Putting a quantum gas through its phases

July 3, 2018, ETH Zurich
Phase diagrams showing the four different regions observed in the experiment: white -- superfluid without photons; red and yellow -- photons in only one of the cavities; blue -- photons in both cavities simultaneously (mixed phase). As the coupling between the orders is increased, the mixed-phase regime (blue) becomes increasingly favourable. Credit: Esslinger group, ETH Zurich (adapted from doi: 10.1038/s41563-018-0118-1)

Physicists at ETH Zurich have developed an experimental platform for studying the complex phases of a quantum gas characterized by two order parameters. With unprecedented control over the underlying microscopic interactions, the approach should lead to novel insight into the properties of a broad range of fundamentally and technologically important materials.

As a physical system undergoes a phase transition, it typically becomes more or less ordered. For instance, when a piece of iron is heated to above the Curie temperature, the strong ferromagnetic alignment of the elementary magnetic dipole moments gives way to much weaker paramagnetic alignment. Such changes are well described in the general framework of order parameters, provided by the Landau theory of phase transitions. However, many materials of current fundamental and technological interest are characterised by more than one order parameter. And here, the situation can become extraordinarily complex, in particular when the different orders interact with one another. The traditional route to gaining an understanding of such complex quantum systems is to carefully explore the response to changes in external conditions and to various probes, and thus to map out the phase diagram of the system. A complementary approach is now presented by Tobias Donner and his team in the group of Tilman Esslinger in the Department of Physics of ETH Zurich. Their approach controls all relevant microscopic parameters of a quantum system governed by two coupled order parameters, and therefore can essentially construct and modify the from the bottom up, as they report in a paper published today in the journal Nature Materials.

Phenomenological models that reproduce the experimentally determined phase diagrams of materials with one or more ordering tendencies have provided deep insight into the behaviour of a variety of systems, such as multiferroics, material that exhibits ferromagnetism and ferroelectric properties, opening the door to new functionality—or certain families of superconductors. However, the microscopic processes underlying the formation of macroscopic order in these systems remain unknown. This gap in understanding limits the predictive power of phenomenological models, and at the same time, makes it difficult to know just how a given material should be modified to obtain desired properties. Hence the appeal of the approach taken by Donner and his colleagues, who started not with a specific system and its phenomenological description, but with a flexible quantum system whose relevant microscopic parameters can be controlled with high accuracy, and be tuned across a broad range of values, enabling the realization of diverse scenarios.

To create such a versatile platform, the team optically trapped a Bose-Einstein condensate (BEC) at the intersection of two optical cavity modes (see the figure). In this configuration, the BEC can crystallise in two different patterns, each of which is associated with a different order parameter. Depending on the experimental setting, the two orders either competed with one another—forcing the system into one of the two patterns (red and yellow)—or to coexist, leading to a new coupled phase (blue), where the two orders do not simply add, but give rise to a more complex spatial arrangement. The extent of this mixed-order can also be controlled to favour regimes of mutual exclusion or of mutual enhancement.

Whereas these particular phases have no known direct role in practical materials, the approach established with these experiments can be modified in the future to simulate properties of that are technologically relevant. In particular, in cuprate high-temperature superconductors, coupled spin and charge order are known to have an important, yet not fully understood role. The sort of experiments now pioneered by the ETH physicists should offer a unique tool to explore such phases starting from a 'clean' quantum system with well-controlled and widely tunable interactions.

Explore further: Quantum cocktail provides insights on memory control

More information: Andrea Morales et al, Coupling two order parameters in a quantum gas, Nature Materials (2018). DOI: 10.1038/s41563-018-0118-1

Related Stories

Quantum cocktail provides insights on memory control

January 26, 2018

Experiments based on atoms in a shaken artificial crystal made of light offer novel insight into the physics of quantum many-body systems, which might help in the development of future data-storage technologies.

A novel test bed for non-equilibrium many-body physics

April 2, 2018

The behavior of electrons in a material is typically difficult to predict. Novel insight comes now from experiments and simulations performed by a team led by ETH physicists who have studied electronic transport properties ...

Mastering metastable matter

March 12, 2018

The phenomenon of metastability, in which a system is in a state that is stable but not the one of least energy, is widely observed in nature and technology. Yet, many aspects underlying the mechanisms governing the behaviour ...

Recommended for you

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.