TunnelSpectroscopy v2

Degree Project 15 hp
Course code: FK6001
Tunneling spectroscopy of superconductors
in high magnetic ﬁelds
Author:
Arash Zeinali
Supervisor:
Vladimir Kraznov
Co-Supervisor:
Taras Golod
September 1, 2014

CONTENTS 1
Contents
1 Introduction 3
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Theory 4
2.1 Superconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 The London equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 Energy gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.3 Phonons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.4 Vortices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.5 Surface superconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.6 Josephson eﬀect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.7 Tunneling theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Experimental Methods 12
3.1 Sample fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.1 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.2 Sputtering-deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.3 Wire-bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.4 SEM/FIB sculpturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4 Results and discussion 19
4.1 Critical temperature and ﬁeld alignment . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.3 Tunneling spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.4 Angular-dependent magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . 27
5 Conclusion 30
A Appendix 32
B Appendix 33
C Errata 35

CONTENTS 2
Abstract
This degree project describes the fabrication of a trilayer Nb/Al-AlOx/Nb superconduc-
tor insulator superconductor (SIS) Josephson junction (JJ) in Albanova Nanofabrication
facility. We study the junction properties in low temperatures and strong fields and preform
two types of measurements to probe bulk properties and electronic spectra of the super-
conductor. This work examines surface superconductivity which is a phenomena appearing
in Type-II superconductors under influence of strong magnetic fields Hc2 < H < Hc3. We
preforme magnetoreistance measurements for extraction of Hc3 and tunneling spectroscopy
for extraction of Hc2. We study the temperature dependence of the upper critical fields and
we calculate the ratio Hc3
Hc3
to see to what extent surface superconductivity prevails. Surface
superconductivity is not well studied and we want to do independent measurement to study
the phenomena further. There are studies reporting that all three phases merge into a single
critical point in the phase plane, we want to find out the temperature for which it happens.
The tunneling barrier of the fabricated junction had bad quality and we preformed mea-
surements on a commercial sample. We show that the ratio between the upper critical fields
decreases with temperature and the ratio approaches 1 already at T≈7 [K], indicating that
the tricritical point is somewhere beyond this temperature.

3
1 Introduction
This project concerns the experimental study of Type-II superconductors and supercon-
ductivity (SC). Superconductivity is a metallic state characterized by zero resistivity and
Meissner effect. Which is the expulsion of magnetic field from the interior of the metal.
SC is a quantum mechanical phenomena that emerges as a result of interaction between
electrons and lattice vibrations of the ions in a metal. Superconductivity requires mutual
attraction between two electrons1
[1] with spin ±1
2 , this happens at low temperatures be-
low a critical temperature Tc. The pair forms a unit called Cooper-pair with integer spin
such particles are Bosons and obey different probability distribution compared to Fermions.
Which in turn allows all the Cooper-pairs to occupy same energy level. Macroscopically we
see this transition as a drastic change in metallic properties[2]. In Sec 2 I try to explain
the fundamentals based on London theory of SC and try to avoid complications that would
have arise from a pure microscopic quantum mechanical treatment of SC. Followed by Sec 3
which explains the sample fabrication and measurement setup. We study electrons tunneling
in JJ through a series of resistivity measurements that are described in Sec 4. In order to
locate the energy gap (see Sec 2.1.2), from which important parameters of the material can
be determined. The energy gap is a measure of the pair binding energy of the Cooper-pair
and it is temperature dependent[1]. The energy gap in SC have similarities the energy gap
in semiconductors and insulators that arise from the Band theory of metals. The energy
gap explains why semiconductors become nonconductive at T=0 since no thermal energy
is involved to excite electrons from valence band to conduction band[3]. In SC the energy
gap is very small in the order of a few [meV] compared to semiconductors where the energy
gap is in the order of [eV]. JJ is the name of a device consisting of an very thin insulating
layer sandwiched between two layers of superconducting materials named after the founder
Brian Josephson who predicted this phenomena. JJ are used in tunneling experiments and
in superconducting quantum interference devices (SQUID). SQUIDs are used to detect very
small magnetic fields.
1.1 Motivation
The aim of this degree project is to learn micro- and nanofabrication techniques used in
fabrication of thin films. In order to fabricate a trilayer Nb/Al-AlOx/Nb superconductor-
insulator-superconductor (SIS) JJ and study its magnetic properties. We are looking at
a phenomena that appears in Type-II superconductors in strong magnetic fields. Where
superconducting state is confined on very thin layer on surface of the electrode at fields
Hc2 < H < Hc3. The upper critical field Hc2 is one of the key parameters in Type-II
superconductors [4]. For instance in high-Tc superconductors (HTS) the magnitude can
help to understand the coupling-mechanism in formation of Cooper-pairs. For HTS it is
immensely hard to extract Hc2 due to the requirement of very strong fields in the order
of ∼ 100 [T] to destroy superconducting state. Even for low-Tc superconductors (LTS)
which is studied in this project the extraction of Hc2 is a huge experimental challenge.
Specially at temperatures near the critical temperature Tc. When the electrodes turn into
normal state and its not possible to probe junction properties. To tackle this problem
we want to fabricate a cross-bridge junction which allows to establish a true four point
measuring probe. The junction design is inspired by the article In situ fabrication of a cross-
bridge Kelvin resistor structure by focused ion beam[5]. This particular geometry eliminates
parasitic impedance of the interconnects which allows accurate measurements of resistance
in nanoscale devices[5]. A similar study has been preformed by Vladimir Krasnov[6] in
which surface superconductivity was observed at Hc2 < H < Hc3 [6] to almost exactly
Hc3(T =2[K])
Hc2(T =2[K]) = 1.69 consistent with theory, it was also observed that the ratio decreased with
increasing temperatures. In this work we are looking closer at the temperature dependence
of the ratio between the upper critical fields. To emphasize the experimental challenge we
are faced, lets take a look on the field dependence of resistance for a Type-II superconductor:
1
Electrons are Fermions

