arXiv subjects

R. C. Myers

Publications and source records attributed to R. C. Myers.

At least 37 records · Page 2Linked to original sources

arXiv2005

Spatial Imaging and Mechanical Control of Spin Coherence in Strained GaAs Epilayers

The effect of uniaxial tensile strain on spin coherence in n-type GaAs epilayers is probed using time-resolved Kerr rotation, photoluminescence, and optically-detected nuclear magnetic resonance spectroscopies. The bandgap, electron spin lifetime, electron g-factor, and nuclear quadrupole splitting are simultaneously imaged over millimeter scale areas of the epilayers for continuously varying values of strain. All-optical nuclear magnetic resonance techniques allow access to the strain induced nuclear quadrupolar resonance splitting in field regimes not easily addressable using conventional optically-detected nuclear magnetic resonance.

cond-mat.mtrl-sci↗

arXiv2005

Structural, electrical, and magneto-optical characterization of paramagnetic GaMnAs quantum wells

Growth of GaMnAs by molecular beam epitaxy is typically performed at low substrate temperatures (250C) and high As overpressures leading to the incorporation of excess As and Mn interstitials, which quench optical signals such as photoluminescence (PL). We report on optical-quality Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells (QWs) with x < 0.2% grown at a substrate temperature of 400C. Electrical and structural measurements demonstrate that this elevated temperature reduces As defects while allowing the substitutional incorporation of Mn into Ga sites. From a combination of Hall and secondary ion mass spectroscopy measurements we estimate that at least 70-90% of the Mn incorporates substitutionally in all samples studied. The incorporation behavior shows both a substrate temperature and QW width dependence. The lower defect density of these heterostructures, compared to typical lower temperature grown GaMnAs, enables the observation of both polarization-resolved PL and coherent electron spin dynamics, from which the conduction band exchange parameter is extracted. No evidence of long-range Mn spin coupling is observed, whereas negative effective Curie temperatures indicate spin heating due to photoexcitation. The electron spin lifetime is maximized for light Mn doping and shows little magnetic field dependence indicating that the Dyakonov-Perel mechanism is dominant in these structures. PL spectra reveal a low energy peak from shallow donors, which because of the paramagnetic behavior of its PL polarization, we ascribe to Mn interstitials.

cond-mat.mtrl-sci↗

arXiv2005

Enhancement of Spin Coherence using Q-factor Engineering in Semiconductor Microdisk Lasers

Semiconductor microcavities offer unique means of controlling light-matter interactions, which have led to the development of a wide range of applications in optical communications and inspired proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities - a new and promising research field - have revealed novel effects such as polarization beats, stimulated spin scattering, and giant Faraday rotation. Here, we study the electron spin dynamics in optically-pumped GaAs microdisk lasers with quantum wells (QWs) and interface-fluctuation quantum dots (QDs) in the active region. In particular, we address the question of how the electron spin dynamics are modified by the stimulated emission in the disks, and observe an enhancement of the spin lifetime when the optical excitation is in resonance with a high quality (Q ~ 5000) lasing mode. This resonant enhancement, contrary to what is expected from the Purcell effect observed in the cavities, is then manipulated by altering the cavity design and dimensions.

cond-mat.mtrl-sci↗

arXiv2005

Spatial imaging of the spin Hall effect and current-induced polarization in two-dimensional electron gases

Spin-orbit coupling in semiconductors relates the spin of an electron to its momentum and provides a pathway for electrically initializing and manipulating electron spins for applications in spintronics and spin-based quantum information processing. This coupling can be regulated with quantum confinement in semiconductor heterostructures through band structure engineering. Here we investigate the spin Hall effect and current-induced spin polarization in a two-dimensional electron gas confined in (110) AlGaAs quantum wells using Kerr rotation microscopy. In contrast to previous measurements, the spin Hall profile exhibits complex structure, and the current-induced spin polarization is out-of-plane. The experiments map the strong dependence of the current-induced spin polarization to the crystal axis along which the electric field is applied, reflecting the anisotropy of the spin-orbit interaction. These results reveal opportunities for tuning a spin source using quantum confinement and device engineering in non-magnetic materials.

cond-mat.mes-hall↗

arXiv2005

Electrical initialization and manipulation of electron spins in an L-shaped strained n-InGaAs channel

An L-shaped strained n-InGaAs channel is used to polarize and subsequently rotate electron spins solely by means of electric fields. Precession of electrically-excited spins in the absence of applied magnetic fields is directly observed by Kerr rotation microscopy. In addition, in-plane and out-of-plane components of the spin polarization in the channel are simultaneously imaged.

