Over the past few years, some materials have become proving grounds for physicists. These materials aren't exactly made of anything special - ordinary particles, protons, neutrons, and electrons. But they are more than just the sum of their parts. These materials have a whole range of interesting properties and phenomena, and sometimes even led physicists to new states of matter - in addition to solid, gaseous and liquid, which we have known from childhood.
One type of material that physicists are particularly worried about is the topological insulator - and, more broadly, the topological phases, the theoretical foundations of which led their inventors to the Nobel Prize in 2016. On the surface of a topological insulator, electrons flow smoothly, but inside they stand motionless. The surface is like a metal conductor and the inside is like a ceramic insulator. Topological insulators have attracted attention for their unusual physics, as well as for their potential applications in quantum computers and so-called spintronic devices that use the spin of electrons and their charge.
This exotic behavior is not always obvious. “You can't just say that, considering a material in the traditional sense, whether it has this kind of properties or not,” says Frank Wilczek, a physicist at MIT and a 2004 Nobel laureate in physics.
What else is a quantum atmosphere?
It turns out that many seemingly ordinary materials can contain hidden, but unusual and, possibly, useful properties. In a recently published paper, Vilchek and Kin-Dong Zhang, a physicist at Stockholm University, proposed a new way to explore such properties: by studying the subtle aura that surrounds the material. They called it the quantum atmosphere.
This atmosphere could reveal some of the fundamental quantum properties of the material that physicists could then measure. If confirmed by experiments, this phenomenon will not only be one of the few macroscopic manifestations of quantum mechanics, says Wilczek, but it will also become a powerful tool for researching new materials.
“If you asked me if something like this could happen, I would say that the idea makes sense,” says Taylor Hughes, a condensed matter theorist at the University of Illinois at Urbana-Champaign. And he adds: "I guess the effect will be very weak." In their new analysis, however, Zhang and Vilchek calculated that, in principle, the quantum atmospheric effect would be within the detectable range.
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Moreover, Wilchek notes, such effects may be detected very soon.
Impact area
The quantum atmosphere, Wilczek explains, is a thin zone of influence around a material. It follows from quantum mechanics that the vacuum is not completely empty; it is filled with quantum fluctuations. For example, if you take two uncharged plates and place them side by side in a vacuum, only quantum fluctuations with wavelengths shorter than the distance between the plates can squeeze between them. But from the outside, fluctuations of all wavelengths will fall on the plates. There will be more energy outside than inside, which will cause the combined force to squeeze the plates together. This is the Casimir effect and is similar to the effect of the quantum atmosphere, Wilczek says.
Just as a plate senses a stronger force as it approaches another, a needle probe will feel the effect of the quantum atmosphere as it approaches a material. “It's like a normal atmosphere,” says Wilchek. "The closer you are to it, the greater its impact." And the nature of this impact depends on the quantum properties of the material itself.
Antimony can act as a topological insulator - material that functions as an insulator everywhere except the surface.
These properties can be very different. Some materials act as separate universes with their own physical laws, as if they are in the multiverse of materials. “A very important idea in modern condensed matter physics is that we have materials at our disposal - say, topological insulators - within which a different set of rules operate,” says Peter Armitage, a condensed matter physicist at Johns Hopkins University.
Some materials act as magnetic monopoles - point magnets with a north pole but no south pole. Physicists have also discovered the so-called fractional electric charge quasiparticles and quasiparticles, which act as their own antimatter and can annihilate.
If similar exotic properties existed in other materials, they could reveal themselves in quantum atmospheres. A whole host of new properties could be uncovered simply by probing the atmospheres of materials, Wilchek says.
To demonstrate their idea, Zhang and Wilchek focused on an unusual set of rules - axion electrodynamics - that can lead to unique properties. Wilchek came up with this theory in 1987 to demonstrate how a hypothetical particle called an axion could interact with electricity and magnetism. (Prior to this, physicists put forward an axion to solve one of the greatest mysteries of physics: why interactions involving a strong force remain the same if particles are replaced by antiparticles and reflected in a mirror, preserving the symmetry of charge and parity (CP-symmetry). Until that day, no one had found any confirmation of the existence of axions, although not so long ago there has been an increased interest in them as candidates for dark matter.
While these rules will not work in most places in the universe, they quite manifest themselves inside a material - such as a topological insulator. “The way electromagnetic fields interact in these new substances, topological insulators, is essentially the same as if they were interacting with a collection of axions,” says Wilczek.
Defects in diamonds
If a material such as a topological insulator obeys the laws of axional electrodynamics, its quantum atmosphere can react to anything that crosses it. Zhang and Vilchek calculated that such an effect would be similar to the manifestation of a magnetic field. In particular, they found that if you put a particular system of atoms or molecules in the atmosphere, their quantum energy levels change. Scientists can measure the change in these levels using standard laboratory methods. “It's an unusual but interesting idea,” says Armitage.
One of these potential systems is a diamond probe with so-called nitrogen-substituted vacancies (NV centers). An NV center is a kind of defect in the crystal structure of a diamond, when a carbon atom of a diamond is replaced by a nitrogen atom, and a place close to nitrogen remains empty. The quantum state of such a system is highly sensitive, which allows NV centers to sense even the weakest magnetic fields. This property makes them powerful sensors that can be used for a wide variety of purposes in geology and biology.
The paper by Zhang and Vilchek, which they submitted to Physical Review Letters, only describes quantum atmospheric influence derived from axionic electrodynamics. To determine what other properties affect the atmosphere, Wilchek says, other calculations need to be done.
Breaking symmetry
In essence, the properties that quantum atmospheres reveal are represented by symmetries. The various phases of a substance, and the properties that correspond to them, can be represented in the form of symmetries. In a solid crystal, for example, the atoms are arranged in a symmetrical lattice that shifts or rotates to form identical crystal patterns. When you heat it up, the bonds break, the lattice structure collapses, the material loses its symmetry and becomes liquid in a sense.
Materials can break other fundamental symmetries, such as the reciprocal time symmetry, which most laws of physics obey. The phenomena can be different if you reflect them in a mirror and break the parity symmetry.
If these symmetries can be broken in the material, then we could observe previously unknown phase transitions and potentially exotic properties. Material with certain symmetry breaking will cause the same breakdown in a probe that passes through the quantum atmosphere, Wilczek says. For example, in a substance that follows axionic thermodynamics, symmetries of both time and parity are broken, but in combination they are not. By touching the atmosphere of the material, you can find out if and to what extent it breaks symmetry.
Wilchek says he has already discussed the idea with the experimenters. Moreover, these experiments are quite feasible, even not in years, but in weeks and months.
If everything works out, the term "quantum atmosphere" will find a permanent place in the lexicon of physicists. Wilczek had previously coined terms such as axions, anions (quasiparticles that can be useful for quantum computing), and time crystals. Quantum atmospheres can also linger.
Ilya Khel
