107 research outputs found
Chiral Amplification of Phosphoramidates of Amines and Amino Acids in Water
The origin of biomolecular homochirality continues to be one of the most fascinating aspects of prebiotic chemistry. Various amplification strategies for chiral compounds to enhance a small chiral preference have been reported, but none of these involves phosphorylation, one of nature's essential chemical reactions. Here we present a simple and robust concept of phosphorylation-based chiral amplification of amines and amino acids in water. By exploiting the difference in solubility of a racemic phosphoramidate and its enantiopure form, we achieved enantioenrichment in solution. Starting with near racemic, phenylethylamine-based phosphoramidates, ee's of up to 95 % are reached in a single amplification step. Particularly noteworthy is the enantioenrichment of phosphorylated amino acids and their derivatives, which might point to a potential role of phosphorus en-route to prebiotic homochirality
Spin-orbit interaction in a dual gated InAs/GaSb quantum well
Spin-orbit interaction is investigated in a dual gated InAs/GaSb quantum
well. Using an electric field the quantum well can be tuned between a single
carrier regime with exclusively electrons as carriers and a two-carriers regime
where electrons and holes coexist. Spin-orbit interaction in both regimes
manifests itself as a beating in the Shubnikov-de Haas oscillations. In the
single carrier regime the linear Dresselhaus strength is characterized by
28.5 meV and the Rashba coefficient is tuned from 75 to
53 meV by changing the electric field. In the two-carriers regime the spin
splitting shows a nonmonotonic behavior with gate voltage, which is consistent
with our band structure calculations
A Tunable Monolithic SQUID in Twisted Bilayer Graphene
Magic-angle twisted bilayer graphene (MATBG) hosts a number of correlated
states of matter that can be tuned by electrostatic doping. Superconductivity
has drawn considerable attention and the mechanism behind it is a topic of
active discussion. MATBG has been experimentally characterized by numerous
transport and scanning-probe experiments. The material has also emerged as a
versatile platform for superconducting electronics, as proven by the
realization of monolithic Josephson junctions. However, even though
phase-coherent phenomena have been measured, no control of the superconducting
phase has been demonstrated so far. Here, we present a Superconducting Quantum
Interference Device (SQUID) in MATBG, where the superconducting phase
difference is controlled through the magnetic field. We observe
magneto-oscillations of the critical current, demonstrating long-range
coherence agreeing with an effective charge of 2e for the superconducting
charge carriers. We tune to both asymmetric and symmetric SQUID configurations
by electrostatically controlling the critical currents through the junctions.
With this tunability, we study the inductances in the device, finding values of
up to 2{\mu}H. Furthermore, we directly observe the current-phase relation of
one of the Josephson junctions of the device. Our results show that
superconducting devices in MATBG can be scaled up and used to reveal properties
of the material. We expect this to foster a more systematic realization of
devices of this type, increasing the accuracy with which microscopic
characteristics of the material are extracted. We also envision more complex
devices to emerge, such as phase-slip junctions or high kinetic inductance
detectors.Comment: Supplementary Information is included in the .pd
Scattering between minivalleys in a moir\'e material
A unique feature of the complex band structures of moir\'e materials is the
presence of minivalleys, their hybridization, and scattering between them. Here
we investigate magneto-transport oscillations caused by scattering between
minivalleys - a phenomenon analogous to magneto-intersubband oscillations - in
a twisted double bilayer graphene sample with a twist angle of 1.94{\deg}. We
study and discuss the potential scattering mechanisms and find an
electron-phonon mechanism and valley conserving scattering to be likely.
Finally, we discuss the relevance of our findings for different materials and
twist angles.Comment: 4 figure
Combined minivalley and layer control in twisted double bilayer graphene
Control over minivalley polarization and interlayer coupling is demonstrated
in double bilayer graphene twisted with an angle of 2.37. This
intermediate angle is small enough for the minibands to form and large enough
such that the charge carrier gases in the layers can be tuned independently.
