Cavendish Laboratory Cavendish Laboratory Semiconductor Physics Group Cavendish Laboratory, University of Cambridge http://www.sp.phy.cam.ac.uk/ Electrically Contacting Nanocrystals Using Graphene The semiconductor industry is in search of new devices the size of molecules (<10 nm) to replace traditional transistors in integrated circuits. Suitable conducting molecules or nanocrystals can be made, but they must be connected to an external circuit. The aim of this project is to fabricate and measure samples in which a layer of nanocrystals has been deposited on a gold electrode with a graphene electrode lowered on top, forming a sandwich structure in which the electrodes are kept apart by the nanocrystals. This will enable studies of the quantum properties of nanocrystals for potential applications. This project is a collaboration with the Optoelectronics Group (Dept. of Physics) and the Electrical Engineering Division of the Dept. of Engineering. Contact: Prof Chris Ford ([email protected]) Optical Single-Electron Spin Detector For the use of electron spin in quantum information processing (QIP), we need to detect the spin state (up/down). This project will develop a spin detection technique for travelling electron wave packets, which would enable the use of electron wave packets as flying qubits interconnecting stationary qubit elements, as well generation of entangled pairs. Spin measurements will use using a lateral p-n junction LED technology. Electrons from a single-electron source will be injected into a p-type region, and the polarisation of emitted photons will be measured. Experiments will be performed both in Cambridge and NPL. This project is a collaboration with the National Physical Laboratory and is supported by an industrial CASE award. Photon released by electron-hole recombination Electron trap Hot electron Valence band Conduction band Contact: Prof Chris Ford ([email protected]) The Quantum Multiplexer The quantum multiplexer is a technology for measuring large arrays (>1000) of quantum devices. We have demonstrated studies of quantum effects in 256 1D wires, and are now extending the multiplexer technology to other types of devices with important applications. Large arrays of quantum dots can be used as an artificial solid state system for quantum simulations and studies of condensed matter phenomena. Large scale parallelisation of single-electron pumps is a promising candidate for a quantum standard of electrical current, analogous to the Josephson voltage standard. H. Al-Taie et al., Appl. Phys. Lett. 102, 243102 (2013) R.K. Puddy et al., Appl. Phys. Lett. 107, 143501 (2015) This work is a collaboration with the University of Cambridge Department of Engineering. Contact: Prof Charles Smith ([email protected]) Topological Materials Nanowire of topological insulator material Sb 2 Te 3 , fabricated by collaborators in the Dept. of Materials, Univ. of Cambridge. States of matter have traditionally been classified by the symmetries they break, e.g., broken translational symmetry in crystals. Recently, states of matter have emerged whose properties are governed by the topology of their band structure, rather than symmetry alone. For example in topological insulators (TIs), the conduction and valence bands are 'inverted'. Remarkably, his leads to conducting states on the surface of the material that cannot be removed by local perturbations. Due to symmetry restrictions these surface states are 'protected' against backscattering. There is great interest in using such topologically protected states towards low-power electronics, spin-based logic, and fault-tolerant quantum communication. Several projects in the SP group are probing both the fundamental physics and potential applications of these topological materials. Contacts: Dr Vijay Narayan ([email protected]), Prof Chris Ford ([email protected]) Contact: Dr Malcolm Connolly ([email protected]) http://connollylab.weebly.com/ Topological Phases in Superconductor-Semiconductor Hybrids In this project you will develop devices and techniques for detecting topological phases of matter that are predicted to emerge from the collective motion of electrons in 2D materials in contact with superconductors. Confirming the existence of these phases would not only dramatically underscore quantum theory, but could also have massive implications for topological qubits for quantum computing. Working with state-of-the-art superconductor deposition systems and semiconductor device fabrication, you will develop devices and use a combination of low- temperature scanning probe techniques, magnetometry, and radiofrequency transport measurements to investigate their properties. THz Excitation of Quantum Devices The SP group has a strong track record of developing both quantum devices (e.g. quantum wires, dots and charge pumps) and sources of THz-frequency radiation. Studies of the interplay between THz EM waves and the quantum behaviour of electrons are difficult because of the lack of suitable THz lasers and the attenuation of THz electrical signals by metallic conductors. This project will couple the SP group’s quantum cascade lasers (QCLs) with quantum devices at sub-kelvin temperatures, using custom-built metallic waveguides (MWGs). Flexible waveguide Detector QCL MWG Contact: Prof. David Ritchie ([email protected]) Molecular-Beam Epitaxy • Growth of layered semiconductor structures, one atomic layer at a time • GaAs, AlGaAs, InAs, GaN etc. Electron Beam Lithograpy • Patterning of nanoscale devices • Length scales less than de Broglie wavelength → quantum behaviour • Quantum dots, photonic crystals etc. Semiconductor Cleanroom • Extensive facilities for making semiconductor devices • Lithography, metallisation, insulators etc. Low-T Measurements • Transport measurements at T down to 30 mK • When k B T < ΔE, quantum effects become apparent • Magnetic fields up to 18 T • Cryogenic scanning probe microscopy Group Facilities THz Optics Laboratory • Development of THz devices and systems