Date: 4th October (Wednesday) 

Time: 4PM

Venue: Over Google Meet 
Video call link: https://meet.google.com/nhp-qcvp-tvf

 

Abstract:  In this talk we focus on the role of electron correlation in exploring the very exotic phase diagram

of Manganites as a function of strain, doping, and electrochemical activity. Selecting Lanthanum
Manganese Oxide LaMnO3 and Lithium Manganese oxide LiMnO2 as our prototypical materials,
both of which have Jahn Teller distorted MnO6 octahedra and Mn in d
4
isoelectronic configuration
we study their phase diagram using a combination of density-functional theory (DFT) and dynamical
mean-field theory (DMFT).
Inspired by the experimental findings of an exotic ferromagnetic insulating state in
LaMnO3/SrTiO3 heterostructures, we calculate the electronic and magnetic state of
LaMnO3/SrTiO3 superlattices with comparable thicknesses [1–3]. Projecting onto the low-energy
subspace of Mn 3d and Ti 3d states, and solving a multi-impurity problem, our approach emphasizes
on local correlations at Mn and Ti sites. We find that a ferromagnetic insulating state emerges due
to intrinsic effects of strong correlations in the system, in agreement with experimental studies. We
also predict that, due to electronic correlations, the emerging 2D electron gas is located at the LMO
side of the interface. This is in contrast to DFT results and also in contrast to previously studied
popular oxide heterostructure systems like LAO/STO [4] that locate the electron gas on the STO

side. We estimate the transition temperature for the paramagnetic to ferromagnetic phase transi-
tion, which may be verified experimentally. Importantly, we also clarify that the epitaxial strain

is a key ingredient for the emergence of the exotic ferromagnetic insulating state. This becomes
clear from calculations on both uniaxial as well as biaxial (epitaxial) strain LaMnO3 system [5],
also showing ferromagnetism which is not seen in the unstrained bulk material, providing a path to
strain tuning of oxides.

Next we explore explore electronic and magnetic states of layered LixMnO2, a Li-ion battery cath-
ode material, as function of states-of-charge x [6]. We observe that an antiferromagnetic insulating

state appears in LiMnO2, with a moderate N ́eel temperature. On delithiation we find various exotic

states emerge such as ferrimagnetic correlated metals, charge-ordered ferromagnetic correlated met-
als with large quasiparticle peak, ferromagnetic metals with small quasiparticle peak, as function

of x, all in high spin states. For x=0.67-0.33, a mix of +3/+4 oxidation states of Mn is observed,
while oxidation state changes from +3 in LiMnO2 to +4 in MnO2. The quasiparticle peaks in the
correlated metals could be attributed to polarons, not captured in DFT studies. We explore the
temperature vs. x phase diagram and conclude that x is a key ingredient for emergence of exotic
correlated phase transitions. We identify the origin of the mixed +3/+4 oxidation state and ascribe
it to the different polaronic quasiparticle weights at different Mn sites. An extremely important
consequence of this charge ordering is that this gives rise to the pathways which eventually lead
to the layered to spinel structural transformation in LiMnO2 which have uptil now prevented the
widespread use of LiMnO2 as a battery cathode. Our study explaining the underlying reason for such
transformation also provides rationale for methods of preventing such structural transformations by
doping methods.

∗ Electronic address: hb595@cam.ac.uk
[1] H. Banerjee and M. Aichhorn, Phys. Rev. B 101, 241112 (2020), URL https://link.aps.org/doi/10.1103/PhysRevB.
101.241112.
[2] H. Banerjee, O. Janson, K. Held, and T. Saha-Dasgupta, Phys. Rev. B 100, 115143 (2019), URL https://link.aps.org/
doi/10.1103/PhysRevB.100.115143.
[3] H. Banerjee, Modern Physics Letters B 34, 2030006 (2020), https://doi.org/10.1142/S0217984920300069, URL https:
//doi.org/10.1142/S0217984920300069.
[4] H. Banerjee, S. Banerjee, M. Randeria, and T. Saha-Dasgupta, Scientific Reports 5, 18647 (2015), ISSN 2045-2322, URL
https://doi.org/10.1038/srep18647.
[5] F. P. Lindner, M. Aichhorn, and H. Banerjee, arXiv p. 2212.01090 (2022).
[6] H. Banerjee, C. P. Grey, and A. J. Morris, arXiv p. 2209.14682 (2022).