Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the ...graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.
Objective
To develop a core outcome set for trials on the treatment of hyperemesis gravidarum (HG).
Design
Identification of outcomes is followed by a modified Delphi survey combined with a consensus ...development meeting and a consultation round.
Setting
An international web‐based survey combined with a consensus development meeting.
Population
Stakeholders including researchers; women with lived experience of HG and their families; obstetric health professionals; and other health professionals.
Methods
We used systematic review, semi‐structured patient interviews, closed group sessions and Steering Committee input to identify potential core outcomes. We conducted two web‐based survey rounds, followed by a face‐to‐face consensus development meeting and a web‐based consultation round.
Main outcome measures
A core outcome set for research on HG.
Results
Fifty‐six potential outcomes were identified. The modified Delphi process was completed by 125 stakeholders, the consensus development meeting by 20 stakeholders and the consultation round by 96 stakeholders. Consensus was reached in ten domains on 24 outcomes: nausea; vomiting; inability to tolerate oral fluids or food; dehydration; weight difference; electrolyte imbalance; intravenous fluid treatment; use of medication for hyperemesis gravidarum; hospital treatment; treatment compliance; patient satisfaction; daily functioning; maternal physical or mental or emotional wellbeing; short‐ and long‐term adverse effects of treatment; maternal death; pregnancy complications; considering or actually terminating a wanted pregnancy; preterm birth; small for gestational age; congenital anomalies; neonatal morbidity and offspring death).
Conclusions
This core outcome set will help standardise outcome reporting in HG trials.
Tweetable
A core outcome set for treatment of hyperemesis gravidarum in order to create high‐quality evidence.
Tweetable
A core outcome set for treatment of hyperemesis gravidarum in order to create high‐quality evidence.
Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field ...also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, two-dimensional systems of electrons exhibit a self-similar recursive energy spectrum. Known as Hofstadter's butterfly, this complex spectrum results from an interplay between the characteristic lengths associated with the two quantizing fields, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical atomic lattices (with periodicities of less than one nanometre) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nanometres) the corresponding fields are too small to overcome disorder completely. Here we demonstrate that moiré superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.
Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, ...depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the ID edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.
We report fabrication of graphene devices in a Corbino geometry consisting of concentric circular electrodes with no physical edge connecting the inner and outer electrodes. High device mobility is ...realized using boron nitride encapsulation together with a dual-graphite gate structure. Bulk conductance measurement in the quantum Hall effect (QHE) regime outperforms previously reported Hall bar measurements, with improved resolution observed for both the integer and fractional QHE states. We identify apparent phase transitions in the fractional sequence in both the lowest and first excited Landau levels (LLs) and observe features consistent with electron solid phases in higher LLs.
The distinct Landau level spectrum of bilayer graphene (BLG) is predicted to support a non-abelian even-denominator fractional quantum Hall (FQH) state similar to the
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state first identified in ...GaAs. However, the nature of this state has remained difficult to characterize. Here, we report transport measurements of a robust sequence of even-denominator FQH in dual-gated BLG devices. Parallel field measurement confirms the spin-polarized nature of the ground state, which is consistent with the Pfaffian/anti-Pfaffian description. The sensitivity of the even-denominator states to both filling fraction and transverse displacement field provides new opportunities for tunability. Our results suggest that BLG is a platform in which topological ground states with possible non-abelian excitations can be manipulated and controlled.
Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization1. Consistent with this, pioneering studies on thin lms reported only superconducting and ...insulating ground states, with a direct transition between the two as a function of disorder or magnetic eld26. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram710, whose nature is intensely debated1117. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe2 in an inert atmosphere. Under a small perpendicular magnetic eld, we induce a transition from superconductor to the quantum metal. We nd a unique power-law scaling with eld in this phase, which is consistent with the Bose-metal model where metallic behaviour arises from strong phase uctuations caused by the magnetic field1114.
Electrons incident from a normal metal onto a superconductor are reflected back as holes--a process called Andreev reflection1, 2, 3. In a normal metal where the Fermi energy is much larger than a ...typical superconducting gap, the reflected hole retraces the path taken by the incident electron. In graphene with low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor-graphene interface owing to the onset of interband Andreev processes, where the effective mass of the reflected holes changes sign4, 5. Here we present measurements of gate-modulated Andreev reflections across the low-disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene-superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro Andreev reflections and interband specular Andreev reflections.
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