band gap

A layered cake of molybdenum disulfide MoS2 sits on the lab bench. It is prepared in the usual way: adhesive tape peels off the top layers to obtain a thin sample. But beneath the tape remains an invisible cavity where the atomic floors are no longer pressed together. It is there that electrons begin to behave differently than in the crystal bulk.

A cartographer receives a sheet with distances between hundreds of unnamed islands. No island is labeled, but the distances repeat, forming triangles and quadrilaterals. From these repetitions, the cartographer reconstructs the map of the archipelago, not knowing which island is which. This is roughly how the new work in molecular spectroscopy works.

A light switch clicks, and the light comes on. But imagine a switch that, with the same press, can either turn on the lamp or leave the room dark. It also remembers what it did last time, and sometimes it seems to flip a coin to decide whether to light up. This kind of behavior was found in a crystal of black phosphorus — a semiconductor just a few atomic layers thick.

A thin silver thread nearly as long as a hair's width, and inside it, pristine quantum laws hold sway. Physicists built a conductor where electrons can only move forward or backward, like on a perfectly straight single-lane road with no shoulders. On such a road, any pothole or pebble would normally destroy the fragile quantum state.

Hit a table, and the sound quickly fades. Elastic waves in solids usually behave like ripples on water: they spread from the source and die out. But there is a way to make them freeze in place — by creating a 'flat band' in the frequency spectrum. This is a range where waves lose their speed and pile up, like cars in a traffic jam.

The quantum world is often compared to a musical instrument. Each system, whether an atom or a chain of particles, sounds at its own frequencies, which physicists call energy levels. In chaotic systems, like an orchestra without a conductor, these frequencies usually fill the entire available range without gaps.

A quantum computer is like an orchestra where each ion is a musician and laser pulses are the conductor's baton. But if someone in the audience rustles, the delicate melody of quantum states goes off key.

A stone falls into a pond, and ripples spread across the water. Long waves and short ones travel at the same speed: that's how ordinary sound behaves in almost any medium. But in quantum liquids, things can be different. Physicists considered a superfluid where particles move along a "flat" energy band.

Sound travels through a medium at a certain speed. If the medium moves faster, the sound gets swept along by the flow — like a wood chip pulled into a whirlpool. In superfluid helium — a frictionless liquid — rapid rotation spawns vortex funnels. Inside them, an acoustic black hole forms: a region from which sound cannot return.

A short laser pulse hits an iodine molecule, and its atoms start to vibrate like a ball on a spring. Scientists track these vibrations with a second, broadband pulse that captures a spectrum at different moments in time. It turns out that vibrations in the excited B state quickly shift in frequency and fade out, and then vibrations appear in the A state.

A single-atom-thick sheet of tungsten telluride is like thin foil: electrons can't pass through because there's a band gap between their possible states, like a moat you can't jump across. Add two more layers, and the moat disappears; the material becomes a metal, and current flows. Add more layers, and the properties change again, as if someone is flipping modes with a single button.

Superconductivity is when electric current flows without losses, like water through a perfectly smooth pipe. It usually occurs at very low temperatures, but there are materials where it appears at higher ones.

Two quantum dots are like two tiny pockets in a piece of silicon where an electron can sit, like a ball in a cup. When the pockets are far apart, each electron lives on its own, and its energy resembles the levels of a solitary atom.

A chain of red phosphorus atoms is like a train where the cars are atoms and the couplers are chemical bonds. Sometimes the couplers are stronger, sometimes weaker, and that affects how electrons run through the train.

A sheet of carbon just one atom thick is placed in a strong magnetic field and cooled to thousandths of a degree above absolute zero. A microwave microscope slides along the sheet's edge, measuring how well that edge conducts electricity.

Layered cuprate crystals resemble a stack of pancakes: each pancake is a plane of copper and oxygen atoms, with spacer layers of other elements in between. Usually, superconductivity is studied in the whole stack, where layers influence each other.

Sound in a thin plate behaves like ripples on water: it travels, reflects off edges, and changes shape. But sound has a peculiar trait: it can pass through places where waves are forbidden to exist. It's as if ripples on a pond suddenly vanished before a stone barrier and reappeared on the other side, without going around it.

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.

High above Antarctica, a balloon carried an instrument that caught electrons from space. These particles come from distant stellar explosions and other powerful events. The instrument measured their energy, like the force of an impact. And in one energy range, there were more impacts than calculations predicted.