1,340 research outputs found
Zero-field spin splitting in InAs-AlSb quantum wells revisited
We present magnetotransport experiments on high-quality InAs-AlSb quantum
wells that show a perfectly clean single-period Shubnikov-de Haas oscillation
down to very low magnetic fields. In contrast to theoretical expectations based
on an asymmetry induced zero-field spin splitting, no beating effect is
observed. The carrier density has been changed by the persistent photo
conductivity effect as well as via the application of hydrostatic pressure in
order to influence the electric field at the interface of the electron gas.
Still no indication of spin splitting at zero magnetic field was observed in
spite of highly resolved Shubnikov- de Haas oscillations up to filling factors
of 200. This surprising and unexpected result is discussed in view of other
recently published data.Comment: 4 pages, 3 figures, submitted to Phys. Rev.
Laetoli's lost tracks: 3D generated mean shape and missing footprints.
The Laetoli site (Tanzania) contains the oldest known hominin footprints, and their interpretation remains open to debate, despite over 35 years of research. The two hominin trackways present are parallel to one another, one of which is a composite formed by at least two individuals walking in single file. Most researchers have focused on the single, clearly discernible G1 trackway while the G2/3 trackway has been largely dismissed due to its composite nature. Here we report the use of a new technique that allows us to decouple the G2 and G3 tracks for the first time. In so doing we are able to quantify the mean footprint topology of the G3 trackway and render it useable for subsequent data analyses. By restoring the effectively 'lost' G3 track, we have doubled the available data on some of the rarest traces directly associated with our Pliocene ancestors
Effect of hydrogen on ground state structures of small silicon clusters
We present results for ground state structures of small SiH (2 \leq
\emph{n} \leq 10) clusters using the Car-Parrinello molecular dynamics. In
particular, we focus on how the addition of a hydrogen atom affects the ground
state geometry, total energy and the first excited electronic level gap of an
Si cluster. We discuss the nature of bonding of hydrogen in these
clusters. We find that hydrogen bonds with two silicon atoms only in SiH,
SiH and SiH clusters, while in other clusters (i.e. SiH,
SiH, SiH, SiH, SiH and SiH) hydrogen is bonded
to only one silicon atom. Also in the case of a compact and closed silicon
cluster hydrogen bonds to the cluster from outside. We find that the first
excited electronic level gap of Si and SiH fluctuates as a function
of size and this may provide a first principles basis for the short-range
potential fluctuations in hydrogenated amorphous silicon. Our results show that
the addition of a single hydrogen can cause large changes in the electronic
structure of a silicon cluster, though the geometry is not much affected. Our
calculation of the lowest energy fragmentation products of SiH clusters
shows that hydrogen is easily removed from SiH clusters.Comment: one latex file named script.tex including table and figure caption.
Six postscript figure files. figure_1a.ps and figure_1b.ps are files
representing Fig. 1 in the main tex
Ethical issues in the use of in-depth interviews: literature review and discussion
This paper reports a literature review on the topic of ethical issues in in-depth interviews. The review returned three
types of article: general discussion, issues in particular studies, and studies of interview-based research ethics. Whilst
many of the issues discussed in these articles are generic to research ethics, such as confidentiality, they often had particular
manifestations in this type of research. For example, privacy was a significant problem as interviews sometimes
probe unexpected areas. For similar reasons, it is difficult to give full information of the nature of a particular interview
at the outset, hence informed consent is problematic. Where a pair is interviewed (such as carer and cared-for) there are
major difficulties in maintaining confidentiality and protecting privacy. The potential for interviews to harm participants
emotionally is noted in some papers, although this is often set against potential therapeutic benefit. As well as
these generic issues, there are some ethical issues fairly specific to in-depth interviews. The problem of dual role is noted
in many papers. It can take many forms: an interviewer might be nurse and researcher, scientist and counsellor, or
reporter and evangelist. There are other specific issues such as taking sides in an interview, and protecting vulnerable
groups. Little specific study of the ethics of in-depth interviews has taken place. However, that which has shows some
important findings. For example, one study shows participants are not averse to discussing painful issues provided they
feel the study is worthwhile. Some papers make recommendations for researchers. One such is that they should consider
using a model of continuous (or process) consent rather than viewing consent as occurring once, at signature, prior
to the interview. However, there is a need for further study of this area, both philosophical and empirical
Liquefaction features produced by the 2010-2011 Canterbury earthquake sequence in southwest Christchurch, New Zealand, and preliminary assessment of Paleoliquefaction features
