425 research outputs found
Kondo physics in tunable semiconductor nanowire quantum dots
We have observed the Kondo effect in strongly coupled semiconducting nanowire
quantum dots. The devices are made from indium arsenide nanowires, grown by
molecular beam epitaxy, and contacted by titanium leads. The device
transparency can be tuned by changing the potential on a gate electrode, and
for increasing transparencies the effects dominating the transport changes from
Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics
appearing in the intermediate region.Comment: 4 pages, 4 figure
Finite bias Cooper pair splitting
In a device with a superconductor coupled to two parallel quantum dots (QDs)
the electrical tunability of the QD levels can be used to exploit non-classical
current correlations due to the splitting of Cooper pairs. We experimentally
investigate the effect of a finite potential difference across one quantum dot
on the conductance through the other completely grounded QD in a Cooper pair
splitter fabricated on an InAs nanowire. We demonstrate that the electrical
transport through the device can be tuned by electrical means to be dominated
either by Cooper pair splitting (CPS), or by elastic co-tunneling (EC). The
basic experimental findings can be understood by considering the energy
dependent density of states in a QD. The reported experiments add
bias-dependent spectroscopy to the investigative tools necessary to develop
CPS-based sources of entangled electrons in solid-state devices.Comment: 4 pages, 4 figure
Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts
Advanced synthesis of semiconductor nanowires (NWs) enables their application
in diverse fields, notably in chemical and electrical sensing, photovoltaics,
or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an
ideal platform for quantum devices, e.g. they may host topological Majorana
states. While the synthesis has been continously perfected, only few techniques
were developed to tailor individual NWs after growth. Here we present three wet
chemical etch methods for the post-growth morphological engineering of InAs NWs
on the sub-100 nm scale. The first two methods allow the formation of
self-aligned electrical contacts to etched NWs, while the third method results
in conical shaped NW profiles ideal for creating smooth electrical potential
gradients and shallow barriers. Low temperature experiments show that NWs with
etched segments have stable transport characteristics and can serve as building
blocks of quantum electronic devices. As an example we report the formation of
a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure
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
Voltage-Controlled Superconducting Quantum Bus
We demonstrate the ability of an epitaxial semiconductor-superconductor
nanowire to serve as a field-effect switch to tune a superconducting cavity.
Two superconducting gatemon qubits are coupled to the cavity, which acts as a
quantum bus. Using a gate voltage to control the superconducting switch yields
up to a factor of 8 change in qubit-qubit coupling between the on and off
states without detrimental effect on qubit coherence. High-bandwidth operation
of the coupling switch on nanosecond timescales degrades qubit coherence
The Influence of Electro-Mechanical Effects on Resonant Electron Tunneling Through Small Carbon Nano-Peapods
The influence of a fullerene molecule trapped inside a single-wall carbon
nanotube on resonant electron transport at low temperatures and strong
polaronic coupling is theoretically discussed. Strong peak to peak fluctuations
and anomalous temperature behavior of conductance amplitudes are predicted and
investigated. The influence of the chiral properties of carbon nanotubes on
transport is also studied.Comment: 17 pages, 3 figures. Replaced with published version. Important
changes. Open access: http://stacks.iop.org/1367-2630/10/04304
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
Anharmonicity of a Gatemon Qubit with a Few-Mode Josephson Junction
Coherent operation of gate-voltage-controlled hybrid transmon qubits
(gatemons) based on semiconductor nanowires was recently demonstrated. Here we
experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson
junctions, a key parameter for their use as a qubit. Anharmonicity is found to
be reduced by roughly a factor of two compared to conventional metallic
junctions, and dependent on gate voltage. Experimental results are consistent
with a theoretical model, indicating that Josephson coupling is mediated by a
small number of highly transmitting modes in the semiconductor junction
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Living With Ambiguity: A Metasynthesis of Qualitative Research on Mild Cognitive Impairment
Purpose of the study: Mild Cognitive Impairment (MCI) is a diagnosis proposed to describe an intermediate state between normal cognitive aging and dementia. MCI has been criticised for its conceptual fuzziness, its ambiguous relationship to dementia, and the tension it creates between medical and sociological understandings of ânormal agingâ.
Design and Methods: We examined the published qualitative literature on experiences of being diagnosed and living with MCI using metasynthesis as the methodological framework.
Results: Two overarching conceptual themes were developed. The first, MCI and myself-in-time, showed that a diagnosis of MCI could profoundly affect a personâs understanding of their place in the world. This impact appears to be mediated by multiple factors including a personâs social support networks, which daily activities are affected, and subjective interpretations of the meaning of MCI. The second theme, Living with Ambiguity, describes the difficulties people experienced in making sense of their diagnosis. Uncertainty arose, in part, from lack of clarity and consistency in the information received by people with MCI, including whether they are even told MCI is the diagnosis.
Implications: We conclude by suggesting an ethical tension is always at play when a MCI diagnosis is made. Specifically, earlier support and services afforded by a diagnosis may come at the expense of a personâs anxiety about the future, with continued uncertainty about how his or her concerns and needs can be addressed
Impacts of COVID-19 restrictions on level of physical activity and health in home-dwelling older adults in Norway
Background - The spread of the coronavirus in spring 2020 led to a lockdown of physical activity (PA) offers. The aim of this study was to investigate how PA, as well as general and mental health, in community-dwelling older adults were affected by the COVID-19 restrictions in Norway.
Methods - Invitation to participate in the study was sent via Facebook and the Norwegian Pensionersâ Association. Inclusion criteria were beingââ„â65 years old and living at home. Participants completed a questionnaire either digitally or on paper in JuneâAugust 2020. The questionnaire included questions on PA, general health, and mental health both before (13th of March) and during lockdown.
Results - We included 565 participants (mean age 74â±â5.3 years, 60.4% female); almost 60% had a university degree, 84% reported performing PA more than once per week, and 20% reported a fall in the previous 12 months. The Wilcoxon signed-rank test indicated that the corona lockdown significantly reduced activity level (Zâ=â-4.918, pâ
Conclusions - In a relatively highly educated and active group of older participants, COVID-19 restrictions still negatively affected level of activity as well as general and mental health. These short-term decreases in activity level and health suggest that preventive actions and increased focus on measures to support older adults in maintaining an active lifestyle are needed
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