Volume 578

  • No. 7796 27 February 2020

    Made to order

    Chirality is a common property of materials in nature, perhaps most readily recognized in molecules that are left- and right-handed mirror images of one another. In condensed-matter physics, crystalline electronic structures can similarly display geometrical chirality, which tends to be predetermined when the crystal lattice is formed. But in some materials, electrons can spontaneously arrange themselves to impart chirality to a structure that was not chiral. This gyrotropic ordering has been described as the quantum equivalent of cholesteric liquid crystals, but it has proved hard to observe. In this week’s issue, Nuh Gedik and his colleagues report they have induced and observed gyrotropic ordering in a transition-metal dichalcogenide — the semimetal 1T-TiSe2. The cover image offers an artist’s interpretation of the team’s method, by which circularly polarized light shone on 1T-TiSe2 while cooling it below the critical temperature results in preferential formation of one chiral domain.

  • No. 7795 20 February 2020

    Twisted nerves

    The cover image shows an artist’s impression of solid tumours clustering near neurons. In this week’s issue, Moran Amit and his colleagues show that head and neck cancer cells can manipulate nearby nerve cells into promoting tumour growth. The researchers find that the cancer cells secrete vesicles containing small RNA molecules (microRNAs) that are taken up by nearby sensory nerve cells. If the cancer cells have a mutant form of the p53 protein, then the vesicles secreted do not feature a microRNA that blocks neuronal growth. As a result, the vesicles promote the proliferation of nerve cells and reprogram them to be adrenergic neurons, which aids tumour growth.

  • No. 7794 13 February 2020

    Triangular momentum

    Electrically driven semiconductor lasers are one of the most common types of laser and are widely used in our daily lives, from optical-fibre communication to laser printing and barcode readers. But the performance of these lasers is affected by semiconductor defects — damage or imperfections introduced during fabrication and packaging. In this week’s issue, Qi Jie Wang, Yidong Chong, Baile Zhang and their colleagues present a terahertz laser based on a quantum cascade laser that suggests such problems might be solved by taking advantage of the topological properties of light. By creating a triangular interface between two differently oriented photonic crystal lattices (as shown on the cover), the authors create topological edge states that can emit terahertz radiation even if there are sharp corners and defects present. As the laser is driven electrically, it could open the way for topological systems to find practical applications.

  • No. 7793 6 February 2020

    Cancer catalogued

    In this week’s issue, the Pan-Cancer Analysis of Whole Genomes (PCAWG) consortium presents the integrative analysis of more than 2,600 whole cancer genomes and their matching normal tissues across 38 tumour types. In a collection of six papers, members of PCAWG examine the evolution and structural variation of cancer genomes. They review genetic events that drive cancer and catalogue mutational signatures. And they report integrative analyses of structural variation, evolutionary trajectories and transcriptome data for many of the genomes and tumour types. Taken together, these papers offer a fresh perspective on the genetic complexity of cancer and provide a valuable resource for the exploration of the biological changes that drive the development of tumours.