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Disrupting crystalline order to restore superfluidity

Theoretical and experimental scientists at the Universität Hamburg have together observed light induced superfluidity. The team of scientists led by Prof. Ludwig Mathey and Prof. Andreas Hemmerich, who are both members of the cluster of excellence CUI, identified a fundamental mechanism for how a typical system with competing phases responds to an external periodic driving. Their work, published in the journal Physical Review Letters, opens a new chapter of solid state physics in which scientists not only measure equilibrium properties of matter, but rather design a non-equilibrium state with desired properties via light control.

A well-ordered quantum phase is transformed by a modulated beam into a superfluid. Illustration: Jayson Cosme

When water is put water in a freezer, water molecules crystallize and form ice. This change from one phase of matter to another is called a phase transition. This transition, and countless others that occur in nature, typically take place at the same fixed conditions, such as the freezing point.

Control of the freezing transition is an essential ingredient for example in making a slushy: to achieve the perfect consistency, constant mixing of the liquid is needed. A slush machine with constantly rotating blades helps prevent water molecules from crystalizing and turning the slushy into a solid block of ice.

The same is true for controlling quantum matter. Rather than forming a normal liquid, like a melted slushy under the sun for too long, quantum matter can form a superfluid. This counterintuitive form of matter was first observed in liquid helium at very low temperatures, less than 2 Kelvin above absolute zero. The helium atoms have a strong tendency to form a crystal, like the water molecules in a slushy, and this restricts the superfluid state of helium to very low temperatures and low pressures.

“But what if you could turn on the blades in your slush machine for quantum matter? What if you could disrupt the crystalline order so that the superfluid could flow freely even at temperatures and pressures where it usually does not?” the scientists wondered. They disrupted crystalline order in a quantum system in a controlled manner by shining light on it that oscillates in time at a specific frequency. Physicists use the term “driving” to describe this kind of periodic change applied to the system – an action performed by the churning blades in a slushy machine.

The researchers studied a gas of cold atoms placed between two highly reflecting mirrors. The mirrors form a cavity, which serves as a resonator for photons as the atoms scatter them multiple times before being detected in experiments. To provide a source of photons, an external pump laser beam is directed at the cloud of atoms.

Computer simulations

Similar to how water can change its phase from liquid to ice, this light-matter system also exhibits a phase transition, a quantum one. Atoms from an initially homogeneous gas spontaneously organize themselves in a checkerboard pattern when the intensity of the pump beam gets sufficiently strong. The self-organization comes at the expense of the superfluid which is suppressed by the crystalline order. This is one of the many examples in nature of competition where one phase wins over the other. The researchers show that with a little bit of “drive”, you can tip the balance in favor of the underdog, in this example, the superfluid phase. “We observe from our computer simulations that a periodic modulation of the pump intensity can destabilize the dominant self-organized phase,” explains lead author Jayson Cosme. “This allows the previously unstable homogeneous phase to reemerge and this restores the superfluid. It is light induced superfluidity.”

Experimental proof

The same team of scientists then indeed observed their prediction in an experiment conducted in the Hemmerich group. “Intuitively one might expect that if we shake the system, all it does is heat up. It was intriguing to see a clear signature of the quantum fluid reemerging,” explains Hemmerich.

The enhancement or suppression of a phase due to an external driving force has also been suggested in other physical systems. For instance, in high-temperature superconductors, laser pulses can melt the equilibrium dominant striped order paving the way for superconductivity to emerge – a phenomenon called light-induced superconductivity. The fundamental mechanism that can help explain this process is still a subject of debate. “We proposed this type of light control of superfluidity to demonstrate the principle that has been hypothesized for light induced superconductivity,” Mathey says. With this finding, cold atom physics demonstrates a general, counterintuitive mechanism of controlling phase transitions in many-body systems.

Jayson G. Cosme, Christoph Georges, Andreas Hemmerich, Ludwig Mathey
“Dynamical Control of Order in a Cavity-BEC System”
Phys. Rev. Lett. 121, 153001 (2018)
DOI: 10.1103/PhysRevLett.121.153001