Quantum tricks to unveil the secrets of topological materials

December 21, 2018, Vienna University of Technology
The spin structure in the atoms in the crystal made of light - it is possible to switch between simple and complex states. Credit: Vienna University of Technology

Electrons are not just little spheres, bouncing through a material like a rubber ball. The laws of quantum physics tell us that electrons behave like waves. In some materials, these electron waves can take on rather complicated shapes. The so-called "topological materials" produce electron states that can be very interesting for technical applications, but it is extremely difficult to identify these materials and their associated electronic states.

TU Wien (Vienna) and several research groups from China have now developed new ideas and implemented them in an experiment. A "crystal " made of is created to hold atoms in a very special geometric pattern. These "light crystals", which have been used in different ways for the manipulation of atoms, can now be used to deliberately drive the system out of equilibrium. By switching between simple and complicated states, the system reveals whether or not it has topologically interesting states. These findings have now been published in the journal Physical Review Letters.

Bread Rolls and Donuts

The importance of topology can easily be seen if we pack too many things into a shopping bag: a bread roll may be slightly crushed and squeezed into a shape similar to a banana. Bread rolls and bananas have the same basic geometric structure, topologically they are the same. On the other hand, a donut has a hole in the middle—its topology is different. Even if it is slightly squeezed, its shape can still be easily distinguished from that of the bread roll.

Optical instruments at TU Wien. Credit: Vienna University of Technology
"It is similar with quantum states," explains Prof. Jörg Schmiedmayer from Vienna Center for Quantum Science and Technology (VCQ) at TU Wien. "Quantum states can have a nontrivial topology which protects them with respect to certain perturbations. That's what makes them so interesting for technology, because you always have to deal with perturbations in every experiment and in every real world technological application." In 2016, the Nobel Prize in Physics for research was awarded for research on topological states of matter, but it is still considered extremely difficult to determine whether or not a certain material allows topologically interesting quantum states.

"Quantum states that are not in equilibrium, are changing rapidly," says Jörg Schmiedmayer. "This dynamics is notoriously difficult to understand, but as we have shown, it is a great way to obtain extremely interesting information about the system." Schmiedmayer cooperated with research teams from China. "The experiment was led by Prof. Shuai Chen, in the research group of Prof. Jian-Wei Pan. Both were once collaborators in my group in Heidelberg, and ever since their return to China, we have been work closely together," says Schmiedmayer. The TU Vienna and the Chinese University of Science and Technology (USTC, Heifei, China) signed a cooperation agreement in 2016, which strengthened research cooperation, especially in the field of physics.

A topologically trivial bandstructure (left), much like a valley, in which a rolling ball approaches the lowest point. The structure on the right is more complex. Credit: Vienna University of Technology
An Imbalance Revealing Material Properties

With the help of interfering light waves, atoms can be held in pre-defined places, creating a regular grid of atoms, similar to a crystal, the atoms taking the roles of the electrons in a solid state crystal. By changing the light, the geometry of the atomic arrangement can be switched, in order to examine how the electron states would behave in a real solid state material.

"With this change, a massive imbalance is suddenly being generated," says Jörg Schmiedmayer. "The states must rearrange and approach a new equilibrium, much like balls rolling down a hill until they find equilibrium in the valley. And during this process we can see clear signatures that tell us whether topologically interesting are to be found or not. "

This is an important new insight for research on topological materials. One could even adapt the artificial light crystals to simulate certain crystal structures and in order to find new topological .

Explore further: Quantum systems: Same, but different

More information: Wei Sun et al. Uncover Topology by Quantum Quench Dynamics, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.121.250403

Related Stories

Quantum systems: Same, but different

November 8, 2018

Remarkable rules have been detected in the apparent chaos of disequilibrium processes. Different systems behave identically in many ways, if they belong to the same "universality class." This means that experiments can be ...

The bizarre world of topological materials

June 27, 2018

In 2016, three physicists received the Nobel Prize for using the mathematical concept of "topology" to explain the strange behavior of certain materials—for example, those that are insulators in their bulk but conductors ...

Ultracold atoms reveal surprising new quantum effects

September 6, 2012

Vienna University of Technology physicists have studied the transition of quantum systems towards thermal equilibrium. They detected an astonishingly stable intermediate state between order and disorder. The results are being ...

Novel insulators with conducting edges

June 1, 2018

Physicists at the University of Zurich are researching a new class of materials: Higher-order topological insulators. The edges of these crystalline solids conduct electric current without dissipation, while the rest of the ...

Recommended for you

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 ...

Trembling aspen leaves could save future Mars rovers

March 18, 2019

Researchers at the University of Warwick have been inspired by the unique movement of trembling aspen leaves, to devise an energy harvesting mechanism that could power weather sensors in hostile environments and could even ...

Quantum sensing method measures minuscule magnetic fields

March 15, 2019

A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well, has been developed by researchers at MIT. The new tool could be useful in applications as diverse as mapping ...

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.