318 research outputs found
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
Transport properties of quantum dots with hard walls
Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation
with an atomic force microscope. This technique, in combination with top gate
voltages, allows us to generate steep walls at the confining edges and small
lateral depletion lengths. The confinement is characterized by low-temperature
magnetotransport measurements, from which the dots' energy spectrum is
reconstructed. We find that in small dots, the addition spectrum can
qualitatively be described within a Fock-Darwin model. For a quantitative
analysis, however, a hard-wall confinement has to be considered. In large dots,
the energy level spectrum deviates even qualitatively from a Fock-Darwin model.
The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure
In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography
A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned.Comment: 4 pages, 3 figure
The dance of the positives and negatives of life: Student wellbeing in the context of #FeesMustFall-related violence
Violence has become a regular occurrence in the context of the South African student movement. However, to date there are no studies that investigate student wellbeing in the context of student movement violence. How are violence and wellbeing experienced in the context of the student movement? What resources do students draw upon to protect or restore their wellbeing during and in the aftermath of violence? This article discusses students’ experiences of violence and wellbeing and particularly the resources that fostered their psychological wellbeing during and after experiences of violence related to the #FeesMustFall protests of 2015/16. The study used a photovoice methodology and worked with groups of former student activists from four universities in South Africa. Student psychological functioning in the midst of adverse circumstances is demonstrated in their ability to make meaning of their experiences by inventing knowledge spaces; creating spaces of inner harmony; having a sense of purpose; and protecting symbolic spaces of hope. It is also shown in their courage to challenge spaces of oppression. This study suggests that the co-production of social space for functioning occurs through an important psychological process of meaning-making that provides direction for student movement activities. Given our findings, it is recommended for student affairs and counselling services to m,./=-09864 facilitate the co-creation of spaces of knowledge and spaces of safety, spaces where students can experience being wanted and connected, and spaces where they can make sense of their experiences in the unfamiliar and potentially alienating higher education space.
 
SSOR Preconditioning of Improved Actions
We generalize local lexicographic SSOR preconditioning for the
Sheikholeslami-Wohlert improved Wilson fermion action and the truncated perfect
free fermion action. In our test implementation we achieve performance gains as
known from SSOR preconditioning of the standard Wilson fermion action.Comment: 3 pages, Latex, 3 figures, Talk presented at Lattice'9
Precision Upsilon Spectroscopy from Nonrelativistic Lattice QCD
The spectrum of the Upsilon system is investigated using the Nonrelativistic
Lattice QCD approach to heavy quarks and ignoring light quark vacuum
polarization. We find good agreement with experiment for the Upsilon(1S),
Upsilon(2S), Upsilon(3S) and for the center of mass and fine structure of the
chi_b states. The lattice calculations predict b-bbar D-states with center of
mass at (10.20 +/- 0.07 +/- 0.03)GeV. Fitting procedures aimed at extracting
both ground and excited state energies are developed. We calculate a
nonperturbative dispersion mass for the Upsilon(1S) and compare with
tadpole-improved lattice perturbation theory.Comment: 8 pages, latex, SCRI-94-57, OHSTPY-HEP-T-94-00
Improved Pseudofermion Approach for All-Point Propagators
Quark propagators with arbitrary sources and sinks can be obtained more
efficiently using a pseudofermion method with a mode-shifted action.
Mode-shifting solves the problem of critical slowing down (for light quarks)
induced by low eigenmodes of the Dirac operator. The method allows the full
physical content of every gauge configuration to be extracted, and should be
especially helpful for unquenched QCD calculations. The method can be applied
for all the conventional quark actions: Wilson, Sheikoleslami-Wohlert,
Kogut-Susskind, as well as Ginsparg-Wilson compliant overlap actions. The
statistical properties of the method are examined and examples of physical
processes under study are presented.Comment: LateX, 26 pages, 10 eps figure
Results of a search for 2-decay of Xe with high-pressure copper proportional counters in Baksan Neutrino Observatory
The experiment for the 2-decay of Xe search with two
high-pressure copper proportional counters has been held in Baksan neutrino
observatory. The search for the process is based on comparison of spectra
measured with natural and enriched xenon. No evidence has been found for
2(2)- and 2(0)-decay. The decay half lifetime limit
based on data measured during 8000 h is Tyr for
2-mode and Tyr for 0-mode (90%C.L.).Comment: 9 pages, 8 figures; talk at the NANP'05 Conference; submitted to
Phys. At. Nuc
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