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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GdHHG
ARPES of the Gd valence bands

Starting Date:

Flexible (earliest March 2016)

Topic:

Ultrafast laser-driven phase transitions of solids are a challenging field of research. After excitation with a femtosecond laser pulse the system is usually far from equilibrium and can therefore not be described on a thermodynamic basis. In order to explain non-equilibrium phase transitions and identify the dominant microscopic mechanisms it is imperative to follow the transient states that are involved in the dynamics.

To this end we will recommission a tabletop high-order harmonic setup for time- and angle-resolved photoelectron spectroscopy in the XUV regime. With photon energies up to 45eV it will be possible to examine the valence bands and outer core levels of the sample system.

Objectives:

-          Recommissioning of a ti:sapphire based high-order harmonic setup

-          Characterization and preparation of in situ grown samples

-          Participation in beamtimes at large scale facilities (BESSY II / RAL)

Gain Experience With:

-          Ultra high vacuum devices

-          Femtosecond laser systems

-          Photoelectron spectroscopy

-          Correlated electron systems

If You Have Any Questions, Do Not Hesitate To Contact:

Björn Frietsch  <b.frietsch[at] fu-berlin.de>

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Sb2Te2S
2PPE on the Dirac cone of the TI Sb2Te2S

Earliest Start: April 2016

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. They cannot be destroyed by adsorption of contaminants. That is why, they open new possibilities for employing the spin information in future electronics (spintronics).
We investigate the topological insulator Bi2Se3, which has two Dirac cones: an occupied one below the Fermi energy and an unoccupied one above the Fermi energy. In two-photon-photoemission experiments, we pump electrons from the occupied into the unoccupied Dirac cone and probe the resulting population. This allows us to measure the lifetimes of excited electrons in such a topological surface state and to learn more about the underlying scattering mechanisms. The experiment is already running in our time-of-flight apparatus. The next step is to do the experiment with spin resolution.

To Do List

  • Installation of a (present) sample-transfer system for ultrahigh vacuum
  • Modification of the sample holder to be compatible with the transfer system
  • Measurement of the spin-dependent electron dynamics (energies, life times, line widths) in two-photon photoemission
  • Concomitant measurements of the 3D band structure (and electron dynamics without spin resolution) are possible with our time-of-flight spectrometer

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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GdSurfaceState
PES of the Gd surface state [Andres et al., PRL 115, 207404 (2015)]

Earliest Start: March 2016

Topic

A magnet can be demagnetized, when it is excited by an intense laser pulse. Such laser-induced magnetic phase transitions may be crucial for the development of future technology. They are the basis of a young research field called femtomagnetism, which is mostly investigated in MOKE and XMCD experiments. That is why the dynamics in the band structure during these phase transitions are up to now not well explored. With our spin- and time-resolved photoemission setup, we can explore the transient changes in the band structure as well as their spin dependence. We already explored the phase transition of gadolinium and iron. The next step is going to similar ferromagnets such as terbium and nickel.

To Do List

  • Installation and commissioning of (already present) terbium and nickel evaporators in our UHV chamber
  • Spin- and time-resolved photoelectron spectroscopy of the laser-induced magnetic phase transitions in Tb and Ni

You Will Learn:

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

If Interested, Write To:

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

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

Earliest Start: August 2016

Topic

Electrons photo-emitted from a sample have a certain energy and angular momentum. Deflection by the appropriate electron optics creates a reciprocal image in the backfocal plane of a cathode lens. This resulting image directly shows the surface-projected band structure inside the crystal owing to momentumconservation in the photoemission process. Our time-of-flight momentum microscope, allows for such energy- and momentum-resolved measurements of the band structure as well as imaging in photoemission electron microscopy (PEEM). It is also equipped with a second flight tube for additional spin-resolution. A LabView software is present to control the electron optics and process images. It will be your task to convert these images into maps of the electronic band structure. The project is ideal to continue as a PhD student after your Master’s thesis.

To Do List

  • Become familiar with our present control software (applies voltages to lenses, processes images)
  • Simple PEEM experiments on test samples to become familiar with the spectrometer
  • Calculation of lens potentials
  • Simulation of electron rays with different voltages applied to lenses
  • Conversion of measured time of flight and point of incidence into energy and momentum of photoemitted electrons
  • Inclusion of the conversion into the present software in LabView (You may also start from scratch in Python or another programming language of your choice, if you like.)

You Will Learn:

  • How to operate laboratory equipment when programming in LabView or Python
  • Physics of photoemission and tracing rays of electrons

If Interested, Write To:

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

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DiracCone
DiracCone

Earliest Start: October 2016

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>

Direct link