Semiconductor Physics Group
Semiconductor Physics Group, Cavendish Laboratory
Dr Rolf Crook
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rc230@cam.ac.uk Office: +44 (0)1223 337465 Lab: +44 (0)1223 337295 Fax: +44 (0)1223 337271 Office: Room 429, Mott building Semiconductor Physics Cavendish Laboratory J J Thomson Avenue Madingley Road, Cambridge CB3 0HE, UK |
Research Interests
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Low-temperature scanning probes. I have developed and demonstrated three low-temperature scanning-probe techniques for the study of quantum phenomena in low-dimensional electronic devices: erasable electrostatic lithography (EEL), Kelvin probe microscopy (KPM), and scanned gate microscopy (SGM). EEL is a device fabrication technique where measurement and lithography are performed in the same environment [1]. Unlike other fabrication routes, the device can be tuned for optimal performance during fabrication. Patterns of charge are drawn on a GaAs surface with a low-temperature scanning probe biased negative. The pattern of charge is projected to a pattern of depletion in a subsurface 2D electron system where the low-dimensional device is defined. Charge is erased either locally using the probe biased positive, or globally by illuminating the device with red light. The figure shows a KPM (electric potential) image of charge previously drawn using EEL with a 1 micron scale bar. SGM uses the scanning probe tip as a local mobile top-gate and the technique has proven particularly useful for imaging electron density, electron flow, and device disorder. |
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Low-dimensional quantum devices. I use scanning probe techniques as the tools to investigate quantum properties of low-dimensional electronic devices such as quantum wires [1,2], quantum dots, and quantum billiards [3]. Scanning probes provide spatial information which is often impossible to obtain using any other technique. For example, EEL was used to fabricate a quantum wire and SGM to tune the wire to be electrically symmetric with respect to a DC bias. Electrical symmetry implies geometric symmetry. Once tuned, an additional plateau at 0.5(2e^2/h) was observed, which is shown in the figure. The additional plateau is understood to be caused by the alignment of electron spin within the quantum wire, meaning the formation of a 1D ferromagnetic phase [2]. This significant result means quantum wires may find a major application as sources and detectors of electron spin in future generations of integrated circuits. |
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Surface acoustic waves. This research is part of the Surface Acoustic Wave (SAW) Quantum Information Processing (QIP) IRC. SAW driven electron transport through a depleted GaAs channel is the basis for a proposed device capable of controlled entanglement and quantum information transfer. The fabrication of such a device will benefit from a detailed understanding of the capture process at the channel entrance and the electron dynamics within the channel. My experiments are designed to obtain spatial information uniquely provided by low-temperature scanning-probe microscopy. Scanned-gate microscopy, which generates images of SAW-induced current, reveals arc-like features confirming that the SAW current is maximized when the maximum potential gradient is minimized [4]. Kelvin-probe microscopy, which generates images of SAW-induced charge, reveals a build up of negative charge at the entrance of the channel when no SAW current flows and a broken line of negative, and occasionally positive, charge when a small SAW current flows. |
R Crook, A C Graham, C G Smith, I Farrer, H E Beere, and D A Ritchie, Nature 424 751 (2003).
[2] Conductance quantization at a half-integer plateau in a symmetric GaAs quantum wire,
R Crook, J Prance, K J Thomas, S J Chorley, I Farrer, D A Ritchie, M Pepper, and C G Smith, Science 312 1359 (2006).
[3] Imaging fractal conductance fluctuations and scarred wave functions in a quantum billiard,
R Crook, C G Smith, A C Graham, I Farrer, H E Beere, and D A Ritchie, Phys. Rev. Lett. 91 246803 (2003).
[4] Scanned-gate microscopy of surface-acoustic-wave induced current,
R Crook, R J Schneble, H E Beere, D A Ritchie, D Anderson, G A C Jones, C G Smith, C J B Ford, and C H W Barnes.
My complete publication list is available below.
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Last modified on 7 November 2006.
