G.Koblmueller

Semiconductor Quantum Nanomaterials

Technical University of Munich

Walter Schottky Institute

Am Coulombwall 4

85748 Garching

Tel. +49 89 289 12779

gregor.koblmueller[at]wsi.tum.de

Group Webpage

Description

Research focus: semiconductor quantum nanomaterials, quantum nanowire photonics, quantum transport

Our research activities are focused on innovative semiconductor nano and quantum materials (III-V quantum nanowires, quantum dots and novel 2D heterostructures) and their use within diverse research areas spanning the fields of nano and quantum electronics, nano-optoelectronics and quantum photonics, as well as information technology and sensing. Along these avenues we pursue all stages from synthesis of nano and quantum materials to simulation and characterization of electronic, optical, structural, and charge carrier properties, as well as to device fabrication and probing device functionalities in regimes that are ruled by mesoscopic and quantum effects.


Quantum Nanowire Integrated Light Sources

Novel quantum-nanowire (NW) cavities on SOI-based platform are not only promising systems as integrated nanolaser sources for future on-chip optical interconnects, but may also enable directly integrated single and entangled states of light that can be efficiently coupled and computed on demand in underlying quantum circuits. One central theme of our work is to synthesize and explore the properties of quantum heterostructures (wells and dots) in NW cavities and further integrate these deterministically on 1D-waveguides (WG).

Our vision towards quantum optical circuits is to thereby exploit chiral optical effects in 1D-WG geometries, which will enable coupling of circularly polarized exciton spin states from NW-quantum dot (QD) emitters to chiral points for bi- and uni-directional propagation of exciton spin-states. Such demonstration to map polarization entanglement into path entanglement may enable large arrays of deterministic NW-QD/WG systems and to realize cascaded quantum states much needed for quantum information processing.

Selected Publications

  • B. Loitsch, D. Rudolph, S. Morkötter, M. Döblinger, G. Grimaldi, L. Hanschke, S. Matich, E. Parzinger, U. Wurstbauer, G. Abstreiter, J. J. Finley, and G. Koblmüller: “Tunable quantum confinement in ultrathin, optically active semiconductor nanowires via reverse reaction growth”, Advanced Materials 27, 2195 (2015).
  • B. Loitsch, M. Müller, J. Winnerl, P. Veit, D. Rudolph, G. Abstreiter, J. J. Finley, F. Bertram, J. Christen, and G. Koblmüller: “Microscopic nature of crystal phase quantum dots in ultrathin GaAs nanowires by nanoscale luminescence characterization”, New Journal of Physics 18, 063009 (2016).
  • T. Stettner, A. Thurn, M. Döblinger, M. O. Hill, J. Bissinger, S. Matich, T. Kostenbader, D. Ruhstorfer, H. Riedl, M. Kaniber, L. J. Lauhon, J. J. Finley, and G. Koblmüller: “Tuning lasing emission towards telecommunication wavelengths in GaAs-(In,Al)GaAs core-multishell nanowires”, Nano Letters 18, 6292 (2018).


Quantum Phenomena in Transport

1D-like quantum NWs and their heterostructures play a substantial role in advanced quantum electronics and quantum information technologies. Ongoing research in these quantum electronic systems aims at investigations of charge carrier dynamics and interaction phenomena, and to investigate the highly relevant, hitherto unexplored non-equilibrium transport in 1D-nano­structures and their devices. Hereby, we study ultra-pure III-V-based NWs with intrinsically very high charge carrier mobility as well as systems with large spin-orbit interaction.

Particularly, we aim at control of the relevant transport regimes (ballistic vs. diffusive) – a capability that allows us to capture several important investigations of many-body correlations, spin-orbit interaction effects, as well as inelastic scattering and electron-phonon interaction effects.

Moving further in the direction of hybrid superconducting-semiconductor topological systems for quantum information processing, we explore proximity-induced local barriers defined between superconducting contacts and NWs to provide a platform for Majorana fermion bound states. This opens unique possibilities in studying unusual non-equilibrium quasiparticle distributions.

Selected Publications

  • S. Morkötter, N. Jeon, D. Rudolph, S. Matich, M. Döblinger, D. Spirkoska, E. Hoffman, J. J. Finley, L. J. Lauhon, G. Abstreiter, and G. Koblmüller: “Demonstration of confined electron gas and steep-slope behavior in delta-doped GaAs-AlGaAs core-shell nanowire transistors“, Nano Letters 15, 3295 (2015).
  • D. Irber, J. Seidl, D. J. Carrad, J. Becker, N. Jeon, B. Loitsch, J. Winnerl, S. Matich, M. Döblinger, Y. Tang, S. Morkötter, G. Abstreiter, J. J. Finley, M. Grayson, L. J. Lauhon, and G. Koblmüller: “Quantum transport and sub-band structure of modulation-doped GaAs/AlAs core-superlattice nanowires“, Nano Letters 17, 4886 (2017).
  • A. V. Bubis, A. O. Denisov, S. U. Piatrusha, I. E. Batov, V. S. Khrapai J. Becker, J. Treu, D. Ruhstorfer, and G. Koblmüller: “Proximity effect and interface transparency in Al/InAs-nanowire/Al-diffusive junctions”, Semiconductor Science and Technology 32, 094007 (2017).

Publications

Charge-neutral nonlocal response in superconductor-InAs nanowire hybrid devices

A.O. Denisov, A.V. Bubis, S.U. Piatrusha, N.A. Titova, A.G. Nasibulin, J. Becker, J. Treu, D. Ruhstorfer, G. Koblmueller, E.S. Tikhonov, V.S. Khrapai

arXiv:2101.02128 (2021).

Show Abstract

Nonlocal quasiparticle transport in normal-superconductor-normal (NSN) hybrid structures probes sub-gap states in the proximity region and is especially attractive in the context of Majorana research. Conductance measurement provides only partial information about nonlocal response composed from both electron-like and hole-like quasiparticle excitations. In this work, we show how a nonlocal shot noise measurement delivers a missing puzzle piece in NSN InAs nanowire-based devices. We demonstrate that in a trivial superconducting phase quasiparticle response is practically charge-neutral, dominated by the heat transport component with a thermal conductance being on the order of conductance quantum. This is qualitatively explained by numerous Andreev reflections of a diffusing quasiparticle, that makes its charge completely uncertain. Consistently, strong fluctuations and sign reversal are observed in the sub-gap nonlocal conductance, including occasional Andreev rectification signals. Our results prove conductance and noise as complementary measurements to characterize quasiparticle transport in superconducting proximity devices.

https://arxiv.org/abs/2101.02128

Breakdown of corner states and carrier localization by monolayer fluctuations in a radial nanowire quantum wells

M. M. Sonner, A. Sitek, L. Janker, D. Rudolph, D. Ruhstorfer, M. Döblinger, A. Manolescu, G. Abstreiter, J. J. Finley, A. Wixforth, G. Koblmueller, H. J. Krenner

Nano Lett. 19 (5), 3336-3343 (2019).

Show Abstract

We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.

DOI:10.1021/acs.nanolett.9b01028

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