Jan Jacob

Cascade of Y-shaped spin filters in InGaAs/InAs/InGaAs quantum wells

An all-electrical semiconductor device for generating and detecting spin-polarized currents with a two-stage cascade of three-terminal spin filters is discussed, where the first stage serves as the spin polarizer and the second stage serves as the detector. Transport simulations show imbalanced output conductance for higher-order subbands, independent of the spin-orbit coupling strength. However, when only the lowest subband is occupied the simulations show that the conductance asymmetry is spin-orbit dependent. Therefore, the conductance asymmetry of the lowest subband can be used to indicate spin polarization. Based on the simulations an experimental device to measure the spin-dependent conductance asymmetry is proposed.

Generation of highly spin-polarized currents in cascaded InAs spin filters

We report the generation of highly spin-polarized currents and their detection in cascaded InAs spin filters via transport measurements below 300 mK. The intrinsic spin-Hall effect is used in the first filter to generate two oppositely spin-polarized currents. From the conductance asymmetry at the outputs of the second filter high spin polarizations are determined in all-electrical measurements. The experiments are in good agreement with ballistic quantum transport simulations, which mimic the double-Y-shaped structure constricted by quantum-point contacts.

Quantized conductance and evidence for zitterbewegung in InAs spin filters

We present measurements of the electron transport in top-gated InAs spin-filter cascades. The cascades consist of a first filter stage that acts as a polarizer, a center wire, and a second filter stage that acts as an analyzer. Conductance quantization indicates quasi-ballistic transport in these rather large devices. Oscillations of the conductances of the second filters outputs with the strength of an in-plane magnetic field perpendicular to the center wire provide evidence of the so-called zitterbewegung and substantiate the interpretation of the conductance imbalance at the second filter as the consequence of a spin polarization.

Direct current-biased InAs spin-filter cascades

We study dc-biased spin-transport in InAs two-stage spin-filter cascades. The cascades allow all-electrical generation and detection of spin-polarized currents in an all-semiconductor device. The application of a dc bias simplifies the interpretation of the experimental results, enhances the signal-to-noise ratio, and paves the way for more definite measurements in magnetic fields.

A versatile LabVIEW and field-programmable gate array-based scanning probe microscope for in operando electronic device characterization

Understanding the complex properties of electronic and spintronic devices at the micro- and nano-scale is a topic of intense current interest as it becomes increasingly important for scientific progress and technological applications. In operando characterization of such devices by scanning probe techniques is particularly well-suited for the microscopic study of these properties. We have developed a scanning probe microscope (SPM) which is capable of both standard force imaging (atomic, magnetic, electrostatic) and simultaneous electrical transport measurements. We utilize flexible and inexpensive FPGA (field-programmable gate array) hardware and a custom software framework developed in National Instruments LabVIEW environment to perform the various aspects of microscope operation and device measurement. The FPGA-based approach enables sensitive, real-time cantilever frequency-shift detection. Using this system, we demonstrate electrostatic force microscopy of an electrically biased graphene field-effect transistor device. The combination of SPM and electrical transport also enables imaging of the transport response to a localized perturbation provided by the scanned cantilever tip. Facilitated by the broad presence of LabVIEW in the experimental sciences and the openness of our software solution, our system permits a wide variety of combined scanning and transport measurements by providing standardized interfaces and flexible access to all aspects of a measurement (input and output signals, and processed data). Our system also enables precise control of timing (synchronization of scanning and transport operations) and implementation of sophisticated feedback protocols, and thus should be broadly interesting and useful to practitioners in the field.

Efficient Algorithms for the Green’s Function Formalism

We present efficient implementations of the non-equilibrium Green’s function method for numeric simulations of transport in semiconductor nanostructures. The algorithms are implemented on CPUs and GPUs using LabVIEW 2011 64-Bit together with the Multicore Analysis and Sparse Matrix Toolkit and the GPU Analysis Toolkit.

InAs Spin Filters Based on the Spin-Hall Effect

We give an overview of the generation of spin-polarized currents in all-semiconductor devices by utilizing the intrinsic spin-Hall effect. Two-staged cascades of Y-shaped three-terminal junctions of narrow quantum wires fabricated from InAs heterostructures with strong Rashba spin–orbit interaction allow all-electrical generation and detection of spin-polarized currents. We compare our low-temperature transport measurements to numerical simulations and find in both highly spin-polarized currents in the case of transport in the lowest one-dimensional subband.