Helium-4 superfluid is a fascinating substance. With properties that seemingly defy normal physics, it leaks straight through glass, bubbles up out of containers, flows around objects and even climbs up walls. As if superfluid helium-4 was not strange enough, in 1941 it was also predicted that it should contain an exotic, particle-like excitation – a quasiparticle – called a roton. After many years of trying to verify this prediction, researchers at the University of California now claim to have successfully created a roton structure in an atomic superfluid of cesium-133.
Initially discovered back in 1937 by Pyotr Kapitsa in Moscow, and independently by John F. Allen and Donald Misener at the University of Toronto, superfluid helium-4 has long been the subject of intricate study regarding its unusual properties. In 1941, in providing a quantum theoretical explanation of Kapitsa’s discovery, future Nobel laureate Lev Landau predicted that a unit of collective motion of atomic particles (described mathematically as if it were a single particle comprising a quantum of vortex movement) should exist within the fluid to explain its quantum superfluidity. This quasiparticle he called a "roton."
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In the many decades since this prediction, scientists have theorized a range of different structures for the roton. However researchers over this period, including Landau himself, Richard Feynman and Philippe Nozières could not agree on a definitive model for the arrangement the roton would take.
"Even nowadays, after seven decades, it remains an issue of interest and controversy," said Cheng Chin, professor of physics at the University of California, and leader of the research team.
In an attempt to produce a way to create artificial rotons to answer the theoretical questions regarding their structure, the University of Chicago researchers first cooled a cylindrical chamber down to a temperature of approximately 15 nano-Kelvin, only a mere fraction above absolute zero (−459.6 °F/−273.15 °C). The scientists then created an optical lattice of interlaced infrared laser beams to capture and hold around 30,000 atoms of cesium-133 within this chamber. Then they did something a little different: they gently shook the whole thing.
"We need about 10 seconds to reach that temperature to prepare a superfluid as our first step," said Professor Chin. "It is a brand new idea that shaking the optical lattice leads to the emergence of the rotons."
The University of Science and Technology in Shanghai, China, and Washington State University also managed to create a roton structure just a few weeks after the University of Chicago research team. However these subsequent roton structures were not created in the same way – they didn’t shake their experiment, they used additional laser beams in a "conventional" manner instead.
"We approached the challenge to create rotons based on a new technology that we recently developed,” said Li-Chung Ha, from the University of Chicago team. “With this technique, we can engineer an excitation spectrum of the atoms."
Viewing and imaging the excitation spectrum feature is one of three pieces of evidence supporting the team's proof of creating of the roton structure. The ability to measure the roton energy and an observation of how the roton excitations affected the superfluidity of the substance – by sweeping a stippled laser pattern across the superfluid – provided the other two pieces of evidence required to confirm the successful creation of the roton structure.
The practical upshot to the realization of a roton structure in a superfluid may be in the realm of superconductors, where there may be improvements in energy efficiency. As such, the researchers have proposed a number of methods to help increase the robustness of superconductors, and atomic superfluids offer experimental means to test these ideas, Professor Chin said.
"Superconductors can transfer energy without dissipation, that is, without energy loss, so a robust superconducting material can find widespread applications everywhere," he stated. "Our experiments provide a new platform to study excitations of a superfluid. They can help us better identify the key issues that limit the robustness of superconductivity."
This research was funded by the National Science Foundation, Army Research Office, and the University of Chicago’s Materials Research Science and Engineering Center. The work was published in the journal Physical Review Letters.
Source: University of Chicago