Prof. Ronan Rappaport, from the Center for Quantum Information at the Hebrew University, one of the leaders of the research: "This is a project that started as a theoretical idea of ours that we were not sure was applicable at all, and after a few years we were able to witness a new phenomenon that we now do not fully understand theoretically"
Physicists around the world, theorists and experimentalists alike, are looking for new techniques to discover and describe unknown phenomena in nature, including new states of aggregation. In the laboratory of Prof. Ronen Rappaport, the head of the department at the Rakeh Institute of Physics and a partner in the Center for Quantum Information at the Hebrew University, together with other researchers from Austria and Germany, showed that with the help of excitons (a type of particle-like) in a strong interaction it is possible to study many-body systems and with their help predict new aggregation states in the future. An article on the subject was recently published in the journal "Physical Review X".
Besides the four well-known states of aggregation (gas, liquid, solid and plasma) other states of aggregation such as superfluid and superconductor appear in nature under extreme conditions at low temperatures. These are phenomena that were discovered in the past thanks to quantum theory and were predicted in many laboratories around the world. The "super" states are unique because they are formed at very low temperatures (close to absolute zero, -273 degrees Celsius) and allow energy to flow through them without loss. A superconductor, for example, is used in a variety of technologies such as MRI, sensing systems and large accelerators in the world.
The research of Prof. Rappaport and his partners showed that it is possible to cause electrons moving within a layered structure of a material, each layer only a few atoms thick, to create a non-trivial mutual reaction: while electrons are all negatively charged and therefore always repel each other electrically, in the new system the researchers succeeded to show that electrons can also be made to attract each other, contrary to the usual intuition. Such a combination of attractive and repulsive forces between the electrons may in the future help in the creation of an electronic supersolid. Evidence for the existence of a supersolid was observed experimentally only recently in laboratories in Italy and Germany in their experiment composed of gases of cold atoms close to absolute zero. The state of supersolid aggregation is considered a very exotic state because the particles that make it up are arranged in a stable periodic structure on the one hand but can flow from place to place within the material like a superfluid. Similar to the states of aggregation from the "super" family, energy is not lost in a liquid because it lacks viscosity. To illustrate, if an eddy current is created within it, it will continue forever. The new way of Prof. Ronen Rappaport and his partners is promising because it will allow researchers and engineers to create the "super-solid" inside chips that are used today in the elite technologies instead of complicated systems of atomic gases, and at much higher temperatures than the researchers who preceded them worked with (at least a million times). In the future, the "super" modes will enable the saving of electrical energy, will enable the construction of strong batteries and will prevent unnecessary air pollution.
Prof. Rappaport notes in this context that: "As a researcher, this work excites me greatly. This is a project that started as a theoretical idea of ours that we weren't sure was applicable at all, and after a few years we were able to witness a new phenomenon that we now don't fully understand theoretically. I believe this is just the beginning of discovering a wealth of exotic phenomena that are still 'hiding' in this material."
A supersolid formed for a short time
Rappaport: "We have the completely non-trivial infrastructure to start looking at exotic phenomena in multi-particle systems. Several years ago there were already scientists who allegedly discovered the phenomenon in liquid helium at a temperature close to absolute zero, but then it turned out that it was a different phenomenon. "
"This year there is already evidence of a supersolid formed for a short time, in an atomic system, but at very, very cold temperatures. One of the required properties of the quantum states of a supersolid is that particles form non-isotropic interactions involving both attraction and repulsion. This is a very interesting phase, and we have taken a significant step in the direction of building an artificial system in which it will be possible to reach this state. It is a semiconductor chip, the same materials with which lasers, diodes and other optical and electronic components are built. In the chip we produce "artificial atoms" - these are actually excited electrons. We flow energy into the chip and this energy excites the electrons from their ground state to a more excited state, and within this state they produce a kind of quasi-atoms called excitons - from the word excitation."
"The resulting system is called a quasi-particle state, that is - multi-particle excitation of many electrons, but it practically behaves like an atom. It is not a real atom, but electrons of atoms that are inside the material but in fact a cloud of electron particles is formed when each of them behaves like an atom. This system has significant differences compared to real atoms because the particles in it are much lighter - the mass is much smaller, which means they are also much larger. Their size is about a hundred times the size of a normal atom and their mass is several thousand times smaller. They reach a super-liquid state - that is, condensation at higher temperatures. Instead of reaching a temperature range of nano Kelvin, we are able to reach a temperature of 2-1 degrees Kelvin. This is still a very low temperature, but much less than fractions of a degree above absolute zero. I estimate that it will be necessary to try different materials, and maybe there will be some that can demonstrate the phenomenon even at room temperature."
"It is still orders of magnitude hotter, relatively, and much simpler than real atoms - the interactions are much stronger, and in addition, we are also able to compress them to a great extent, a situation where phenomena that cannot be seen with atoms begin to be discovered."
Exotic phase
"A supersolid is an exotic phase. Usually we are used to the phase of a crystalline material, meaning the atoms are arranged. If I know where one atom is I can know where all the others are and the particles are arranged in a certain symmetry. On the other hand, we have another phase which is the superfluid phase. Superfluid particles are in the opposite state - when the helium gas is cooled to a low temperature, you get a superfluid. The particles lose their exact spatial arrangement and a uniform material is obtained - the particles can flow like a liquid. It is called a superfluid because it can flow without friction, which does not exist in a normal liquid state. "
"Al-solid - this is a non-intuitive phase because it is a combination of both together. On the one hand, it breaks the symmetry and arranges itself in a kind of crystal, but in a crystal, nothing flows on its surface - it is a solid substance and the particles inside it do not move."
"The atoms arrange themselves in space, but on the other hand there is a flow inside the solid. For example, if there are defects in the solid, for example if an atom is missing in a certain place (position 157, for example), then we get a lattice in which there are defects. In a solid these defects can move inside the crystal like a flowing particle since these defects do not encounter any resistance (friction). When a defect moves from position 157 to position 158 it means that an atom that was in position 158 has moved to position 157. This situation where the defects can move without friction means that there are atoms in this solid that can move without any resistance. The atoms that move advance like in a liquid and get a substance that is ordered and breaks the symmetry, a situation that does not exist in a normal liquid, but there is still a lack of uniformity in terms of location."
"What we were able to do is produce these artificial atoms and show that we have the basic conditions that can allow in the future - in the coming years - observations of quantum phenomena such as supersolids. We wanted there to be a non-trivial interaction, which includes both repulsion and attraction, between the particles, and we were able to do this using thin layers of electrons that have electrical interactions."
More of the topic in Hayadan:
