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.
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.
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.
Advancements in genetic testing have led to Usher syndrome now being diagnosed at a much earlier age than in the past, enabling the provision of early intervention and support to children and ...families. Despite these developments, anecdotal reports suggest there are substantial gaps in the services and supports provided to parents of children with Usher syndrome. The current study investigated the support needs of parents of children with Usher syndrome Type 1 when their child was aged 0 to 5 years.
Purposive sampling was used, and six semi-structured interviews were conducted with Australian parents of children with Usher syndrome, Type 1. Data was analysed using modified reflexive thematic analysis.
Four key themes were identified as being central to the support needs of parents of children with Usher syndrome aged 0 to 5 years. (1) Social Needs referred to parents' need for various sources of social support, (2) Informational Needs described the lack of information parents received regarding Usher syndrome from treating professionals, (3) Practical Needs included supports needed to assist parents in managing the day-to-day tasks of caring for a child with a disability, and (4) Emotional Needs represented the emotional support (both formal and informal) that parents needed to be a positive support to their child.
Findings provide rich information for relevant support groups, policy makers, individual healthcare professionals, and professional governing bodies regarding the education of stakeholders and the development and implementation of best-practice treatment guidelines.
A quantum phase transition from an antiferromagnetic to a ferromagnetic state is now measured in graphene bilayers. This observation supports the idea that bilayer graphene can sustain ...counter-propagating spin-polarized edge modes in analogy to the quantum spin Hall effect seen in topological insulators.
Electronic systems with multiple degenerate degrees of freedom can support a rich variety of broken symmetry states. In a graphene Landau level (LL), strong Coulomb interactions and the fourfold ...spin-valley degeneracy lead to an approximate SU(4) isospin symmetry. At partial filling, exchange interactions can break this symmetry, manifesting as further Hall plateaus outside the normal integer sequence. Here we report the observation of a number of these quantum Hall isospin ferromagnetic (QHIFM) states, which we classify according to their real spin structure using tilted field magnetotransport. The large activation gaps confirm the Coulomb origin of all the broken symmetry states, but the order depends strongly on LL index. In the high-energy LLs the Zeeman effect is the dominant aligning field, leading to real spin ferromagnets hosting skyrmionic excitations at half filling, whereas in the 'relativistic' zero LL lattice scale interactions drive the system to a spin unpolarized state. PUBLICATION ABSTRACT
Transferring graphene onto hexagonal boron nitride enables high-mobility multiterminal quantum Hall devices to be built. This makes it possible to study graphene's unique fractional quantum Hall ...behaviour more easily and more directly than previously. A study reports multiterminal measurements of the FQHE in high-mobility graphene devices fabricated on hexagonal boron nitride substrates.
It is widely recognized that adaptive control of the power supply is one of the most effective variables to achieve energy-efficient computation. Most on-chip dc-dc conversion systems have relied on ...buck converters with off-chip LC filters. In this paper, we describe the development of a software controllable, fully integrated on-chip dc-dc downconversion system that combines switched-capacitor voltage dividers and linear regulators to efficiently regulate from 2.5 V down to about 0.65 V. The use of switched-capacitor supplies offers better efficiencies than what is achievable with linear regulators alone.
A major goal of cancer research has been to identify genes that contribute to cancer formation. The similar pathology between zebrafish and human tumors, as well as the past success of large-scale ...genetic screens in uncovering human disease genes, makes zebrafish an ideal system in which to find such new genes. Here, we show that a zebrafish forward genetic screen uncovered multiple cell proliferation mutants including one mutant, crash&burn (crb), that represents a loss-of-function mutation in bmyb, a transcriptional regulator and member of a putative proto-oncogene family. crb mutant embryos have defects in mitotic progression and spindle formation, and exhibit genome instability. Regulation of cyclin B levels by bmyb appears to be the mechanism of mitotic accumulation in crb. Carcinogenesis studies reveal increased cancer susceptibility in adult crb heterozygotes. Gene-expression signatures associated with loss of bmyb in zebrafish are also correlated with conserved signatures in human tumor samples, and down-regulation of the B-myb signature genes is associated with retention of p53 function. Our findings show that zebrafish screens can uncover cancer pathways, and demonstrate that loss of function of bmyb is associated with cancer.
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