55 research outputs found
Mapping the wavefunction of transition metal acceptor states in the GaAs surface
We utilize a single atom substitution technique with spectroscopic imaging in
a scanning tunneling microscope (STM) to visualize the anisotropic spatial
structure of magnetic and non-magnetic transition metal acceptor states in the
GaAs (110) surface. The character of the defect states play a critical role in
the properties of the semiconductor, the localization of the states influencing
such things as the onset of the metal-insulator transition, and in dilute
magnetic semiconductors the mechanism and strength of magnetic interactions
that lead to the emergence of ferromagnetism. We study these states in the GaAs
surface finding remarkable similarities between the shape of the acceptor state
wavefunction for Mn, Fe, Co and Zn dopants, which is determined by the GaAs
host and is generally reproduced by tight binding calculations of Mn in bulk
GaAs [Tang, J.M. & Flatte, M.E., Phys. Rev. Lett. 92, 047201 (2004)]. The
similarities originate from the antibonding nature of the acceptor states that
arise from the hybridization of the impurity d-levels with the host. A second
deeper in-gap state is also observed for Fe and Co that can be explained by the
symmetry breaking of the surface.Comment: 19 pages, 6 figure
Atom-by-Atom Substitution of Mn in GaAs and Visualization of their Hole-Mediated Interactions
The discovery of ferromagnetism in Mn doped GaAs [1] has ignited interest in
the development of semiconductor technologies based on electron spin and has
led to several proof-of-concept spintronic devices [2-4]. A major hurdle for
realistic applications of (Ga,Mn)As, or other dilute magnetic semiconductors,
remains their below room-temperature ferromagnetic transition temperature.
Enhancing ferromagnetism in semiconductors requires understanding the
mechanisms for interaction between magnetic dopants, such as Mn, and
identifying the circumstances in which ferromagnetic interactions are maximized
[5]. Here we report the use of a novel atom-by-atom substitution technique with
the scanning tunnelling microscope (STM) to perform the first controlled atomic
scale study of the interactions between isolated Mn acceptors mediated by the
electronic states of GaAs. High-resolution STM measurements are used to
visualize the GaAs electronic states that participate in the Mn-Mn interaction
and to quantify the interaction strengths as a function of relative position
and orientation. Our experimental findings, which can be explained using
tight-binding model calculations, reveal a strong dependence of ferromagnetic
interaction on crystallographic orientation. This anisotropic interaction can
potentially be exploited by growing oriented Ga1-xMnxAs structures to enhance
the ferromagnetic transition temperature beyond that achieved in randomly doped
samples. Our experimental methods also provide a realistic approach to create
precise arrangements of single spins as coupled quantum bits for memory or
information processing purposes
Fusion Assessment by MRI in Comparison With CT in Anterior Lumbar Interbody Fusion: A Prospective Study
Copyright The Author(s) 2018
Creative Commons Non Commercial No Derivs CC BY-NC-ND: This article is distributed under the terms of the Creative Commons Attribution-Non Commercial-NoDerivs 4.0 License (http://www.creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, reproduction and distribution of the work as published without adaptation or alteration, without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).Study Design:
Prospective cohort study.
Objectives:
To evaluate the role of magnetic resonance imaging (MRI) in evaluation of fusion status following anterior lumbar interbody fusion (ALIF) and compare agreement and confidence in assessing fusion or its absence on MRI to the current standard computed tomography (CT).
Methods:
A prospective follow up of patients undergoing surgery by 2 spine surgeons between 2012 and 2015 at a single institution. Fusion was assessed at different time points in these patients by 2 independent musculoskeletal radiologists. Fusion was analyzed in coronal and sagittal planes using both imaging modalities, with confidence being attributed on a scale of 0 to 3. Assessors were blinded to patient data.
Results:
Fourteen patients (25 levels) with mean follow-up of 10.2 months (range 2.4-20.3 years) and age of 41 years (range 20.7-61.5 years) were assessed. MRI within the interbody cage in coronal (κ = .58) and sagittal (κ = .50) planes had the highest interobserver agreement. CT anterior to the cage in coronal (κ = .48) and sagittal (κ = .44) planes, as well as within the cage in coronal (κ = .50) and sagittal planes (κ = .44) showed moderate agreement. Confidence anterior to the interbody cage using MRI scan was reduced when compared with remaining angles and imaging modalities.