Liquefaction features and the geologic environment in which they formed were carefully studied at two sites near Lincoln in southwest Christchurch. We undertook geomorphic mapping, excavated trenches, and obtained hand cores in areas with surficial evidence for liquefaction and areas where no surficial evidence for liquefaction was present at two sites (Hardwick and Marchand). The liquefaction features identified include (1) sand blows (singular and aligned along linear fissures), (2) blisters or injections of subhorizontal dikes into the topsoil, (3) dikes related to the blows and blisters, and (4) a collapse structure. The spatial distribution of these surface liquefaction features correlates strongly with the ridges of scroll bars in meander settings. In addition, we discovered paleoliquefaction features, including several dikes and a sand blow, in excavations at the sites of modern liquefaction. The paleoliquefaction event at the Hardwick site is dated at A.D. 908-1336, and the one at the Marchand site is dated at A.D. 1017-1840 (95% confidence intervals of probability density functions obtained by Bayesian analysis). If both events are the same, given proximity of the sites, the time of the event is A.D. 1019-1337. If they are not, the one at the Marchand site could have been much younger. Taking into account a preliminary liquefaction-triggering threshold of equivalent peak ground acceleration for an Mw 7.5 event (PGA7:5) of 0:07g, existing magnitude-bounded relations for paleoliquefaction, and the timing of the paleoearthquakes and the potential PGA7:5 estimated for regional faults, we propose that the Porters Pass fault, Alpine fault, or the subduction zone faults are the most likely sources that could have triggered liquefaction at the study sites. There are other nearby regional faults that may have been the source, but there is no paleoseismic data with which to make the temporal link
Overturning established chemoselectivities : selective reduction of arenes over malonates and cyanoacetates by photoactivated organic electron donors
The prevalence of metal-based reducing reagents, including metals, metal complexes, and metal salts, has produced an empirical order of reactivity that governs our approach to chemical synthesis. However, this reactivity may be influenced by stabilization of transition states, intermediates, and products through substrate-metal bonding. This article reports that in the absence of such stabilizing interactions, established chemoselectivities can be overthrown. Thus, photoactivation of the recently developed neutral organic superelectron donor 5 selectively reduces alkyl-substituted benzene rings in the presence of activated esters and nitriles, in direct contrast to metal-based reductions, opening a new perspective on reactivity. The altered outcomes arising from the organic electron donors are attributed to selective interactions between the neutral organic donors and the arene rings of the substrates
Spin relaxation in (110) and (001) InAs/GaSb superlattices
We report an enhancement of the electron spin relaxation time (T1) in a (110)
InAs/GaSb superlattice by more than an order of magnitude (25 times) relative
to the corresponding (001) structure. The spin dynamics were measured using
polarization sensitive pump probe techniques and a mid-infrared, subpicosecond
PPLN OPO. Longer T1 times in (110) superlattices are attributed to the
suppression of the native interface asymmetry and bulk inversion asymmetry
contributions to the precessional D'yakonov Perel spin relaxation process.
Calculations using a nonperturbative 14-band nanostructure model give good
agreement with experiment and indicate that possible structural inversion
asymmetry contributions to T1 associated with compositional mixing at the
superlattice interfaces may limit the observed spin lifetime in (110)
superlattices. Our findings have implications for potential spintronics
applications using InAs/GaSb heterostructures.Comment: 4 pages, 2 figure
Silica in Protoplanetary Disks
Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope show prominent narrow
emission features indicating silica (crystalline silicon dioxide). Silica is
not a major constituent of the interstellar medium; therefore, any silica
present in the circumstellar protoplanetary disks of TTS must be largely the
result of processing of primitive dust material in the disks surrouding these
stars. We model the silica emission features in our spectra using the opacities
of various polymorphs of silica and their amorphous versions computed from
earth-based laboratory measurements. This modeling indicates that the two
polymorphs of silica, tridymite and cristobalite, which form at successively
higher temperatures and low pressures, are the dominant forms of silica in the
TTS of our sample. These high temperature, low pressure polymorphs of silica
present in protoplanetary disks are consistent with a grain composed mostly of
tridymite named Ada found in the cometary dust samples collected from the
STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks
may arise from incongruent melting of enstatite or from incongruent melting of
amorphous pyroxene, the latter being analogous to the former. The high
temperatures of 1200K-1300K and rapid cooling required to crystallize tridymite
or cristobalite set constraints on the mechanisms that could have formed the
silica in these protoplanetary disks, suggestive of processing of these grains
during the transient heating events hypothesized to create chondrules.Comment: 47 pages, 9 figures, to appear in the 1 January, 2009 issue of the
Astrophysical Journa
Multiple determination of the optical constants of thin-film coating materials
The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh. All samples of each material were prepared in a single deposition run. Brief descriptions are given of the various methods used for determination of the optical constants of these coating materials. The measurement data are presented, and the results are compared. The mean of the variances of the Sc2O3refractive-index determinations in the 0.40–0.75-nm spectral region was 0.03. The corresponding variances for the refractive index and absorption coefficient of Rh were 0.35 and 0.26, respectively
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