Fermi-Edge Singularitäten im resonanten Transport durch II-VI QuantenpunkteUniversität WürzburgAm Hubland, D-97074

Michael Rüth, Anatoliy Slobodskyy, Charles Gould, Georg Schmidt, Laurens W. Molenkamp

Physikalisches Institut (EP3) Spintronics

Outline

• Project and status

• Magnetic structures

• FES in non-magnetic structures

• Conclusion and outlook

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

layer structure and transport mechanism

• II-VI double barrier heterostructure• (10x10) μm² pillars• approximately 104 quantum dots• transport dominated by a few dots at low bias voltages

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

nonmagnetic (Zn,Be,Mn)Se tunnel barrierszero 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)

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

20

0

0

2 δ/π/πδα

μEθμE

ξI i

α

nonmagnetic (Zn,Be,Mn)Se tunnel barrierstemperature 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 varioustemperatures on one curve accordingto theory

nonmagnetic (Zn,Be,Mn)Se tunnel barrierszero 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

nonmagnetic (Zn,Be,Mn)Se tunnel barriersmagnetic 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

nonmagnetic (Zn,Be,Mn)Se tunnel barriersemitter 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

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

outlook

• Grow new samples and follow up on project program

• Coupled dots• Magnetic/Non-magnetic barriers• ...

I/V measurements in magnetic fieldinfluence of applied fields