Conclusions:
The study demonstrates that MRI may be a useful tool in the assessment of fusion following ALIF with results comparable to CT, and that it may have a useful role in select patients especially considering marked radiation exposure reduction
Iodine-125 brachytherapy for brain tumours - a review
Iodine-125 brachytherapy has been applied to brain tumours since 1979. Even though the physical and biological characteristics make these implants particularly attractive for minimal invasive treatment, the place for stereotactic brachytherapy is still poorly defined
Calling where it counts:Subordinate pied babblers target the audience of their vocal advertisements
For territorial group-living species, opportunities to reproduce on the natal territory can be limited by a number of factors including the availability of resources within a territory, access to unrelated individuals, and monopolies on reproduction by dominant group members. Individuals looking to reproduce are therefore faced with the options of either waiting for a breeding opportunity to arise in the natal territory, or searching for reproductive opportunities in non-natal groups. In the cooperatively breeding Southern pied babbler, Turdoides bicolor, most individuals who achieve reproductive success do so through taking up dominant breeding positions within non-natal groups. For subordinate pied babblers therefore, searching for breeding opportunities in non-natal groups is of primary importance as this represents the major route to reproductive success. However, prospecting (where individuals leave the group to search for reproductive opportunities within other groups) is costly and individuals rapidly lose weight when not part of a group. Here we demonstrate that subordinate pied babblers adopt an alternative strategy for mate attraction by vocal advertisement from within their natal territories. We show that subordinates focus their calling efforts on the edges of their territory, and specifically near boundaries with neighbouring groups that have potential breeding partners (unrelated individuals of the opposite sex). In contrast to prospecting, calling individuals showed no body mass loss associated with this behaviour, suggesting that calling from within the group may provide a 'cheap' advertisement strategy. Additionally, we show that subordinates use information regarding the composition of neighbouring groups to target the greatest number of potential mating partners
Atom-by-Atom Substitution of Transition Metals in Gallium Arsenide and Visualization of Hole-Mediated Interactions
105 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006.We also present the first controlled atomic scale study of the interactions between isolated Mn impurities mediated by electronic states in GaAs. High-resolution STM measurements provide visualization of the GaAs electronic states that participate in Mn-Mn interactions. We quantify the interaction strengths between Mn pairs as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga1-xMnxAs structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD
Atom-by-Atom Substitution of Transition Metals in GaAs and Visualization of Hole-Mediated Interactions
The discovery of ferromagnetism in Mn doped InAs and GaAs has ignited interest in the development of semiconductor technologies based on the electron spin. A major hurdle remaining for realistic applications of ferromagnetic semiconductors, such as Ga(1-x)Mn(x)As, is their below room-temperature ferromagnetic transition temperature. Enhancing ferromagnetism in semiconductors requires understanding the mechanisms for interactions between magnetic dopants and identifying the circumstances that maximize ferromagnetic interactions.
In this thesis, we present a novel atom-by-atom substitution technique with the scanning tunnelling microscope (STM) to controllably incorporate transition metal dopants into GaAs. We compare the electronic states of isolated single acceptors in an identical configuration - Ga sites in the top layer of a GaAs surface. The acceptor levels and anisotropic shape of the hole states for manganese, iron, cobalt; and zinc are determined with STM topography and spectroscopy. The manganese acceptor has a deeper acceptor level than the nonmagnetic
zinc acceptor. The iron and cobalt acceptors have two acceptor levels that are complementary in their spatial distribution. We discuss the influence of the GaAs band structure and the p-d hybridization on the hole states. In addition, we probe the Mn acceptor in n-type and p-type GaAs to understand the role of tip-induced band bending in our experiments.
We also present the first controlled atomic scale study of the interactions between isolated Mn impurities mediated by electronic states in GaAs. High-resolution STM measurements provide visualization of the GaAs electronic states that participate in Mn-Mn interactions. We quantify the interaction strengths between Mn pairs as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga(1-x)Mn(x)As structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples.U of I Onlythesis/dissertatio
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