D. D. Awschalom

Design, fabrication, and performance of integrated miniature SQUID susceptometers

The design, construction, and performance of miniature SQUID (superconducting quantum interference device) susceptometers is discussed. Spins (in units of mu /sub beta /) per square root Hz has been identified as an important figure of merit. Simple expressions for S/sub n/ (spin sensitivity) in miniature SQUID susceptometers are developed and the implications of dimensional scaling explored. The details of several existing and proposed designs are reviewed, including versions that utilize commercial SQUIDs. With thin-film DC SQUIDs, S/sub n/ values of a few thousand spins/ square root Hz have already been obtained, and it is projected that values of a few hundred will be achieved soon. >

Low-noise modular microsusceptometer using nearly quantum limited dc SQUIDs

A flexible combination of superconducting integrated circuits was used to construct a low‐temperature magneto‐optic microsusceptometer utilizing a dc superconducting quantum inteference device (SQUID) detector operating near the quantum limit (coupled energy sensitivity of 1.7ℏ). Miniature pick‐up loop assemblies on transparent substrates were joined by superconducting interchip connections to a thin‐film dc SQUID, which is in turn read out by a second dc SQUID connected to room‐temperature electronics. Measurements on an 8.5‐μm‐diam titanium dot evaporated directly into the pick‐up loop demonstrate a spin sensitivity of ∼103 spins/(Hz)1/2 at T=290 mK.

Direct deposition of magnetic dots using a scanning tunneling microscope

A scanning tunneling microscope has been used to directly deposit nanometer‐scale structures into the input coil of a planar dc superconducting quantum interference device microsusceptometer. Iron pentacarbonyl was used as the source gas for the deposits, yielding dots with diameters ranging from 10 to 30 nm and heights from 30 to 100 nm. Measurements on the particles at low temperatures show them to be magnetic and reveal macroscopic spin properties.

Picosecond studies of electron confinement in simple colored glasses

Time‐resolved optical measurements at low temperatures reveal finite size effects for excitons trapped in semiconductor microcrystallites embedded in a simple borosilicate glass. The excitation lifetimes are significantly shorter than those in comparable bulk material, with a marked dependence on crystallite size. The results are interpreted as an effect of three‐dimensional confinement with ‘‘box’’ dimensions comparable to the exciton diameter.

Low‐temperature magnetic spectroscopy with a dc SQUID

New magneto‐optical methods combining integrated dc superconducting quantum interference device and fiber optic technology have allowed energy‐ and polarization‐dependent magnetization measurements to be made on a 10‐μm‐diam sample of Cd1−xMnxTe. Direct optically induced magnetization data probe the energetics of magnetic polaron formation and reveal the ultimate spin orientation with respect to the incident photon angular momentum.

Integrated magnetic spectroscopy of dilute magnetic semiconductors (invited)

An integrated dc SQUID magnetic spectrometer has been developed to obtain direct high‐resolution measurements of optically induced magnetization in a 10‐μm‐diam sample of Cd0.8Mn0.2Te. The static and picosecond dynamics of the magnetic response have been studied and are seen to be strikingly dependent on the energy and polarization of the optical excitation. The entire sample magnetization changes upon illumination, and the disturbed spins relax via an efficient spin coupling to the lattice.

Optical studies of diluted magnetic semiconductor superlattices

Diluted magnetic semiconductors exhibit a variety of interesting and unique optical phenomena generated by the strong spin exchange interaction between charge carriers and the magnetic ions in the materials. Recent successes in epitaxial growth of these compounds have enabled study of the effects of quantum confinement, and of size dependence of the magnetic interactions, on the physics of these systems. This review of optical studies discusses static and time-resolved measurements, both of luminescence and of optically induced magnetization. We focus on the wide-gap, II–VI zincblende crystal Cd1−xMnxTe which is, apart from the magnetic species, much like GaAs.

Picosecond magnetic spectroscopy with integrated DC SQUIDs

Advanced VLSI (very large scale integration) technology was used to fabricate ultraminiature integrated SQUID (superconducting quantum interference device) susceptometers. With the appropriate design parameters, the sensitivity of the devices approaches the quantum limit. The use of integrated circuits in conjunction with pulsed optical techniques allows magnetic systems to be probed with a picosecond time resolution. The response can be mapped out as a function of the energy of the optical excitation, providing detailed spectroscopic information. Applying these techniques to the study of II-VI dilute magnetic semiconductors has yielded new insight into the mechanics of magnetic polaron formation and the dynamics of the magnetic spins. First experiments were carried out on a small approximately 10*10*1- mu m/sup 3/ single crystal platelet of Cd/sub 0.8/Mn/sub 0.2/Te. The results of the time-averaged magnetic spectroscopy at two different temperatures are presented, displaying the magnetic response to optical excitation at constant intensity from a photon energy of 1.83 to 2.0 eV. >

Picosecond photoluminescence excitation spectroscopy of GaAs/AlGaAs quantum wells

A synchronous pair of pulsed dye lasers is used to obtain continuous photoexcitation energy spectra of quantum well luminescence at fixed time delays following carrier generation, with picosecond time resolution. A measurement of the energy‐dependent initial luminescence polarization of photoexcited carriers in GaAs/AlGaAs quantum wells, from a pulsed optical pumping experiment, is presented as an illustration of the technique.

Transient grating dynamics in Cd1−xMnxTe diluted magnetic semiconductor superlattices

The dynamics of carriers excited from both bulk and superlattice samples of Cd1−xMnxTe have been analyzed using a picosecond transient grating optical technique in a reflection geometry. Two distinct relaxation times were observed from the decays of orientational and carrier density gratings, yielding an upper bound on the ambipolar mobility of 6000 cm2/(Vu2009s). Measurements have been performed as functions of sample temperature, photon energy, and grating period.