cond-mat.mes-hall↗

arXiv2005

Antiferromagnetic s-d exchange coupling in GaMnAs

Measurements of coherent electron spin dynamics in Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells with 0.0006% < x < 0.03% show an antiferromagnetic (negative) exchange bewteen s-like conduction band electrons and electrons localized in the d-shell of the Mn2+ impurities. The magnitude of the s-d exchange parameter, N0 alpha, varies as a function of well width indicative of a large and negative contribution due to kinetic exchange. In the limit of no quantum confinement, N0 alpha extrapolates to -0.09 +/- 0.03 eV indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs. Measurements of the polarization-resolved photoluminescence show strong discrepancy from a simple model of the exchange enhanced Zeeman splitting, indicative of additional complexity in the exchange split valence band.

cond-mat.mtrl-sci↗

arXiv2005

Electron spin interferometry using a semiconductor ring structure

A ring structure fabricated from GaAs is used to achieve interference of the net spin polarization of conduction band electrons. Optically polarized spins are split into two packets by passing through two arms of the ring in the diffusive transport regime. Optical pumping with circularly polarized light on one arm establishes dynamic nuclear polarization which acts as a local effective magnetic field on electron spins due to the hyperfine interaction. This local field causes one spin packet to precess faster than the other, thereby controlling the spin interference when the two packets are combined.

cond-mat.mes-hall↗

arXiv2004

Optoelectronic control of spin dynamics at near-THz frequencies in magnetically doped quantum wells

We use time-resolved Kerr rotation to demonstrate the optical and electronic tuning of both the electronic and local moment (Mn) spin dynamics in electrically gated parabolic quantum wells derived from II-VI diluted magnetic semiconductors. By changing either the electrical bias or the laser energy, the electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic field of 3 T and at a temperature of 5 K. The corresponding range of the electrically-tuned effective electron g-factor is an order of magnitude larger compared with similar nonmagnetic III-V parabolic quantum wells. Additionally, we demonstrate that such structures allow electrical modulation of local moment dynamics in the solid state, which is manifested as changes in the amplitude and lifetime of the Mn spin precession signal under electrical bias. The large variation of electron and Mn-ion spin dynamics is explained by changes in magnitude of the sp−d exchange overlap.

cond-mat.mes-hall↗

arXiv2004

Control of Electron Spin Coherence Using Landau Level Quantization in a Two-Dimensional Electron Gas

Time-resolved optical measurements of electron spin dynamics in modulation doped InGaAs quantum wells are used to explore electron spin coherence times and spin precession frequencies in a regime where an out of plane magnetic field quantizes the states of a two-dimensional electron gas into Landau levels. Oscillatory features in the transverse spin coherence time and effective g-factor as a function of applied magnetic field exhibit a correspondence with Shubnikov-de Haas oscillations, illustrating a coupling between spin and orbital eigenstates. We present a theoretical model in which inhomogeneous dephasing due to the population of different Landau levels limits the spin coherence time and captures the essential experimental results.

cond-mat.mes-hall↗

arXiv2004

Spin transfer and coherence in coupled quantum wells

Spin dynamics of optically excited electrons confined in asymmetric coupled quantum wells are investigated through time resolved Faraday rotation experiments. The inter-well coupling is shown to depend on applied electric field and barrier thickness. We observe three coupling regimes: independent spin precession in isolated quantum wells, incoherent spin transfer between single-well states, and coherent spin transfer in a highly coupled system. Relative values of the inter-well tunneling time, the electron spin lifetime, and the Larmor precession period appear to govern this behavior.

cond-mat.mes-hall↗

arXiv2004

Pinning a Domain Wall in (Ga,Mn)As with Focused Ion Beam Lithography

We utilize a focused beam of Ga+ ions to define magnetization pinning sites in a ferromagnetic epilayer of (Ga,Mn)As. The nonmagnetic defects locally increase the magneto-crystalline anisotropy energies, by which a domain wall is pinned at a given position. We demonstrate techniques for manipulating domain walls at these pinning sites as probed with the giant planar Hall effect (GPHE). By varying the magnetic field angle relative to the crystal axes, an upper limit is placed on the local effective anisotropy energy.

cond-mat.mes-hall↗

arXiv2004

Current induced electron spin polarization in strained semiconductors

The polarization of conduction electron spins due to an electrical current is observed in strained nonmagnetic semiconductors using static and time-resolved Faraday rotation. The density, lifetime, and orientation rate of the electrically-polarized spins are characterized by a combination of optical and electrical methods. In addition, the dynamics of the current-induced spins are investigated by utilizing electrical pulses generated from a photoconductive switch. These results demonstrate the possibility of a spin source for semiconductor spintronic devices without the use of magnetic materials.

