Study the spectrum of excitation of a trapped dipolar supersolid

Supersolids, solid materials with superfluid properties (that is, in which a substance can escape with zero viscosity), have recently been the subject of numerous physics studies. Supersolids are paradoxical phases of matter in which two distinct and somewhat antithetic orders coexist, giving rise to a material that is both crystalline and superfluid.

Planned for the first time in the late 1960s, supersolidity has gradually become the focus of a growing number of research studies, sparking debate in different scientific fields. Several years ago, for example, a team of researchers published controversial results identifying this phase of solid helium, which were later refuted by the authors themselves.

A key problem of this study was that it did not take into account the complexity of helium and the unreliable observations it can sometimes produce. Moreover, in the case of atoms, the interactions are generally very strong and regular, which complicates the task of this phase.

Diptic quantum gases lie at the opposite end of structures such as solid helium, as they consist of ultra-cold magnetic atoms in the cooled gas phase at a temperature of nanokelvins. Therefore, in these gases, the interactions between the atoms are weak, but they are also long-range and adjustable with externally controlled magnetic fields.

Because of their high degree of tunability, a few years ago, quantum gases began to appear more frequently in theoretical propositions of supersolidity. The first experiments using gases coupled to light fields showed states with supersolid properties, but in these states, the solid remained incompressible.

Finally, a few months ago, three research groups on ultra-cold gases of highly magnetic atoms (a German group led by Tilman Pfau, an Italian group led by Giovanni Modugno and a group of researchers based at the 39, University of Innsbruck and by the Institute for Quantenoptik und Quanteninformation directed by Francesca Ferlaino), has simultaneously published state observations with supersolid properties.

"We have been able to prove that under particular interaction conditions, the magnetic gas undergoes a phase transition to a supersolid state, showing both spontaneous density modulation (crystal) and overall phase coherence (the superfluid) "Researchers informed Phys.org by email. "Remarkably, the supersolid properties really derive from naked interparticle interactions, which have a strong dipole-dipole contribution."

Building on these previous results, the research team led by Francesca Farlaino conducted a new study on the excitation spectrum of a trapped dipolar supersolid, collecting new interesting observations. This study is an important step in discovering how the supersolid state of matter reacts to excitations.

"To explore supersolidity, it is important to prove that the superfluid and crystalline nature of a system reacts differently to perturbations," the researchers explained. "More generally, in quantum physics, any system has intrinsic excitation modes describing how it responds to a disturbance.For example, a pinch guitar string only responds at a given frequency, producing a its clear, that a trained ear could recognize note by estimating the characteristics of the string.The same goes for a quantum system; its spectrum of excitation reveals intimate information about its intrinsic character. supersolids can provide a new and deeper understanding of this intriguing phase. "

The responses observed by the researchers correspond to the theoretical predictions associated with supersolids, suggesting that they have successfully observed a supersolid state. Their article, published in Letters of physical examination, focuses specifically on the spectrum of elementary excitations of a dipole Bose gas placed in a 3D anisotropic trap while it undergoes the transition between superfluid and supersolid.

"We have taken a significant step forward in studying the response to system excitations," the researchers told Phys.org. "The way a system responds tells you a lot about the system itself, just think of an external excitation in which you throw a stone at a system and how different the answer is if you throw that stone at the sea ​​or on a wall This is just an example: instead of throwing a stone, we study the compressibility of the system. "

In their study, Ferlaino and his colleagues basically probed the modes of excitation of the supersolid state produced from an quantum gas of erbium atoms in a shaped trap. cigar makes light by changing the value of an external magnetic field. In this experimental setup, the density modulation appeared spontaneously along the trap, while the system remained fluid.

The researchers then generally excited the system by disrupting the trap in the same direction as that in which the density modulation had occurred. This resulted in the excitation of distinct modes that they probed by observing the evolution of the gas-wave interference behavior of the gas with itself (obtained by dilating it) over time.

"In our work, we identify the different elementary modes of excitation by applying a statistical analysis without model called principal component analysis on the temporal evolution of the models that we observed," the researchers said. "Our most significant observation is that the simultaneous existence of the two orders – crystal and superfluid – in a supersolid is reflected in remarkable properties of its elemental excitation spectrum, which we have deepened in our work."

Previous studies suggest that at the thermodynamic limit (that is, in infinite systems), the existence of crystalline and superfluid properties produces two branches in the excitation spectrum, each of which is associated with one of the orders. This results in modes that are respectively vibrations of the crystalline structure or superfluid flow. In their study, Ferlaino and his colleagues have shown, theoretically and experimentally, that this key feature of the supersolid spectrum occurs in laboratory systems where only a few crystalline sites are present.

"Experimentally, we observed that the response of the system to our overall excitation scheme goes from one to several excited modes when the system goes from a normal superfluid to a supersolid, reflecting the multiplicity of the branch of excitation in the system, "explained the researchers. "It is important to note that a class of excited modes has a decreasing energy cost when one sinks deeper into the supersolid diet, ie when the superfluid character This behavior characterizes the modes inducing a superfluid flow in the network of droplets. "

The researchers found that in the Bose-Einstein condensate regime, the system they examined had an ordinary quadrupole oscillation, while it produced an intriguing two-frequency response in the supersolid regime. This response is associated with the two spontaneously broken system symmetries.

The study by Ferlaino and his colleagues provides evidence of the possibility of superfluid flow in the supersolid state, while its solid elasticity is noticeable. However, to verify their observations, researchers should also prove the irrationality of the superfluid flow, for example by observing eddies. It is one of the many things that they hope to accomplish in their future work.

"The history of supersolid dipole gas is still incomplete and many chapters remain to be written," the researchers said. "For example, how does the superfluid fraction evolve along the phase diagram, what is the nature of superfluid flux in such a system, and how does the system react to local rotation or disturbance?" What are the other characteristics? that one can grasp the excitation spectrum of the supersolid, regarding both its solid elasticity and its superfluid fraction? these are just some of the interesting directions that we could explore in the future. "

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