528 research outputs found
High-mobility two-dimensional hole gases in III-V semiconductor heterostructures: growth and transport properties
In this work, we investigated very high quality carbon-doped two-dimensional hole gases (2DHGs). The first part deal with high-mobility GaAs/AlGaAs quantum wells (QWs). Optimizing the heterostructure design, the hole mobility was extremely increased. Quantum Hall effect, photoconductivity effect, Rashba spin splitting, fractional quantum Hall effect (revealing interesting anisotropy in the thermally activated transport) and the band structure were investigated. In the second part, we studied InAs/InGaAs/InAlAs QWs with high spin-orbit coupling. A great success was the preparation of a carbon p-type doping in QWs with high indium content. A conductivity type inversion from p- to n-type with changing composition was observed. The heterostructures exhibit weak-antilocalization, hole-hole interaction effect and strong transport anisotropy. The spin splitting can be engineered providing small changes in the structure design. Both topics are of major interest for spintronics research
Identifying the needs of communities in rural Uganda: A method for determining the ‘User-Perceived Value’ of rural electrification initiatives
This research paper describes the use of a ‘User-Perceived Value Game’ to explore the value of development initiatives as perceived by villagers in 119 interview settings in seven Ugandan villages. Based on the findings from the game, a ‘User-Perceived Value Framework’ is developed, consisting of 64 value categories. This is depicted graphically as a ‘User-Perceived Value Wheel’ supported by a ‘Key Phrase Wheel’, both can be modified using computer-assisted software developed by one of the authors. The aim is to understand the reasons why something is perceived by the end user to be important. This will lead to an improved understanding of how a development initiative can be better tailored for lower-income markets. The initiative can then be marketed appropriately, which will result in user acceptance because the initiative will be perceived to have personal value to the user and therefore the user will care for its upkeep. The paper concludes with a brief application of the ‘User-Perceived Value Wheel’ to demonstrate how this tool can be used to better understand the true sustainability drivers behind rural electrification development initiatives.Engineering and Physical Sciences Research Council (Grant ID: EP/K503009/1), Qualcomm European Research Studentships in Technology (Grant ID: 1068
Editorial Cross-cultural Engagements in Decolonial Times: Subverting Euro-centric Structures, Epistemologies and Ontologies
Since 2016, I have been an active member of the Decolonising SOAS WG and in the last two years, I have been leading at SOAS an initiative on decolonising research in coordination with the SOAS Research & Enterprise Office
Resonant spin amplification of hole spin dynamics in two‐dimensional hole systems: experiment and simulation
Spins in semiconductor structures may allow for the realization of scalable quantum bit arrays, an essential
component for quantum computation schemes. Specifically, hole spins may be more suited for this purpose than electron
spins, due to their strongly reduced interaction with lattice nuclei, which limits spin coherence for electrons in quantum dots.
Here, we present resonant spin amplification (RSA) measurements, performed on a p-modulation doped GaAs-based quantum
well at temperatures below 500 mK. The RSA traces have a peculiar, butterfly-like shape, which stems from the initialization
of a resident hole spin polarization by optical orientation. The combined dynamics of the optically oriented electron and hole
spins are well-described by a rate equation model, and by comparison of experiment and model, hole spin dephasing times of
more than 70 ns are extracted from the measured data
Optical polarization of localized hole spins in p-doped quantum wells
The initialization of spin polarization in localized hole states is
investigated using time-resolved Kerr rotation. We find that the sign of the
polarization depends on the magnetic field, and the power and the wavelength of
the circularly polarized pump pulse. An analysis of the spin dynamics and the
spin-initialization process shows that two mechanisms are responsible for spin
polarization with opposite sign: The difference of the g factor between the
localized holes and the trions, as well as the capturing process of dark
excitons by the localized hole states.Comment: 4 pages, 2 figure
Controlling hole spin dynamics in two‐dimensional hole systems at low temperatures
With the recent discovery of very long hole spin decoherence times in GaAs/AlGaAs heterostructures of more than 70 ns
in two-dimensional hole systems, using the hole spin as a viable alternative to electron spins in spintronic applications seems
possible. Furthermore, as the hyperfine interaction with the nuclear spins is likely to be the limiting factor for electron spin
lifetimes in zero dimensions, holes with their suppressed Fermi contact hyperfine interaction due to their p-like nature should
be able to show even longer lifetimes than electrons. For spintronic applications, electric-field control of hole spin dynamics
is desirable.
Here, we report on time-resolved Kerr rotation and resonant spin amplification measurements on a two-dimensional hole
system in a p-doped GaAs/AlGaAs heterostructure. Via a semitransparent gate, we tune the charge density within the sample.
We are able to observe a change in the hole g factor, as well as in the hole spin dephasing time at high magnetic fields
Categorical generalisations of quantum double models
We show that every involutive Hopf monoid in a complete and finitely
cocomplete symmetric monoidal category gives rise to invariants of oriented
surfaces defined in terms of ribbon graphs. For every ribbon graph this yields
an object in the category, defined up to isomorphism, that depends only on the
homeomorphism class of the associated surface. This object is constructed via
(co)equalisers and images and equipped with a mapping class group action. It
can be viewed as a categorical generalisation of the ground state of Kitaev's
quantum double model or of a representation variety for a surface. We apply the
construction to group objects in cartesian monoidal categories, in particular
to simplicial groups as group objects in SSet and to crossed modules as group
objects in Cat. The former yields a simplicial set consisting of representation
varieties, the latter a groupoid whose sets of objects and morphisms are
obtained from representation varieties.Comment: 46 page
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