cond-mat.mes-hall↗

arXiv2003

Tunable spin polarization in III-V quantum wells with a ferromagnetic barrier

We demonstrate the epitaxial growth of optical-quality electrically-gated III-V ferromagnetic quantum structures. Photoluminescence spectroscopy reveals that initially unpolarized photoexcited holes in a GaAs quantum well become spin-polarized opposite to the magnetization of an adjacent digital ferromagnetic layer in the AlGaAs barrier. A vertical bias is used to tune the spin polarization from -0.4 to 6.3 percent at T = 5 K and B = 1 kG during which the luminescence becomes quenched, indicating that the polarization is mediated by wave function overlap between heavy holes in the quantum well and Mn-ions in the barrier. Polarization is observed under negligible current flow and is insensitive to the initial spin orientation of the carriers, differentiating the effect from both electrical and optical spin injection.

cond-mat.mtrl-sci↗

arXiv2003

Local Manipulation of Nuclear Spin in a Semiconductor Quantum Well

The shaping of nuclear spin polarization profiles and the induction of nuclear resonances are demonstrated within a parabolic quantum well using an externally applied gate voltage. Voltage control of the electron and hole wave functions results in nanometer-scale sheets of polarized nuclei positioned along the growth direction of the well. RF voltages across the gates induce resonant spin transitions of selected isotopes. This depolarizing effect depends strongly on the separation of electrons and holes, suggesting that a highly localized mechanism accounts for the observed behavior.

cond-mat.mes-hall↗

arXiv2003

Independent Electronic and Magnetic Doping in (Ga,Mn)As Based Digital Ferromagnetic Heterostructures

Ferromagnetic semiconductors promise the extension of metal-based spintronics into a material system that combines widely tunable electronic, optical, and magnetic properties. Here, we take steps towards realizing that promise by achieving independent control of electronic doping in the ferromagnetic semiconductor (Ga,Mn)As. Samples are comprised of superlattices of 0.5 monolayer (ML) MnAs alternating with 20 ML GaAs and are grown by low temperature (230 C) atomic layer epitaxy (ALE). This allows for the reduction of excess As incorporation and hence the number of charge-compensating As-related defects. We grow a series of samples with either Be or Si doping in the GaAs spacers (p- and n-type dopants, respectively), and verify their structural quality by in situ reflection high-energy electron diffraction (RHEED) and ex situ x-ray diffraction. Magnetization measurements reveal ferromagnetic behavior over the entire doping range, and show no sign of MnAs precipitates. Finally, magneto-transport shows the giant planar Hall effect and strong (20%) resistance fluctuations that may be related to domain wall motion.

cond-mat.mtrl-sci↗

arXiv2000

World-Volume Potentials on D-branes

By evaluating string scattering amplitudes, we investigate various low energy interactions for the massless scalars on a nonabelian Dirichlet brane. We confirm the existence of couplings of closed string fields to the world-volume scalars, involving commutators of the latter. Our results are consistent with the recently proposed nonabelian world-volume actions for Dp-branes.

hep-th↗

arXiv2000

Fractional Branes and the Entropy of 4D Black Holes

We reconsider the four dimensional extremal black hole constructed in type IIB string theory as the bound state of D1-branes, D5-branes, momentum, and Kaluza-Klein monopoles. Specifically, we examine the case of an arbitrary number of monopoles. Consequently, the weak coupling calculation of the microscopic entropy requires a study of the D1-D5 system on an ALE space. We find that the complete expression for the Bekenstein-Hawking entropy is obtained by taking into account the massless open strings stretched between the fractional D-branes which arise in the orbifold limit of the ALE space. The black hole sector therefore arises as a mixed Higgs-Coulomb branch of an effective 1+1 dimensional gauge theory.

hep-th↗

arXiv2000

A Naturally Small Cosmological Constant on the Brane?

There appears to be no natural explanation for the cosmological constant's small size within the framework of local relativistic field theories. We argue that the recently-discussed framework for which the observable universe is identified with a p-brane embedded within a higher-dimensional `bulk' spacetime, has special properties that may help circumvent the obstacles to this understanding. This possibility arises partly due to several unique features of the brane proposal. These are: (1) the potential such models introduce for partially breaking supersymmetry, (2) the possibility of having low-energy degrees of freedom which are not observable to us because they are physically located on a different brane, (3) the fundamental scale may be much smaller than the Planck scale. Furthermore, although the resulting cosmological constant in the scenarios we outline is naturally suppressed by weak coupling constants of gravitational strength, it need not be exactly zero, raising the possibility it could be in the range favoured by recent cosmological observations.

hep-th↗