Quantum exciton found in magnetic van der Waals material NiPS3
Exciton of many-body origin in van der Waals antiferromagnet NiPS3 points
a new direction in quantum information

Date:
July 20, 2020
Source:
Institute for Basic Science
Summary:
Things can always be done faster, but can anything beat
light? Computing with light instead of electricity is seen as
a breakthrough to boost the computer speeds. Transistors, the
building blocks of data circuits, require to switch electrical
signals into light in order to transmit the information via a
fiber-optic cable. Optical computing could potentially save the
time and energy used to be spent for such conversion. In addition
to the high-speed transmission, outstanding low-noise properties
of photons make them ideal for exploring quantum mechanics. At
the heart of such compelling applications is to secure a stable
light source, especially in a quantum state.



FULL STORY ========================================================================== Things can always be done faster, but can anything beat light? Computing
with light instead of electricity is seen as a breakthrough to boost
the computer speeds. Transistors, the building blocks of data circuits,
require to switch electrical signals into light in order to transmit the information via a fiber- optic cable. Optical computing could potentially
save the time and energy used to be spent for such conversion. In addition
to the high-speed transmission, outstanding low-noise properties of
photons make them ideal for exploring quantum mechanics. At the heart
of such compelling applications is to secure a stable light source,
especially in a quantum state.


==========================================================================
When light is shone onto electrons in a semiconductor crystal, a
conduction electron can combine with a positively charged hole in the semiconductor to create a bound state, the so-called exciton. Flowing
like electrons but emitting light when the electron-hole pair
gets back together, excitons could speed up the overall data
transmission circuits. In addition, plenty of exotic physical phases
like superconductivity are speculated as phenomena arising from
excitons. Despite the richness of exotic theoretical predictions and its
long history (first reported in the 1930's), much of the physics regarding excitons has been mostly about its initial concept of "simple" binding
of an electron and a hole, rarely updated from the findings in the 1930s.

In the latest issue of the journal Nature, a research team led by
Professor PARK Je-Geun of the Department of Physics and Astronomy,
Seoul National University -- previously Associate Director of the Center
for Correlated Electron Systems within the Institute for Basic Science
(IBS, South Korea) - - found a new type of exciton in magnetic van der
Waals material NiPS3. "To host such a novel state of an exciton physics,
it requires a direct bandgap and most importantly, magnetic order with
strong quantum correlation. Notably, this study makes it the latter
possible with NiPS3, a magnetic van der Waals material, an intrinsically correlated system," notes Professor PARK Je-Geun, corresponding author
of the study. Prof. Park's group reported the first realization of exact
2D magnetic van der Waals materials using NiPS3 in 2016.

Using the same material, they have demonstrated that NiPS3 hosts a
completely different magnetic exciton state from the more conventional
excitons known to date. This exciton state is intrinsically of many-body origin, which is an actual realization of a genuine quantum state. As
such, this new work signals a significant shift in the vibrant field of research in its 80 years of history.

All of this unusual exciton physics in NiPS3 began with bizarrely high
peaks spotted in early PL (photoluminescence) experiments done in 2016
by Prof.

CHEONG Hyeonsik of Sogang University. It was soon followed by another
optical absorption experiment done by Prof. KIM Jae Hoon of Yonsei
University. Both sets of optical data clearly indicated two points of significant importance: one is the temperature dependence and the other extremely narrow resonant nature of the exciton.

To understand the unusual findings, Prof. Park used a resonant inelastic
X-ray scattering technique, known as RIXS, together with Dr. Ke-Jin Zhou
at the Diamond Facilities, the UK. This new experiment was critical to
the success of the overall project. First, it confirmed the existence
of the 1.5 eV exciton peak beyond any doubt. Secondly, it provided an
inspiring guide on how we could come up with a theoretical model and
the ensuing calculations. This connection between the experiment and the
theory played a pivotal role for them to crack the big puzzle in NiPS3.

Using the analytical process shown above, Dr. KIM Beom Hyun and
Prof. SON Young-Woo of the Korea Institute for Advanced Study carried
out massive theoretical many-body calculations. By exploring massive
quantum states totaling 1,500,000 in the Hilbert space, they concluded
that all the experimental results could be consistent with a particular
set of parameters.

When they compared the theoretical results with the RIXS data (Fig. 3-a),
it was clear that they came to a full understanding of the very unusual
exciton phase of NiPS3. At last, the team could theoretically understand
the magnetic exciton state of many-body nature, i.e., a genuine quantum
exciton state.

There are several vital distinctions to be made about the quantum magnetic exciton discovered in NiPS3 as compared with the more conventional
exciton found in other 2D materials and all the other insulators
having an exciton state. First and foremost, the excitons found in
NiPS3 is intrinsically a quantum state arising from a transition from
a Zhang-Rice triplet to a Zhang- Rice singlet. Second, it is almost a resolution-limited state, indicative of some kind of coherence present
among the states. For comparison, all the other exciton states reported
before are from extended Bloch states.

It is probably too early for us to make any definite predictions; it
might as well bring on the future of the related field of magnetic van der Waals researches, not to mention our lives. However, it is clear even at
this moment that "The quantum nature of the new exciton state is unique
and will attract a lot of attention for its potentials in the field of
quantum information and quantum computing, to name only a few. Our work
opens an interesting possibility of many magnetic van der Waals materials having similar quantum exciton states," explains Professor Park.


========================================================================== Story Source: Materials provided by Institute_for_Basic_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Soonmin Kang, Kangwon Kim, Beom Hyun Kim, Jonghyeon Kim, Kyung
Ik Sim,
Jae-Ung Lee, Sungmin Lee, Kisoo Park, Seokhwan Yun, Taehun Kim,
Abhishek Nag, Andrew Walters, Mirian Garcia-Fernandez, Jiemin Li,
Laurent Chapon, Ke-Jin Zhou, Young-Woo Son, Jae Hoon Kim, Hyeonsik
Cheong, Je-Geun Park.

Coherent many-body exciton in van der Waals antiferromagnet NiPS3.

Nature, 2020; DOI: 10.1038/s41586-020-2520-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200720112323.htm

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