MESA+ University of Twente
Mesa+ Meeting

Mesa+ - Meeting

Thematic Sessions


Early diagnostics of diseases, chaired by Loes Segerink & Albert van den Berg


Unconventional electronics, chaired by Wilfred van der Wiel & Guus Rijnders


Storage and conversion of renewable energy, chaired by prof.dr. Guido Mul & Mark Huijben


Water, chaired by Rob Lammertink & Wiebe de Vos

Unconventional Electronics (ROOM 6)

Chairs: Wilfred van der Wiel (NanoElectronics) & Guus Rijnders (Inorganic Materials Science)


One of the greatest successes of the 20th century has been the development of digital computers. Conventional computation is based on Turing’s abstract model of a machine, now referred to as the Turing machine. Based on the ideas of Turing, Von Neumann proposed a computer architecture, which forms the foundation for all digital computing. In this computational paradigm, physical components are constructed into logic gates, from which the Von Neumann architecture is built. During the last decades computers have become more and more powerful by integrating increasingly many and smaller components on chips. It is becoming very hard and extremely expensive to continue this miniaturisation. Current transistors consist of only a handful of atoms. It is a major challenge to produce chips in which the millions of transistors have the same characteristics, and thus to make the chips operate properly. Another drawback is that their energy consumption is reaching unacceptable levels. It is obvious that we have to look for alternative, unconventional directions.

In this session, we will address questions as Have we reached the end of the Digital Age? Can we use molecular self-assembly for future electronics? and Can we exploit the computational power of nanoscale materials to realize revolutionary new computer architectures? The audience is invited to participate in a lively discussion following the four short presentations.



Introduction by Wilfred van der Wiel


Jorrit de Boer (QTM)

Majorana bound states in Weyl semimetals


Matthias Brauns (NE)

Hole spins in Ge-Si nanowires


Paul Kelly (CMS)

Giant room temperature interface spin Hall and inverse spin Hall effects


Ray Hueting (SC)

Negative-Capacitance Field-Effect Transistor: The Balancing Act





Majorana bound states in Weyl semimetals

Jorrit de Boer MSc. (Quantum Transport in Matter)

Observation of a 4π-Periodic supercurrent in 3D Dirac/Weyl semimetal based Josephson junctions

3D Dirac semimetals are a class of materials with a 3-dimensional linear dispersion in the bulk. When either time reversal symmetry or inversion symmetry is broken, a 3D Dirac semimetal can turn into a Weyl semimetal. One Dirac cone splits into two Weyl cones with opposite chirality at opposite momenta k0 and -k0. Such a 3D Dirac semimetal can be found in Bi-based materials at the transition point from a band insulator to a topological insulator. For Sb-doped Bi, the phase diagram is well known and the transition point is ~3%.

By mechanical exfoliation and standard e-beam lithography, we fabricated various devices on Bi0.97Sb0.03 flakes. The high field results of Hall bar shaped samples consistently show a negative longitudinal magnetoresistance (LMR) with B parallel to I. This Adler-Bell-Jackiw anomaly is a strong indication of a Weyl semimetal.

We investigated the Nb/Bi0.97Sb0.03/Nb junctions in different regimes by making junctions with different lengths. We observed normal Fraunhofer patterns and Shapiro steps in long junctions, and irregular Fraunhofer patterns and a missing n=1 Shapiro step at low RF in short junctions. The missing n=1 Shapiro step is considered to be one of the major indications of the presence of Majorana bound states.

Hole spins in Ge-Si nanowires

Dipl.-Phys. Matthias Brauns (NanoElectronics)

We grow monocrystalline and nearly defect-free Ge-Si core-shell nanowires, which we use to fabricate devices with ohmic contacts to the nanowire and local gates. The low defect density allows us to electrostatically define quantum dots of various lengths in the wires. We can control quantum dot lengths from ~50 to ~400 nm and the longer dots can be split into a double dot in a controlled way from strong to very weak interdot tunnel coupling, indicating a very clean system.

Using these devices for spintronics experiments, we observe a strongly anisotropic g-factor in excited-state magnetospectroscopy measurements on single quantum dots. Forming a double quantum dot, we also observe Pauli spin blockade, a key ingredient for spin-based quantum computation. These findings demonstrate that Ge-Si nanowires are an ideal platform for experiments leading to quantum computation applications using spin-orbit states as well as Majorana bound states.

Giant room temperature interface spin Hall and inverse spin Hall effects

Prof.dr. Paul Kelly (Computational Materials Science)

The spin Hall angle (SHA) is a measure of the efficiency with which a transverse spin current is generated from a charge current by the spin-orbit coupling and disorder in the spin Hall effect (SHE). In a study of the SHE for a Pt|Py (Py=Ni80Fe20) bilayer using a first-principles scattering approach, we find a SHA that increases monotonically with temperature and is proportional to the resistivity for bulk Pt. By decomposing the room temperature SHE and inverse SHE currents into bulk and interface terms, we discover a giant interface SHA that dominates the total inverse SHE current with potentially major consequences for applications.

To study bulk Pt, we set up a scattering geometry consisting of two crystalline semi-infinite Pt leads sandwiching a scattering region of length LPt of disordered Pt with atoms displaced from their equilibrium positions by populating phonon modes. For the resistivity and spin-flip diffusion length, this approach has been shown to yield essentially perfect agreement with experiment. We study the SHE by calculating local longitudinal and transverse charge and spin current densities in the scattering region so that both intrinsic and extrinsic contributions are naturally included. To study interface effects, we model a Py|Pt bilayer by matching 9x9 interface unit cells of Py to 37x37 unit cells of Pt [3] including both lattice and spin disorder in Py. This presentation will review the computational procedures that make these calculations possible.

Negative-Capacitance Field-Effect Transistor: The Balancing Act Ray Hueting (Semiconductor Components)

In many electronics devices, in particular portable electronics (e.g. tablets and smart-phones), reducing the power consumption has been quite cumbersome. Part of this can be attributed to the operation of the conventional transistor, the key component of the microchip. Recently there has been a wide interest from both the industry and academics in adopting the so-called negative-capacitance field-effect transistor (NC-FET) for tackling this issue. In this talk, the basic background of this NC-FET will be explained and a brief overview will be given.