O.M. Stewart Colloquium

Every Monday, at 4 PM the department of Physics and Astronomy hosts the O. M. Stewart Colloquium, in rm 120, Physics Bldg.

Refreshments are served starting at 3:30 PM in the Physics Library (rm 223, second floor).

Spring 22
Date Speaker/Title/Abstract
5/23/22 Prof. Shaffique Adam
Strong correlations in twisted moiré materials

Moiré patterns are well known in the visual arts and textile industries -- the term comes from the textured patterns seen in mohair silk fabrics.  It arises whenever two periodic structures are superimposed giving new periodicities.  Such translational symmetry breaking atomic positions is at the heart of condensed matter physics and twisting two atomically thin materials on top of each other yields designer materials where the material properties like bandwidth, electron velocity, and band topology can be controllably altered. 


Less than 15 years after the first isolation of two dimensional materials, our experimental colleagues are now able to tune the twist angle between adjacent atomic monolayers to within 0.1 degrees allowing, for example, the change in electronic bandwidth from 10,000 K in monolayer graphene to less than 10 K in twisted bilayer graphene.  Moreover, there are more than 1,000 possible “easily exfoliatable” materials to play with, giving billions of designer band structures. In the past decade, we have achieved a good handle on building non-interacting models for these materials, however, we do not have a good way to include Coulomb interactions in such moiré van der Waals systems.  I will discuss our successful work on adding Coulomb interactions to the bands of monolayer graphene [1] where we found that contrary to expectation, the interactions are strong.  However, this is masked by the effective competition between the short-range (i.e. “Hubbard-U”) and long-range (i.e. “Coulomb tail”) interaction [2]. I will speculate on how similar ideas might be applied to twisted bilayer graphene [3].     


[1] Hokin Tang, Jia Ning Leaw, Joao Rodrigues, Igor Herbut, Pinaki Sengupta, Fakher Assaad, and Shaffique Adam, " The role of electron-electron interactions in two-dimensional Dirac fermions", Science 361 570 (2018).


[2] Jia Ning Leaw, Ho-Kin Tang, Maxim Trushin, Fakher F Assaad, Shaffique Adam, “Universal Fermi-surface anisotropy renormalization for interacting Dirac fermions with long-range interactions”, Proc. Natl. Acad. Sci. (USA) 116 26431 (2019).


[2] Girish Sharma, Maxim Trushin, Oleg Sushkov, Giovanni Vignale, and Shaffique Adam

“Superconductivity from collective excitations in magic-angle twisted bilayer graphene”, Phys. Rev. Research, Rapid Comm. 2, 022040 (2020).


† This work is supported by the Singapore National Science Foundation Investigator Award (NRF-NRFI06-2020-0003).

4/25/22 Prof. László Forró, Marquez Chair Professor of Physics and Director of Stavropoulos Center for Complex Quantum Matter University of Notre Dame, USA
Novel photovoltaic perovskites: beyond solar cells

Novel photovoltaic perovskites, the organo-metallic lead halides (e.g. CH3NH3PbI3), have revolutionized the field of solar cells by their high photon to electron conversion efficiency η of 25%.  But due to their chemical and structural tunability (one can grow crystals from nanometer size to 1000 cm3), they offer to study a wealth of exciting physical phenomena and open further possibilities for applications. To illustrate them, a selected set of measurements will be reported together with some device prototypes.

4/11/22 Prof. Christopher Arendse
Sn-Pb binary and mixed-halide perovskite thin films by low-pressure chemical vapor deposition

Since its first application as light absorbing materials in photovoltaic technology, perovskite solar cells (PSCs) have achieved a remarkable certified record power conversion efficiency (PCE) of over 25% in just over a decade. However, hybrid perovskite absorbers still face the issue of chemical instability as they degrade under continued exposure to moisture, light illumination, and UV light and are unstable at high temperatures. These instabilities are related to the deposition method used and the intrinsic properties of the material. We have demonstrated the deposition of pure, polycrystalline, smooth, and compact MAPbI3 perovskite films, using a sequential low-pressure chemical vapor deposition (LPCVD) method in a single reactor. This material was incorporated into a planar single-junction PSC (with no additives or additional interfacial engineering) that was fabricated, stored and tested under open-air conditions, yielding a best PCE of 11.7%. The solar cell maintains 85% of its performance up to 13 days in the open air with a relative humidity up to 80%.

This LPCVD method was further developed to produce mixed-halide and Sn-Pb perovskite thin films. We will report on the deposition procedure of these thin films and its resultant structural, compositional and optical properties. Furthermore, the impact of Cl-doping on the PSC performance will be discussed.

3/10/22 Prof. Thomas Curtright, University of Miami
Quantum Mechanics in Phase Space