Download Study of the superconducting proximity effect in

Superconductivity in Two-Dimensional Crystals
M S El Bana1, 2* and S J Bending1
1Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
2Department of Physics, Ain Shams University, Cairo, Egypt
4. Two steps of E-beam lithography for graphene / NbSe2:
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/SiO2 substrates. Further experiments have been performed to study
superconductivity in single and few-layer NbSe2 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
–
–
Electrode mask (inner features) Ti-Al / Cr-Au (10/50 nm)
Outer bond pads Cr-Au (20/250 nm)
EBL Patterning 1
Deposition 1
50 μm
50 μm
EBL Patterning 2
Deposition 2
20 μm
10 μm
•Two superconducting electrodes and a non-superconducting link (graphene).
•Proximity effect due to diffusion of Cooper pairs.
Preliminary Results
Bipolar charge carriers in Graphene Devices
I+
V+
V-
Graphene
0.28
R (K)
SiO2
100 μm
electrons
holes
0.32
0.24
0.20
I-
Vg
Dirac point
0.16
Si substrate
-2
-1
0
1
2
Vg (V)
• Graphene Device with Ti (10nm)/ Al (50 nm) electrodes.
• Electrodes spacing's are 500 nm, 750 nm and 1000 nm.
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.
Weak link
Micromagnetic measurements of NbSe2 flakes
7.0
SC
SC
M [G]
3.5
I s  I c sin 
(a)
Hp
7.0
0.0
T = 6.5 K
3.5
T = 6.6 K
-3.5
T = 6.8 K
M [G]
T=7K
-140
-70
0
70
140
H [Oe]
0.0
Josephson junction with 2D massless Dirac fermions
T = 6.5 K
T = 6.6 K
-3.5
T = 6.8 K
T=7K
Device Fabrication
-20
-10
0
10
20
H [Oe]
50 μm
1. Patterning alignment marks on Si/SiO2 chips by standard photolithographic
(b)
0.8
techniques.
2. Mechanical exfoliation of graphene.
HP [G]
0.6
0.4
TC = 7.13 K
0.2
6.56
6.72
6.88
7.04
T [K]
a) ‘Local’ magnetisation curves for an NbSe2 flake at various
temperatures.
b) The penetration field, Hp, as a function of temperature.
Repeat cleavage
Si Substrate with 300 nm of SiO2
3. Identifications of the number of layers of graphene / NbSe2 by
interference colours under optical microscope.
Graphene
NbSe2
Optical image of the Hall probe array used to make
‘local’ magnetisation measurements (top) and a
schematic of the electrical set-up used (bottom).
Future Work
• Study of the superconducting proximity effect in single and few-layer graphene
flakes.
• Investigation of superconductivity in few-unit cell NbSe2.
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).
6.33 μm
Acknowledgement
50 μm
50 μm
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 .