semimetal

A thin layer of indium arsenide lies hidden under a superconducting film, like a voice beneath the noise of a crowd. In such a hybrid, current takes the path of least resistance and flows almost entirely through the metal, leaving the semiconductor inaudible to standard probes.

Current in an ordinary metal flows like a crowd of people down a straight street: each electron is on its own, and if you turn on a magnetic field, the whole crowd shifts slightly sideways. This shift is called the Hall effect.

The magnetic framework of the EuMnSb2 crystal resembles scaffolding: manganese atoms form rigid vertical pillars, while europium atoms act as movable crossbars. Until now, there was no consensus on the angle at which these crossbars rest and how they rotate when a magnetic field presses on the scaffold.

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 sheet of paper with magnetic properties: that's roughly what the new material 1T-CrS₂ is like. Its layers are held together by weak forces, like the pages of a notebook, but each layer is a true magnet.

In a crystal, electrons are not distributed randomly. Their waves obey the lattice symmetry, like ripples on water taking the shape of a hoop thrown into it. The new work shows that at high-symmetry points, this ripple must have nodes — places where electron density is strictly zero.

Electrons in a solid usually either run freely (metal) or are locked in place (insulator). But sometimes strong mutual repulsion locks them into a halfway state: charges stand still, while spins, tiny magnetic arrows, continue to live their own life. This is called a Mott insulator.

Two sheets of tungsten diselenide (not carbon, but a compound of tungsten and selenium) are rotated relative to each other by a tiny angle, about 5 degrees. This creates a moiré pattern: an enlarged periodic lattice where electrons behave very differently than in a regular crystal.

Electrons in a crystal are like cars on city streets: some race down avenues, others get stuck in traffic jams. In certain lattices, motion freezes entirely — velocity stops depending on energy, and electrons become "flat." This isn't a traffic jam but a special rhythm of the network where waves cancel each other out.

Two paper-thin sheets of a semiconductor are stacked and slightly rotated — by about three to four degrees. The atomic lattices don't match, creating a large-scale pattern — a moiré, like the ripples from two washboards shifted by a small angle. In this pattern, electrons move quite differently than in a regular crystal.

In familiar metals like copper, electrons move almost independently, like cars on an open highway. Resistance arises from rare collisions and can be described by simple formulas.

A crystal is like a city: if the streets are crooked, traffic jams. In a crystal, the streets are atomic rows, and the traffic is the flow of electrons. Any tiny imperfection, impurity, or atomic displacement makes electrons scatter. Scientists from several labs have learned to grow crystals so that the "streets" are almost perfectly straight. The method is called horizontal vapor transport.

Honeycombs are built from identical cells, but tweak the shape of just one, and the whole structure gains new properties. Researchers did something similar to create a magnet without a single metal atom. They took flat carbon molecules shaped like triangles, triangulenes, and arranged them in a hexagonal lattice, just as bees build honeycombs.

An ordinary crystal is like an apartment building: electrons move through shared hallways and staircases, influencing each other through the walls. Now, imagine removing all the walls, leaving just a single line of apartments. How would the residents behave? Hydrogen has nothing to do with it, but the collective behavior of electrons takes center stage.

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.

A mosaic floor made of many small tiles, each turned in a different direction. It seems that any pattern trying to cross such a floor would get scrambled and vanish. That's how people usually think about disorder in crystals: if atoms aren't arranged in a perfect lattice, delicate quantum effects should fall apart.

When you flip a switch, the light doesn't come on immediately — electrons in the wires need time to 'get going' and move together. In ordinary metals and semiconductors, current builds up gradually, like water in a hose when you first open the tap.

The dance floor is packed with people, each moving on their own. Suddenly, some of them break into pairs (spinning around, barely noticing their neighbors) while the rest line up in chains. That's roughly how atoms behave in the metal CsCr₃Sb₅, a member of the kagome metals, whose crystal lattice resembles a woven basket.

A thin wire of the new material conducts electricity without loss when cooled to three degrees above absolute zero. It's like water flowing through a pipe without any friction against the walls. The material is called PtPb3Bi, and its hallmark is an internal structure resembling a bundle of threads: electrons run freely along a thread but struggle to jump between threads.

A woven basket made of willow twigs holds its shape thanks to the interweaving. A similar pattern, triangles linked into a flat network, has been found in a new crystal with the formula Cs₃V₉Te₁₃. Here, vanadium atoms form two sets of triangles that pierce each other like threads in fabric. This pattern is not just pretty: it forces electrons to move along special "flat" energy pathways.

