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The evolution of the superconductivity as a function of film thickness and doping is systematically studied in FeSe films. A high-temperature superconducting phase is found to arise in multilayer films. The recent discovery of possible high-temperature ( T c ) superconductivity over 65 K in a monolayer FeSe film on SrTiO 3 (refs  1 , 2 , 3 , 4 , 5 , 6 ) triggered a fierce debate on how superconductivity evolves from bulk to film, because bulk FeSe crystal exhibits a T c of no higher than 10 K (ref.  7 ). However, the difficulty in controlling the carrier density and the number of FeSe layers has hindered elucidation of this problem 4 , 8 . Here, we demonstrate that deposition of potassium onto FeSe films markedly expands the accessible doping range towards the heavily electron-doped region. Intriguingly, we have succeeded in converting non-superconducting films with various thicknesses into superconductors with T c as high as 48 K. We also found a marked increase in the magnitude of the superconducting gap on decreasing the FeSe film thickness, indicating that the interface plays a crucial role in realizing the high-temperature superconductivity. The results presented provide a new strategy to enhance and optimize T c in ultrathin films of iron-based superconductors.

A topological insulator has surface metallic states that are topologically protected by time-reversal symmetry. Tin telluride is now shown to be a ‘topological crystalline insulator’, in which the surface metallic state is instead protected by the mirror symmetry of the crystal. A topological insulator is an unusual quantum state of matter, characterized by the appearance, at its edges or on its surface, of a gapless metallic state that is protected by time-reversal symmetry 1 , 2 . The discovery of topological insulators has stimulated the search for other topological states protected by other symmetries 3 , 4 , 5 , 6 , 7 , such as the recently predicted 8 topological crystalline insulator (TCI) in which the metallic surface states are protected by the mirror symmetry of the crystal. Here we present experimental evidence for the TCI phase in tin telluride (SnTe), which has been predicted to be a TCI (ref.  9 ). Our angle-resolved photoemission spectra show the signature of a metallic Dirac-cone surface band, with its Dirac point slightly away from the edge of the surface Brillouin zone in SnTe. Such a gapless surface state is absent in a cousin material, lead telluride, in line with the theoretical prediction.

Realization of topological superconductors (TSCs) hosting Majorana fermions is a central challenge in condensed-matter physics. One approach is to use the superconducting proximity effect (SPE) in heterostructures, where a topological insulator contacted with a superconductor hosts an effective p -wave pairing by the penetration of Cooper pairs across the interface. However, this approach suffers a difficulty in accessing the topological interface buried deep beneath the surface. Here, we propose an alternative approach to realize topological superconductivity without SPE. In a Pb(111) thin film grown on TlBiSe 2 , we discover that the Dirac-cone state of substrate TlBiSe 2 migrates to the top surface of Pb film and obtains an energy gap below the superconducting transition temperature of Pb. This suggests that a Bardeen-Cooper-Schrieffer superconductor is converted into a TSC by the topological proximity effect. Our discovery opens a route to manipulate topological superconducting properties of materials. Realizing topological superconductivity is essential for applicable fault-tolerant quantum computation. Here, Trang et al . report migration of Dirac-cone from TlBiSe 2 substrate to top surface of superconducting Pb film due to topological proximity effect, suggesting realization of topological superconductivity.

The three-dimensional topological insulator is a quantum state of matter characterized by an insulating bulk state and gapless Dirac cone surface states. Device applications of topological insulators require a highly insulating bulk and tunable Dirac carriers, which has so far been difficult to achieve. Here we demonstrate that Bi 2-x Sb x Te 3-y Se y is a system that simultaneously satisfies both of these requirements. For a series of compositions presenting bulk-insulating transport behaviour, angle-resolved photoemission spectroscopy reveals that the chemical potential is always located in the bulk band gap, whereas the Dirac cone dispersion changes systematically so that the Dirac point moves up in energy with increasing x , leading to a sign change of the Dirac carriers at x ~0.9. Such a tunable Dirac cone opens a promising pathway to the development of novel devices based on topological insulators. The surface electronic structure of topological insulators is characterized by a so-called Dirac cone energy dispersion. This study shows that by tuning the compositions in the compound Bi 2−x Sb x Te 3−y Se y one can control the precise features of its Dirac cone structure while keeping it a bulk insulator.

Thecosome pteropods form an important part of marine food webs, especially in polar ecosystems, and are the focus of research on ocean acidification. Although the larval stages of species in the genus Limacina often form major components of zooplankton communities, little is known of their population dynamics. We report high Limacina spp. abundance in March 2000 during surface zooplankton community sampling via a Continuous Plankton Recorder (CPR; 270-µm mesh) in a large area within the seasonal ice zone of the Southern Ocean. Regions with high Limacina spp. abundances extended to 600 nautical miles (ca 1110 km). Annual variability in Limacina spp. abundance and shell size is evaluated using North Pacific standard net (100-µm mesh) data from the same area and sampling periods (March) from 1997 to 2006. Although the relative total abundance of Limacina spp. in 2000 was the highest in the study period, its overall abundance was lower than the mean value for that period. Mean shell size for most years ranged 160–300 µm, while a relatively large mean size (444.7 µm) occurred in 2000. We conclude that a CPR with 270-µm mesh could catch large Limacina individuals that dominated in March 2000. The timing of reproduction and growth of the new generation may influence Limacina abundance throughout the sampling area.

