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<jats:title>Numerics converging on stripes</jats:title> <jats:p> The Hubbard model (HM) describes the behavior of interacting particles on a lattice where the particles can hop from one lattice site to the next. Although it appears simple, solving the HM when the interactions are repulsive, the particles are fermions, and the temperature is low—all of which applies in the case of correlated electron systems—is computationally challenging. Two groups have tackled this important problem. Huang <jats:italic>et al.</jats:italic> studied a three-band version of the HM at finite temperature, whereas Zheng <jats:italic>et al.</jats:italic> used five complementary numerical methods that kept each other in check to discern the ground state of the HM. Both groups found evidence for stripes, or one-dimensional charge and/or spin density modulations. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1161" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aak9546">1161</jats:related-article> , p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1155" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aam7127">1155</jats:related-article> </jats:p>

<jats:title>Numerics converging on stripes</jats:title> <jats:p> The Hubbard model (HM) describes the behavior of interacting particles on a lattice where the particles can hop from one lattice site to the next. Although it appears simple, solving the HM when the interactions are repulsive, the particles are fermions, and the temperature is low—all of which applies in the case of correlated electron systems—is computationally challenging. Two groups have tackled this important problem. Huang <jats:italic>et al.</jats:italic> studied a three-band version of the HM at finite temperature, whereas Zheng <jats:italic>et al.</jats:italic> used five complementary numerical methods that kept each other in check to discern the ground state of the HM. Both groups found evidence for stripes, or one-dimensional charge and/or spin density modulations. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1161" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aak9546">1161</jats:related-article> , p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1155" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aam7127">1155</jats:related-article> </jats:p>

<jats:title>Gravity gets into the earthquake game</jats:title> <jats:p> Earthquakes generate large movements of mass, which slightly change the gravitational field. Unlike the elastic waves that propagate from the earthquake, the gravity perturbations travel at the speed of light. Vallée <jats:italic>et al.</jats:italic> have finally observed these gravity perturbations in seismometer records from the great Tohoku earthquake in Japan in 2011. The signal would have allowed an accurate magnitude estimation in minutes, rather than hours, for this catastrophic earthquake. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1164" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao0746">1164</jats:related-article> </jats:p>

<jats:title>Calcium can breach benzene's defenses</jats:title> <jats:p> Calcium plays a major, multifaceted role in biology and mineralogy. In organic chemistry, though, it is largely overlooked and overshadowed by the carbon compounds of its cousins lithium and magnesium. Wilson <jats:italic>et al.</jats:italic> now report that the element was just biding its time: Several organocalcium compounds that they prepared can alkylate benzene by displacing a hydride, with no need for a more conventionally reactive leaving group such as chloride (see the Perspective by Mulvey). This surprising, previously elusive reaction attests to the unusual nucleophilicity of the carbons bound to calcium. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1168" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao5923">1168</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1132" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aaq1314">1132</jats:related-article> </jats:p>

<jats:title>Fine-tuned for image formation</jats:title> <jats:p> We typically think of eyes as having one or more lenses for focusing incoming light onto a surface such as our retina. However, light can also be focused using arrays of mirrors, as is commonly done in telescopes. A biological example of this is the scallop, which can have up to 200 reflecting eyes that focus light onto two retinas. Palmer <jats:italic>et al.</jats:italic> find that spatial vision in the scallop is achieved through precise control of the size, shape, and packing density of the tiles of guanine that together make up an image-forming mirror at the back of each of the eyes. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1172" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aam9506">1172</jats:related-article> </jats:p>

<jats:title>Putting photons to work</jats:title> <jats:p> Interacting quantum particles can behave in peculiar ways. To understand that behavior, physicists have turned to quantum simulation, in which a tunable and clean system can be monitored as it evolves under the influence of interactions. Roushan <jats:italic>et al.</jats:italic> used a chain of nine superconducting qubits to create effective interactions between normally noninteracting photons and directly measured the energy levels of their system. The interplay of interactions and disorder gave rise to a transition to a localized state. With an increase in the number of qubits, the technique should be able to tackle problems that are inaccessible to classical computers. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1175" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao1401">1175</jats:related-article> </jats:p>

<jats:title>A plasmonic route for mixing waves</jats:title> <jats:p> Nonlinear optics typically requires photons to interact over distances spanning hundreds or thousands of wavelengths. Nonlinear optical devices therefore tend to be bulk components. Nielsen <jats:italic>et al.</jats:italic> used a polymer material with a high nonlinear coefficient that they embedded within a plasmonic cavity to show that the interaction length scale could be reduced dramatically. The plasmonic cavity focused the light down to the nanoscale, providing an intense electromagnetic field that induced the nonlinear process of four-wave mixing in the polymer. The technique provides a versatile platform for compact nonlinear optical devices. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1179" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao1467">1179</jats:related-article> </jats:p>

<jats:title>Lighting the way to drug labeling</jats:title> <jats:p> It is important during drug development to study how candidate compounds get absorbed and broken down biologically. One common technique for tracking a drug's fate is to label its molecular framework with heavier isotopes of hydrogen (either deuterium or tritium). Loh <jats:italic>et al.</jats:italic> developed a light-promoted protocol to install these labels on alkyl carbons adjacent to nitrogen. The technique relies on incorporation of the heavy isotope into a thiol from a convenient heavy water source through acid-base chemistry. Next, a photoredox catalyst strips a hydrogen atom equivalent from the carbon, and the thiol engages in radical chemistry to transfer the deuterium or tritium in its place. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1182" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aap9674">1182</jats:related-article> </jats:p>

<jats:title>Going with the grain boundaries</jats:title> <jats:p> Bulk defects in a metal, such as grain boundaries, can create regions of increased strain at its surface that could affect its catalytic activity. Mariano <jats:italic>et al.</jats:italic> studied the electroreduction of CO <jats:sub>2</jats:sub> to CO on polycrystalline gold films, a reaction that competes with H <jats:sub>2</jats:sub> evolution. By annealing the films to create larger grains, they could change the types and distribution of grain boundaries at the surface. Scanning electrochemical cell microscopy revealed that the dislocation density correlated with CO <jats:sub>2</jats:sub> electroreduction activity, but such defects had no effect on H <jats:sub>2</jats:sub> evolution. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1187" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao3691">1187</jats:related-article> </jats:p>

<jats:title>Minimizing losses at interfaces</jats:title> <jats:p> Among the issues facing the practical use of hybrid organohalide lead perovskite solar cells is the loss of charge carriers at interfaces. Hou <jats:italic>et al.</jats:italic> show that tantalum-doped tungsten oxide forms almost ohmic contacts with inexpensive conjugated polymer multilayers to create a hole-transporting material with a small interface barrier. This approach eliminates the use of ionic dopants that compromise device stability. Solar cells made with these contacts achieved maximum efficiencies of 21.2% and operated stably for more than 1000 hours. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6367" page="1192" related-article-type="in-this-issue" vol="358" xlink:href="10.1126/science.aao5561">1192</jats:related-article> </jats:p>