4
R [Ohm]
H [T]
Rn
b)a)
Figure 1: A sketch of R(H) with and without flux-flow. a) flux-flow is associated (see Section
2.1.4) with dissipation of energy therefore the R(H) can have a rather complicated form. b) the
ideal but unrealistic case where vortices are pinned to the surface of the superconductor
In Figure 1 we see that in the ideal case where all the vortices are pinned, it would be
easy to extract Hc2 from the field corresponding to when R = 0 anymore, then trace tail up
to Hc3 where resistance become normal. However in the general case a) it is not possible to
say where Hc2 is. And this is the big question that needs to be answered.
2 Theory
2.1 Superconductivity
Superconductivity was discovered in 1911 by Heike Kamerlingh-Onnes [7] three years he
first managed to liquify He-4. He was studying the temperature dependence of resistivity
in mercury when he made an unexpected discovery, resistivity dropped instantly at T ≈ 4
[K]. At that time it was known that there was an temperature independent contribution to
resistivity due to crystal impurities, leading to a residual resistance ρ0 at zero temperature[8].
Temperature for which the phase transformation occurs is called critical temperature Tc. It
was later discovered that there are more critical transition quantities.
Figure 2: Superconducting phase occurs inside the critical surface. The critical magnetic field
is denoted Hc and the critical current density is Jc. [9]

2.1 Superconductivity 5
In the Figure 2 we see that the maximum Hc is obtained at lowest temperature and for
higher temperatures it is described by the empirecal formula[7]:
Hc(T) = Hc(T = 0) 1 − (T/Tc)2
(2.1)
One interesting aspect of the superconducting state is that it cannot be understood as an
ideal conductor which is a metal with zero resistivity. These states have different properties
in presence of magnetic fields. This fact is important because scientist believed this was the
case for nearly 20 years after the discovery of superconducting state. An ideal conductor
trap magnetic flux to its interior depending on its magnetic history upon cooling (ideal skin
effect)[8, 7]. Superconductors however always2
expels magnetic field from the interior by
creating surface currents which generate field that screens the external field[8]. Screening
of magnetic field is called the Meissner effect. From a thermodynamical perspective this
means that B = 0 and ρ = 0 can be treated as intrinsic properties at H < Hc[8]. However
the screening is not complete the field penetrates the surface of the superconductor length
scales of about 100 [nm][8]. The length scale is known as the penetration depth λ.
2.1.1 The London equations
The first theory that explained the relation between the electric field, current,magnetic field
and penetration depth was developed by London brothers. It assumes a two fluid model
consisting of two types of electrons and that the normal electrons are short circuited[8],
meaning that the normal component do not generate an electric field. The total density
of electrons consist of two parts, normal- and superconducting electrons described by n =
ns(T) + nn(T)[8] to conserve the number of particles.
Figure 3: Temperature dependence of electron concentrations[8]
If one assumes that ns is constant inside the sample, then the superconducting electrons
can be described by Newton’s second law:
nsm
dvs
dt
= nseE =⇒ E = Λ
dJs
dt
(2.2)
Here we used Js = nsevs and Λ = m
nse2 . Applying Maxwells equations × E = −∂B
∂t and
× H = Js to eq 2.3 we get the second London equation:
H + λ2
× ( × H) = 0 (2.3)
Where λ2
= mc2
µ0nse2 , λ is the penetration depth of the field. With equation 2.3 and two
boundary conditions one can work out the variation of the field inside the sample. Then use
Maxwell equation to work out the variation of current inside the sample.
2
True for Type-I superconductors

