High-quality epitaxial Ge layers for GaAs/Ge/GaAs heterostructures were grown in situ in an arsenic-free environment on (100) off-oriented GaAs substrates using two separate molecular beam epitaxy (MBE) chambers, connected via vacuum transfer chamber. The structural, morphological, and band offset properties of these heterostructures are investigated. Reflection high energy electron diffraction studies exhibited (2 Â 2) Ge surface reconstruction after the growth at 450 C and also revealed a smooth surface for the growth of GaAs on Ge. High-resolution triple crystal x-ray rocking curve demonstrated high-quality Ge epilayer as well as GaAs/Ge/(001)GaAs heterostructures by observing Pendell€ osung oscillations and that the Ge epilayer is pseudomorphic. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of $0.45 nm and room temperature photoluminescence spectroscopy exhibited direct bandgap emission at 1583 nm. Dynamic secondary ion mass spectrometry depth profiles of Ga, As, and Ge display a low value of Ga, As, and Ge intermixing at the Ge/GaAs interface and a transition between Ge/GaAs of less than 15 nm. The valence band offset at the upper GaAs/Ge-(2 Â 2) and bottom Ge/(001)GaAs-(2 Â 4) heterointerface of GaAs/Ge/GaAs double heterostructure is about 0.20 eV and 0.40 eV, respectively. Thus, the high-quality heterointerface and band offset for carrier confinement in MBE grown GaAs/Ge/GaAs heterostructures offer a promising candidate for Ge-based p-channel high-hole mobility quantum well field effect transistors. V

A little more than a decade ago, MIT initiated a program of Freshman Advisor Seminars (FAS) in response to the need for improved academic advising and intellectual diversity in the freshman year. FAS build on a "very small group" learning model by bringing eight advisees together with an advisor (normally a faculty member) and an upperclass associate advisor for a weekly, informal seminar on one of a wide variety of leader-chosen topics. FAS have become the dominant mode of first year advising at MIT, with approximately 90% of the freshman class enrolled in 135 different seminars during the 1996-97 Academic Year. This paper introduces the MIT FAS program, sets forth its origins and its goals, and presents a case study of its implementation within one MIT academic department. Some lessons learned are extracted from the study.

.; Kini, R. N.; Gifford, A.; et al., "Relaxation of photoinduced spins and carriers in ferromagnetic InMnSb films," Appl. Phys. Lett. 90, 143109 (2007); http:// dx

Lanthanide-doped upconverting nanoparticles (UCNPs) have the ability to convert low energy photons into high energy photons, making this material appealing for a variety of scientific pursuits, from solar energy conversion to bioimaging.

We measured time-resolved differential transmission in InMnAs for different pump/probe schemes as a function of temperature, laser fluence, and external magnetic field. We observed tunability of the carrier relaxation time. In addition, we found that the sign of the differential transmission changed as a function of probe wavelength. The electronic structure for InMnAs was calculated for B = 0, using an eight-band k·p model, which includes conduction and valence band mixing as well as coupling of electrons and holes to the magnetic Mn impurities. This allows us to explain some of the carrier dynamics and the sign changes in the differential transmission.