Varsha's Excellent Qualifying Exam and Kagome Trimer Halides Paper

Varsha had her qualifying exam - the first in the group! - and had one of the best any of her committee members had seen. She is co-advised in experiment with Sara Skrabalak, so her work involves both theory on correlated electrons, as well as synthesis and theory for photoluminescent, layered & lone pair oxyhalide materials. The qualifying exam in our department is a long event: 1 hr talk+questions, followed by 1.5-2 hrs of questions from the committee members.

Her work has also been published here: https://pubs.rsc.org/en/content/articlehtml/2025/tc/d5tc01981h
Some more pictures below:

Varsha flanked by her proud (and at least in my case, nervous) advisors before her talk.

Post-exam ice cream. I do not, in fact, recommend cherry wasabi as an ice cream flavor.

Group picture! Mai, Madison, Akhond, Alex, Varsha, Bipasa

Skrabalak and Georgescu Groups Receive NSF DMR Solid State Chemistry Grant to study Photoluminescent Bismuth Lone Pair Materials

Happy to share that Sara Skrabalak and myself have been awarded a new NSF DMR Solid State Chemistry grant for our collaborative project on photoluminescent oxyhalides with bismuth lone pairs! 🎉

This achievement would not have been possible without the incredible contributions of our students 🎉 . From left to right:

-Emily Ward (undergraduate, co-advised with Sara, now pursuing her Ph.d at Princeton) introduced these materials and the role of lone pairs to our group, then developed the first Wannier-based Hamiltonian and representation of lone pairs in a solid ( https://lnkd.in/gb7_fMBA )
-Varsha Kumari (Ph.D. student, co-advised) laid the foundation for much of the theoretical framework behind our proposal, taught me about photoluminescence in these materials, and began her own direction in both simulations and measurements.
-Nayana Christudas Beena (in Sara’s group, who is applying for postdocs, so you should reach out to her 😊 ) has driven forward the experimental exploration of these materials while making our team discussions more rigorous, as well as helped mentor Emily since she started college.
-Connor Schulte (in Sara's group) contributed important preliminary results that helped shape this direction.

I am especially grateful to Emily and Varsha for trusting me enough to work with me before I even had a research group of my own, and for setting the stage for the outstanding students who have since joined our group.

This project allows us to continue supporting and training excellent students , who are pushing science forward! 🎉

Varsha Attends IEEE Ferroschool at Oak Ridge National Labs

Congratulations (again) to Varsha for being accepted and attending the IEEE Ferroschool at Oak Ridge National Labs. She is learning about a wide range of theoretical, experimental and AI-related tools for ferroelectric & polar materials study and discovery. We can’t wait to hear what she will learn.

Congratulations Emily and Good Luck at Princeton

Congratulations Emily! Emily is the first undergraduate student to graduate from our group. She did a remarkable amount of work of high complexity during her time in the group, including the first paper accurately modeling lone pairs in a solid using Wannier functions (https://iopscience.iop.org/article/10.1088/2515-7639/adc33e/meta ). We will miss her high energy and kindness, and wish her good luck during her Ph.D at Princeton.

Group presents at PINDU, November 2024

Group Member Presentations at Conferences In November

Our group members presented at multiple conferences this Fall! In November, Varsha, Emily, Carina and Bipasa all presented their posters at PINDU (see picture above), the Purdue, Indiana and Notre Dame inorganic chemistry conference. Later on, Varsha and Bipasa presented their work at the Fall Meeting of the Materials Research Society in Boston. Varsha and Bipasa each presented a poster, and Varsha also gave a talk. Varsha also won a travel award from the department supporting her travel and recognizing her high quality work on the theoretical study of trimer halide materials. Congratulations all!

PARADIM Johns Hopkins University/Cornell University Summer School Talk

You can now watch the PARADIM Summer School talk I gave online at Johns Hopkins University. The goal with this talk was to introduce students to concepts they may need to understand the literature in the field of correlated electron materials, with a focus on superconductors, particularly cuprate and nickelate based ones; I discuss topics including what correlated electron materials are, Jahn-Teller distortions, different types of orbital models, and introduce in a non-technical way how to read a dynamical mean field theory paper and its spectral functions, and what a self-energy is.

