Research Highlights

Ferroelectricity and Ferroelectric Materials

Hybrid Improper Ferroelectric Phase Diagram

Ferroelectricity, the emergence of a spontaneous and switchable electric dipole moment in a crystal, is a promising area that is important for both academic interest and a whole range of applications. First principles methods have been historically very important and successful in not only understanding ferroelectricity, but also designing new ferroelectric compounds. Recently, our group performed work on the seemingly contradictory coexistence of free carriers with ferroelectricity. Contrary to the long standing expectation that free carriers always suppress ferroelectricity, we showed that there is a regime in oxides where degenerate doping can not only enhance but also induce a structural polarization.

Our current work on ferroelectrics, funded by the Office of Naval Research, aims to discover and design new ferroelectric materials and elucidate the driving forces of ferroelectricity in them. We showed that layered antiperovskites, which are perovskite-like compounds where the cationic and anionic sites are interchanged, can display ferroelectricity. The uncommon charge states of anions in these compounds make the coupling between the crystal and electronic structures particularly interesting. 

On a separate project, we are also studying the effect of ferroelectric supports on the catalytic properties of metallic electrodes. We aim to exploit the methods and concepts developed for the study of ferroelectric heterostructures (such as negative and interfacial capacitances) to design a metal-ferroelectric-metal system which enables significant modulation of adsorption energies to go beyond the Sabatier limit. 

Recent References: 

  • "Free carrier induced ferroelectricity in layered perovskites", Li, Birol, Phys. Rev. Lett. 127, 087601 (2021) [Journal] [arXiv]
  • "Suppressing The Ferroelectric Switching Barrier in Hybrid Improper Ferroelectrics", Li, Birol, npj Comput. Mater. 6, 168 (2020) [Journal] [arXiv]
  • "Stable and switchable electric polarization in two dimensions", Birol, Nature 560, 174 (2018) (News & Views) [Journal]
  • "'Ferroelectric' Metals Reexamined: Fundamental Mechanisms and Design Considerations for New Materials", Benedek, Birol, J. Mater. Chem. C 4, 4000 (2016) [Journal] [arXiv]
  • "Atomic-Scale Imaging of Competing Ferroic States in a Ruddlesden-Popper Layered Oxide", Stone, Ophus, Birol, et al., Nat. Comm. 7, 12572 (2016) [Journal]


Charge Density Waves

Charge Density Wave in Kagome

We recently initiated a new research thrust to theoretically study the charge density wave phases in different families of 2D and 3D compounds. These materials undergo a phase transition at an ordering temperature due to an instability driven by the electrons, and in this ordered phase the translational symmetry of the crystal structure is reduced. While there is a large number of compounds with well studied charge density wave phases, new compounds with novel behavior is being discovered every year. Our current efforts focus on a family of vanadium based Kagome metals, where we are attempting to achieve the most comprehensive classification of possible charge density wave states using group and representations theories. These materials are likely to host imaginary charge density wave states (also known as loop-current phases) which we are studying using magnetic space groups, which was never attempted before. The use of group theory in conjunction with first principles density functional theory also enables us to help explain the experimental observations by our collaborators, including muon spin relaxation and Raman spectroscopy results. 

Recent References:

  • "Two types of charge order in the superconducting kagome material  CsV3Sb5", Gupta, Das, Mielke, Ritz, Hotz, Yin, Tu, Gong, Lei, Birol, Fernandes, Guguchia, Luetkens, Khasanov, Under Review [arXiv]
  • "Charge density wave order in kagome metal AV3Sb5 (A= Cs, Rb, K)", Wu, Ortiz, Tan, Wilson, Yan, Birol, Blumberg, Under Review [arXiv]
  • "Electrons go loopy in a family of superconductors", Christensen, Birol, Nature 602, 216 (2022) (News & Views) [Journal]
  • "Theory of the charge-density wave in AV3Sb5 kagome metals", Christensen, Birol, Andersen, Fernandes, Phys. Rev. B 104, 214513 (2021) [Journal] [arXiv]
  • "Revealing the competition between charge-density wave and superconductivity in CsV3Sb5 through uniaxial strain", Qian, Christensen, Hu, Saha, Andersen, Fernandes, Birol, Ni, Phys. Rev. B 104, 144506 (2021) [Journal] [arXiv]

Magnetism and Magnetic Materials

Magnetic and Multiferroic Materials

It took more than two millennia to discover antiferromagnetism after ferromagnetism. Fortunately, new and exciting magnetic phenomena are being discovered at a much higher rate nowadays. In our group, we study the magnetic and multiferroic materials using first principles methods to understand and extract the parameters of their magnetic Hamiltonians, and then use a range of other tools (mean field, linear spin wave, exact diagonalization, etc.) to solve these models. We are also interested in the underlying structural chemistry that gives rise to particular magnetic phenomena. 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.

Recent References:

  • "First-principles characterization of the magnetic properties of Cu2(OH)3Br", Gautreau, Saha, Birol, Phys. Rev. Mat. 5, 024407 (2021) [Journal] [arXiv]
  • "Unraveling the origin of phases of the S=1 Kagome antiferromagnet Na2Ti3Cl8", Paul, Chung, Birol, Changlani, Phys. Rev. Lett. 124, 167203 (2020) [Journal] [arXiv]
  • "Nature of the magnetic interactions in Sr3NiIrO6", Birol, Haule, Vanderbilt, Phys. Rev. B 98, 134432 (2018) [Journal] [arXiv]


Correlated Materials Design

The idea of first principles materials by design has been around for a while: Thanks to the advances in computational approaches, 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. This being said, first principles design efforts are limited by the weaknesses of the 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 with predictive power on 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 recent work includes the discovery of the potential of SrNbO3 as a ultraviolet transparent conducting electrode, which was featured in NSF Discovery File series because of its potential to help fight COVID19. We have also shown how the inter-cationic charge transfer in the double perovskite Sr2VNbO6 leads to enhanced correlations with possible Hund's metallicity.

Recent References:

  • "Cation Order Control of Correlations in Double Perovskite Sr2VNbO6", Paul, Birol, Phys. Rev. Research 2, 033156 (2020) [Journal] [arXiv] 
  • "SrNbO3 as Transparent Conductor in the Visible and Ultraviolet Spectrum", Park, Roth, Oka, Hirose, Hasegawa, Paul, Pogrebnyakov, Gopalan, Birol, Engel-Herbert, Communications Physics 3, 102 (2020) [Journal]
  • "Strain tuning of plasma frequency in vanadate, niobate, and molybdate perovskite oxides", Paul, Birol, Phys. Rev. Mat. 3, 085001 (2019) [Journal] [arXiv]
  • "Applications of DFT+DMFT in Materials Science", Paul, Birol, Annu. Rev. Mater. Res. 49, 31 (2019) [Journal] [arXiv]