Niels Bohr

Light strikes a metal plate, and electrons fly out. This is the photoelectric effect, and it works only if the light has enough energy to cross the threshold that holds electrons inside.

Electronic charge is a tumbleweed that moves camp depending on energy. In tungsten selenide (WSe₂) crystals, this is seen with utmost clarity: at a bias of −1.4 V, near the K point of the Brillouin zone, the maxima of tunneling current sit in the centers of hexagons, where there are no atoms.

Niels Bohr insisted: you cannot simultaneously know which path a photon took and see interference. For a long time, this was considered a limitation of knowledge — we simply cannot obtain both types of information at once.

An ordinary insulator conducts no current, neither in its interior nor along its edges. A topological insulator is a different story: it remains insulating inside but conducts electricity without loss along its edges.

Topological phases usually hide inside cryostats. Superconducting circuits, ultracold atoms, laser pulses: all of them shield quantum order from thermal noise.

In 1976, Gerard 't Hooft noticed that the vacuum of a gauge theory is not as simple as it seems: it possesses a topological structure that manifests itself in a nonzero topological susceptibility.

Imagine a stack of pancakes where each pancake is magnetized, but the magnetization directions in adjacent pancakes are opposite. That's exactly how this new magnetic material is arranged: in each atomic layer, all magnetic moments — the atom's 'arrows'—point in one direction, while the next layer is magnetized in the opposite direction.

A quantum particle, an inhabitant of the elementary particle world, resembles a coin spinning in the air: neither heads nor tails, but a blend of possibilities. Yet when it lands on the table, the outcome becomes definite. What serves as the table in a measurement?

Two pieces of graphite rub against each other, and the resistance vanishes as if an invisible film of oil separated them. Previously, this was only seen under a microscope, on tiny contact patches.

A thin sheet of carbon, resembling an intricate mesh of triangles, hexagons, and octagons, lies on a substrate. By itself, it is not superconducting: electrons move through it haphazardly, bumping into atoms. But stretch the sheet evenly, like canvas on a frame, and the picture changes.

Spacetime at ultra-short distances resembles a digital screen: it's not smooth but made of tiny 'pixels.' This minimum length was calculated by Max Planck, and Heisenberg's uncertainty principle explains why the world cannot be continuous: precise measurement always…

Electrons in a conductor usually behave like a gas: they dart around chaotically, collide, and don't hold formation. But if you spread them far apart and cool them down, their mutual repulsion can force them into place, like attendees in an overcrowded hall who settle into every other seat.

A sheet of germanane just one atom thick sits on a piece of germanium, and current races across it like a highway with no traffic lights. Germanane is a flat lattice of germanium atoms covered with hydrogen, a cousin of graphene.

A quantum computer is like an orchestra that can only perform in complete silence. The slightest noise, and the delicate melody falls apart. For quantum devices, heat is that noise: even faint infrared radiation from nearby components heats up the qubits and erases their quantum states.

A quantum sensor is like a telegraph line transmitting a hypersensitive message. The slightest disturbance distorts the signal. It used to be thought that accurate measurement required an error-free 'telegraphist' — a perfect error-correction system. But in practice, all elements are noisy. The new approach allows for errors at every step: in sending, verification, and reception.

A piece of chromium exposed to air gets covered in invisible armor, an oxide film, within a minute. That film protects the metal from rust, but for a battery it's a disaster: the film blocks chromium ions, so no current flows. Scientists mixed five metals — chromium, bismuth, copper, tin, and nickel.

A graphene sheet is placed in a solution of sodium sulfate — an ordinary salt, similar to table salt. Near the surface, water stops being uniform: it arranges into layers, like a stack of pancakes. Between and within these layers, ions, charged particles from the dissolved salt, distribute themselves.

Graphene is a sheet of carbon one atom thick. If you cut it into narrow strips, you get graphene nanoribbons. Their properties depend on how the strip is cut: just as from a single piece of fabric you can sew both a smooth tablecloth and a pleated skirt. In the new work, chemists assembled a ribbon of asymmetric shape.

A graphene transistor works like a microscopic switch: a molecule lands on its surface, and the current changes. But a bare graphene surface catches nothing. You need a "bait"—antibodies that recognize only viral proteins.

A cube of potassium tantalate behaves like an ordinary dielectric: it doesn't store an electric field. But grow this material as a thin film on a substrate of another crystal, and everything changes.

A crystal one atom thick is like a sheet of paper you can write on with electricity. Such a material promises to become the foundation for tiny, fast microchips. But before putting it into production, you need to know exactly what it's made of. And that's where the usual formula falls apart.

