Uncovering unexpected properties in a complex quantum material

Date:
February 17, 2022
Source:
University of Pennsylvania
Summary:
A new study describes previously unexpected properties in a
complex quantum material, findings that have implications for
future quantum devices.



FULL STORY ==========================================================================
A new study describes previously unexpected properties in a complex
quantum material known as Ta2NiSe5. Using a novel technique developed
at Penn, these findings have implications for developing future quantum
devices and applications. This research, published in Science Advances,
was conducted by University of Pennsylvania graduate student Harshvardhan Jogand led by professor Ritesh Agarwal in collaboration with professor
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 challenge is the ability to only 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 challenge, materials need
to be efficient at quantum entanglement, which occurs when the states
of qubits remain linked no matter their distance from one another,
as well as coherence, or when a system can maintain this entanglement.

In this study, Jog looked at 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 in the past several years,
this technique 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 method
has only been applied in materials without inversion symmetry, whereas Ta2NiSe5does haveinversion 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 see that there was a signal
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 challenge with 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 was hypothesized, they
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 from both experiment and theory, an essential component of the success of this project, 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 also have similar properties. On the theory side, Mele is studying
how prevalent this phenomena 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
that range 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 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." ========================================================================== Story Source: Materials provided by University_of_Pennsylvania. Original written by Erica K.

Brockmeier. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Harshvardhan Jog, Luminita Harnagea, Eugene J. Mele, Ritesh Agarwal.

Exchange coupling-mediated broken symmetries in Ta 2 NiSe 5 revealed
from quadrupolar circular photogalvanic effect. Science Advances,
2022; 8 (7) DOI: 10.1126/sciadv.abl9020 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220217122352.htm

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