Research Highlights

The idea of first principles materials by design has been around for a while: Thanks to both the advances in both the computational methods and their implementations, it is possible to not only explain experimental observations and build structure-property relationships for solid state materials, but we can also predict properties of yet to be synthesized materials and help guide the synthesis effort.

First principles design efforts are limited by the available computational methods, and due to the short comings of LDA/GGA, the standard Kohn-Sham Density Functional Theory (DFT) is not suitable for strongly correlated systems. In our group, we use the state of the art Dynamical Mean Field Theory (DMFT) method to approach strongly correlated systems. When interfaced with DFT, DMFT becomes a very strong tool that can not only reproduce but can also predict the properties of transition metal systems with a wide range of correlations from weakly correlated metals to Mott insulators. DFT+DMFT enables us to perform strongly correlated materials by design to take advantage of electronic correlations to obtain materials with superior macroscopic properties. 

Examples of our results includes the discovery of the Jeff=1/2 Mott-insulating state in Ir and Rh fluorides, and the study of vanadate, niobate, and moblybdate oxides as correlated transparent conductors.

References:
"Jeff=1/2 Mott-Insulating State in Ir and Rh Fluorides", Birol, Haule, Phys. Rev. Lett. 114, 096403 (2015) [Journal] [arXiv]
"Strain Tuning of Plasma Frequency in Vanadate, Niobate, and Molybdate Perovskite Oxides", Paul, Birol (2019)

Spectral Function of Rb2MF6

After the discovery of ferromagnetism, it took more than two millenia to discover antiferromagnetism. Fortunately, new and exotic magnetic phases and materials are predicted and verified more frequently nowadays. We study the magnetic hamiltonians of novel materials and the structural chemistry that gives rise to them using first principles methods. For example, our work has shown how a combination of strong spin-orbit coupling, the orbital configuration, and the face-sharing polyhedral geometry gives rise to a radically anisotropic exchange in Sr3NiIrO6; and how there exists ring exchange terms in addition to the well known biqadratic couplings in the breathing-Kagome compound Na2Ti3O8

References:
"Nature of the magnetic interactions in Sr3NiIrO6", Birol, Haule, Vanderbilt, Phys. Rev. B 98, 134432 (2018) [Journal] [arXiv]
"Unraveling the origin of phases of the S=1 kagome antiferromagnet Na2Ti3Cl8", Paul, Chung, Birol, Changlani (2019)

Breathing Kagome - Na2Ti3O8

 

One focus of our research is the structural phase transitions in oxides. As an example of such a phase transition, ferroelectricity is a popular research topic for both academic interest and possible applications. First principles methods have been historically very important and successful in not only understanding Ferroelectricity, but also designing new ferroelectric compounds.

We have been studying the layered perovskites of SrTiO3, the Srn+1TinO3n+1 Ruddlesden-Popper phases, since ~ 2010. Our original prediction for the emergence of ferroelectricity under strained thin films was verified, but other predictions, such as the important interfacial rumpling and the emergence of other ferroic phases was not.

In our recent work, our collaborators explored the same compounds using TEM with sub-angstrom resolution. There is a high level of agreement between the theory and the experiment, and the novel ferroic phases predicted in 2011 are now experimentally observed. 

References: 
"Atomic-Scale Imaging of Competing Ferroic States in a Ruddlesden-Popper Layered Oxide", Stone, Ophus, Birol, et al., Nat. Comm. 7, 12572 (2016) [Journal]
"Exploiting Dimensionality and Defect Mitigation to Create Tunable Microwave Dielectrics", Lee, Orloff, Birol, et al., Nature 502, 532 (2013) [Journal] [News&Views]
"Interface control of emergent ferroic order in Ruddlesden-Popper Srn+1TinO3n+1", Birol, Benedek, Fennie, Phys. Rev. Lett. 107, 257602 (2011) [Journal] [arXiv]

 

STO - RP

Recent experimental observation of a ferroelectric phase transition in metallic LiOsO3 challenged the view that ferroelectricity is not compatible with the presence of free carriers in a crystal. With our colleague, Nicole Benedek from Cornell, we questioned the validity of this idea using Density Functional Theory as well as a survey of the Inorganic Crystal Structure Database, and showed that there is no fundamental incompatibility between metallicity and polar distortions of the crystal structure.

References: 
'Ferroelectric' Metals Reexamined: Fundamental Mechanisms and Design Considerations for New Materials, Benedek, Birol, J. Mater. Chem. C 4, 4000 (2016) [Journal] [arXiv]

Metallic_FE