**Complex oxides: Rhenates and Osmates**

**
**Having enjoyed the excitement of research on cuprates and high temperature superconductivity, the resonating valence bond state, and pseudogap behavior, my group is now moving toward a new class of materials, the 5d transition metal oxides that combine the dual thrusts of spin-orbit and correlation effects. In addition to the iridates which are also being explored by many other groups, what is unique about our effort is a search for novel phenomena among the rhenates and osmates that allow for a variety of d-electron counts. Our theoretical efforts are coupled with experiments within the Center for Emergent Materials (CEM) funded by NSF-MRSEC.

All of these systems admit a fascinating interplay between spin, orbital and charge degrees of freedom, and have the potential of producing not only new phases of matter by entangling these degrees of freedom but also promise the possibility of tuning or control. For example strain, which directly couples to the orbital freedom, can be used as a handle to affect charge ordering or magnetic behavior in many oxide compounds.

# Double perovskites:

The double perovskites (DPs) are an interesting class of materials closely related to the perovskite family which show an amazingly rich variety of properties by combining two transition metal ions.

Double perovskites with the general formula A_{2}BB’O_{6} is a composite of two different ternary perovskites ABO_{3} and AB’O_{3} arranged in a 3D checker board pattern. The additional flexibility of choosing two different transition metal ions in double perovskites opens up many new avenues of material exploration, like the juxtaposition of strong spin-orbit coupling and strong interaction by combining 5d and 3d transition metals. Already the range of properties span metals to band insulators, and multi-band Mott insulators, as well as ferromagnets, antiferromagnets, ferroelectrics, multiferroics, and spin liquids.

## Sr_{2}FeMoO_{6} (SFMO):

The most widely studied double perovskite Sr_{2}FeMoO_{6} (SFMO) has the rare combination of a half metallic ground state with a ferromagnetic transition temperature Tc =420K which is well above room temperature. The potential for applications in spintronics is tremendous. We have developed a good theoretical understanding of half-metallic double perovskites including the effects of disorder. The main results of our work are:

- We derived an effective spin Hamiltonian for half-metallic double perovskites which is different from any known magnetic Hamiltonian by generalizing the Anderson-Hasegawa analysis to double perovskites. It provides an efficient and accurate way to study magnetic properties including the the effects of disorder
- We have linked the spin polarization at the chemical potential which is the crucial quantity for spintronics applications to the magnetization. Spin polarization is very difficult to measure and by showing that it is proportional to the magnetization as a function of temperature, we have provided an efficient way for estimating spin polarization.
- Even though SFMO is widely believed to be a half-metal, only indirect experimental evidence like magnetoresistance and optical absorption measurements are available. We have made precise predictions for APRES which can be used to directly observe the half-metallic nature of SFMO
- Finally, we have studied the effects of thermal fluctuations and disorder on the optical conductivity. We showed that weak disorder does not affect the half-metallicity. We also explained for the first time the origin of a secondary peak in optical conductivity seen in experiments and related it to deviations from half-metallicity.

**Publications:
**

- O. Erten, O.N. Meetei, A. Mukherjee, M. Randeria, N. Trivedi and P. Woodward, “Theory of Half-Metallic Ferrimagnetism in Double Perovskites”, Phys. Rev. Lett. 107, 257201 (2011)
- O.N. Meetei, O. Erten, M. Randeria, N. Trivedi and P. Woodward, “Theory of High Tc Ferrimagnetism in a Multi-orbital Mott Insulator”, Phys. Rev. Lett. 110, 087203 (2013)
- O.N. Meetei, O. Erten, A. Mukherjee, M. Randeria, N. Trivedi and P. Woodward, “Theory of Half-Metallic Double Perovskites I: Double Exchange Mechanism”, Phys. Rev. B 87, 165104 (2013)
- O. Erten, O.N. Meetei, A. Mukherjee, M. Randeria, N. Trivedi and P. Woodward, “Theory of Half-Metallic Double Perovskites II: Effective Spin Hamiltonian and Disorder Effects”, Phys. Rev. B 87, 165105 (2013)
- J. Janczak, O.N. Meetei, M. Randeria and N. Trivedi, “Predictions for Spin Resolved Spectral Function and Optical Conductivity in Half-metallic Double Perovskites” (to be submitted)

## Sr_{2}CrOsO_{6} (SCOO):

SCOO is an interesting material with the highest T_{C} (725K) among all perovskites with a net moment and an unusual non-monotic magnetization as a function of temperature. Theoretically, it had many unserovled puzzles like 1) why is it an insulator? 2) why is there a net moment? 3) why is the Tc so high and 4) why is magnetization non-monotonic? In our work, we explained all of the puzzles. We showed that

- SCOO is a multi-orbital Mott insulator. We derived a new Mott criterion for double perovskites with half-filled bands: the geometric mean of onsite interaction strengths on Cr and Os competes with the kinetic energy. SCOO is on the insulating side.
- The net moment comes from magnetic frustrations arising from next nearest neighbor exchange interaction between Os ions and not because of spin orbit coupling as suggested in earlier work.
- The non-monotonic magnetization is also due to magnetic frustration.
- The combination of strong interaction on Cr and weak interaction on Os puts SCOO close to the Mott transition and this leads to the unusually high T
_{C}.

**Publications:**

- O.N. Meetei, O. Erten, M. Randeria, N. Trivedi and P. Woodward, “Theory of High Tc Ferrimagnetism in a Multi-orbital Mott Insulator”, Phys. Rev. Lett. 110, 087203 (2013)

**Theory of strain controlled magnetotransport and stabilization of the ferromagnetic insulating phase in manganite thin films**

Recent advances in the controlled growth of transition metal oxides has made it possible to produce atomically sharp interfaces between materials. If the two materials are not lattice matched, one of them will be strained. Here we studied the effects of strain on half-doped manganite thin films by proposing a realistic microscopic model of strain and solving this model with a powerful Monte Carlo technique. We demonstrated that strain can be used to reach a ferromagnetic charge-ordered insulating state, which has previously been inaccessible in experiments. We also showed that strain is a useful “control knob” over the magnetoresistance response in the film.

**Publications:**

- Anamitra Mukherjee, William S. Cole, Nandini Trivedi, Mohit Randeria, Patrick Woodward, PRL
**110**157201 (2013)