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, 1101-1105 (2009). [2] Heersche H B,
et al., Nature 446, 56-59 (2007). [3] Kanda A, et al., Physica C
470, 1477-1480 (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. 6.33 m NbSe 2 60 m
Graphene 50 m 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 m VgVg I-I- I+I+ V-V- V+V+ Si substrate SiO 2
Graphene 100 m 50 m 20 m 10 m EBL Patterning 2 EBL Patterning
1Deposition 1 Deposition 2