with a soft interaction is Gay-Berne particles. Recently, elastic properties were calculated [89,90]. Using the classical Car-Parrinello scheme, the interactions between charged rods have been considered [91]. Concerning phase transitions, the sohd-fluid equihbria for hard dumbbells that interact additionally with a quadrupolar force was considered [92], as was the nematic-isotropic transition in a fluid of dipolar hard spherocylinders [93]. The influence of an additional attraction on the phase behavior of hard spherocylinders was considered by Bolhuis et al. [94]. The gelation transition typical for clays was found in a system of infinitely thin disks carrying point quadrupoles [95,96]. In confined hquid-crystalline films tilted molecular layers form near each wall [97]. Chakrabarti has found simulation evidence of critical behavior of the isotropic-nematic phase transition in a porous medium [98]. 

Due to new opportunities and continued computer min-iaturizations (e.g., laptops replacing desktops and desktops getting smaller), the HDD is in transition to smaller form factors. The 2.5 in. and smaller disk sizes are rapidly growing. In 2004, a quarter of HDD shipments were 2.5 in. or smaller. By 2008 these smaller form factor drives could count for about one-half of HDD shipments. 

The present chapter will focus on the practical, nuts and bolts aspects of this particular CA approach to modeling. In later chapters we will describe a variety of applications of these CA models to chemical systems, emphasizing applications involving solution phenomena, phase transitions, and chemical kinetics. In order to prepare readers for the use of CA models in teaching and research, we have attempted to present a user-friendly description. This description is accompanied by examples and hands-on calculations, available on the compact disk that comes with this book. The reader is encouraged to use this means to assimilate the basic aspects of the CA approach described in this chapter. More details on the operation of the CA programs, when needed, can be found in Chapter 10 of this book. 

The sources most commonly used so far consisted of sintered disks containing about 100 mg ZnO enriched with 90% Zn. The disks were irradiated with 12 MeV deuterons or 30 MeV He particles, to yield the 78 h activity of Ga, and then annealed by heating in oxygen to 700-1,000 K for about 12 h and cooling down slowly (about 50 K h ) to room temperature. A Nal scintillation counter, 2-3 mm thick, is suitable for the detection of the 93 keV y-rays. Because of the relatively high transition energy, both source and absorber are generally kept at liquid helium temperature. 

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