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2014 – PhD in Physics at University of Basel in the group of Prof. C. Schönenberger
2011-2013 M.Sc. in Nanosciences at the University of Basel in the group of Prof. D. Zumbühl.

Master thesis: “Hydrogen plasma etching of graphene: Towards crystallogrpahic zigzag edges”

2008-2011 B.Sc. in Nanosciences at the University of Basel


Graphene spintronics

Proximity spin-orbit in graphene through transition metal dichalcogenides

Superconducting tunnel spectroscopy

Graphene pn-junction physics

List of Publications

Please see Google Scholar for a full list.

  • Spin transport in two-layer-CVD-hBN/graphene/hBN heterostructures
    M. Gurram, S. Omar, S. Zihlmann, P. Makk, Q. C. Li, Y. F. Zhang, C. Schönenberger, and B. J. van Wees.
    Phys. Rev. B, 97:45411, jan 2018. [DOI] arXiv:1712.00815

    We study room-temperature spin transport in graphene devices encapsulated between a layer-by-layer-stacked two-layer-thick chemical vapor deposition (CVD) grown hexagonal boron nitride (hBN) tunnel barrier, and a few-layer-thick exfoliated-hBN substrate. We find mobilities and spin-relaxation times comparable to that of SiO2 substrate-based graphene devices, and we obtain a similar order of magnitude of spin relaxation rates for both the Elliott-Yafet and D’Yakonov-Perel’ mechanisms. The behavior of ferromagnet/two-layer-CVDhBN/ graphene/hBN contacts ranges from transparent to tunneling due to inhomogeneities in the CVD-hBN barriers. Surprisingly, we find both positive and negative spin polarizations for high-resistance two-layer-CVDhBN barrier contacts with respect to the low-resistance contacts. Furthermore, we find that the differential spininjection polarization of the high-resistance contacts can be modulated by dc bias from −0.3 to +0.3 V with no change in its sign, while its magnitude increases at higher negative bias. These features point to the distinctive spin-injection nature of the two-layer-CVD-hBN compared to the bilayer-exfoliated-hBN tunnel barriers.

  • Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2/Gr/hBN heterostructures
    S. Zihlmann, A. W. Cummings, J. H. Garcia, M. Kedves, K. Watanabe, T. Taniguchi, C. Schönenberger, and P. Makk.
    Phys. Rev. B, 2018. [DOI] arXiv:1712.05678

    Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit coupling and its relaxation mechanism remained unknown. We show for the first time an increased spin-orbit coupling close to the charge neutrality point in graphene, where topological states are expected to appear. Single layer graphene encapsulated between the transition metal dichalcogenide WSe2 and hBN is found to exhibit exceptional quality with mobilities as high as 100 000 cm2/Vs. At the same time clear weak anti-localization indicates strong spin-orbit coupling and a large spin relaxation anisotropy due to the presence of a dominating symmetric spin-orbit coupling is found. Doping dependent measurements show that the spin relaxation of the in-plane spins is largely dominated by a valley-Zeeman spin-orbit coupling and that the intrinsic spin-orbit coupling plays a minor role in spin relaxation. The strong spin-valley coupling opens new possibilities in exploring spin and valley degree of freedoms in graphene with the realization of new concepts in spin manipulation.

  • Contactless Microwave Characterization of Encapsulated Graphene p-n Junctions
    V. Ranjan, S. Zihlmann, P. Makk, K. Watanabe, T. Taniguchi, and C. Schönenberger.
    Phys. Rev. Applied, 7(5):54015, may 2017. [DOI] arXiv:1702.02071

    Accessing intrinsic properties of a graphene device can be hindered by the influence of contact electrodes. Here, we capacitively couple graphene devices to superconducting resonant circuits and observe clear changes in the resonance-frequency and -widths originating from the internal charge dynamics of graphene. This allows us to extract the density of states and charge relaxation resistance in graphene p-n junctions without the need of electrical contacts. The presented characterizations pave a fast, sensitive and non-invasive measurement of graphene nanocircuits.

  • Microwave Photodetection in an Ultraclean Suspended Bilayer Graphene p–n Junction
    M. Jung, P. Rickhaus, S. Zihlmann, P. Makk, and C. Schönenberger.
    Nano Letters, 16(11):6988-6993, nov 2016. [DOI] arXiv:1702.01529

    We explore the potential of bilayer graphene as a cryogenic microwave photodetector by studying the microwave absorption in fully suspended clean bilayer graphene p–n junctions in the frequency range of 1–5 GHz at a temperature of 8 K. We observe a distinct photocurrent signal if the device is gated into the p–n regime, while there is almost no signal for unipolar doping in either the n–n or p–p regimes. Most surprisingly, the photocurrent strongly peaks when one side of the junction is gated to the Dirac point (charge-neutrality point CNP), while the other remains in a highly doped state. This is different to previous results where optical radiation was used. We propose a new mechanism based on the phototermal effect explaining the large signal. It requires contact doping and a distinctly different transport mechanism on both sides: one side of graphene is ballistic and the other diffusive. By engineering partially diffusive and partially ballistic devices, the photocurrent can drastically be enhanced.

  • Spin transport in fully hexagonal boron nitride encapsulated graphene
    M. Gurram, S. Omar, S. Zihlmann, P. Makk, C. Schönenberger, and B. J. van Wees.
    Physical Review B, 93(11):115441, mar 2016. [DOI] arXiv:1603.04357

    We study fully hexagonal boron nitride (hBN) encapsulated graphene spin valve devices at room temperature. The device consists of a graphene channel encapsulated between two crystalline hBN flakes: thick-hBN flake as a bottom gate dielectric substrate which masks the charge impurities from Si_{O2}/Si substrate and single-layer thin-hBN flake as a tunnel barrier. Full encapsulation prevents the graphene from coming in contact with any polymer/chemical during the lithography and thus gives homogeneous charge and spin transport properties across different regions of the encapsulated graphene. Further, even with the multiple electrodes in-between the injection and the detection electrodes which are in conductivity mismatch regime, we observe spin transport over 12.5 $\mu$m-long distance under the thin-hBN encapsulated graphene channel, demonstrating the clean interface and the pinhole-free nature of the thin hBN as an efficient tunnel barrier.

  • Role of hexagonal boron nitride in protecting ferromagnetic anostructures from oxidation
    S. Zihlmann, P. Makk, C. A. F. Vaz, and C. Schönenberger.
    2D Materials, 3(1):11008, feb 2016. [DOI] arXiv:1509.03087

    Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using x-ray photoemission electron microscopy because of its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.

  • Guiding of Electrons in a Few-Mode Ballistic Graphene Channel
    P. Rickhaus, M. -H. Liu, P. Makk, R. Maurand, S. Hess, S. Zihlmann, M. Weiss, K. Richter, and Schönenberger Richter (Uni. C. -. in cooperation with group Regensburg).
    Nano Letters, 15(9):5819-5825, sep 2015. [DOI] arXiv:1509.02653

    In graphene, the extremely fast charge carriers can be controlled by electron-optical elements, such as waveguides, in which the transmissivity is tuned by the wavelength. In this work, charge carriers are guided in a suspended ballistic few-mode graphene channel, defined by electrostatic gating. By depleting the channel, a reduction of mode number and steps in the conductance are observed, until the channel is completely emptied. The measurements are supported by tight-binding transport calculations including the full electrostatics of the sample.