Metal insulator transitions (MITs) in oxides are an intriguing problem from both a fundamental materials physics and an applied technology perspective. Though the precise roles of electron correlations and lattice distortions on the phase transition remains an active area of research, many recent theoretical studies have suggested intimate interplay among the orbital splitting/polarization, correlation effects, and Peierls dimerization in the 3d1 system. Occupied states have been probed by x ray photoelectron spectroscopy (XPS), and a rough structure of unoccupied 3d-like states have been deduced by O K-edge x-ray absorption measurements. NbO2, a 4d1 system, like VO2 crystallizes in a distorted rutile type structure with Nb dimers and undergoes a temperature induced MIT, albeit at a considerably higher temperature of ~1083 K. It is commonly accepted that because 4d orbital valence states are more dispersed in both space and energy, Mott physics is less important in 4d transition metal oxides than in 3d ones. Along this line of reasoning, it is perhaps surprising that the insulating state of NbO2 persists to higher temperatures than that of VO2.
A proposed explanation for this difference is that the Peierls effect in NbO2 is stronger due to larger Nb metal-metal overlap of 4d orbitals, leading to greater orbital splitting between occupied d|| states and the unoccupied eg-states; however, given the many attempts to revise and improve theoretical and computational studies of VO2, the physical and electronic properties of NbO2 also should be examined more thoroughly. Currently, there are few experimental studies that provide insight into the electronic structure of NbO2.
There have been diffraction, calorimetry, electrical, and magnetic studies on bulk NbO2, which have shown that it transforms from a high temperature rutile structure metal to a low temperature Nb dimerized diamagnetic insulator at ~1083 K. Recently epitaxial NbO2 thin films have been grown on (0001) Al2O3, (111) MgO, (111) MgAl2O4, and (111) perovskite oxide substrates. The key to achieving epitaxy of (100) rutile type compounds is exploiting substrate surfaces with eutactic planes.
Epitaxial NbO2 films were grown on (0001) Al2O3 by DC reactive sputtering of a Nb metal target at 650°C, 200W, 10 mTorr, 7.5 sccm O2, and 42.5 sccm Ar. An epitaxial (010) VO2 film was grown on (0001) Al2O3 by RF sputtering a V2O5 ceramic target at 450°C, 150W, 5 mTorr, 1.3 sccm O2, and 48.7 sccm Ar. Deposition conditions were optimized to both achieve stoichiometric phases as well as film smoothness for reliable ellipsometry measurements. X-ray reflectivity was used to measure the film thickness, and x-ray diffraction was used for phase and orientation determination.
Raman spectroscopy was performed in a confocal microscope using a 532 nm laser source; a filter prevents the collection of signals <170 cm-1. Electrical transport measurements were performed in the van der Pauw geometry; contact pads of 5 nm of Ti and then 50 nm of Au were sputtered on the films. Ex situ XPS scans were taken with Al K-alpha radiation and with an electron flood gun that prevents charging, and the samples were grounded to the spectrometer. The energy scale of the XPS data is referenced so that Au4f7/2 peak is at 84.0 eV.
The real optical conductivity of V1, N1, and N2 are shown in Figs. 1(a) (c). Sum rule analysis [Fig. 1(d)] can be performed on optical conductivity spectra to determine the effective number of electrons neff per formula unit of NbO2 accounting for optical excitation from 0 to a cutoff energy of E.
This work was done by Shriram Ramanathan of Harvard University for the Air Force Research Laboratory. AFRL-0285
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