What are Supersolids?

A supersolid is a strange type of matter that behaves like a solid with a fixed shape but also flows without friction like a superfluid, a special liquid that can flow without any resistance, meaning it never slows down or loses energy.

A supersolid is a phase of matter that uniquely combines the properties of solids and superfluids. It simultaneously exhibits crystalline order (characteristic of solids) and superfluidity (quantum mechanical fluid motion without viscosity). This paradoxical state was theoretically predicted in the late 1960s…

by Andrei F. Andreev (1939–present) and Ilya M. Lifshitz (1917–1982), and Gordon V. Chester (1929–2018). They proposed that quantum mechanical effects, particularly zero-point energy1 and the Bose-Einstein condensation (BEC) of vacancies, could lead to superfluidity in a crystalline lattice2.

Crystalline order refers to the regular, repeating pattern that atoms or molecules form in a solid, like how the pieces of a puzzle fit together in a perfect grid. In a crystal, each atom is in a fixed position, and the whole structure looks the same if you zoom in far enough. Superfluidity, on the other hand, is a special property of certain liquids that allows them to flow without any friction or resistance, like how water can flow through a narrow pipe without slowing down. When these two ideas are combined, as in a supersolid, you get a material that has both the rigid, repeating structure of a crystal and the flowing properties of a superfluid, which is a strange and fascinating state of matter.

The Bose-Einstein condensation (BEC) of vacancies refers to a theoretical phenomenon in which atomic vacancies (missing atoms in a crystalline lattice) behave as quantum particles and undergo Bose-Einstein condensation. In a quantum crystal, particularly one made of helium-43, the vacancies can be treated as bosonic quasiparticles4 due to the indistinguishability of the atoms.

At extremely low temperatures, these vacancies can delocalize and form a macroscopic quantum state, exhibiting properties similar to a superfluid within the solid matrix. The BEC of vacancies suggests that quantum mechanical effects can dominate even in a seemingly classical solid state, leading to fascinating hybrid behavior.

Supersolidity is intriguing because it challenges the classical dichotomy between fluidity and rigidity. The existence of such a phase provides new insights into the interplay between quantum mechanics and condensed matter systems. Liquid helium, particularly helium-4,

was initially considered the most promising system for realizing a supersolid phase due to its well-known superfluid properties. Experiments in the early 2000s by Moses Chan and Eun-Seong Kim at Penn State suggested evidence for supersolidity in solid helium-4. Their torsional oscillator5 experiments showed a drop in rotational inertia at low temperatures, which was interpreted as the onset of a supersolid phase.

However, this interpretation was later contested. Further studies revealed that the observed effects might be due to elastic changes in the solid helium lattice rather than genuine supersolidity. This led to a reassessment of the role of crystal defects, grain boundaries, and dislocations in contributing to the observed phenomena. Supersolidity was more convincingly demonstrated in ultracold quantum gases6.

Bose-Einstein condensates (BECs) of atoms, such as rubidium or dysprosium7, provide an ideal platform for observing supersolid behavior. The interplay of long-range interactions (e.g., dipole-dipole interactions8) and quantum fluctuations can stabilize a density-modulated phase that behaves as a supersolid.

In 2017, independent research teams at ETH Zurich and the University of Stuttgart observed supersolid properties in dipolar BECs. They used atomic gases of dysprosium and erbium9 cooled to near absolute zero. By fine-tuning the interaction parameters via external magnetic fields,

these systems exhibited both crystalline order (in density modulations) and superfluid flow. Another landmark experiment was conducted using optical cavities10. Here, atoms confined in a cavity and coupled to light fields showed supersolid-like behavior due to the self-organization of atoms in a periodic pattern driven by light-matter interactions.

The theoretical description of supersolids involves combining the frameworks of solid-state physics and quantum many-body theory. Two key mechanisms are central to the formation of supersolids:
  • Vacancy Condensation: In traditional solids, vacancies (empty lattice sites) are generally localized. In a quantum solid, however, vacancies can delocalize and form a Bose-Einstein condensate, leading to superfluidity within the solid matrix.
  • Interaction-Induced Modulations: In systems like dipolar BECs, supersolidity arises from competition between short-range repulsion and long-range interactions. This interplay creates periodic density modulations (solid-like behavior) while maintaining phase coherence (superfluidity).

The Gross-Pitaevskii equation11, a mean-field description of Bose-Einstein condensates, has been extended to incorporate these features. Quantum Monte Carlo simulations12 and density functional theories have further advanced the understanding of supersolidity in both helium and ultracold atomic systems.

