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R K Kawakami

Publications and source records attributed to R K Kawakami.

6 recordsLinked to original sources
PubMed2006

Impurity band conduction in a high temperature ferromagnetic semiconductor.

The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.

Journal Article↗
PubMed2004

Negative intrinsic resistivity of an individual domain wall in epitaxial (Ga,Mn)As microdevices.

Magnetic domains, and the boundaries that separate them (domain walls, DWs), play a central role in the science of magnetism. Understanding and controlling domains is important for many technological applications in spintronics, and may lead to new devices. Although theoretical efforts have elucidated several mechanisms underlying the resistance of a single DW, various experiments report conflicting results, even for the overall sign of the DW resistance. The question of whether an individual DW gives rise to an increase or decrease of the resistance therefore remains open. Here we report an approach to DW studies in a class of ferromagnetic semiconductors (as opposed to metals) that offer promise for spintronics. These experiments involve microdevices patterned from monocrystalline (Ga,Mn)As epitaxial layers. The giant planar Hall effect that we previously observed in this material enables direct, real-time observation of the propagation of an individual magnetic DW along multiprobe devices. We apply steady and pulsed magnetic fields, to trap and carefully position an individual DW within each separate device studied. This protocol reproducibly enables high-resolution magnetoresistance measurements across an individual wall. We consistently observe negative intrinsic DW resistance that scales with channel width. This appears to originate from sizeable quantum corrections to the magnetoresistance.

Journal Article↗
PubMed2003

Giant planar Hall effect in epitaxial (Ga,Mn)as devices.

Large Hall resistance jumps are observed in microdevices patterned from epitaxial (Ga,Mn)As layers when subjected to a swept, in-plane magnetic field. This giant planar Hall effect is 4 orders of magnitude greater than previously observed in metallic ferromagnets. This enables extremely sensitive measurements of the angle-dependent magnetic properties of (Ga,Mn)As. The magnetic anisotropy fields deduced from these measurements are compared with theoretical predictions.

Journal Article↗
PubMed2001

Ferromagnetic imprinting of nuclear spins in semiconductors.

We examine how a ferromagnetic layer affects the coherent electron spin dynamics in a neighboring gallium arsenide semiconductor. Ultrafast optical pump-probe measurements reveal that the spin dynamics are unexpectedly dominated by hyperpolarized nuclear spins that align along the ferromagnet's magnetization. We find evidence that photoexcited carriers acquire spin-polarization from the ferromagnet, and dynamically polarize these nuclear spins. The resulting hyperfine fields are as high as 9000 gauss in small external fields (less than 1000 gauss), enabling ferromagnetic control of local electron spin coherence.

Journal Article↗
PubMed1997

Predicting soft tissue changes in maxillary impaction surgery: a comparison of two video imaging systems.

The purpose of this retrospective study was to investigate the accuracy of two video imaging systems, Orthognathic Treatment Planner (OTP) and Prescription Portrait (Portrait), in predicting soft tissue profile changes after maxillary impaction surgery. Computer-generated line drawing predictions were compared with actual postsurgical profiles. Neither program was very accurate with vertical measures and lower lip contour. Portrait was more accurate at pronasale, inferior labial sulcus, and pogonion in the y-axis direction (P < 0.05). Video image predictions produced from the presurgical photographs were rated by orthodontists, surgeons, and lay people, who compared the predictions with the actual postsurgical photographs using a visual analog scale. Portrait's prediction images were scored higher than OTP's for five of eight areas. Orthodontists were most critical of the lips and the overall appearance. Lay people were most critical of the chin and submental areas.

Adult↗