PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...
PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...
PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...
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Introduction 3<br />
The temperature and doping dependent metal-insulator transitions in these materials<br />
are found to be closely related to the unique electronic structure derived from<br />
the Mn 3d and O 2p hybridized orbitals of MnO 6 octahedra. The 3d orbitals of Mn<br />
in the MnO 6 octahedra, which are split by the crystal field into t 2g and e g states,<br />
are further split by the Jahn-Teller distortion. With charge carrier doping, many of<br />
them show well defined charge ordering (CO) and/or orbital ordering (OO) at low<br />
temperatures, especially when x equals a commensurate value (e.g x = 0.5). The CO<br />
state where the Mn 3+ (t 3 2ge 1 g) and Mn 4+ (t 3 2g) ions arranged like in a checkerboard was<br />
found to bear a strong influence on the one electron band width (W) of the e g band<br />
and the transfer interaction of the e g holes (electrons).<br />
The work presented in this thesis consists of the investigations of the electronic<br />
structure of Pr 1−x Ca x MnO 3 using different spectroscopic techniques as photoemission<br />
spectroscopy, inverse photoemision spectroscopy and x-ray absorption spectroscopy<br />
etc. The thesis is structured as follows : Following this overview, the Chapter 1 gives<br />
a general idea about the physical properties (structural, electronic and magnetic) and<br />
the mechanisms associated with the CMR manganites. Chapter 2 describes the principles<br />
and operations of the experimental techniques used for this thesis work. Chapter<br />
3 presents the electronic structure of Pr 0.67 Ca 0.33 MnO 3 at different temperatures<br />
using ultra-violet photoelectron and x-ray absorption spectroscopy. The electronic<br />
structure of the Pr 1−x Ca x MnO 3 series at different temperatures using ultra-violet<br />
photoemission and inverse photoemission spectroscopy are discussed in Chapter 4.<br />
In Chapter 5, the electronic structure of Ca 0.86 Pr 0.14 MnO 3 at different temperatures<br />
using high resolution photoemission and x-ray absorption spectroscopy are discussed.<br />
Chapter 6 gives the summary and conclusions of this thesis.<br />
1.2 Colossal magnetoresistance<br />
Colossal magnetoresistance is a property of some materials, which is the gigantic<br />
decrease of resistance by the application of magnetic field. The magnetoresistance<br />
can be defined as,<br />
MR =<br />
R(H) − R(0)<br />
, (1.1)<br />
R(0)<br />
where R(H) and R(0) are the resistances with and without magnetic field H respectively.<br />
Expressing the results as a percentage (i.e multiplying by an additional factor<br />
100), it has been shown by Jin et. al. [10] that the MR value as large as -100,000