11,429 research outputs found
Tunneling between Dilute GaAs Hole Layers
We report interlayer tunneling measurements between very dilute
two-dimensional GaAs hole layers. Surprisingly, the shape and
temperature-dependence of the tunneling spectrum can be explained with a Fermi
liquid-based tunneling model, but the peak amplitude is much larger than
expected from the available hole band parameters. Data as a function of
parallel magnetic field reveal additional anomalous features, including a
recurrence of a zero-bias tunneling peak at very large fields. In a
perpendicular magnetic field, we observe a robust and narrow tunneling peak at
total filling factor , signaling the formation of a bilayer quantum
Hall ferromagnet.Comment: Revised to include additional data, new discussion
A preliminary discussion of gravitational physics experiments for the Spacelab era
An overview of past, present, and proposed future experiments in gravitational physics is given. These experiments are concerned with the measurement of relativistic gravity effects to test theories of gravitation. Certain experiments which could be performed on shuttle and Spacelab missions and the potential of Spacelab for gravitation physics research are discussed
Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems
We report measurements and calculations of the spin-subband depopulation,
induced by a parallel magnetic field, of dilute GaAs two-dimensional (2D) hole
systems. The results reveal that the shape of the confining potential
dramatically affects the values of in-plane magnetic field at which the upper
spin subband is depopulated. Most surprisingly, unlike 2D electron systems, the
carrier-carrier interaction in 2D hole systems does not significantly enhance
the spin susceptibility. We interpret our findings using a multipole expansion
of the spin density matrix, and suggest that the suppression of the enhancement
is related to the holes' band structure and effective spin j=3/2.Comment: 6 pages, 4 figures, substantially extended discussion of result
Spin precession and alternating spin polarization in spin-3/2 hole systems
The spin density matrix for spin-3/2 hole systems can be decomposed into a
sequence of multipoles which has important higher-order contributions beyond
the ones known for electron systems [R. Winkler, Phys. Rev. B \textbf{70},
125301 (2004)]. We show here that the hole spin polarization and the
higher-order multipoles can precess due to the spin-orbit coupling in the
valence band, yet in the absence of external or effective magnetic fields. Hole
spin precession is important in the context of spin relaxation and offers the
possibility of new device applications. We discuss this precession in the
context of recent experiments and suggest a related experimental setup in which
hole spin precession gives rise to an alternating spin polarization.Comment: 4 pages, 2 figures, to appear in Physical Review Letter
Optical orientation of electron spins in GaAs quantum wells
We present a detailed experimental and theoretical analysis of the optical
orientation of electron spins in GaAs/AlAs quantum wells. Using time and
polarization resolved photoluminescence excitation spectroscopy, the initial
degree of electron spin polarization is measured as a function of excitation
energy for a sequence of quantum wells with well widths between 63 Ang and 198
Ang. The experimental results are compared with an accurate theory of excitonic
absorption taking fully into account electron-hole Coulomb correlations and
heavy-hole light-hole coupling. We find in wide quantum wells that the measured
initial degree of polarization of the luminescence follows closely the spin
polarization of the optically excited electrons calculated as a function of
energy. This implies that the orientation of the electron spins is essentially
preserved when the electrons relax from the optically excited high-energy
states to quasi-thermal equilibrium of their momenta. Due to initial spin
relaxation, the measured polarization in narrow quantum wells is reduced by a
constant factor that does not depend on the excitation energy.Comment: 12 pages, 9 figure
Hole spin dynamics and hole factor anisotropy in coupled quantum well systems
Due to its p-like character, the valence band in GaAs-based heterostructures
offers rich and complex spin-dependent phenomena. One manifestation is the
large anisotropy of Zeeman spin splitting. Using undoped, coupled quantum wells
(QWs), we examine this anisotropy by comparing the hole spin dynamics for high-
and low-symmetry crystallographic orientations of the QWs. We directly measure
the hole factor via time-resolved Kerr rotation, and for the low-symmetry
crystallographic orientations (110) and (113a), we observe a large in-plane
anisotropy of the hole factor, in good agreement with our theoretical
calculations. Using resonant spin amplification, we also observe an anisotropy
of the hole spin dephasing in the (110)-grown structure, indicating that
crystal symmetry may be used to control hole spin dynamics
Shuttle time and frequency transfer experiment
A proposed space shuttle experiment to demonstrate techniques for global high precision comparison of clocks and primary frequency standards is described. The experiment, using transmitted microwave and pulsed laser signals, compared a hydrogen maser clock onboard the space shuttle with a clock in a ground station in order to demonstrate time transfer with accuracies of 1 nsec or better and frequency comparison at the 10 to the -14th power accuracy level
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