Using a dual-gated geometry we identify and control all possible combinations
of minivalley polarization via the population of the two bilayers. An applied
displacement field opens a band gap in either of the two bilayers, allowing us
to even obtain full minivalley polarization. In addition, the wavefunctions of
the minivalleys are mixed by tuning through a Lifshitz transition, where the
Fermi surface topology changes. The high degree of control makes twisted double
bilayer graphene a promising platform for valleytronics devices such as valley
valves, filters and logic gates.Comment: Supplemental Material included in PD
Light-Induced Mechanistic Divergence in Gold(I) Catalysis:Revisiting the Reactivity of Diazonium Salts
In a systematic study of the Au-catalyzed reaction of o-alkynylphenols with aryldiazonium salts, we find that essentially the same reaction conditions lead to a change in mechanism when a light source is applied. If the reaction is carried out at room temperature using a AuI catalyst, the diazonium salt undergoes electrophilic deauration of a vinyl AuI intermediate and provides access to substituted azobenzofurans. If the reaction mixture is irradiated with blue LED light, C−C bond formation due to N2-extrusion from the diazonium salt is realized selectively, using the same starting materials without the need for an additional photo(redox) catalyst under aerobic conditions. We report a series of experiments demonstrating that the same vinyl AuI intermediate is capable of producing the observed products under photolytic and thermal conditions. The finding that a vinyl AuI complex can directly, without the need for an additional photo(redox) catalyst, result in C−C bond formation under photolytic conditions is contrary to the proposed mechanistic pathways suggested in the literature till date and highlights that the role of oxidation state changes in photoredox catalysis involving Au is thus far only poorly understood and may hold surprises for the future. Computational results indicate that photochemical activation can occur directly from a donor–acceptor complex formed between the vinyl AuI intermediate and the diazonium salt
Hard superconducting gap and diffusion-induced superconductors in Ge-Si nanowires
We show a hard induced superconducting gap in a Ge-Si nanowire Josephson
transistor up to in-plane magnetic fields of mT, an important step
towards creating and detecting Majorana zero modes in this system. A hard
induced gap requires a highly homogeneous tunneling heterointerface between the
superconducting contacts and the semiconducting nanowire. This is realized by
annealing devices at C during which aluminium inter-diffuses and
replaces the germanium in a section of the nanowire. Next to Al, we find a
superconductor with lower critical temperature ( K) and a
higher critical field ( T). We can therefore selectively
switch either superconductor to the normal state by tuning the temperature and
the magnetic field and observe that the additional superconductor induces a
proximity supercurrent in the semiconducting part of the nanowire even when the
Al is in the normal state. In another device where the diffusion of Al rendered
the nanowire completely metallic, a superconductor with a much higher critical
temperature ( K) and critical field ( T) is
found. The small size of diffusion-induced superconductors inside nanowires may
be of special interest for applications requiring high magnetic fields in
arbitrary direction
Kondo effect and spin-orbit coupling in graphene quantum dots
The Kondo effect is a cornerstone in the study of strongly correlated
fermions. The coherent exchange coupling of conduction electrons to local
magnetic moments gives rise to a Kondo cloud that screens the impurity spin.
Whereas complete Kondo screening has been explored widely, realizations of the
underscreened scenario - where only some of several Kondo channels participate
in the screening - remain rare. Here we report the observation of fully
screened and underscreened Kondo effects in quantum dots in bilayer graphene.
More generally, we introduce a unique platform for studying Kondo physics. In
contrast to carbon nanotubes, whose curved surfaces give rise to strong
spin-orbit coupling breaking the SU(4) symmetry of the electronic states
relevant for the Kondo effect, we study a nominally flat carbon material with
small spin-orbit coupling. Moreover, the unusual two-electron triplet ground
state in bilayer graphene dots provides a route to exploring the underscreened
spin-1 Kondo effect
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