2.1.2 Energy gap
An important discovery that led the way to a more precise understanding of superconduc-
tivity was the discovery of the energy gap which is in the order ∆ ∼ kBTc[10]. First evidence
of energy gap came from specific heat measurements, a sudden peak in cv was observed at
Tc. From where the specific heat decayed exponentially.
Figure 4: cn = γT from the Sommerfeld theory (normal state) and cs ≈ γTcae−b Tc
T (supercon-
ducting state) [11]
The exponential decay of the specific heat below Tc shows that thermal energy is involved
in excitation of electrons from ground state to conduction band, exponential form stems from
the fact that the excitations are governed by Fermi-Dirac statistics. The parameter b in cs
shown in Figure 2 was estimated from experiments to ∼ 1.5, which implies that a minimum
energy (per particle) of ∆ ∼ 1.5kBTc[10] is required to bridge the gap. Later Bardeen
Cooper and Schrieffer (BCS) showed that the ground state is occupied by Cooper-pairs,
consisting of two electrons with opposite momenta and opposite spin[10]. The attraction is
mediated by a weak electron-phonon interaction. Required pair-breaking energy was shown
to be:
Eg(T = 0) = 2∆(T = 0) = 3.528kBTc (2.4)
Which is roughly a factor 2 larger than the value extracted from the specific heat mea-
surements. The formation of Cooper-pairs is one of few macroscopic quantum phenomena
we directly can observe as the coherent behavior of the electrons in a superconductor.
2.1.3 Phonons
London equations explained some aspects of superconductivity but do not explain the mi-
croscopic picture of superconductivity. It later became clear with experiments that lattice
structure plays an important role[8]. For example the two different phases of mercury Hg(α)
and Hg(β) which have different crystal structure also have different Tc and Hc. Another
evidence is the isotope effect, which is an effect of how Tc changes for different isotopes
obeying TcMα
= const. For many superconductors α ∼ 0.5 [8].
The temperature dependence of the energy gap according to BCS theory shows that
Tc ∝ ΩD[8]. Where ΩD is the maximum frequency of the oscillating ions. For different
isotopes it is found that ΩD ∝ 1√
M
. Isotope effect is one indication that phonons are involved
in formation of Cooper-pairs. Phonons being the energy quantum of lattice vibrations.
2.1.4 Vortices
In the end of 1940s it became clear that London theory wrongly predicted that supercon-
ductors have negative surface energy σns < 0[8]. Which means that a superconductor in
magnetic field become more energetically favorable if it is turned to mixture of normal and
superconducting regions[7]. This contradiction was later resolved by Abrikosov[8]. It was
shown that superconductors can be divided in to two groups referred as Type-I and Type-II.

