T. Maitra

Proposed orbital ordering in MnV2O4 from first-principles calculations.

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Proposed Orbital Ordering inMnV2O4from First-Principles Calculations

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Effects of fe substitution on the electronic, transport, and magnetic properties of ZnGa2O4 : A systematic ab initio study

We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide-band-gap spinel

An extended Falicov-Kimball model on a triangular lattice

\mathrm{Zn}{\mathrm{Ga}}_{2}{\mathrm{O}}_{4}

Magnetic properties of doped GdI2

. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange-correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.

Effect of spin-orbit coupling on magnetic and orbital order in MgV2O4.

The combined effect of frustration and correlation in electrons is a matter of considerable interest lately. In this context a Falicov-Kimball model on a triangular lattice with two localized states, relevant for certain correlated systems, is considered. Making use of the local symmetries of the model, our numerical study reveals a number of orbital ordered ground states, tuned by the small changes in parameters while quantum fluctuations between the localized and extended states produce homogeneous mixed valence. The inversion symmetry of the Hamiltonian is broken by most of these ordered states leading to orbitally driven ferroelectricity. We demonstrate that there is no spontaneous symmetry breaking when the ground state is inhomogeneous. The study could be relevant for frustrated systems like GdI2, NaTiO2 (in its low-temperature C2/m phase) where two Mott localized states couple to a conduction band.

A ground state phase diagram of a spinless, extended Falicov–Kimball model on the triangular lattice

Abstract. Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI2  and its doped variant GdI2Hx we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.

The nature of itineracy in CoV2O4: a first-principles study

Recent measurements on MgV(2)O(4) single crystals have reignited the debate on the role of spin-orbit (SO) coupling in dictating the orbital order in vanadium spinel systems. Density functional theory calculations were performed using the full-potential linearized augmented-plane-wave method within the local spin density approximation (LSDA), Coulomb correlated LSDA (i.e. LSDA + U), and with SO interaction (LSDA + U + SO) to study the magnetic and orbital ordering in the low temperature phase of MgV(2)O(4). It is observed that, in the experimental antiferromagnetic phase, the spin-orbit coupling affects the orbital order differently in alternate V-atom chains along the c-axis. This observation is consistent with the experimental prediction that the effect of spin-orbit coupling is intermediate between those in the cases of ZnV(2)O(4) and MnV(2)O(4).

Phase transitions in a spinless, extended Falicov–Kimball model on the triangular lattice

Correlated systems with hexagonal layered structures have come to the fore with renewed interest in cobaltates, transition metal dichalcogenides and GdI(2). While superconductivity, unusual metal and possible exotic states (prevented from long-range order by strong local fluctuations) appear to come from frustration and correlation working in tandem in such systems, they freeze at a lower temperature to crystalline states. The underlying effective Hamiltonian in some of these systems is believed to be the Falicov-Kimball model and therefore, a thorough study of the ground state of this model and its extended version on a non-bipartite lattice is important. Using a Monte Carlo search algorithm, we identify a large number of different possible ground states with charge order as well as valence and metal-insulator transitions. Such competing states, close in energy, give rise to complex charge order and other broken symmetry structures as well as the phase segregations observed in the ground state of these systems.

Thermodynamic studies of the two dimensional Falicov-Kimball model on a triangular lattice

Inspired by recent experiments, we have theoretically explored the nature of itineracy in CoV2O4 under pressure and investigated, using first-principles density functional theory calculations, whether it has any magnetic and orbital ordering. Our calculations indicate that there could be two possible routes for obtaining the experimentally observed pressure induced metallicity in this system. One is via the spin–orbit interaction coupled with Coulomb correlation, which can take the system from a semiconducting state at ambient pressure to a metallic state under high pressure. The other mechanism, as indicated by our GGA + U calculations, is based on the presence of two kinds of electrons in the system: localized and itinerant. An effective Falicov–Kimball model could then possibly explain the observed insulator to metal transition. Comparison of the two scenarios with existing experimental observations leads us to believe that the second scenario offers a better explanation for the mechanism of the insulator to metal transition in CoV2O4 under pressure.