1. Fermi-Edge Singularitäten im resonanten Transport durch II-VI Quantenpunkte Universität Würzburg Am Hubland, D-97074 Michael Rüth, Anatoliy Slobodskyy, Charles Gould, Georg Schmidt, Laurens W. Molenkamp Physikalisches Institut (EP3) Spintronics

2. Outline Project and status Magnetic structures FES in non-magnetic structures Conclusion and outlook

3. Project and status Goal Investigation of tunneling transport through self assembled II-VI quantum dots in semimagnetic barriersmagnetic tunneling barriers Setback Several consecutive breakdowns in the MBE interrupted by calibration periods Status MBE up and running again New III-V chamber running (18 months after installation) Working RTDs fabricated All presented experiments performed on samples from the beginning of the project

4. layer structure and transport mechanism II-VI double barrier heterostructure (10x10) μm² pillars approximately 10 4 quantum dots transport dominated by a few dots at low bias voltages

5. magnetic (Zn,Be,Mn)Se tunnel barriers Further samples show clear zero field spin splitting (left)  mediated ferromagnetic interaction with Mn ions in barriers Slight variation in layer thickness and smaller Mesa lead to cleaner (not sharper) resonance  Transistion 0D -> 2D is observed

6. magnetic (Zn,Be,Mn)Se tunnel barriers 100x100 μm² 10x10 μm² Further samples show clear zero field spin splitting (left)  mediated ferromagnetic interaction with Mn ions in barriers Slight variation in layer thickness and smaller mesa lead to higher PVR (not sharper resonance) without zero field splitting (right)  Transistion 0D -> 2D is observed

7. nonmagnetic (Zn,Be,Mn)Se tunnel barriers zero field measurements – FES signal resonance has shape of a Fermi-Edge-Singulariy enhanced tunneling current RTD with self assembled InAs dots showing FES enhancement – Frahm et al. Phys. Rev. B 72, 5375 (2006)

8. Fermi Edge Singularity in resonant tunneling Geim et al. `93 – tunneling between a localized level and a 2DEG interaction between tunneling electrons and the Fermi sea in the contacts two competing effects: attraction of contact electrons to the emtpy state in the localized level Fermi sea shake-up (appearance of hole potential in localized level modifies many particle wave function – suppression of tunneling rate) Description with one body scattering potential with two phase shifts

9. nonmagnetic (Zn,Be,Mn)Se tunnel barriers temperature dependence and theory tunneling enhancement reduces with increasing temperature resonance still visible at 45 K with a PVR of nearly 2 theory (Frahm et al. 2006) reproduces data up to 45K rescaling collapses data from various temperatures on one curve according to theory

10. nonmagnetic (Zn,Be,Mn)Se tunnel barriers zero field measurements – FES signal superimposed FES peaks + finestructure at low temperatures finestructure not included at low temperatures Marcus & Zumbuhl: resonant tunneling in an open qdot

11. nonmagnetic (Zn,Be,Mn)Se tunnel barriers magnetic field measurements (perpendicular to layer stack @~100mK) complex structure in perpendicular field Resonances run to lower voltages with increased field no field dependence in parallel setup Parallel setup @1.6K

12. nonmagnetic (Zn,Be,Mn)Se tunnel barriers emitter Landau fan fit disordered 2D-like emitter states produce multiple landau fan like structures (intersecting levels on magnetic field – energy – plane) main features can be fitted with an emitter landau fan Additional features cannot be explained with only one landau fan or by adding Darwin-Fock like behavior of the conducting dot level only one visible resonance at B=0T  no other conducting dot levels

13. summary magnetic tunneling barriers Zero field spin splitting could be reproduced for 0D and vanishes in 2D nonmagnetic tunneling barriers resonant tunneling through self assembled CdSe quantum dots in a II-VI device Current enhancement by a fermi edge singularity Isolated signla of a conducting quantum dot level FES theory reproduces data for all temperatures Disordered 2D-like emitter with visible landau level structure in perpendicular magnetic field

14. outlook Grow new samples and follow up on project program Coupled dots Magnetic/Non-magnetic barriers...

15. I/V measurements in magnetic field influence of applied fields