What is magnetic phase transition

Inconclusive quanta

In ytterbium nickel phosphide there is a quantum-critical point between ferromagnetism and the non-magnetic state that was previously not considered possible

What does melting ice have in common with high-temperature superconductivity? Nothing, and yet there is a strange connection. This great, unsolved puzzle of physics and other quantum phenomena have to do with so-called phase transitions, which also include the melting of ice. However, it is “quantum phase transitions” that are closely linked to such quantum phenomena. They exist at the very bottom of the temperature scale, at absolute zero. Physicists from the Max Planck Institute for Chemical Physics of Solids in Dresden have now created an exotic material: At extremely low temperatures, it does not know whether it should undergo a phase transition into a magnetic state or not. It is at a quantum critical point. Such strange states are considered to be the key to a better understanding of exotic phenomena such as high-temperature superconductivity, in which a material loses its electrical resistance even at relatively high temperatures.

At the moment we experience almost every day that snow and ice melt into water or, conversely, water freezes into ice. If you look closely, such a change in properties is dramatic; in physics it is called a phase transition. In general, a rising temperature first causes melting and later evaporation; this applies to water as to other materials. As in any solid, the molecules in ice are neatly parked in their crystalline positions. The supply of thermal energy releases them from these parking positions and drives them into an increasingly teeming molecular traffic: the ice melts into liquid water. Phase transitions are therefore transitions between states with different degrees of order.

Manuel Brando's “Extreme Conditions Competence Group” at the Max Planck Institute for Chemical Physics of Solids in Dresden is also interested in phase transitions. However, these “quantum phase transitions” are as extreme as they are puzzling. In order to research them, physicists have to expose their samples to extremely low temperatures. We also know the phenomenon that the Dresdeners use as a laboratory mouse, so to speak, from everyday life. It is magnetism, even in its sensually tangible form, which is precisely called ferromagnetism - from the Latin word ferrum for iron. Ferromagnetism is also based on a certain form of order.

Many exotic phenomena are linked to a quantum critical point

In iron, the phase transition from the non-magnetic (“paramagnetic”) to the ferromagnetic state occurs below 770 degrees Celsius or 1043 K (K stands for the Kelvin temperature scale). If water freezes to ice, the temperature display will hang until the water has given off so much heat that it has completely solidified. This energy-intensive temperature hanger is called the first-order phase transition. The development of ferromagnetism, on the other hand, is characterized by cooling without such a hanger. This behavior, which glides over the temperature scale, is the characteristic of a second-order phase transition.

The phenomenon that the Dresden physicists are on the trail also has to do with such second-order phase transitions. It's called the quantum critical point, and it actually only exists at absolute temperature zero. "At a quantum-critical point, the strange thing is that it has an effect at much higher temperatures," says doctoral student Alexander Steppke: "Many exotic phenomena are linked to it." One of them is the still puzzling high-temperature superconductivity, which is relatively warm Temperature clusters up to 135 Kelvin (minus 138 degrees Celsius) feels comfortable.

A quantum critical point is generally characterized by the fact that the boundary between two different quantum phases disappears. In the case of the Dresdeners, the sample can no longer decide at this point whether it wants to be non-magnetic or ferromagnetic. In principle, with such a quantum phase transition, thermal energy is no longer allowed to provide the drive as with ice, because such transitions only exist at absolute temperature zero. That is 0 Kelvin or minus 273.15 degrees Celsius.

Foreign atoms in the crystal lattice put the material under negative pressure

So the Dresdeners have to use another lever, and that is the pressure. However, a mechanical press is ruled out. “First of all, we need enormous pressures in the gigapascal range,” says Steppke. With such pressure, industry presses carbon into diamond. “Secondly, we also need these pressures in negative form,” the physicist continues: “We have to forcefully relax the sample, so to speak.” This is only possible with “chemical pressure”. For this purpose, the Dresden-based company specifically builds foreign atoms into the crystal of their samples, which lower the pressure in the three-dimensional crystal lattice. The trick is not to change the other properties of the samples despite this deliberate contamination.

For their experiment, Manuel Brando's group had to create a world record. To do this, her colleague Christoph Geibel and his “Material Development Competence Group” at the institute had to design an unprecedented material. Unlike iron, for example, it only becomes ferromagnetic in the vicinity of absolute temperature zero. Thanks to their experience, Brando says: “The new material ytterbium nickel phosphide has the lowest Curie temperature that has ever been observed!” This temperature, named after the French Nobel laureate in physics, Pierre Curie, describes the point at which the phase transition to ferromagnet takes place. But what creates the magnetism?

Spins can still rotate even at absolute zero

Magnetic atoms in the crystal are responsible for this, here the ytterbium. Electrons sit on such atoms and behave like rotatable, tiny elementary magnets. Their “spins”, which ensure their micro-magnetism, sense each other. In the case of ferromagnets, all these spins rotate in one direction, and their collective order provides for the magnetism on a large scale. For ytterbium nickel phosphide, or YbNi for short4P2, this ferromagnetic transition is now so close to absolute temperature zero that another famous quantum effect strikes: the Heisenberg uncertainty relation. Actually, in this extreme cold, no phase transition would be possible. Because where there is no thermal energy, all movement freezes. The electron spins could no longer switch back and forth between ferromagnetism and non-magnetic disorder. "Because of the uncertainty relation, however, their energy is not exactly determined," says Steppke, "and that's why they can turn."

With this sophisticated experiment, the Dresdeners succeeded for the first time in observing a quantum-critical point in the transition between ferromagnetism and the non-magnetic state in a metal. Current theories had previously ruled out its existence and now need to be improved. The basic researchers from Dresden hope that with such experiments they can also help solve the riddle of high-temperature superconductivity.

RW / PH