Master Theses

We currently have 6 topics to offer for a Master or Bachelor thesis. Please take a look at the list below and contact us, if you are interested. Our offers for PhD positions can be found in the FU Stellenanzeiger.

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iToF
Cross section of the spectrometer [Schönhense et al., J. Electron Spectrosc., 200 (2015), pp. 95–119]

Earliest Start: February 2017

Topic

Electrons photo-emitted from a sample can either be used to get a microscopic image of the sample or to map the electronic bandstructure. In our momentum microscope, we can do both. By changing the voltages of the electron lenses, we easily switch from real space to momentum space and back. Analyzing the energy and angular momentum of the sample then allows to map the complete bandstructure in 3 dimension.

In the real space imaging, we use the instrument as a photoemission electron microscope (PEEM). When doing the bandstructure mapping, the drift tube of the microscope operates as a time-of-flight spectrometer (ToF). Additionally, the momentum microscope is equipped with a second flight tube for spin-resolution, which gives us a fourth dimesion to measure in.

During your master thesis, we will do the first measurements in the ToF operation mode of this instrument and start to set up a spin filter. The PEEM mode is already established and was used to image the photo-switching of molecular monolayers of azobenzene on gold. To start the ToF operation, a small laser setup has to be adjusted first to provide light pulses for the flight-time measurement. The investigations on switching molecules are planned to be continued during your thesis followed by first measurements on molecular spin-interfaces.

To Do List

  • Adjust the pulsed laser setup comprising a Ti:Sapphire oscillator and a second harmonic generation
  • First measurements on metallic samples (Cu, Au) for calibration
  • Prepare molecular monolayers
  • Measurements of the molecular switching behavior
  • (Set up a new spin filter)

You Will Learn:

  • Almost everything about photoemission
  • Working with short-pulsed lasers and ultrahigh vaccum
  • A lot about molecular dynamics
  • Some Labview programming to improve and extend the measurement software

If Interested, Write To:

Beatrice Andres <andres[at]physik.fu-berlin.de>

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DiracCone
DiracCone

Earliest Start: April 2017

Topic

Topological insulators are a new class of material. They are insulating in the bulk, but conducting at the surface. The conducting surface states form a so-called Dirac cone and possess a peculiar spin texture. The Dirac surface states are induced by spin-orbit coupling as well as protected by the time-reversal symmetry. Breaking time-reversal symmetry in a topological insulator with ferromagnetic perturbation has been predicted to lead to many exotic quantum phenomena, such as the quantum anomalous Hall effect, topological magnetoelectric effect, as well as image magnetic monopole.

The Dirac surface states show a linear energy-momentum dispersion relation and are protected by time reversal symmetry. Upon a time reversal operation, which lets the system to evolve backward in time, the electron wave vector k and the spin will flip the sign. The helical surface states of a topological insulator are invariant under such operation since the opposite spin channels are locked to the opposite momenta. In the presence of magnetic field or magnetic impurities, however, this invariant or symmetry will be broken. This is what we plan to do by evaporating nickel onto the topological insulator Bi2Se3.

To Do List

  • Installation and commissioning of a (present) nickel evaporator in our UHV chamber
  • Two-photon photoemission spectroscopy of the 3D band structure on the topological insulator Bi2Se3 with and without Ni adatoms (in our time-of-flight spectrometer)
  • Measurements of the electron dynamics with and without Ni adatoms

You Will Learn:

  • How to operate a short-pulsed laser
  • Working with ultrahigh vacuum
  • Analysis and interpretation of photoemission spectra
  • Physics of topological insulators

If Interested, Write To:

Sophia Ketterl <schu[at]zedat.fu-berlin.de> or Beatrice Andres <andres[at]physik.fu-berlin.de>

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