M S El Bana 1, 2* and S J Bending 1 1 Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK 2 Department of Physics, Ain Shams University, Cairo, Egypt Superconductivity in Two-Dimensional Crystals Abstract Since the first isolation of graphene in 2004, the subject of two-dimensional crystals has become of enormous interest worldwide. Several theoretical [1] and experimental [2, 3] works have addressed the problems of superconductivity and the superconducting proximity effect in graphene. Initial experiments have focused on a study of the superconducting proximity effect in single and few-layer graphene flakes. Devices with superconducting Al electrodes have been realized by micromechanical cleavage techniques on Si/SiO 2 substrates. Further experiments have been performed to study superconductivity in single and few-layer NbSe 2 flakes exfoliated from bulk single crystals. Our investigations will focus on the dependence of the critical temperature on the number of layers as well as the superconducting properties in an applied magnetic field. In this extreme two-dimensional limit we would expect superconductivity to be destroyed by the unbinding of thermally excited vortex-antivortex pairs, and such samples will provide a critical test of the Berezinskii-Kosterlitz-Thouless transition. Device fabrication steps will be described and preliminary results are presented. Graphene Josephson Junctions Device Fabrication 1. Patterning alignment marks on Si/SiO 2 chips by standard photolithographic techniques. 2. Mechanical exfoliation of graphene. Two superconducting electrodes and a non-superconducting link (graphene). Proximity effect due to diffusion of Cooper pairs. Graphene Device with Ti (10nm)/ Al (50 nm) electrodes. Electrodes spacing's are 500 nm, 750 nm and 1000 nm. SC Weak link Josephson junction with 2D massless Dirac fermions 4. Two steps of E-beam lithography for graphene / NbSe 2 : – Electrode mask (inner features) Ti-Al / Cr-Au (10/50 nm) – Outer bond pads Cr-Au (20/250 nm) Study of the superconducting proximity effect in single and few-layer graphene flakes. Investigation of superconductivity in few-unit cell NbSe 2. Future Work Bibliography [ 1] Feigel'man M V et al., Solid State Communications 149, (2009). [2] Heersche H B, et al., Nature 446, (2007). [3] Kanda A, et al., Physica C 470, (2010). Preliminary Results Bipolar charge carriers in Graphene Devices In these graphs the influence of gating on the resistance of two different samples at room temperature is shown. The position of the Dirac point as well as the symmetry of the electron and hole regions are influenced by extrinsic doping effects. Micromagnetic measurements of NbSe 2 flakes 3. Identifications of the number of layers of graphene / NbSe 2 by interference colours under optical microscope μm NbSe 2 60 μm Graphene 50 μm Repeat cleavage Si Substrate with 300 nm of SiO 2 Acknowledgement I would like to thank the Egyptian government and Ain Shams University for funding this work as well as financial support from EPSRC under grant nos. EP/G036101/1. Optical image of the Hall probe array used to make ‘local’ magnetisation measurements (top) and a schematic of the electrical set-up used (bottom). a) ‘ Local’ magnetisation curves for an NbSe 2 flake at various temperatures. b) The penetration field, H p, as a function of temperature. HpHp 50 μm VgVg I-I- I+I+ V-V- V+V+ Si substrate SiO 2 Graphene 100 μm 50 μm 20 μm 10 μm EBL Patterning 2 EBL Patterning 1Deposition 1 Deposition 2