It became clear that three different length scales can be associated with superconductors,
mean free path l, penetration depth λ and coherence length ξ[12]. Materials can be charac-
terized by a dimensionless constant κ = λ
ξ its magnitude determines the Type-.
Figure 5: Type-I: κ < 1√
2
; σns > 0. Type-II: κ > 1√
2
σns < 0 [8, 11, p. 70]
Type-II superconductors have a second critical field, the region between Hc1 < H < Hc2
is referred to as the Mixed state. In this state the external field penetrates the bulk as
quantized magnetic vortices[8], magnetic field passes through the bulk in form of a long
cylinder parallel to the external field[7]. The magnetic flux through each vortex is Φ0.
Φ0 =
h
2e
(2.5)
As the external field is increased the vortex density is increased. When the distance between
the vortices become ∼ λ, repulsive force between the vortices will appear due to difference in
Bernoulli pressures in region between the vortices[8]. Eventually the vortices form triangular
lattice structure is formed.
Figure 6: Shaded area normal core radius ∼ ξ, circulating super current within an area of radius
∼ λ [8, 7]
At Hc2 the vortices become so close that the normal region overlap. Overlap causes
phase transition to normal state, this happens when [8]:
Hc2 =
Φ0
2πξ2(T)
(2.6)
The relation above can be used to calculate the coherence length which is roughly about
∼ 10 [nm] for Nb at low T. Mixed state superconductivity is not characterized by zero
resistivity. Applying external current sets the vortices in motion due to Lorentz force3
3
Other forces are involved Magnus Force and viscous forces

exerted on the vortex. Vortex motion is the main cause for dissipation of energy[8]. It is
therefore important to increase the vortex pinning force for high current applications[8] by
creating crystal defects such as grain boundaries and voids to trap the vorticies.
2.1.5 Surface superconductivity
Consider a thin superconducting film with a thickness less than λ, with its surface aligned
parallel to external field. Studies have shown that a thin superconducting layer with thick-
ness ∼ ξ(T) can be sustained above Hc2[8, 7]. An expression for the third critical field is
obtained from thin film solutions of the second London equation 2.3 together with expres-
sion of the critical field of thin films Hc = 2
√
6Hcm
λ
d [7]. Where Hc is critical field of thin
film and Hcm is the thermodynamic critical field and d is the film thickness. We see that a
decrease of film thickness increases the critical field. Numerical calculations shows that[7]:
Hc3 = 1.69Hc2 (2.7)
Figure 7: H-T phase diagram of Type-II superconductors [8]
A good way of testify bulk properties of superconductors are through magnetoresistance
measurements[6]. The idea is to decrease the external field strength reaching Hc3 from
above. Assuming that there is no or at least very little magnetoresistance in normal state,
atleast for modest field strengths. We want investigate the temperature dependence of the
upper critical fields near Tc. The temperature dependence of Hc3
Hc2
is not completely known
since it is immensely hard to extract Hc2 near Tc. However there are reports showing that
the ratio decreases with increasing temperature [6]. Other reports indicates that there is
tricitical point in the region near Tc at T=8 [K] for a Nb sample with a larger Tc than the
one used in this experiment[13, 14].
2.1.6 Josephson effect
The Josephson effect is the name of the tunneling phenomena that appears when two su-
perconductors are separated by a weak-link. Effects are known as DC- and AC-Josephson
effects. Brian Josephson predicted these effects when he considered tunneling of electrons
between two superconductors[7]. In general there are many ways of constructing the weak
link, here we consider the example of when two superconductors are separated by a thin
insulating oxide layer with thickness ∼ 1 − 2 [nm] [7].

Figure 8: A SIS junction the shaded region is an insulating layer. SR refers to superconductor
on the right side. [8]
In thermal equilibrium wave function ΨL = |ΨL| eiΘ1
and ΨR = |ΨR| eiΘ2
have the
same amplitude since the metals have the same temperature. However the phase of the
wave functions are arbitrary[8]. The wave functions ΨL and ΨR can tunnel through the
barrier and interfere with each other. Interference merges the two wave functions to a single
phase coherent wave function describing the whole system[8]. All electron pairs occupy the
same quantum level and become super fluid like and able to pass through the ionic lattice
without dissipation. The DC-effect relates supercurrent Is through the junction to the phase
difference θ = Θ2 − Θ1 of the wave functions:
Is = Ic sin θ (2.8)
A sufficiently small current lesser than the critical current Ic can pass through the junc-
tion without dissipation by means of tunneling. The second effect describes what happens
when a finite voltage is applied between two electrodes. Supercurrent will start to oscillate
resulting in emission of high frequency electromagnetic waves from the junction [8].
ωJ =
2eV
(2.9)
It was later discovered that Ic coupled to external magnetic field. Producing similar
diffraction pattern produced by single slit experiments from optics. However in this case it
is the interference between the wave functions in the insulating region causing the Fraunhofer
modulation [8]. This is often used experimentally to confirm if there is a proper tunneling
barrier.
Figure 9: Experimentally observed dependence of the maximum supercurrent through a Sn-
SnOx-Sn junction on magnetic field [7]