Footnotes
  1. Zero-point energy is the lowest possible energy that a physical system, like an atom or molecule, can have, even when it’s at absolute zero temperature where all motion is supposed to stop. This happens because of the strange rules of quantum mechanics, which say particles are always in motion, even in their most stable state. It’s like the system can never be completely still because of the constant “buzzing” of quantum uncertainty. This energy is what keeps helium from freezing at normal atmospheric pressure, even at extremely low temperatures. ↩︎
  2. Superfluidity in a crystalline lattice happens when certain parts of a solid, like missing atoms (vacancies), start behaving like a superfluid, which means they can move through the lattice without any resistance. Even though the solid itself is rigid and holds its shape, these quantum vacancies act like a special kind of liquid that flows smoothly and freely within the solid. This unusual behavior can only occur under extreme conditions, like very low temperatures, where quantum effects dominate. It’s a fascinating mix of solid and liquid properties in the same material. ↩︎
  3. Helium-4 is a common form of the element helium, which is a light, colorless gas used in balloons and scientific experiments. Each helium-4 atom is made up of two protons, two neutrons, and two electrons, making it very stable. At extremely low temperatures, helium-4 becomes a liquid and shows amazing quantum behaviors, like flowing without any friction in its superfluid state. This unique property makes it a favorite for studying quantum physics and strange states of matter, like supersolids. ↩︎
  4. Bosonic quasiparticles are tiny energy waves or particle-like things that act like bosons, which are particles that can bunch together and move in sync, unlike normal particles that avoid each other. These quasiparticles appear inside materials when many atoms or electrons interact in a way that makes them behave as a single quantum entity. Examples include phonons (vibrations in a solid) and magnons (tiny waves in a magnet). Since they follow the same rules as real bosons, they can form strange quantum states like Bose-Einstein condensates, where they all act as one. ↩︎
  5. A torsional oscillator is a device that twists back and forth like a swinging door but in a very precise and controlled way. Scientists use it to study tiny changes in materials by measuring how easily something rotates when attached to it. In experiments with helium-4, torsional oscillators were used to look for supersolidity—if part of the helium became a superfluid inside the solid, it would reduce the resistance to twisting, making the oscillator move slightly faster. This tool helps physicists detect strange quantum effects that are otherwise invisible. ↩︎
  6. Ultracold quantum gases are gases made up of atoms that are cooled down to temperatures just above absolute zero, which is the coldest temperature possible. At these extremely low temperatures, the atoms move so slowly that they start to act in very strange ways, following the rules of quantum mechanics instead of the normal rules of everyday physics. This allows scientists to study behaviors like superfluidity and Bose-Einstein condensation, where atoms can group together into a single “super-atom.” These gases are used to explore fundamental quantum phenomena that are usually too hard to see in normal conditions. ↩︎
  7. Rubidium and dysprosium are both chemical elements, but they’re quite different. Rubidium is a soft, silvery metal that is part of the alkali metals group, similar to sodium and potassium. It’s highly reactive and doesn’t appear much in nature, but scientists use it in things like atomic clocks and certain types of experiments with lasers and ultracold gases. Dysprosium, on the other hand, is a rare, heavy metal that’s part of the lanthanide series. It’s used in high-performance magnets and certain medical devices. Both elements are interesting because their atoms can be cooled to ultra-low temperatures, which makes them useful in experiments exploring quantum physics, like creating Bose-Einstein condensates and studying supersolidity. ↩︎
  8. Dipole-dipole interactions occur when two particles, like atoms or molecules, each have a “magnetic” or “electric” field around them, similar to how a magnet has a north and south pole. These particles attract or repel each other depending on the alignment of their fields. Imagine two magnets: if you put opposite poles together, they pull toward each other, and if you put like poles together, they push away. In atoms, this interaction is crucial when particles have electric or magnetic dipoles (like in certain atoms such as dysprosium), and it plays a big role in shaping the behavior of gases at very low temperatures, helping to create unique quantum states like supersolids. ↩︎
  9. Erbium is a rare, shiny metal that is part of the lanthanide series of elements, often found in minerals and used in various technologies. It’s not something you encounter often in everyday life, but it’s useful in specialized areas like fiber optics and lasers because it can emit light at specific wavelengths. Erbium is also used in some medical imaging techniques. In experiments with ultracold gases, erbium atoms are particularly interesting because they have strong magnetic properties that make them behave in unique ways when cooled to extremely low temperatures, allowing scientists to study fascinating quantum phenomena. ↩︎
  10. Optical cavities are special setups used to trap light between two mirrors or other reflective surfaces. The light bounces back and forth between these mirrors, creating a controlled environment where scientists can study how light interacts with matter. Think of it like a tiny, high-tech “light box” where the light is contained and focused. In physics experiments, optical cavities are used to trap atoms or manipulate the behavior of light in ways that wouldn’t happen in open space, enabling researchers to create and study unique quantum states of matter, like those seen in supersolids. ↩︎
  11. The Gross-Pitaevskii equation is a mathematical formula used to describe how ultra-cold gases of atoms behave at extremely low temperatures, especially in states like Bose-Einstein condensates. It’s kind of like a guidebook for understanding how a group of atoms in these special quantum states move and interact with each other. The equation takes into account the wave-like nature of atoms at low temperatures and includes factors like their attraction or repulsion, helping scientists predict the behavior of quantum gases in experiments. It’s an essential tool for studying phenomena like superfluidity and the formation of quantum phases. ↩︎
  12. Quantum Monte Carlo simulations are computer-based methods used to study how tiny particles, like atoms or electrons, behave in complex systems governed by quantum mechanics. The “Monte Carlo” part refers to using random sampling, like drawing names out of a hat, to simulate the behavior of these particles. This method is helpful when solving quantum problems that are too difficult for traditional calculations. It allows scientists to make predictions about how particles interact, how systems like ultracold gases behave, or how materials will act at very small scales. ↩︎
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Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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