Discover unexpected properties in a complex quantum material

A new study describes previously unexpected properties in a complex quantum material known as Ta2NiSe5. Using a new technique developed at Penn, these findings have implications for the development of future quantum devices and applications. This research, published in Scientists progresswas conducted by Harshvardhan Jogand, a graduate student from the University of Pennsylvania, led by Prof. Ritesh Agarwal in collaboration with Prof. Eugene Mele and Luminita Harnagea from the Indian Institute of Science Education and Research.

While the field of quantum information science has seen progress in recent years, the widespread use of quantum computers is still limited. One of the challenges is the ability to use only a small number of “qubits”, the unit that performs calculations in a quantum computer, as current platforms are not designed to allow multiple qubits to “talk to each other. In order to meet this challenge, materials must be efficient at quantum entanglement, which occurs when qubit states remain linked regardless of their distance from each other, as well as coherence, or when a system can maintain this entanglement.

In this study, Jog examined Ta2NiSe5, a material system that has a strong electronic correlation, which makes it attractive for quantum devices. A strong electronic correlation means that the atomic structure of the material is related to its electronic properties and the strong interaction that occurs between electrons.

To study Your2NiSe5, Jog used a modification of a technique developed in Agarwal’s lab known as the circular photogalvanic effect, where light is designed to carry an electric field and is able to probe different properties of materials. Developed and iterated over the past few years, this technique has revealed insights into materials such as silicon and Weyl semimetals in ways not possible with conventional physics and materials science experiments. .

But the challenge of this study, says Agarwal, is that this method has only been applied in materials without inversion symmetry, whereas Ta2NiSe5 has inversion symmetry, Jog “wanted to see if this technique could be used to study materials that have inversion symmetry which, in a conventional sense, should not produce this answer,” says Agarwal.

After connecting with Harnagea to get high quality samples of Ta2NiSe5, Jog and Agarwal used a modified version of the circular photogalvanic effect and were surprised to see that a signal was produced. After conducting additional studies to make sure this wasn’t an error or an experimental artifact, they worked with Mele to develop a theory that might help explain these unexpected results.

Mele says the challenge with developing a theory was that what was hypothesized about the symmetry of Ta2NiSe5 did not agree with the experimental results. Then, after finding a previous theoretical paper that suggested the symmetry was less than assumed, they were able to develop an explanation for this data. “We realized that if there was a low-temperature phase where the system would spontaneously shear, it would, suggesting that this material was deforming into this other structure,” Mele says.

By combining their experimental and theoretical expertise, a key element in the success of this project, the researchers discovered that this material broke symmetry, a finding that has important implications for the use of this material and others. materials in future devices. Indeed, symmetry plays a fundamental role in classifying the phases of matter and ultimately in understanding their downstream properties.

These results also provide a platform to find and describe 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 need to think about symmetries because it has huge implications,” says Agarwal.

If there is a “long trip” left before Ta2NiSe5 can be integrated into quantum devices, researchers are already making progress in evaluating this phenomenon. In the lab, 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 also have similar properties. On the theory side, Mele investigates the prevalence of this phenomenon in other material systems and develops suggestions for other materials that experimenters can study in the future.

“What we’re seeing here is a response that shouldn’t be happening but is happening under these circumstances,” Mele says. “Expanding the space of structures you have where you can enable these effects that are theoretically forbidden is really important. It’s not the first time this has happened in spectroscopy, but whenever it happens, it’s an interesting thing.”

In addition to 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 characteristics, entanglement and macroscopic coherence that are crucial for future quantum applications ranging from 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 perform many complex simulations,” explains Agarwal. “It could be that, if we can understand these kinds of systems, they could become natural platforms for doing large-scale quantum simulation.”

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