Abstract
The work presented in this thesis comprises experimental investigations of the pressure
response of electrical-transport behaviour of two topical transition metal oxides exhibiting
charge order (CO), viz., Fe2OBO3 and LuFe2O4, as well as that of hybridization-gapped
semiconducting FeSi. This was realized by utilizing a diamond anvil cell (DAC) to generate
static pressure up to 30–35 GPa as a non-thermal control parameter. The three Fe-based
materials are examples of systems in which strong electron-electron interactions govern their
physical properties. Pressure is a perfect tool to investigate the competition amongst different
ground states, as the application of pressure modifies the lattice parameters thus changing the
relative magnitudes of pertinent electron interactions (3d bandwidth w, on-site repulsion U and
inter-site repulsion V ) without necessarily introducing any disorder.
The resistivity pressure measurements on Fe2OBO3 at variable cryogenic temperatures
reveal two distinct pressure regimes, namely, low-pressure (P < 6 GPa) behaviour and that at
high-pressure (P > 16 GPa). In the low-pressure regime, electrical transport properties are well
characterized by 3D Mott variable range hopping (M-VRH), whereas in the high-pressure
phase Efros-Shklovskii variable range hopping (ES-VRH) seems more appropriate. Some
level of disorder must be present in both pressure regimes. The localization length calculated
within the framework of the M-VRH formulation, increased from ~2.5 Å to ~4.2 Å when
pressure is increased from ~2 GPa to ~22 GPa, respectively. These values are similar to the
unit-cell parameters, which is an indication that electron confinement prevails throughout.
Results reported in this work are complementary to magnetic-electronic (Fe Mössbauer)
pressure studies, which revealed disruption of site-centered CO at P ~16 GPa [G.R. Hearne et
al., Phys. Rev. B 86, 195134 (2012)]. This involved formation of electronic symmetric dimers
from Fe2+⟺Fe3+ electron hopping. In the low-pressure phase, the wall formations of CO
nanodomains are deemed to be the source of disorder. On the other hand, in the high-pressure
phase rapid Fe2+⟺Fe3+ electron exchange and the associated fluctuating potential contribute
to the disorder.
Two samples of multiferroic LuFe2O4 (LFO) with different stoichiometries were
investigated. These include highly-stoichiometric (HS-LFO) and off-stoichiometric
(OS-LFO) samples. Similar electrical-transport behaviour has been observed in each of these
up to elevated pressures of 25–30 GPa. Two pressure regimes were also identified in both
samples, i.e., low-pressure (P < 5 GPa) and high-pressure (P > 8 GPa) phases, respectively.
The value increased from ~13 Å at ~1 GPa to ~20 Å at ~25 GPa for HS-LFO, and from
~15 Å at ~1 GPa to ~22 Å at ~30 GPa for OS-LFO. Previous Mössbauer pressure studies
[G.R. Hearne et al., Phys. Rev. B 93, 105101 (2016)] evidenced site-centered CO in the
low-pressure phase, and electronic asymmetric-dimers and bond-centered CO in the
high-pressure phase, in conjunction with structural studies [J. Rouquette et. al., Phys. Rev.
Lett. 105, 237203 (2010)] which also evidenced a new CO state in the high-pressure phase. In
the low-pressure phase, M-VRH is the best model for describing the conduction mechanism
of charge carriers in both HS-LFO and OS-LFO. Here the wall dynamics of CO nanodomains
created in the a-b plane constitute the source of disorder. The ES-VRH model best accounts
for transport properties of the two samples in the high-pressure regime. Formation of...
Ph.D. (Physics)