Using a novel technique developed at the University of Pennsylvania, researchers have uncovered previously unexpected properties in a complex quantum material known as Ta2NiSe5. These findings have implications for developing future quantum devices and applications. This research, reported in a paper in Science Advances, was conducted by University of Pennsylvania graduate student Harshvardhan Jog and led by Ritesh Agarwal, in collaboration with Eugene Mele and Luminita Harnagea from the Indian Institute of Science Education and Research.

While the field of quantum information science has experienced progress in recent years, the widespread use of quantum computers is still limited. One current issue is only being able to use a small number of ‘qubits’, the unit that performs calculations in a quantum computer, because current platforms are not designed to allow multiple qubits to ‘talk’ to one another. In order to address this issue, materials need to be efficient at quantum entanglement, which occurs when the states of qubits remain linked no matter their distance from one another, and coherence, which refers to how long a system can maintain this entanglement.

In this study, Jog investigated Ta2NiSe5, a material system that has strong electronic correlation, making it attractive for quantum devices. Strong electronic correlation means that the material’s atomic structure is linked to its electronic properties and the strong interaction that occurs between electrons.

To study Ta2NiSe5, Jog used a modification of a technique developed in the Agarwal lab known as the circular photogalvanic effect, where light is engineered to carry an electric field and is able to probe different material properties. Developed and iterated over the past few years, this effect has revealed insights about materials such as silicon and Weyl semimetals in ways that are not possible with conventional physics and materials science experiments.

But the challenge in this study, says Agarwal, is that this technique has only been applied to materials without inversion symmetry, whereas Ta2NiSe5 does have inversion symmetry. Jog “wanted to see if this technique can be used to study materials which have inversion symmetry, which, from a conventional sense, should not be producing this response,” says Agarwal.

After connecting with Harnagea to obtain high-quality samples of Ta2NiSe5, Jog and Agarwal used a modified version of the circular photogalvanic effect and were surprised to find that a signal was being produced. After conducting additional studies to ensure that this was not an error or an experimental artifact, they worked with Mele to develop a theory that could help explain these unexpected results.

Mele says that the main challenge in developing a theory was that what was hypothesized about the symmetry of Ta2NiSe5 did not align with the experimental results. Then, after finding a previous theory paper that suggested that the symmetry was lower than what had been hypothesized, the researchers were able to develop an explanation for these data. “We realized that, if there was a low temperature phase where the system would spontaneously shear, that would do it, suggesting that this material was deforming to this other structure,” says Mele.

By combining their expertise in both experiment and theory, the researchers found that this material had broken symmetry, a finding that has significant implications on using this and other materials in future devices. This is because symmetry plays a fundamental role in classifying phases of matter and, ultimately, in understanding what their downstream properties will be.

These results also provide a platform for finding and describing similar properties in other types of materials. “Now, we have a tool that can probe very subtle symmetry breaking in crystalline materials. To understand any complex material, you have to think about symmetries because it has huge implications,” says Agarwal.

While there remains a ‘long journey’ before Ta2NiSe5 can be incorporated into quantum devices, the researchers are already making progress on evaluating this phenomenon further. In the laboratory, Jog and Agarwal are interested in studying additional energy levels within Ta2NiSe5, looking for potential topological properties and using the circular photogalvanic method to study other correlated systems to see if they might have similar properties. On the theory side, Mele is investigating how prevalent this phenomenon might be in other material systems and is developing suggestions for other materials for experimentalists to study in the future.

“What we’re seeing here is a response that shouldn’t occur but does under these circumstances,” says Mele. “Expanding the space of structures that you have, where you can turn on these effects that are nominally forbidden, is really important. It’s not the first time that’s ever happened in spectroscopy, but, whenever it does occur, it’s an interesting thing.”

Along with presenting a new tool for studying complex crystals to the research community, this work also provides important insights into the types of materials that can provide two key features – entanglement and macroscopic coherence – that are crucial for future quantum applications such as medical diagnostics, low-power electronics and sensors.

“The long-term idea, and one of the biggest goals of condensed matter physics, is to be able to understand these highly entangled states of matter because these materials themselves can do a lot of complex simulation,” says Agarwal. “It could be that, if we can understand these types of systems, they can become natural platforms to do large-scale quantum simulation.”

This story is adapted from material from the University of Pennsylvania, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

“Now, we have a tool that can probe very subtle symmetry breaking in crystalline materials. To understand any complex material, you have to think about symmetries because it has huge implications.”Ritesh Agarwal, University of Pennsylvania