A new study demonstrates previously unanticipated features in a complicated quantum material known as Ta2NiSe5. These findings, which were made possible by a new technique developed at Penn, have implications for the development of future quantum devices and applications. This research, published in Science Advances, was conducted by graduate student Harshvardhan Jog and led by Professor Ritesh Agarwal in collaboration with Penn’s Eugene Mele and Luminita Harnagea from the Indian Institute of Science Education and Research.
While the
subject of quantum information science has undergone growth in recent years,
the mainstream application of quantum computers is still limited. One problem
is the ability to only employ a small number of “qubits,” the unit that
conducts computations in a quantum computer, because existing platforms are not
intended to allow numerous qubits to “talk” to one another. In order to meet
this difficulty, materials need to be efficient at quantum entanglement, which
happens when the states of qubits remain connected no matter their distance
from one another, as well as coherence, or when a system can sustain this
entanglement.
In this
work, Jog looked at Ta2NiSe5, a material system that exhibits excellent
electrical correlation, making it interesting for quantum devices. Strong
electronic correlation indicates that the material’s atomic structure is
connected to its electronic characteristics and the strong interaction that
occurs between electrons.
To explore
Ta2NiSe5, Jog employed a variation of a technique developed in the Agarwal lab
known as the circular photogalvanic effect, where light is manufactured to
carry an electric field and is able to test diverse material characteristics.
This approach, which has been developed and refined over several years, has
revealed information about materials such as silicon and Weyl semimetals in
ways that are not achievable with traditional physics and materials science
research.
But the
challenge in this study, says Agarwal, is that this method has only been
applied in 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.
Jog and
Agarwal employed a modified version of the circular photogalvanic effect after
connecting with Harnagea to collect high-quality Ta2NiSe5 samples and were
startled to observe that a signal was generated. They collaborated with Mele to
build a hypothesis that may help explain these surprising results after
conducting more research to assure that this was not an error or an
experimental artifact.
Mele
states that the issue with creating a theory was that what was postulated about
the symmetry of Ta2NiSe5 did not fit with the experimental data. Then, after
locating a previous theoretical work that revealed that the symmetry was lower
than what was expected, they were able to provide an explanation for these
results. “We realised 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
integrating their experience from both experiment and theory, a crucial
component of the success of this effort, the researchers determined that this
material has broken symmetry, a finding that has substantial ramifications on
employing this and other materials in future devices. This is because symmetry
plays a vital role in categorising phases of matter and, eventually, in knowing
what their downstream characteristics will be.
These findings can also be used to find and describe similar properties in other types of materials. “Now, we have a technology that can investigate extremely minute symmetry breakdown 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” until Ta2NiSe5 can be included into quantum
devices, the researchers are already making headway on studying this phenomena
further. In the lab, Jog and Agarwal are interested in looking for potential
topological qualities in extra energy levels inside Ta2NiSe5, as well as using
the circular photogalvanic approach to look at other associated systems to see
if they have comparable properties. On the theory side, Mele is exploring how
frequent similar phenomena might be in other material systems and is making
proposals for new materials for experimentalists to examine in the future.
“What we’re seeing here is a response that shouldn’t exist but does under these circumstances,” says Mele. "It's critical to expand the set of structures you have, where you may switch on these effects that are theoretically disallowed." It's not the first time something has happened in spectroscopy, but it's always interesting when it occurs."
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 concept, and one of the major aspirations of condensed matter physics, is to be able to comprehend these extremely entangled states of matter because these materials themselves can conduct a lot of intricate simulation,” adds Agarwal. “It might be that, if we can understand these sorts of systems, they can become natural platforms to undertake large-scale quantum simulation.”
References:
- Harshvardhan Jog et al, Exchange coupling–mediated broken symmetries in Ta 2 NiSe 5 revealed from quadrupolar circular photogalvanic effect, Science Advances (2022). DOI:10.1126/sciadv.abl9020
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