A sheet of graphene one atom thick is like a flexible metal mesh through which electrons zip almost unimpeded. Stack two such sheets with a slight twist, and the material's properties change dramatically: it can become an insulator that blocks current.

A sheet of graphene one atom thick is twisted at a small angle, and it starts behaving like a layered pie with layers slightly rotated relative to each other. At a certain twist angle, this material becomes a superconductor: electrons begin to move in pairs, like dancers in a waltz, without losing energy. But what exactly happens inside has remained a mystery.

In the woven basket pattern, where the strands interlace into triangles, some electrons get stuck in place, like coins wedged in the gaps of the weave. Other electrons roam freely across the whole basket, like light filling a room. Physicists have long sought the moment when these two types of electrons start to influence each other, and now it has been found in the material CsCr6Sb6.

A piece of silver antimony telluride is like a sponge: it readily soaks up heat and turns it into electricity. Such materials are called thermoelectrics, and they already work in space probes and car refrigerators. But if you squeeze this sponge, it starts behaving like a superconductor — current flows with zero resistance.

A metal is a sea of electrons that move almost freely, colliding only occasionally. But in some materials, this sea suddenly becomes viscous and unpredictable: electrons stop behaving like independent particles. Physicists call this state 'non-Fermi liquid,' and it remains a puzzle.

A quantum computer is like an orchestra where every musician must hit their note without going flat. But superconducting qubits share a common ailment: random charges accumulate on their electrodes, and the 'note' drifts. Engineers call this charge offset drift, and usually battle it for years.

A crystal of osmium dioxide, OsO₂, is like a stubborn teenager: under ordinary conditions it behaves like a typical metal, but press hard enough and its character changes.

A quantum computer is like an orchestra where every musician must come in exactly on the beat. The faintest rustle in the hall throws off the violinist, and the melody falls apart. In silicon chips, that rustle is electrical noise: random fluctuations of charge inside the material. They make quantum bits err.

Two-dimensional bismuth on a silicon substrate is a promising material for future electronics. Its edges conduct current without loss, like a highway with no traffic lights. But bismuth itself is delicate: air and moisture quickly degrade its properties.

A quantum bit is like a spinning top that must keep spinning steadily for a long time. In germanium, researchers have learned to set such tops spinning, but the floor around them is always trembling: electrical noise knocks the spin off course.

An electron in a metal is usually like a person in rush hour: it constantly bumps into others and loses its way. But if you remove the obstacles, it can fly in a straight line, like a car on an empty night highway. This kind of motion is called ballistic — from the word "ballista," an ancient catapult that hurled a stone without interference.

The popping lid of a jam jar is the hero of this story. Press it, and with a loud click it flips from a convex to a concave shape. Exactly the same happens with a hinged beam inside each cell of a new mechanical lattice. Scientists assembled these snapping beams into a hexagonal grid, like honeycomb, but made of elastic plates. When one beam is sharply compressed, it snaps and pushes its neighbor.

An ordinary rubber pad dampens vibration the way a thick blanket muffles sound: the thicker and heavier, the better. Engineers have long dreamed of a material that is light yet "eats" oscillations as well as a massive damper.

Imagine a superhighway where cars zoom along with barely a slowdown: that's what a layer of ultraclean semiconductor looks like, where electrons travel vast distances without a single collision. Scientists have built a transistor in which electrons run twice as fast as in any similar device before.

The classic BCS theory of superconductivity, developed by John Bardeen, Leon Cooper, and Robert Schrieffer, describes the mechanism by which electrons pair up into Cooper pairs and move without resistance.

In NaAlSi, as it cools, electrons form fleeting ripples — stripes that appear and vanish, like patterns on water stirred by wind. These stripes intertwine with superconductivity, a state where current flows without resistance. Typically, for this, electrons pair up with uniform binding energy.

Diamond is not just a gemstone. This crystal can conduct electricity when free charge carriers, electrons and holes, appear in it. But how fast do they move? In scientific papers, numbers vary by tens of times. One lab reports a mobility of 1000 cm²/(V·s), another 4000.

Metals are full of free electrons: they dart between atoms like a swarm of bees around a hive. If you poke such a metal with a thin magnetic filament, the electrons start to feel its field. A new calculation shows they don't just go around the field; they trap it inside, like a cork sealing water in a bottle.

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.

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.