102 research outputs found
Photo-assisted Andreev reflection as a probe of quantum noise
Andreev reflection, which corresponds to the tunneling of two electrons from
a metallic lead to a superconductor lead as a Cooper pair (or vice versa), can
be exploited to measure high frequency noise. A detector is proposed, which
consists of a normal lead--superconductor circuit, which is capacitively
coupled to a mesoscopic circuit where noise is to be measured. We discuss two
detector circuits: a single normal metal -- superconductor tunnel junction and
a normal metal separated from a superconductor by a quantum dot operating in
the Coulomb blockade regime. A substantial DC current flows in the detector
circuit when an appropriate photon is provided or absorbed by the mesoscopic
circuit, which plays the role of an environment for the junction to which it
couples. Results for the current can be cast in all cases in the form of a
frequency integral of the excess noise of the environment weighted by a kernel
which is specific to the transport process (quasiparticle tunneling, Andreev
reflection,...) which is considered. We apply these ideas to the measurement of
the excess noise of a quantum point contact and we provide numerical estimates
of the detector current.Comment: 19 pages, 11 figure
Transport through a single Anderson impurity coupled to one normal and two superconducting leads
We study the interplay between the Kondo and Andreev-Josephson effects in a
quantum dot coupled to one normal and two superconducting (SC) leads. In the
large gap limit, the low-energy states of this system can be described exactly
by a local Fermi liquid for the interacting Bogoliubov particles. The phase
shift and the renormalized parameters for the Bogoliubov particles vary
depending on the Josephson phase between the two SC leads. We explore the
precise features of a crossover that occurs between the Kondo singlet and local
Cooper-pairing states as the Josephson phase varies, using the numerical
renormalization group approach.Comment: 4 pages, 4 figures, contribution to SCES 201
Spin-polarized transport through weakly coupled double quantum dots in the Coulomb-blockade regime
We analyze cotunneling transport through two quantum dots in series weakly
coupled to external ferromagnetic leads. In the Coulomb blockade regime the
electric current flows due to third-order tunneling, while the second-order
single-barrier processes have indirect impact on the current by changing the
occupation probabilities of the double dot system. We predict a zero-bias
maximum in the differential conductance, whose magnitude is conditioned by the
value of the inter-dot Coulomb interaction. This maximum is present in both
magnetic configurations of the system and results from asymmetry in cotunneling
through different virtual states. Furthermore, we show that tunnel
magnetoresistance exhibits a distinctively different behavior depending on
temperature, being rather independent of the value of inter-dot correlation.
Moreover, we find negative TMR in some range of the bias voltage.Comment: 9 pages, 7 figures, accepted in Phys. Rev.
Molecular states in carbon nanotube double quantum dots
We report electrical transport measurements through a semiconducting
single-walled carbon nanotube (SWNT) with three additional top-gates. At low
temperatures the system acts as a double quantum dot with large inter-dot
tunnel coupling allowing for the observation of tunnel-coupled molecular states
extending over the whole double-dot system. We precisely extract the tunnel
coupling and identify the molecular states by the sequential-tunneling line
shape of the resonances in differential conductance.Comment: 5 pages, 4 figure
Defining and controlling double quantum dots in single-walled carbon nanotubes
We report the experimental realization of double quantum dots in
single-walled carbon nanotubes. The device consists of a nanotube with source
and drain contact, and three additional top-gate electrodes in between. We show
that, by energizing these top-gates, it is possible to locally gate a nanotube,
to create a barrier, or to tune the chemical potential of a part of the
nanotube. At low temperatures we find (for three different devices) that in
certain ranges of top-gate voltages our device acts as a double quantum dot,
evidenced by the typical honeycomb charge stability pattern.Comment: 9 pages, 3 figure
Validity and worth in the science curriculum: learning school science outside the laboratory
It is widely acknowledged that there are problems with school science in many developed countries of the world. Such problems manifest themselves in a progressive decline in pupil enthusiasm for school science across the secondary age range and the fact that fewer students are choosing to study the physical sciences at higher levels and as careers. Responses to these developments have included proposals to reform the curriculum, pedagogy and the nature of pupil discussion in science lessons. We support such changes but argue from a consideration of the aims of science education that secondary school science is too rooted in the science laboratory; substantially greater use needs to be made of out-of-school sites for the teaching of science. Such usage should result in a school science education that is more valid and more motivating and is better at fulfilling defensible aims of school science education. Our contention is that laboratory-based school science teaching needs to be complemented by out-of-school science learning that draws on the actual world (e.g. through fieldtrips), the presented world (e.g. in science centres, botanic gardens, zoos and science museums) and the virtual worlds that are increasingly available through information and communications technologies (ICT)
