Studies of the Band Structure of NbO by X‐Ray Photoelectróh Spectroscopy

Abstract

I n conjunction with ongoing measurements on electrical properties [l to 31 and band structure calculations [4] for NbO, the X-ray photoelectron spectra (XPS) of this compound have been measwed. No prior work of this type seems to have been reported for this material.Single crystals of high perfection and purity were grown as described elsewhere [5]. The experimental procedures and requisite precautions for the XPS mearurements have been discussed in publications pertaining to SrTiO, [6] and V20, [7]. The purity of the oxide and, more particularly, the absence of any other oxide of NbO, was checked by examining the 0-1s and Nb3d peaks. It was found that although immediately after surface cleaning by argon ion bombardment, the oxygen peak was narrow and reasonably symmetric, it became asymmetric and broad after several hours of run required for mapping the conduction band. However, this problem was minimized to a considerable extent, when the sample inlet tube of the machine was enclosed in a dry box maintained in an atmosphere of dry nitrogen. The sample was subjected to Ar ion bombardment to etch the surface.The instrument calibration was done by measuring the binding energy of 4fvl2 level of gold from a thin layer evaporated on to the sample and setting this value equal to 83.8 eV. All the measured levels were thus referenced to the Fermi level of gold and hence, indirectly, to that of NbO.

Electronic signature of the vacancy ordering in NbO(Nb3O3)

“…VB photoemission spectra have been reported previously for very low energy incident photons with hν = 30, 68 and 83.8 eV [18,19]. Considering the enormous energy difference of the incident photons with the present experiments, the peaks around -1.5 eV and -7 eV energies, along with a shoulder around -5.5 eV are qualitatively similar with those earlier reports [18,19].…”

“…It should be noted here that the presence of defectfree f cc-Nb 4 O 4 was previously already deemed hypothetical based on the finding that the Nb 3 O 3 structure is more stable than the Nb 4 O 4 by about 1 eV per NbO unit [15,16]. Also, the comparison of the DOS to the then available spectroscopic data supported this notion [14,18]. To confirm that such is still the case even when considering self-interaction corrections, we performed hybrid functional calculations for Nb 4 O 4 as well, with the same lattice constant a = 4.21 Å as Nb 3 O 3 .…”

“…VB photoemission spectra have been reported previously for very low energy incident photons with hν = 30, 68 and 83.8 eV [18,19]. Considering the enormous energy difference of the incident photons with the present experiments, the peaks around -1.5 eV and -7 eV energies, along with a shoulder around -5.5 eV are qualitatively similar with those earlier reports [18,19]. Yet, we are now able to discern additional features due to the improved energy resolution and larger variation of the photo-ionization cross-sections, which help us to present a more precise analysis of the electronic structure of niobium monoxide as discussed below.…”

“…Density functional based band structure calculations have been performed [12,[14][15][16] to explain the properties of niobium monoxide [17][18][19][20] and it was found that the Nb 3 O 3 structure is more stable than the Nb 4 O 4 by about 1 eV per NbO unit [15,16]. Our objective here is to test experimentally the accuracy of band structure calculations and to identify the bands in the measured spectra that are characteristic for Nb 3 O 3 , thereby proving that the vacancies are ordered and that band formation is an essential ingredient for the formation of vacancies in niobium monoxide.…”

Kondo effect and superconductivity in niobium with iron impurities

“…Thus it is likely the layer of Nb that is in intimate contact with SiO layer is converted to NbO which then reduces the of the top layer of Nb by the proximity effect 62 . It should be noted that NbO is itself an interesting quantum material 63 , 64 .…”