Completely filled band configuration leads to insulating phase (band insulator). It is now realized that large electron-electron Coulomb repulsion strength may lead to insulating phase in a partially filled systems, which are called Mott insulators. In addition, disorder in the lattice due to defects or impurities can also lead to insulating phase due to localization effect.

Electron correlation strength is expected to reduce with the increase in radial extension. In parallel, large radial extension leads to a large coupling with the lattice. Various recent studies show that 4d and 5d transition metal oxides exhibit varieties of interesting and unusual properties due to the interplay of electron correlation and lattice coupling. We studied some of these systems employing high resolution photoemission spectroscopy to explore the origin of various unusual phenomena such as Non-Fermi liquid behavior in 4d systems, unusual insulating phase in 5d systems, charge density wave state in insulators etc.

Interestingly, 5d transition metal oxides such as BaIrO₃, Y₂Ir₂O₇ etc. are highly insulating. However, they possess finite density of states (DOS) at the Fermi level. These DOS exhibit Fermi-Dirac like temperature dependence but does not follow the dependence expected for disordered systems.

A New Scenario seems necessary

Non Fermi liquid behavior is often found experimentally in a phase in the proximity of quantum critical point, where such ground state is related to magnetic instability. Evidence of the proximity of such quantum critical behavior has been observed in the recent studies in the high temperature superconductors. Various studies suggest that the existence of low-dimensionality in these systems leads to charge fractionalization and hence non Fermi liquid behavior manifests as has been shown in one-dimensional systems possessing decoupled charge and spin excitations.

We studied a three dimensional oxide CaRuO₃ exhibiting non-Fermi liquid behavior, while a similar system SrRuO₃ is a ferromagnet. High resolution photoemission spectra exhibit signature of particle-hole asymmetry. In addition, they show some oscilattions in the lineshape that matches with the phonon excitation energies observed in Raman Spectra. These results indicate the parameters such as particle-hole asymmetry and electron-phonon coupling may need considerations in the study of NFL behavior.

Europhys. Lett. 78, 17002 (2005).

Magnetism depends on two entities; (i) magnetic moment and (ii) delocalized electrons that couple them. Almost all the magnetic materials consist of elements having partially filled f (Eu, Gd, Tb, Dy etc.) or d ( Fe, Co, Ni etc.) bands among highest occupied energy bands. Small radial extension of f-orbitals in rare-earths makes them highly correlated and possess strong local character, which leads to finite magnetic moment. In these materials, coupling between the moments occurs via the interaction of these moments with the conduction electrons. The d electrons have both; magnetic moment due to strong electron-electron Coulomb repulsion and itineracy that mediates magnetic coupling. Surprisingly, CaB₆ exhibits ferromagnetism (Curie temperature > 600 K), despite the absence of partially filled d or f band element!

It is found that the non-magnetic impurities such as carbon/oxygen do not generate magnetic moment in CaB₆. The vacancy in boron sublattice leads to the formation of an impurity band near Fermi level. Such impurity states have B 2p character and are located close to the vacancy site. Exchange splitting of these states is finite leading to magnetic moment similar to experimental estimations. This suggests that vacancies/defects usually expected in semiconductors introduce local character in low density conduction electrons that may gives rise to finite moment leading to interesting magnetism.

Europhys. Lett. 82, 67006 (2008).

Candidates with good academic record and Ph.D. in Physical Sciences may apply for post doctoral position (PDF) to Prof. Kalobaran Maiti