Semiconductor Physics Group
Semiconductor Physics Group, Cavendish Laboratory
Dr Malcolm ConnollyE-mail: firstname.lastname@example.org
Office phone number: +44 (0)1223 337332
Fax: +44 (0)1223 766130
Secretary: +44 (0)1223 337482
Office: Room 335A, Mott Building
Graphene Single Electron Pumps
Working with the Quantum Detection group at the National Physical Laboratory (NPL), the UK’s National Measurement Institute, I have demonstrated that the world’s first graphene single-electron pump (SEP) provides the speed required to create a practical realisation of the quantum ampere .
The present definition of the ampere dates from 1960 and is vulnerable to drift and instability due to damage associated with the artefact kilogram. This definition is clearly not sufficient to meet the accuracy needs of present and certainly future routine electrical metrology. Consequently, the highest global metrology authority, the Conférence Générale des Poids et Mesures, has proposed that the ampere be re-defined in terms of the electron charge. SEPs are frontrunners in this race to redefine the ampere. They create a flow of individual electrons by shuttling them from a source lead into a quantum dot – a particle holding pen – and emitting them one at a time at a well-defined rate to the drain. A good SEP shifts precisely one electron at a time in order to make the current accurate, and also pumps them out quickly to make it sufficiently large. Up to now the development of a practical SEP has been very much a two-horse race. Tuneable barrier pumps use traditional semiconductors and have the advantage of speed, while the hybrid turnstile utilises superconductivity and has the advantage that many can be put in parallel. Our recent work gives traditional metallic pumps, thought to be not worth pursuing, a new lease of life by fabricating them out of the world’s latest supermaterial - graphene. The devices were fabricated using the state of the art clean room and electron beam facilities here in Cambridge and then measured at NPL using their low-temperature high precision setup for measuring currents.
Quantum Nanoanalytics of Two-Dimensional Atomic Crystal Heterostructures
We have developed a low-temperature scanning probe technique for characterising the electronic behaviour of a new breed of devices which could revolutionise modern electronics [2,3,7].
The huge demand for portable multi-functional electronics has driven the size of silicon transistors close to their ultimate size limit. Without a revolutionary redesign to allow more efficient heat removal, further miniaturisation will require new materials and device concepts to implement logic based on binary switching. The International Roadmap for Semiconductors (ITRS) is promoting a diverse range of approaches to this problem. One solution involves two-dimensional crystals, such as graphene and layered transition metal oxides and chalcoginides, which can be sown together and stacked on top of one another to create atomically thin artifical materials with customised properties. They combine non-trivial electronic, optical, and mechanical properties in incredibly compact architectures, are immanently scalable because they can be grown using wafer-scale techniques, and are compatible with existing silicon processing.
Quantum Nanoanalytics involves moving the tip of a low-temperature atomic force microscope in close proximity to the surface of such devices and monitoring the effect this has on the source-drain current. At low temperature, inelastic scattering is reduced and phase coherence increases the sensitivity of electron trajectories to the presence of the tip, allowing us to extract important device parameters such as the local carrier density and mobility. Our speciality involves drawing charge on the insulating component of a heterostructure to sculpt the potential seen by the electrons as they travel between contacts . This technique allows us to draw floating-gate defined components such as quantum dots, split gates, and tunnel barriers for protoyping new quantum device concepts.
Topologically Confined Vortex Matter
I am interested in characterising and manipulating superconducting vortices in mesocopic devices using magnetometry and electrical transport measurements. Superconducting vortices are point-like topological defects in the macroscopic wavefunction which characterises a condensate of superconducting electrons at low temperature. Aside from the fundamental interest in their collective behaviour, one major driver for acquiring control over vortices stems from their ability to support non-Abelian anyonic excitations known as Majorana fermions (MFs). By braiding the world lines of MFs at the core of vortices it is possible to switch between different ground states of a topologically protected qubit, the building block required for fault tolerant quantum computing. My PhD work, carried out with Prof. Simon Bending at the University of Bath, used scanning Hall probe microscopy and magneto-optical imaging to determine high stability “magic number” configurations of vortices in mesoscopic superconductors [8-10]. I am now developing techniques for gaining a handle over vortices in graphene and topological insulators proximity coupled to s-wave superconductors .
 M. R. Connolly, K. L. Chiu, S. P. Giblin, M. Kataoka, J. D. Fletcher, C. Chua, J. P. Griffiths, G. A. C. Jones, V. I. Fal'ko, C. G. Smith, T. J. B. M. Janssen, Gigahertz quantized charge pumping in graphene quantum dots, Nature Nanotechnology (2013) (in press).
 M. R. Connolly, R. K. Puddy, D. Logoteta, P. Marconcini, M. Roy, J. Griffths, G. A. C. Jones, P. Maksym, M. Macucci, C. G. Smith, Unraveling quantum Hall breakdown in bilayer graphene with scanning gate microscopy, Nano Lett. 12 (11), 5448 (2012).
 M. R. Connolly, E. D. Herbschleb, R. K. Puddy, M. Roy, D. Anderson, G. A. C. Jones, P. Maksym, and C. G. Smith, Reading and writing charge on graphene devices, Appl. Phys. Lett. 101, 023505 (2012).
 K. L. Chiu, M. R. Connolly, A. Cresti, C. Chua, S. J. Chorley, F. Sfigakis, S. Mi- lana, A. C. Ferrari, J. P. Griffiths, G. A. C. Jones, and C. G. Smith, Single-particle probing of edge-state formation in a graphene nanoribbon, Phys. Rev. B 85, 205452 (2012).
 M. R. Connolly, K. L. Chiu, A. Lombardo, A. Fasoli, A. C. Ferrari, D. Anderson, G. A. C. Jones, and C. G. Smith, Tilted potential induced coupling of localized states in a graphene nanoconstriction, Phys. Rev. B 83, 115441 (2011).
 M. R. Connolly and C. G. Smith, Nanoanalysis of graphene layers using scanning probe techniques, Phil. Trans R. Soc. A 368, 5379-5389 (2010).
 M. R. Connolly, K. L. Chiou, C. G. Smith, D. Anderson, G. A. C. Jones, A. Lombardo, A. Fasoli, A. C. Ferrari, Scanning gate microscopy of current-annealed single layer graphene, Appl. Phys. Lett. 96, 113501 (2010).
 M. R. Connolly, M. V. Milosevic, S. J. Bending, J. R. Clem, and T. Tamegai. Continuum vs. discrete flux behaviour in large mesoscopic BSCCO disks, Europhys. Lett. 85, 17008 (2009).
 M. R. Connolly, M. V. Milosevic, S. J. Bending, and T. Tamegai. Magneto-optical imaging of flux penetration into arrays of BSCCO microdisks, Phys. Rev. B 78, 132501 (2008).
 M. R. Connolly, S. J. Bending, A. N. Grigorenko, and T. Tamegai. Anisotropic pancake vortex transport in the crossing lattices regime of BSCCO single crystals, Phys. Rev. B 72 224504 (2005).
 C. L. Richardson, S. D. Edkins, G. R. Berdiyorov, L. Covaci, C. J. Chua, J. P. Griffiths, G. A. C. Jones, M. R. Buitelaar, C. G. Smith, M. R. Connolly, Detection of flux-quantized vortices in ordered graphene Josephson junction arrays, (2013) (in preparation).