fermi level

Picture an orchestra in which violins and trumpets play different parts but together create a powerful sound. In a crystal of the layered magnet Fe5GeTe2 doped with nickel, tiny regions of a different phase, Fe6GeTe2 nanoprecipitates, take on the role of those instruments. It is they, not the nickel itself, that keep the material magnetic at 478 kelvin, that is, at 205 degrees Celsius.

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.

Tuning a radio receiver to a station that changes frequency as you cross a phase boundary: that is how you might picture the challenge facing the researchers. In the metal YbRh2Si2, electrons behave as "heavy" particles: their effective mass is hundreds of times that of a free electron.

Hot water after a shower, warm air from ventilation, the heated walls of an engine — all this is heat below 100°C. Usually, it dissipates without doing any useful work. Now it can be captured and turned into electricity. Inside the new device is a wire made of a nickel-titanium alloy. This material remembers its shape: when heated, it contracts; when cooled, it returns to its original shape.

In the quantum world, particles sometimes behave like acrobats on invisible tightropes. An instanton field is a brief vortex that appears and vanishes in an instant. When a fermion (a particle like an electron) enters such a vortex, its energy does not stay put. Instead, it smoothly slides from one level to another, like a ball rolling down a hill.

In a tight row of atoms, spaced closer than the wavelength of light, a remarkable state hides. The excitation is distributed among all particles so that their radiation cancels itself out. This quantum 'silence' is an ideal vault for a photon: it can be stored there hundreds of times longer than usual.

The wind carries pollen, and each grain moves as if on its own. But once the air swirls into a vortex, the pollen trajectories can no longer be separated: they are tangled. This is roughly how electrons behave in a strange metal: their motion is so strongly coupled to soft fluctuations of the medium that the usual particle carriers disappear.

The furnace is white-hot, but the ceramic powder remains loose. Turn on the electric field, and within seconds the material settles, hardens, and turns into a solid monolith. This technique is called flash sintering (from the English "flash"), and it saves hours of firing and gigawatts of energy. But what exactly happens to the atoms at the moment of the flash remained a mystery.

A molecule works like a marketplace: electron shoppers enter through two gates (contacts) and head for stalls with the most attractive goods — the chemically active spots. Previously, current was thought to spread evenly, but it turns out up to 90% of all flow rushes through these 'hot spots'. Reactive centers become superconducting highways inside the molecule.

The thinner the ferroelectric film, the stronger the depolarizing field — a dictate of classical physics. Imagine a pencil balancing on a fingertip: the shorter it is, the harder to keep it upright. Nanoscale polarization likewise strives to vanish.

Using laser spectroscopy — a method akin to listening to the echo from a crystal — physicists examined the layered material. They found regions where electrons move freely (Fermi pockets) and noticed that each such region behaves in its own way.

A rubber band that gets thicker the harder you pull it. In electricity, this behavior is called negative capacitance: the more charge accumulates on the plates, the lower the voltage. A regular capacitor works the opposite way — voltage rises with charge.

The atomic nucleus is not a solid ball but a dense swarm of tiny particles called quarks, glued together by a special glue (the strong force). In light nuclei, such as calcium, this swarm is simply arranged: the numbers of the two quark types, up and down, are almost equal, like the white and black keys in an octave.

A copper wire under current is a familiar sight, but inside it something unexpected is happening. A strong electric current doesn't just heat the metal: it knocks atoms out of their rightful places, leaving behind tiny voids, as if a few balls were suddenly removed from a tightly packed arrangement.

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.

Radio waves that oscillate less than once per second pass through the earth and water with almost no loss. Such signals are used to communicate with submarines and to search for minerals. But a conventional antenna for them would need to be hundreds of meters long, and its sensitivity leaves much to be desired.

A power transistor made of silicon carbide operates under voltages thousands of times higher than what comes out of a wall outlet. If the field inside it becomes uneven, the device burns out. Previously, this field was invisible: it is too strong for ordinary sensors, and the device must remain under voltage.

A solar flare is like an explosion in the atmosphere: bright, fast, with different 'voices.' X-rays come from electrons that have been accelerated and then abruptly slowed down, losing energy. But in 2017, detectors on the Fermi satellite caught a different signal — gamma rays that didn't fit this picture.

An oven with three racks, each baking a different dish: that's the image that comes to mind when you look at the galaxy I Zw 18. The top rack is blazing at 21,200 kelvins, the middle one at 17,290, and the bottom at 16,170.

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.

The law discovered by Wolfgang Pauli forbids identical particles from occupying the same spot. Hence, in cold matter, a Fermi sea is born: particles, like water, fill energy levels from bottom to top. The shape of the sea's surface — including 'islands' of emptiness — dictates the properties of the whole system.

The dance floor where magnetic moments and conduction electrons take turns leading and following: that is the Kondo lattice in the crystal V5S8.

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.