This paper develops empirical relationships to estimate FAA/EASA and MIL-3013B rules-compliant take-off field performance for single and multi-engine aircraft. Recent experience with modern aircraft flight manuals revealed that popular empirical legacy methods are no longer accurate; improvements in tires and brakes lead to significantly shorter certified distances. This work relies upon a survey of current operational aircraft and extensive numerical simulations of generic configurations to support the development of a collection of new equations to estimate take-off performance for single and multi-engine aircraft under dry and wet conditions. These relationships are individually tailored for civilian and U.S. Military rules; they account for the superior capability of modern braking systems and the implications of minimum-control speed on the certified distance.

The genetic aetiology of autism remains elusive. Occasionally, individuals with Cowden syndrome (a cancer syndrome) and other related hamartoma disorders such as Bannayan-Riley-Ruvalcaba syndrome, Proteus syndrome, and Proteus-like conditions, are characterised by germline PTEN mutations, and may have neurobehavioural features resembling autism as well as overgrowth and macrocephaly. Therefore, we undertook PTEN gene mutation analysis in 18 subjects mainly prospectively ascertained with autism spectrum disorder and macrocephaly. Of these 18 autistic subjects (13 males and five females; ages 3.1–18.4 years) with a head circumference range from 2.5 to 8.0 standard deviations above the mean, three males (17%) carried germline PTEN mutations. These three probands had previously undescribed PTEN mutations: H93R (exon 4), D252G (exon 7), and F241S (exon 7). They had the larger head circumference measurements amongst all our study subjects. The three residues altered in our patients were highly evolutionarily conserved. We suggest that PTEN gene testing be considered for patients with autistic behaviour and extreme macrocephaly. The gene findings may impact on recurrence risks as well as medical management for the patient.

Recently discovered genome-wide rare copy number variants (CNVs) have unprecedented levels of statistical association with many developmental neuropsychiatric disorders, including schizophrenia, autism spectrum disorders, intellectual disability and attention deficit hyperactivity disorder. However, as CNVs often include multiple genes, causal genes responsible for CNV-associated diagnoses and traits are still poorly understood. Mouse models of CNVs are in use to delve into the precise mechanisms through which CNVs contribute to disorders and associated traits. Based on human and mouse model studies on rare CNVs within human chromosome 22q11.2, we propose that alterations of a distinct set of multiple, noncontiguous genes encoded in this chromosomal region, in concert with modulatory impacts of genetic background and environmental factors, variably shift the probabilities of phenotypes along a predetermined developmental trajectory. This model can be further extended to the study of other CNVs and may serve as a guide to help characterize the impact of genes in developmental neuropsychiatric disorders.

The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z 2 -type topological insulator—a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either ‘strong’ or ‘weak’ 1 , 2 , and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions 3 – 5 . By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals 6 – 10 . Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi 4 I 4 . Notably, the crystal has naturally cleavable top and side planes—stacked via van der Waals forces—which have long been desirable for the experimental realization of the WTI state 11 , 12 . As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase—viewed as quantum spin Hall insulators stacked three-dimensionally 13 , 14 —will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering. Angle-resolved photoemission spectroscopy is used to characterize the surface states of bismuth iodide, providing experimental evidence of a weak topological insulator state that had previously been only theoretically predicted.

Introducing the concept of topology has revolutionized materials classification, leading to the discovery of topological insulators and Dirac–Weyl semimetals 1 – 3 . One of the most fundamental theories underpinning topological materials is the Su–Schrieffer–Heeger (SSH) model 4 , 5 , which was developed in 1979—decades before the recognition of topological insulators—to describe conducting polymers. Distinct from the vast majority of known topological insulators with two and three dimensions 1 – 3 , the SSH model predicts a one-dimensional analogue of topological insulators, which hosts topological bound states at the endpoints of a chain 4 – 8 . To establish this unique and pivotal state, it is crucial to identify the low-energy excitations stemming from bound states, but this has remained unknown in solids because of the absence of suitable platforms. Here we report unusual electronic states that support the emergent bound states in elemental tellurium, the single helix of which was recently proposed to realize an extended version of the SSH chain 9 , 10 . Using spin- and angle-resolved photoemission spectroscopy with a micro-focused beam, we have shown spin-polarized in-gap states confined to the edges of the (0001) surface. Our density functional theory calculations indicate that these states are attributed to the interacting bound states originating from the one-dimensional array of SSH tellurium chains. Helices in solids offer a promising experimental platform for investigating exotic properties associated with the SSH chain and exploring topological phases through dimensionality control. Unusual electronic states derived from topological helix chains were observed that support the emergent bound states in elemental tellurium.