fermi-edge singularitäten im resonanten transport durch ii-vi quantenpunkte universität würzburg...
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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• ...