Superconductivity in Two-Dimensional Crystals

kitefleaUrban and Civil

Nov 15, 2013 (3 years and 9 months ago)

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M S El Bana
1
,

2
*

and S J Bending
1

1
Department of Physics, University of Bath, Claverton Down, Bath BA
2 7
AY, 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 (
10
nm)/ Al (
50
nm) electrodes.



Electrodes spacing's are
500
nm,
750
nm and
1000
nm.


SC

SC

Weak link




sin
c
s
I
I
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

-2
-1
0
1
2
0.16
0.20
0.24
0.28
0.32
Dirac point
holes
electrons


V
g
(V)
R (K

)
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
μ
m

NbSe
2

Graphene

50
μm

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/G
036101
/
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).




-20
-10
0
10
20
-3.5
0.0
3.5
7.0
(a)
H [Oe]
M [G]
T = 6.5 K
T = 6.6 K
T = 6.8 K
T = 7 K
-140
-70
0
70
140
-3.5
0.0
3.5
7.0
H [Oe]
M [G]
T = 6.5 K
T = 6.6 K
T = 6.8 K
T = 7 K
6.56
6.72
6.88
7.04
0.2
0.4
0.6
0.8
(b)
T
C
= 7.13 K
T
[K]
H
P
[G]
a)


Local’ magnetisation curves for an NbSe
2

flake at various
temperatures.

b)

The penetration field, H
p
, as a function of temperature.

H
p

50
μm

V
g

I
-

I
+

V
-

V
+

Si substrate

SiO
2


Graphene

100
μm

50
μm

50
μm

20
μm

10
μm

EBL Patterning 2

EBL Patterning 1

Deposition 1

Deposition 2