We employ photoelectron spectroscopy to probe the electronic structure directly. In this technique,
photons are used to excite the electrons from different energy levels, the process is known as Photoelectric
effect. If the energy of the photoexcited electrons due to the absorption of the incident photons
is sufficiently large, these excited
electrons are ejected through the sample surface and are detected by an electron detectors.
Depending on the energy of the excitation source and the detection process, the technique is named as
XPS (x-ray photoemission spectroscopy), UPS (ultra-violet photoemission spectroscopy), ARPES
(angle-resolved photoemission spectroscopy), SRPES (spin-resolved photoemission spectroscopy) etc.
LABORATORY (Experiment)
High resolutions photoelectron spectrometer (HRPES and ARPES).
It consists of a sample analysis chamber (SAC)
containing monochromatic photon sources and R4000 Gammadata Scienta Analyser. An open cycle He-cryostat is installed
to achieve temperature variation in the range of 4 K - 350 K.
The sample preparation chamber is contains sample surface cleaning facilities (scraper, cleaver, sputter ion gun etc.)
and a Low Energy Electron Diffractometer (LEED) /Auger electron spectrometer (AES) to see the crystal orientation and surface
cleanliness.
Energy resolution:
The energy resolution is found to be about 1.4 meV in ARPES and UPS studies with photon energies 21.2 eV and 40.8 eV.
The energy resolution for XPS measurements is close to 300 meV.
Angle resolution:
The angle resolution is also found close to 0.3 deg. for the angle range of 30 deg. For a smaller angle range, the resolution is about 0.1 deg.
We prepare our samples in the Laboratory of our colleagues in TIFR. The single crystals are obtained from our collaborators in TIFR as well as from abroad. We have active collaborations with the colleagues in TIFR.
It consists of a sample analysis chamber (SAC)
containing monochromatic photon sources and R4000 Gammadata Scienta Analyser. An open cycle He-cryostat is installed
to achieve temperature variation in the range of 4 K - 350 K.
The sample preparation chamber is contains sample surface cleaning facilities (scraper, cleaver, sputter ion gun etc.)
and a Low Energy Electron Diffractometer (LEED) /Auger electron spectrometer (AES) to see the crystal orientation and surface
cleanliness.
Energy resolution: The energy resolution is found to be about 1.4 meV in ARPES and UPS studies with photon energies 21.2 eV and 40.8 eV.
The energy resolution for XPS measurements is close to 300 meV.
Angle resolution:
The angle resolution is also found close to 0.3 deg. for the angle range of 30 deg. For a smaller angle range, the resolution is about 0.1 deg.
We prepare our samples in the Laboratory of our colleagues in TIFR. The single crystals are obtained from our collaborators in TIFR as well as from abroad. We have active collaborations with the colleagues in TIFR.
Spin resolved photoelectron spectroscopy (SRPES)
We have installed a spin-resolved photoemission spectrometer, where one can identify spin of the
photo-electrons from a magnetized ferromagnetic material.
Two photographs of the spin resolved spectrometer installed in our laboratory is shown here.
Since the electrons are charged particles, the influence of strong Lorenz force makes it impossible
to build a spin detector based on Stern-Gerlach kind of setup. Therefore, the detectors in a spin-resolved
photoemission spectrometer is built based on Mott scattering -
the asymmetry of scattered electrons from a metal foil reflects the spin polarization of the incident electron
beam due to the spin orbit coupling - the detector is called a Mott detector.
A photograph of the spin detector (Mott detector) installed in our laboratory is shown here.
We have installed a spin-resolved photoemission spectrometer, where one can identify spin of the
photo-electrons from a magnetized ferromagnetic material.
Two photographs of the spin resolved spectrometer installed in our laboratory is shown here.
Since the electrons are charged particles, the influence of strong Lorenz force makes it impossible
to build a spin detector based on Stern-Gerlach kind of setup. Therefore, the detectors in a spin-resolved
photoemission spectrometer is built based on Mott scattering -
the asymmetry of scattered electrons from a metal foil reflects the spin polarization of the incident electron
beam due to the spin orbit coupling - the detector is called a Mott detector.
A photograph of the spin detector (Mott detector) installed in our laboratory is shown here.
High Resolution Electron Energy Loss Spectroscopy (HREELS)
A picture of the high resolution electron energy loss spectrometer (HREELS) installed in the SRPES spectrometer
is shown here. This will be used to study the electron-phonon, electron-magnon, electron-plasmon couplings
those might be important in determining material properties.
Energy resolution achieved so far is about 2 meV as shown in the left figure.
Preliminary experimental results on Mg10B2 at room temperature is shown here. Distinct signature of phonon excitations can be seen in the spectrum.
A picture of the high resolution electron energy loss spectrometer (HREELS) installed in the SRPES spectrometer
is shown here. This will be used to study the electron-phonon, electron-magnon, electron-plasmon couplings
those might be important in determining material properties.
Energy resolution achieved so far is about 2 meV as shown in the left figure. Preliminary experimental results on Mg10B2 at room temperature is shown here. Distinct signature of phonon excitations can be seen in the spectrum.
LABORATORY (Theory)
Photoemission spectroscopy directly provides the density of
single-electron excitations from any system. Thus, the spectral
energetics correspond to the eigen-spectrum of the final (N-1)
particle states, while the ground state wave function of the
N-particle state influences the intensities of various
transitions. In an uncorrelated system, the eigen-spectrum of the
final states is also the eigen-spectrum of the initial state and in
such cases a single particle description of the system is
sufficient to describe the electronic structure. In general,
however, the electron-electron interaction strength is quite large
in systems of interest, such as the transition metals or the rare
earths. In such cases, the final state spectrum is not the same as
that of the initial N-particle state and the spectroscopic result
cannot be directly used to elucidate the ground state properties.
This is most effectively illustrated by the comparison of the
calculated Density of States (DOS) within an ab initio but effectively
single-particle, band structure theory and the experimental
spectrum. Keeping this in view, we calculate the electronic band structure
within the local density approximations (LDA) using Full potential Linearized
Augmented Plan Wave (FLAPW) method (Wien2k).
When the band structure results within LDA fails to describe the experimental electronic spectra, indicating a presence of strong correlation effects, we tried to introduce the electron electron Coulomb interactions within these calculations (LDA+U). For materials containing heavier elements, it is often necessary to include spin-orbit coupling in the calculations.
It is often found that above prescription is not able to capture the experimental findings. In such cases, it is necessary to analyze such spectroscopic results in terms of model-many body calculations based on the Anderson impurity model or the multi-band Hubbard model to understand the ground state properties of the system.
In addition, we collaborate with various theoretical groups around the world to theoretically probe the material properties.
When the band structure results within LDA fails to describe the experimental electronic spectra, indicating a presence of strong correlation effects, we tried to introduce the electron electron Coulomb interactions within these calculations (LDA+U). For materials containing heavier elements, it is often necessary to include spin-orbit coupling in the calculations.
It is often found that above prescription is not able to capture the experimental findings. In such cases, it is necessary to analyze such spectroscopic results in terms of model-many body calculations based on the Anderson impurity model or the multi-band Hubbard model to understand the ground state properties of the system.
In addition, we collaborate with various theoretical groups around the world to theoretically probe the material properties.
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Candidates with good academic record and Ph.D. in Physical Sciences may apply for post doctoral position (PDF) to Prof. Kalobaran Maiti