Electron Quadruplets Suggest New Physics, Applications

Novel electron pairs of pairs may be commonplace and even find use in sensors

2 min read
Close-up of gold wires attached to a yellow material with a strip of grey and blue blobs

The iron-based superconductor material, Ba1−xKxFe2As2, is mounted for experimental measurements.

Vadim Grinenko and Federico Caglieris

Experiments have now revealed a new state of matter—electron quadruplets—which researchers suggest may one day lead to new kinds of sensors as well as untold other novel applications.

The new discovery invites comparisons to the mechanisms underlying superconductors, materials that conduct electricity without dissipating energy. Superconductivity relies on electrons not repelling each other as they do in ordinary materials, but instead forming weakly bonded duos known as Cooper pairs, which can flow with zero resistance.

Nearly 20 years ago, study senior author Egor Babaev, a theoretical physicist now at the KTH Royal Institute of Technology in Stockholm, Sweden, and his colleagues suggested it was also possible for electrons to form quartets. They later predicted electron quadruplets could form within materials such as barium potassium iron arsenide.

"This is a new state of matter, that we believe is no less interesting than superconductivity and superfluidity," Babaev says.

The researchers suggested these "quartic phases" could arise before materials achieved superconducting states, when temperature and other conditions prevented the condensation of Cooper pairs but allowed the formation of electron foursomes.

"At that time our theoretical works were considered something impossible by most people and did not have any resonance in the community," Babaev says.

The first experimental glimpses of this novel state happened accidentally in 2017, when study lead author Vadim Grinenko, an experimental physicist at the Technical University of Dresden in Germany, and his colleagues discovered superconductivity in barium potassium iron arsenide. They discovered thermal, electrical and magnetic anomalies they could not account for even though "a great effort was taken to make better measurements to get rid of that impossible effect," Grinenko recalls.

After Babaev and Grinenko met by chance at a 2018 conference in Stockholm, Babaev realized Grinenko potentially discovered electron quadruplets. Substantiating their findings enough for a scientific journal to accept took more than three more years of research, they recall.

"In our work, we report the first experimental realization of the quadrupling state," Grinenko says.

One exotic property of the quartic state is "there are spontaneously forming flows that produce local magnetic fields," Babaev says. Such spontaneous currents and magnetic fields are not seen with Cooper pairs and typical superconductors, he notes.

It remains uncertain what applications this new discovery might hold, if any. However, the ways in which quartets of electrons can move in relation to each other can be significantly more complex than seen with pairs of electrons, so "we expect that a lot of new physics will be revealed, inevitably resulting in new applications," Babaev says. Grinenko does add the unconventional properties that appear with electron quartets could potentially find use in sensors.

The researchers predict there are many more materials where electron quadruplets can form. "Detection of this state is subtle and requires complex studies, and therefore it may have been overlooked in some already known materials," Grinenko says. He adds that electron quadruplets could be a quite general phenomenon, especially in two-dimensional films.

The scientists detailed their findings online Oct. 18 in the journal Nature Physics.

The Conversation (1)
FB TS30 Oct, 2021

Is there really any concrete evidence that Cooper pairs actually exists?Why that theory cannot explain all superconductors, if superconductivity really happens that way?What if, (all kinds of) superconductivity is just electrons creating a (2D/3D) superfluid state?

The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.