Figure 9 shows a periodicity in magnetic field ∆H where Imax = 0, this happens when
the total flux through the junction Φ = nΦ0[8]. From which it is possible calculate the
period by ∆H = Φ0/A where A=L(t+λ+λ) is the area, t is the thickness of the insulating
layer and λ is the penetration depth and L is the length of the junction.
2.1.7 Tunneling theory
Tunneling experiments is a direct way of examining the spectrum of the quasi-particles. For
this discussion its sufficient to think of the quasi-particles as unpaired Coper-pairs [7]. The
DOS of a superconductor is [8]:
Ns(E) = N(0)
E
√
E2 − ∆2
(2.10)
where N(0) is the DOS in normal state and E is the energy. It follows DOS has a BCS
singularity at E = ∆. The excitations of quasi-particles from ground state obey Fermi-dirac
statistics, since we consider normal electrons. An expression tunneling current from left to
right [8] is:
IL→R =
2π ∞
−∞
|T|
2
NL(E)fL(E)
nL
NR(E)(1 − fR(E)
nR
dE (2.11)
Where |T|
2
is the tunneling matrix which gives the magnitude of the probability flux,
NL,R(E) is the density of states for left and right side, fL,R is the probability distribution
and nL,R is the fraction of filled and empty states on each side of junction. By switching
indices in eq 2.11 one can compute the net current across the junction[8, 10]:
I = IL→R − IR→L =
2π ∞
−∞
|T|
2
NL(E)NR(E) [fL(E) − fR(E)] dE (2.12)
Applying voltage across the junction will shift the Fermi level by fR,L(E+eV ) depending
on the polarity of the voltage. This shift causes a current to flow from one side to another.
From eq 2.12 one can extract tunneling current between two normal metals, assuming that
the DOS on both sides is constant at the Fermi level energy[8]:
INN = σN V (2.13)
σN is the tunneling conductance in a NIN junction. The current voltage characteris-
tic show Ohmic feature for tunneling between normal electrodes. For tunneling between
superconductors the characteristic is remarkably different. However numerical calculations
is needed to compute I-V characteristic[10]. Since it requires the BCS DOS which is not
constant. It can be shown that differential (tunneling) conductance dI
dV [8]:
dI
dV
= σD ∝ NS(E) (2.14)

Figure 10: SIS tunneling characteristic between two different superconductors, ∆1,2 is the energy
gap corresponding to respective superconductor separated by the barrier
From the Figure 10 it is see that no tunneling current can flow until V = ∆1+∆2
e at
T = 0. However when T > 0 there is another peak at V = ∆1−∆2
e which comes from ther-
mally excited electrons[10]. These two distinct peaks in the I-V is used to extract ∆1(T) and
∆2(T). In Josephson junctions the gap of the two electrodes is the same therefore only one
peak is observed in the σ(V ). From the I-V curves we can extract the spectrum of the quasi-
particles in the superconductor. This type of tunneling spectroscopy measurements was first
preformed by Giaever, which confirmed the BCS theory of conventional superconductors[8].

12
3 Experimental Methods
3.1 Sample fabrication
The sample fabrication was conducted in AlbaNova Nanofabrication Facility. Each stage
of the sample fabrication was supervised by Taras Golod. Parts of the fabrication was
preformed by Taras such as FIB-Sculpturing. The micro- and nano fabrication techniques
used in this project consist of three steps. The first step is to transfer a desired pattern to
the substrate (silicon wafer) which is the design of the electrodes for the junctions. Then
we deposited thin films on to the substrate using magnetron sputtering in order to create
the tri-layer SIS junction structure. Finally the sample is loaded into SEM/FIB for fine 3D
sculpturing of the junction to get the desired geometry. Figure 11 shows a schematic of the
sample fabrication steps:
Figure 11: a) Photolithography: UV-exposure on positive photoresist. b) Development: removal
of UV-exposed photoresist. c) Magnetron sputtering: Deposition of thin films and oxidation of
Aluminum surface forming the junction barrier. d) Liftoff: Remove remaining photoresist with
acetone in ultra-sonic cleaner bath. [Figure provided by Lena Merkulova]
3.1.1 Photolithography
Photolithography is a method used to transfer pattern to substrate by using light sensitive
chemical usually positive or negative photoresist. The exposed areas of the photoresist
undergoes a photochemical reaction where the absorbed photons breaks the polymer chains
(for positive resist). The broken chains can be removed with a developer (MF139). The
base of the sample is a silicon wafer which has an oxidized surface. A big peace of silicon
wafer is cut down into ∼ 5x5 [mm] sized peaces. The peaces are cleaned with acetone and
water under a workbench which circulates clean air through HEPA filters. A thin layer of
photoresist is applied to the substrate by spin coating at the rate ∼ 4000 RPM during 1
minute. Which gives a photoresist thickness of ∼ 1.6 [µm] see Figure 12. Then the substrate
is placed on a hot plate at 100 [◦
C] during 1 min to remove any remaining solvents on the
sample.

