milliKelvin Photoluminescence Imaging Combined with Spectroscopy – milliPICS

Magneto-spectroscopy is widely used to investigate the fundamental properties of various material systems giving access to the electronic structure, excitonic effects and many body interactions. Moreover, certain optical spectroscopy techniques, such as micro–photoluminescence (uPL), can be used as a local probe of the system giving access to short range interactions or the influence of the local environment, for example the charge of the investigated object. In addition the spin degree of freedom can be probed and even manipulated using optical techniques which has potential applications in the fields of spintronics and quantum computing.

The integer and fractional quantum Hall effects (QHE) in a two dimensional electron gas (2DEG) have been investigated since more than thirty years. Nevertheless, the research field remains highly competitive with a large number of groups around the world working on this topic. Optical methods such as absorption, photoluminescence have been already widely applied to 2DEG’s. In particular, polarization resolved measurements give a direct handle to the spin degree of freedom, thus allowing the detailed investigation of spin related phenomena. Absorption is particularly powerful technique since it allows us to probe the ground state of the system and therefore gives access to phase transitions in the ground state, for example the formation of skyrmions (spin textures), bubble or stripe phases (the equivalent of charge density or spin density waves). The complementary technique of emission (PL) gives access to the electrons in occupied Landau levels below Fermi energy.

In this project a unique micro-photoluminescence (mPL) system will be developed which is capable of taking spectroscopically resolved images, in-situ at ultra low (mK) temperatures and in high magnetic fields. The setup, which comprises a monomode fiber coupled to microscope objective with piezo x-y-z translation stages, will be directly inserted into the mixing chamber of a top loading dilution refrigerator with a base temperature of 10mK and equipped with a 16/18T superconducting magnet.

The system will be used to image and locally probe the novel integer and fractional quantum Hall many body ground states e.g. the quantum Hall ferromagnet at filling factor v=1, the Moore-Reed state at v=5/2, stripe and bubble phases at higher half integer filling factors and the breakdown of the quantum Hall effect at integer filling factors. The rich quantum Hall physics displayed in GaAs based 2D systems is a direct result of the small bare Zeeman energy so that the many body interactions almost always dominates over the single particle physics. A better understanding of the physics involved is important both from a fundamental and an applied point of view. For example, at filling factor v=5/2 the fermions are thought to be described by the Moore-Reed wave function and obey non Abelian statistics which makes it potentially useful for quantum computation. Equally, the breakdown of the quantum Hall effect at high electrical currents limits the accuracy of metrology measurements of the resistance standard which is now defined from the value of the quantized Hall plateau.

In parallel, a uPL system based on free beam optics will be developed for use in pulsed magnetic fields. Design constraints linked to the pulsed field environment prohibit the use of piezo translation stages. The ultimate goal is to measure the PL of individual nano objects e.g. graphene nanoribbons, carbon nanotubes, GaAs nano wires, in pulsed fields up to 80T. As a first approach in this project graphene nanoribbons will be measured in dc and pulsed magnetic fields using both uPL systems.