**Date/Time**

Date(s) - Feb 12 2019

*2:00 PM - 3:00 PM*

**Location**

Room 130, McCullough Building

**Category(ies)**

**First principles investigation on quantum materials**

**Subhasish Mandal**

Postdoctoral Research Associate,

Dept. of Physics & Astronomy,

Rutgers University, Piscataway, NJ

https://sites.google.com/site/mandalsubhasish/

Computer simulations based on first principles calculations play a central role in helping us understand, predict, and engineer physical, chemical, and electronic properties of technologically relevant materials. This can solve many problems towards building faster, smaller and cheaper devices for processing and storing information as well as for saving energy. Many of these processes involve electron excitations and strong local magnetic fluctuation that the ‘standard model’ of electronic structure, Density Functional Theory (DFT), can’t capture properly. In this context, I will highlight my work in two popular approaches that go beyond the standard DFT. Dynamical Mean Field Theory (DMFT) in combination with DFT has recently been successful for detailed modeling of the electronic structure of many complex materials with strong electron correlation. To give an example, I will show the anomalous properties of the iron-based superconductors in both bulk and monolayer phases, which have their origin in strong Hund’s coupling [1]. In this context, I will discuss how computed electron-phonon coupling could be enhanced in the presence of electron correlation in FeSe. This has been recently verified in a femto-second coherent locked-in photoemission spectroscopy experiment [2,3]. Another *ab-initio *method that goes beyond the limit of DFT is GW-approximation, which is extensively used to compute excited states of electrons in solids. So far, most of the GW calculations have been confined to small unit-cell of bulk-like materials due to the extreme computational demand of the approach. I will discuss my collaborative effort toward developing a high scalable, open-source GW software to compute electronic excited states more efficiently for petascale architectures using the Charm++ parallel framework [4,5]. At the end, I will briefly discuss topological crystalline insulators, which are a new class of topological materials where electronic surface states are topologically protected along certain crystallographic directions by crystal symmetry. I will discuss how without any external perturbation, both massless Dirac fermions protected by the crystal symmetry and massive Dirac fermions with crystal symmetry breaking can coexist on a single surface [6].

References:

1. **S. Mandal**, P. Zhang, S. Ismail-Beigi, K. Haule; “How correlated is the FeSe/SrTiO3 system?”. *Phys. Rev. Lett*. 119, 067004 (2017).

2. **S. Mandal**, R. E. Cohen, and K. Haule; “Strong Pressure Dependent Electron-Phonon Coupling in FeSe”. *Phys. Rev. B Rapid Communications *89, 220502(R) (2014).

3. S. Gerber *et al.*; “Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser” *Science *357, 71 (2017).

4. M. Kim*. **S. Mandal*** *et al. *“Scalable GW software for quasiparticle properties using OpenAtom” *arXiv:1810.07772 *(2018).

5. http://charm.cs.illinois.edu/OpenAtom/

6. O. E. Dagdeviren*, **S. Mandal* ***et al. Phys. Rev. Mat. 2, 114205, (2018).*