According to the laws of physics, nothing can outrun light, otherwise for some observers the effect would occur before the cause. Artur Ekert and Andrzej Dragan showed that this situation creates an uncertainty resembling quantum indeterminacy.

If you force water through a narrow neck, the flow speeds up right at the edge, and a whirlpool forms behind the obstacle. Electrons in ultra-clean two-dimensional materials, such as a thin layer of graphene, behave in much the same way.

If you dump a handful of coins on a table, the surface becomes bumpy. But add more coins, and they spread out into a flat layer on their own. Something similar happens when you deposit ultra-thin films of individual molecules onto a substrate. At first, islands appear on the surface — molecules cluster together like people in a queue. The film gets rougher and rougher.

A graphene ribbon nine atoms wide is like a track where electrons race, as water rushes down a narrow trough. Gamma radiation is a stream of particles with enormous energy, capable of punching through metal and plastic.

An ordinary laser amplifies light into a bright beam. Run the process in reverse, and you get an anti-laser — a device that sucks in light without a trace. But such a machine is finicky: even a slight disorder can ruin the absorption. Physicists have found a way to make the anti-laser invulnerable.

A thin film of metal oxide lies on a surface with a tiny pit. If you gently compress the film from the sides, it won't crumple — instead, it will bow downward into the pit or bulge upward like a hump. Both positions are stable, like a canning jar lid that is either concave or convex and doesn't flip back and forth on its own.

In a superconductor, electrons move in pairs, like dance partners. Usually, through a junction between two superconductors, one pair passes at a time. But add a third contact, and the dance gets more complex: two pairs can move together, forming a foursome. It's this four-part resonance that has been seen for the first time in a graphene junction with three terminals.

An ordinary octane molecule is a chain of eight carbon atoms, like a flexible wire. On a hot platinum surface (above 600 K), it doesn't just lie still: it starts to squirm. First, the chain bends so that its ends come closer together.

Magnetic materials can store oscillatory energy, like a tuning fork that keeps ringing after being struck. When such a material is placed in a microwave cavity, its oscillations (magnons) start interacting with light (photons).

Steel is not just a piece of iron. Inside it, like in a layered pie, different structures, phases, coexist. In ultra-strong steel for cars, there are three such phases: soft ferrite, strong martensite, and ductile austenite. Medium-manganese steel owes its strength precisely to this trio.

A light pulse, like a lump of clay, keeps its shape when passing through an ordinary material. But if you put a special layer just one atom thick in its path, the pulse starts to spread: its spectrum widens.

In a thin layer of a crystal, electrons behave differently than in an ordinary piece of metal. They are cramped, collide with each other more often, and feel the shape of the lattice more strongly. Usually, such states are created artificially by stacking layers of different materials. But sometimes nature does it all by itself.

In layers of molybdenum telluride twisted at a specific angle, electrons behave unusually: they can split into parts. This is akin to a whole pie suddenly being cut into perfectly equal slices, even though there's only one knife. In ordinary materials, the electron's charge is indivisible, but in these layers, under strong cooling, states emerge where the charge splits into three parts.

A thin film of ruthenium oxide Sr₃Ru₂O₇ grown on a foreign crystalline substrate behaves like a tightly stretched rubber sheet: the substrate either compresses it or stretches it.

A thin film of a complex oxide is like a transfer tattoo: you need to carefully peel it off one surface and stick it onto another without tearing it. Previously, water-soluble layers were used for this, but they either dissolved too slowly or damaged the film itself. Now a new 'glue' has been proposed: barium oxide. It disappears in water within minutes, leaving the film intact.

Heat one end of a conductor and keep the other cold, and electrons start flowing from hot to cold. Usually, this creates a voltage along the conductor, like water running down a tilted trough. But in some materials, electrons veer sideways, and the voltage appears across the conductor.

A thin nanowire of α-Bi₄Br₄ — a compound of bismuth and bromine — conducts current not like an ordinary wire. Inside, electricity moves only along the edges, like water in a trough, not touching the bottom or sides. This behavior is called topological insulation: the interior does not conduct, but electrons glide losslessly along the surface or edges.

A thin film of iron oxide SrFeO3 sits on a substrate that slightly stretches it. Inside the film, the magnetic moments of atoms twist into a spiral, like a spiral staircase. The strain changed that staircase: the step became shorter, and the axis itself tilted. The reason lies in oxygen.

Twisted layers of atoms in a material form a pattern of tiny regions, domains, each of which can be electrified in one of two directions, like a compass needle pointing north or south. Previously, such materials were studied only as a whole, measuring thousands of domains at once, and their collective behavior was like a choir where individual voices are not heard.