Quantized Conduction on Domain Walls of a Magnetic Topological Insulator
In a paper published in Science, researchers from the University of Tokyo and RIKEN (Japan) have studied “Quantized conduction on domain walls of a magnetic topological insulator” using an attoAFM/MFM in a 3He-cryostat down to 500 mK. In their paper, Yasuda et al. designed and fabricated magnetic domains in the quantum anomalous Hall state, and proved the existence of the chiral one-dimensional edge conduction along the domain walls through transport measurements.
This discovery would permit fully electrical control of the mobile domain walls and chiral edge states, which may lead to the realization of low-power-consumption spintronic, memory and quantum information processing devices in the future.
Yasuda et al., Science 358, 1311–1314 (2017)
Dynamic Visualization of Nanoscale Vortex Motion using attoSTM in an attoLiquid3000
Matias Timmermans and co-workers invented an innovative technique removing the lack of temporal resolution in STM imaging. They used an attoSTM in an attoLIQUID3000 3He cryostat to measure and study vortex motion in 2H-NbSe2 on a much shorter time scale. By applying a small AC magnetic field they induced a periodic movement of the vortices. The external perturbation results in a distinct smearing of the vortex in the images. Instead of collecting several consecutive images, the tunnelling current is recorded at each point over three cycles of the excitation. The exceptional thermal and spatial stability of the attoSTM in the attoLiquid3000 allows further analysis of the time dependence of this signal at each point. Using an additional lock-in technique more details and understanding of the vortex motion is revealed. By mapping the first and a second harmonic of the tunnelling signal (see upper figures), they were able to visualize changes of the vortex lattice when the vortex density is increased by increasing the DC magnetic field.
In a next step, they used the AC excitation as a time reference to track the motion of individual vortices in time. This results in time resolved snapshots of the vortex motion, which allows them to construct a movie frame by frame. This visualization procedure is unprecedented and promises a much better understanding of the dynamical behaviour of the superconducting condensate (see lower figures). Contrary to the expectation the vortex does not move in a line but follows a circular motion, due to a potential created by atoms and/or vortices.
Imaging fractional incompressible stripes in integer quantum Hall effect
Nicola Paradiso, Stefan Heun, and co-workers measured the fractional quantum Hall effect in quantum point contacts using an attoAFM III in an attoLIQUID3000 at very low temperature (300 mK) and high magnetic field (≈8 T). They demonstrated that fractional features were unambiguously observed in every integer quantum Hall constriction. These ground-breaking Scanning Gate Microscopy experiments pave the way to a better understanding of the role of fractional phases in the field of coherent quantum transport!
Imaging Fractional Incompressible Stripes in Quantum Hall Systems
In newer measurements, the group performed SGM measurements at the temperature and magnetic field conditions required to observe the fractional quantum Hall effect. The goal is to image for the first time the presence of fractional incompressible stripes, i.e. the existence of an inner structure within the integer edge channel. The measurements were performed at bulk filling factor n = 1 (B = 8.23 T, T = 300 mK). The corresponding SGM map in the region close to the QPC center is depicted in the lower Figure (a). Analogously to the n = 4 case, one expects to find plateaus when the local electron phase is gapped, i.e. when the local filling factor n* equals a robust fraction. The scan profile depicted in the right figure reveals a clear shoulder for Gsd = e2
(Data and images were generously provided by N. Paradiso, S. Heun et al., NEST, CNR-INFM and Scuola Normale Superiore, Pisa, Italy.)
This measurement was realized with the attoLIQUID3000.
N. Paradiso, et al., Phys. Rev. Lett. 108, 246801 (2012)
Scanning Gate Microscopy at 300 mK
In this measurement, an attoAFM III was operated inside an attoLIQUID3000 cryostat at 300 mK in scanning gate microscopy mode (SGM) - investigating the trajectory and interaction of edge channels of a split-gate quantum point contact (QPC) device in the Quantum Hall (QH) regime. By scanning the SGM tip over the surface of the QPC at constant height and by simultaneously measuring and plotting the source-drain current, conductance maps were obtained. The image to the left is an example of such a conductance map depicting the characteristic branched-flow of electrons at zero magnetic field, which in turn shows electron interference fringes and the actual electron path (T = 400 mK, 2DEG density n2D = 3.37 x 1011 cm-2).
(Data and images were generously provided by S. Heun et al., NEST, CNR-INFM and Scuola Normale Superiore, Pisa, Italy.)
Scanning Tunneling Spectroscopy and Vortex Imaging on NbSe2 with attoAFM III / STM I at 315 mK
Scanning Tunneling Spectroscopy (STS) is a useful tool to characterize material properties, especially on superconductors at ultra low temperatures. In a series of experiments STS measurements as well as vortex imaging on NbSe2 have been performed at a temperature of only 315 mK. The tests show excellent stability of the combined attoAFM/STM microscope setup as well as the possibility to apply stable voltages in the micro-Volt range.