You can watch the video here, and timestamps should guide you: https://www.youtube.com/watch?v=aVJXcH6XzJs

Here are the timestamps with the different chapters, before you click:

0:00 Intro from Tyrel McQueen 0:25 What you should get from this talk 1:40 Correlated Electron Materials in a Nutshell 2:45 Flat Bands: Kagome and Moire 3:45 Type 1 vs 2 Superconductors 4:30 Mott Physics 6:25 Electron or lattice physics? 7:00 Mott insulator 8:50 Correlated Metals 9:30 DFT vs ARPES in correlated metals: Hubbard bands and quasiparticles 10:35 DMFT, Subsidiary Bosons 11:02 DMFT Spectral function NdNiO2 and CaCuO2 11:55 Self Energy 14:45 Self-Energy and Spectral functions in SrVO3 15:03 FermiSee, simple Fermi Liquid Tool 15:57: p-d models intro 22:50 things to keep in mind when reading DMFT papers 23:30 local ionic environment and effective models 25:22 Tune Tc in cuprates via apical oxygen 27:26 Nickelate and cobaltate heterostructures for superconductivity 30:29 Machine Learning Status

Chemical Bonding Concepts in Solids

I was recently reading Nobel Chemistry laureate, Roald Hoffman’s, really nice book on bonding in solids and thought that the explanations for certain concepts (like Peierls distortion and Bloch wavefunctions) are so much clearer than the way I was taught in physics, and also make it easier for me to teach. So I decided to make this video, introducing some useful concepts from this book as well as that I use in my work on a regular basis, such as antibonding orbitals, p-d models and Wannier models. You can click on the timestamps in my comment on the video to watch the most interesting parts for you: https://www.youtube.com/watch?v=x4ihhQrS6lA

If you are a condensed matter physicist or materials scientist and never took an inorganic chemistry class, you will likely find this useful (like I did). If terms terms like Wannier functions, antibonding and p-d model are a bit confusing, you'll probably also find this useful.

I discuss chemical bonding concepts as relevant for the Peierls distortion, bonding/antibonding (antibonding orbitals destabilize materials), Bloch wavefunctions, supercells/band folding, low energy vs p-d models, Wannier functions and the LCAO (linear combination of atomic orbitals) approximation.

An Introduction to Correlated Electron Materials - youtube video

I’ve made this introductory video for people who are interested in correlated electron materials and are either new to the field or may want to learn something new. I cover things ranging from Jahn-Teller and trigonal distortions, Peierls distortion, band renormalization in correlated metals and the importance of beyond-Density Functional Theory methods, spin frustration, and so on. This is the result of A LOT of work, and I’ve found both new students, and some experienced scientists who might not be familiar to one aspect or another of the phenomenology presented here have found it interesting. So, please enjoy: https://www.youtube.com/watch?v=sp_f8UlVJOE

Trigonal Symmetry and its Electronic Effects in 2D Halides and Other Materials

This paper is now published in PRB.

One of the difficulties I’ve found in the literature when looking at transition metal halides and dihalides, was to understand what mechanism makes these materials insulating: it’s pretty hard to understand why a 3-fold degenerate t2g orbital basis would lead to an insulating material, for an orbital filling of 1, 2, 4, 5. This is the model many people work with, however, and the result are often fancy explanations for pretty simple behavior.

The explanation is quite simple: the basis is not degenerate to begin with - and due to the broken symmetry inherent in a 2D material, it can’t be. I explain here how to reliably build a basis that conforms to the appropriate trigonal symmetry, and provide some simple matlab scripts to do so. This has important implications: particularly in the potential spin-liquid material RuCl3, which many people model as having a 3-fold degenerate orbital basis as the basis for their models. This class of materials - and more generally, edge and face-connected octahedra - have their crystal field splittings parametrized by ligand-ligand bond-length distances rather than metal-ligand bond length differences. I explain this in this 10 min talk, as well as in a paper with James Rondinelli and Andy Millis.

Youtube video here and PRB here.