3.1 Sample fabrication 13
1
2
3
Figure 12: 1) LTD model 4000 photoresist spinner. 2) Chemicals from left to right MF139
(developer), aceton and water 3) Hotplate
Figure 13: Projection mask aligner 1) mercury lamp, 2) mask space with alignment screws 3)
Focusing lenses 4) Microscope used for alignment of mask onto substrate 5) sample space
The substrate is placed in the sample space and is exposed with UV light for 23 seconds
and put into a developer for 20-30 seconds to remove photoresist from the exposed areas.
Finally the sample is placed in a optical microscope to examine if the pattern is transferred
properly.

3.1.2 Sputtering-deposition
AJA Orion sputtering system[15] was used for deposition of thin films. Magnetron sputtering
is a process used to deposit thin films on to a substrate. A simplified explanation of the
process follows: Inert gas is pumped into a vacuum chamber. Magnetic fields are generated
over the target material (deposition material) by coils. The magnetic field traps electrons
on top of a target material. The electrons are produced by the plasma generated from
the strong electric field produced near the electrode. Argon atoms eventually collides with
the electrons which ionize the atom. The positive charged ion are then accelerated to the
target which is placed on top of a negatively charged electrode, the collision of the ion
”sputters” away material from the target. The sputtered atoms are deposited on to the
rotating substrate see figure 14. By varying the argon pressure in the chamber one can vary
the sputtering rate[16]. All the sputtering parameters can be found in Appendix A.
Figure 14: Magnetron sputtering: top left picture argon gas pumped into vacuum chamber.
The white lines are the magnetic field lines, red dot is the argon gas and blue dot is an trapped
electron. Right figure: Argon ion (purple dot) is accelerated to target which releases target
material. The purple glow on top of target is from the recombination of argon ion with electrons
forming a neutral atom the surplus energy of the system is emitted as photons. [Figure modified
originally from [15]]
In Figure 14 we see an illustration from the inside of the sputtering chamber. To estimate
the sputtering rate a series of calibration depositions were performed during 10 minutes for
both Nb and Al target. The deposited layer were analyzed in a surface profiler (KLA Tencor
P-15 surface profiler) to measure the thickness of the films. The insulating tunneling barrier
between the superconducting electrodes is formed on top of Al layer because Nb does not
form oxide well. The average height of Al deposition during calibration run was measured to
283.6 [˚A], from which its possible to calculate the sputtering rate 283.6
600 ≈ 0.47 [˚A/s]. The Nb
average height was measured to 641.6 [˚A] with sputtering rate 1.07 [˚A/s]. Sputtering time
can be estimated from the sputtering rate. Thickness of the bottom and top Nb electrode
is ∼ 200 [nm] respectively. For Al layer we tried thickness of 10 and 50 [nm]. After the
sputtering deposition is completed a liftoff process is preformed to remove any remaining
photoresist. The substrate is placed in a beaker with aceton and put in to ultra-sonic cleaner
bath.
3.1.3 Wire-bonding
The sample is glued to a printed circuit board (PCB) see Figure 15 b) aluminum wires are
soldered from sample to PCB to establish a true four point probe using Kulicke & Soffa
Digital Wedge Bonder[17]. Two contacts are used to measure voltage and two for sending

