How Do Remarkable Properties of Matter Emerge from Complex

How do Remarkable Properties of Matter Emerge from Complex Correlations of
the Atomic or Electronic Constituents and How Can We Control These Properties?
Progress on Grand Challenge
What parts of the Grand Challenge have been solved?
We have made much progress, especially in the following areas:
New materials with properties driven by correlated
electron phenomena: include 2D materials such as
graphene, MoS2 etc with new phenomena including
formation of stable trions as an example.
b. Understanding the role of electron correlations in the
generation of multiple excitons from single photon
excitation in molecular systems complete with new
chemical design principles.
Remaining Challenge
• Does enough remain to be grand?
Clearly the general grand challenge remains and the potential
to impact the Energy needs of the world remains.
• Is it tractable on the decadal scale or longer?
Yes this should be tractable, but we can hope to see movement
toward interface to the real world of energy creation, control,
and use for the betterment of humankind.
New Horizons for Grand Challenge
Has the focus/scope of the Grand Challenge evolved?
a. The issue of control or active control of the properties of
correlated electron systems remains.
b. We have made tremendous progress in the realm of 2D
systems not only in creating and characterizing new materials
but in building controlled heterostructures in devices which
provide electronic interconnection to the real world. Yet the
potential for actively controlling the characteristics of these
devices for application to Energy problems remains.
Refreshed Grand Challenge?
• Is a new statement of the Grand Challenge needed?
The statement could be broadened toward: How can
we use these remarkable properties? How can we
manipulate materials in order to explicitly control the
material properties? How can we assemble these
materials into structures or devices which will allow
external control and utilization of these properties?
• Should the Grand Challenge be retired?
Submitted by: James Yardley
Affiliation: Columbia University
Excitons in atomically thin transition metal dichalcogenides
Scientific Achievement
We have determined experimentally the energies of the excited
excitonic states of the fundamental optical transition in
monolayer tungsten disulfide. From these we establish a large
exciton binding energy of 0.32 eV and observe a pronounced
deviation from the usual hydrogenic behavior. We explain these
results using a microscopic theory in which the non-local nature
of the dielectric screening modifies the Coulomb interaction.
Significance and Impact
Monolayer tungsten disulfide is part of the family of new directgap semiconductors useful for atomically thin electronic and
photovoltaic devices. The properties of these materials are found
to be strongly influenced by the excitonic effects. Our study of
the excitonic properties provides a fundamental insight in the
nature of the Coulomb interaction in two-dimensional crystals.
Research Details
Top: Reflectance derivative spectrum of the monolayer
tungsten disulfide with an illustration of the exciton level
structure; Bottom: experimental and calculated exciton
energy levels and a spatially resolved image of the 2s state
A. Chernikov, T. C. Berkelbach, H. M. Hill, A. Rigosi, Y. Li, Ö. B. Aslan, D. R.
Reichman, M. S. Hybertsen, T. F. Heinz: "Anomalous excitons in atomically
thin materials." submitted manuscript
 We experimentally determined the energies of the excited
states of the excitons in monolayer WS2
 We have observed an unusually large exciton binding energy of
0.32 eV as well as strong deviations of the excitonic properties
from the conventional hydrogen model
 We have used microscopic theory to explain our findings in
terms of non-uniform dielectric environment, characteristic for
two-dimensional materials

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