12,819 research outputs found
Automated Identification and Differentiation of Spectrally Similar Hydrothermal Minerals on Mars
Early telescopic observations corroborated hydration related absorptions on Mars in the infrared. Images from the Viking missions led to speculation of hydrothermal alteration and were followed by two missions which mapped the spatial variability of the ~ 3 m hydration feature. Since then, the Compact Reconnaissance Imager for Mars (CRISM) has provided high spatial resolution (up to 18m) spectral identification of a suite of hydrothermal and diagenetic minerals which have illuminated a range of formation mechanisms. Presence/absence and spatial segregation or mixing of minerals like prehnite, epidote, chlorite amphiboles, and mixed-layer Fe/Mg smectite-chlorite provide valuable evidence for the geologic setting of deposits on Earth, and these phases are often used as temperature and aqueous chemistry indicators in terrestrial systems. Mapping the distribution of these phases will help to answer whether Mars had widespread conditions favorable for low-grade metamorphism and diagenesis, or only focused hydrothermal systems in areas of high heat flow. Further characterizing the chemistry and structure of these phases will then help to answer how most of the widespread Fe/Mg phyllosilicates formed, further defining early geochemical cycling and climate. A fully automated approach for accurate mapping of important hydrothermal mineral phases on Mars has been a challenge. Due to overlapping features in the M-OH region (~2.2-2.4 m), the strongest absorption features of chlorite, prehnite, and epidote in the short-wave infrared are difficult to distinguish from one another and from the most commonly occurring hydrated silicates on Mars, Fe/Mg smectites. Weaker absorptions are present in both prehnite and epidote which help to distinguish them from chlorite and smectites, but their relative strength in the presence of noise and spatial mixing is often too low to confidently identify them without the noise suppression and feature enhancement methods described here. The spectral signatures of mixed-layer Fe/Mg smectite-chlorite and partially chloritized Fe/Mg smectites have not yet been adequately assessed. Here we evaluate the effectiveness of two empirical and statistical methods for identifying and differentiating these phases using CRISM data
Single wall carbon nanotube double quantum dot
We report on two top-gate defined, coupled quantum dots in a semiconducting
single wall carbon nanotube, constituting a tunable double quantum dot system.
The single wall carbon nanotubes are contacted by titanium electrodes, and
gated by three narrow top-gate electrodes as well as a back-gate. We show that
a bias spectroscopy plot on just one of the two quantum dots can be used to
extract the addition energy of both quantum dots. Furthermore, honeycomb charge
stability diagrams are analyzed by an electrostatic capacitor model that
includes cross capacitances, and we extract the coupling energy of the double
quantum dot.Comment: Published in Applied Physics Letters 4 December 2006.
http://link.aip.org/link/?APL/89/23211
Critical Current 0- Transition in Designed Josephson Quantum Dot Junctions
We report on quantum dot based Josephson junctions designed specifically for
measuring the supercurrent. From high-accuracy fitting of the current-voltage
characteristics we determine the full magnitude of the supercurrent (critical
current). Strong gate modulation of the critical current is observed through
several consecutive Coulomb blockade oscillations. The critical current crosses
zero close to, but not at, resonance due to the so-called 0- transition in
agreement with a simple theoretical model.Comment: 5 pages, 4 figures, (Supplementary information available at
http://www.fys.ku.dk/~hij/public/nl_supp.pdf
Can women have children and a career? IV evidence from IVF treatments
This paper introduces a new IV strategy based on IVF (in vitro fertilization) induced fertility variation among childless women to estimate the causal effect of having children on their career. For this purpose, we use administrative data on IVF treated women in Denmark. Because observed chances of IVF success do not depend on labor market histories, IVF treatment success provides a plausible instrument for childbearing. Our IV estimates indicate that fertility effects on earnings are: (i) negative, large, and long-lasting; (ii) driven by fertility effects on hourly earnings and not so much on labor supply; and (iii) much stronger at the extensive margin than at the intensive margin
Texture formation in FePt thin films via thermal stress management
The transformation variant of the fcc to fct transformation in FePt thin films was tailored by controlling the stresses in the thin films, thereby allowing selection of in- or out-of-plane c-axis orientation. FePt thin films were deposited at ambient temperature on several substrates with differing coefficients of thermal expansion relative to the FePt, which generated thermal stresses during the ordering heat treatment. X-ray diffraction analysis revealed preferential out-of-plane c-axis orientation for FePt films deposited on substrates with a similar coefficients of thermal expansion, and random orientation for FePt films deposited on substrates with a very low coefficient of thermal expansion, which is consistent with theoretical analysis when considering residual stresses
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
Time-varying Learning and Content Analytics via Sparse Factor Analysis
We propose SPARFA-Trace, a new machine learning-based framework for
time-varying learning and content analytics for education applications. We
develop a novel message passing-based, blind, approximate Kalman filter for
sparse factor analysis (SPARFA), that jointly (i) traces learner concept
knowledge over time, (ii) analyzes learner concept knowledge state transitions
(induced by interacting with learning resources, such as textbook sections,
lecture videos, etc, or the forgetting effect), and (iii) estimates the content
organization and intrinsic difficulty of the assessment questions. These
quantities are estimated solely from binary-valued (correct/incorrect) graded
learner response data and a summary of the specific actions each learner
performs (e.g., answering a question or studying a learning resource) at each
time instance. Experimental results on two online course datasets demonstrate
that SPARFA-Trace is capable of tracing each learner's concept knowledge
evolution over time, as well as analyzing the quality and content organization
of learning resources, the question-concept associations, and the question
intrinsic difficulties. Moreover, we show that SPARFA-Trace achieves comparable
or better performance in predicting unobserved learner responses than existing
collaborative filtering and knowledge tracing approaches for personalized
education
Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots
We study low-temperature transport through carbon nanotube quantum dots in
the Coulomb blockade regime coupled to niobium-based superconducting leads. We
observe pronounced conductance peaks at finite source-drain bias, which we
ascribe to elastic and inelastic cotunneling processes enhanced by the
coherence peaks in the density of states of the superconducting leads. The
inelastic cotunneling lines display a marked dependence on the applied gate
voltage which we relate to different tunneling-renormalizations of the two
subbands in the nanotube. Finally, we discuss the origin of an especially
pronounced sub-gap structure observed in every fourth Coulomb diamond
Singlet-Triplet Physics and Shell Filling in Carbon Nanotube Double Quantum Dots
An artifcial two-atomic molecule, also called a double quantum dot (DQD), is
an ideal system for exploring few electron physics. Spin-entanglement between
just two electrons can be explored in such systems where singlet and triplet
states are accessible. These two spin-states can be regarded as the two states
in a quantum two-state system, a so-called singlet-triplet qubit. A very
attractive material for realizing spin based qubits is the carbon nanotube
(CNT), because it is expected to have a very long spin coherence time. Here we
show the existence of a gate-tunable singlet-triplet qubit in a CNT DQD. We
show that the CNT DQD has clear shell structures of both four and eight
electrons, with the singlet-triplet qubit present in the four-electron shells.
We furthermore observe inelastic cotunneling via the singlet and triplet
states, which we use to probe the splitting between singlet and triplet, in
good agreement with theory.Comment: Supplement available at:
http://www.fys.ku.dk/~hij/public/singlet-triple_supp.pd
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