A major electronics upgrade for the H.E.S.S. Cherenkov telescopes 1-4
The High Energy Stereoscopic System (H.E.S.S.) is an array of imaging
atmospheric Cherenkov telescopes (IACTs) located in the Khomas Highland in
Namibia. It consists of four 12-m telescopes (CT1-4), which started operations
in 2003, and a 28-m diameter one (CT5), which was brought online in 2012. It is
the only IACT system featuring telescopes of different sizes, which provides
sensitivity for gamma rays across a very wide energy range, from ~30 GeV up to
~100 TeV. Since the camera electronics of CT1-4 are much older than the one of
CT5, an upgrade is being carried out; first deployment was in 2015, full
operation is planned for 2016. The goals of this upgrade are threefold:
reducing the dead time of the cameras, improving the overall performance of the
array and reducing the system failure rate related to aging. Upon completion,
the upgrade will assure the continuous operation of H.E.S.S. at its full
sensitivity until and possibly beyond the advent of CTA. In the design of the
new components, several CTA concepts and technologies were used and are thus
being evaluated in the field: The upgraded read-out electronics is based on the
NECTAR readout chips; the new camera front- and back-end control subsystems are
based on an FPGA and an embedded ARM computer; the communication between
subsystems is based on standard Ethernet technologies. These hardware solutions
offer good performance, robustness and flexibility. The design of the new
cameras is reported here.Comment: Proceedings of the 34th International Cosmic Ray Conference, 30 July-
6 August, 2015, The Hague, The Netherland
Double quantum dot with integrated charge sensor based on Ge/Si heterostructure nanowires
Coupled electron spins in semiconductor double quantum dots hold promise as
the basis for solid-state qubits. To date, most experiments have used III-V
materials, in which coherence is limited by hyperfine interactions. Ge/Si
heterostructure nanowires seem ideally suited to overcome this limitation: the
predominance of spin-zero nuclei suppresses the hyperfine interaction and
chemical synthesis creates a clean and defect-free system with highly
controllable properties. Here we present a top gate-defined double quantum dot
based on Ge/Si heterostructure nanowires with fully tunable coupling between
the dots and to the leads. We also demonstrate a novel approach to charge
sensing in a one-dimensional nanostructure by capacitively coupling the double
dot to a single dot on an adjacent nanowire. The double quantum dot and
integrated charge sensor serve as an essential building block required to form
a solid-state spin qubit free of nuclear spin.Comment: Related work at http://marcuslab.harvard.edu and
http://cmliris.harvard.ed
Decreased body mass index in the preclinical stage of autosomal dominant Alzheimerâs disease
The relationship between body-mass index (BMI) and AlzheimerÂŽs disease (AD) has been extensively investigated. However, BMI alterations in preclinical individuals with autosomal dominant AD (ADAD) have not yet been investigated. We analyzed cross-sectional data from 230 asymptomatic members of families with ADAD participating in the Dominantly Inherited Alzheimer Network (DIAN) study including 120 preclinical mutation carriers (MCs) and 110 asymptomatic non-carriers (NCs). Differences in BMI and their relation with cerebral amyloid load and episodic memory as a function of estimated years to symptom onset (EYO) were analyzed. Preclinical MCs showed significantly lower BMIs compared to NCs, starting 11.2 years before expected symptom onset. However, the BMI curves begun to diverge already at 17.8 years before expected symptom onset. Lower BMI in preclinical MCs was significantly associated with less years before estimated symptom onset, higher global AÎČ brain burden, and with lower delayed total recall scores in the logical memory test. The study provides cross-sectional evidence that weight loss starts one to two decades before expected symptom onset of ADAD. Our findings point toward a link between the pathophysiology of ADAD and disturbance of weight control mechanisms. Longitudinal follow-up studies are warranted to investigate BMI changes over time
Decreased body mass index in the preclinical stage of autosomal dominant Alzheimer\u27s disease
The relationship between body-mass index (BMI) and AlzheimeĆs disease (AD) has been extensively investigated. However, BMI alterations in preclinical individuals with autosomal dominant AD (ADAD) have not yet been investigated. We analyzed cross-sectional data from 230 asymptomatic members of families with ADAD participating in the Dominantly Inherited Alzheimer Network (DIAN) study including 120 preclinical mutation carriers (MCs) and 110 asymptomatic non-carriers (NCs). Differences in BMI and their relation with cerebral amyloid load and episodic memory as a function of estimated years to symptom onset (EYO) were analyzed. Preclinical MCs showed significantly lower BMIs compared to NCs, starting 11.2 years before expected symptom onset. However, the BMI curves begun to diverge already at 17.8 years before expected symptom onset. Lower BMI in preclinical MCs was significantly associated with less years before estimated symptom onset, higher global AÎČ brain burden, and with lower delayed total recall scores in the logical memory test. The study provides cross-sectional evidence that weight loss starts one to two decades before expected symptom onset of ADAD. Our findings point toward a link between the pathophysiology of ADAD and disturbance of weight control mechanisms. Longitudinal follow-up studies are warranted to investigate BMI changes over time
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