current through the cross like junction.
AB
C
D
E
F
G
H K L M N P
R
S
T
U
V
WX
2 3 4
a)
b)
Figure 15: a) Figure from optical microscope after the liftoff process, dark regions are SiOx and
the bright area is the sputtered Nb/Al-AlOx/Nb layers. The junctions will be located at the
intersection between the two perpendicular electrodes in the middle and labeled from 1-6 from
left to right. b) Sample glued on PCB after soldering.
Junction # I+ I− V+ V−
2 E F D C
3 T S M N
4 U V X W
Table 1: Contact schematics for all three junctions
The PCB will be mounted on a sample carrier to be inserted into a cryostat. The
contacts are connected to the measurement system through twisted pair wires (AB, CD,
etc) to reduce noise induced by magnetic field outside the cryostat.
3.1.4 SEM/FIB sculpturing
The 3D sculpturing was done in FEI Nova 200 dual beam system which has scanning elec-
tron microscope (SEM) and focused ion beam (FIB) integrated in the same system. The
advantage of this system that one can simultaneously monitor the sample with the SEM
and use the FIB etching capability for sculpturing[18]. The FIB accelerates Gallium ions to
an energy of ∼ 30 [eV] and focus the beam to the sample through magnetic lenses to the
target[18]. Gallium ions sputters away the uppermost ions from the sample surface[18]. It
is possible to vary the ion current to do fine or rough milling, the advantage of high ion
current is the reduced milling time[18]. The disadvantage is bad focusing. For fine milling
ion beam current 10 [pA] is used and 100 [pA] for rough milling.

A) B)
C)
1)
2)
3) 4)
Figure 16: A) FEI Nova 200 is a dual beam system 1) electron column 2) ion column 3) vaccum
chamber B) Monitoring system C) Inside the vacuum chamber 4) Sample holder: rotable (360
deg) and tiltable (-10 to 52 deg)
The 3D sculpturing is preformed by tilting and rotating the sample holder with respect
to the ion column. Sample holder tilted at 52 degrees correspond to sample being perpen-
dicular to ion beam. The maximum angle for side milling is 62 degrees when the sample
is mounted on a flat sample holder. Both flat and tilted sample holder (45 deg) were used
during sculpturing. The later to increase the milling angle. Three different square junctions
were sculptured with sides ranging from ∼ 1-2[µm]. Complete description of the junction
geometry and fabrication can be found in Ref [5].

3.2 Measurement setup 17
A)
1
2
3
4
Figure 17: SEM image of junction 4. A) Square junction with side ∼ 1.2 [µm]. Voltage drop
can be measured from 1 to 3 and current sent from 2 to 4 for tunneling experiment.
In the figure above under electrode 2 there is a region with slightly darker color, this is
the Al layer sputtered on top of Nb bottom layer. Electrode 3 and 4 are cut from the sides
while 1 and 2 are cut from below using a tilted angle of 62 degree. It is important that
the side cuts extend across the insulating barrier to force the current to pass through the
tunneling barrier rather than through the electrodes.
3.2 Measurement setup
The sample is cooled in a 17 T cryostat with a base temperature of 1.8 K. This cryostat is a
closed cycled[19] which means that it recycles the warmer exhaust vapour to a gas reservoir
then condenses the gas back to the Helium pot[19]. To acquire sufficient cooling power it
is required to have continuous gas flow through the system, the pressure in the VTI can
be read on via the pressure gauge the optimal the pressure is 14 [mbar]. Thermometers,
Hall Probe and sample wires is connected to the measurement system on the left side of the
cryostat see Figure 18.

3.2 Measurement setup 18
1a)
1b)
2)3)
4)
5)
Figure 18: 17 T cryostat from Cryogenic Ltd: 1a) Sample is loaded into cryostat with dipstick
from above. 1b) Closeup of 1a): Sample mounted into rotatable sample holder (+20 to -120
deg). Behind rotating stage there are two Hall probes parallel and perpendicular to field and
thermometers. 2) He-4 reservoir and compressors. Inside 3) there are superconducting magnets
generating field along dipstick 4) Needle valve to regulate amount liquid in helium pot. 5) VTI
pressure gauge
1)
2)
3)
4)
5)
6)
Figure 19: Measurement system: 1) Temperature controller 2) magnet power supply 3) Cryostat
computer 4) Measurement computer 5) Live monitoring of ongoing experiment 6) Preamplifiers

19
Lock-in measurements is preformed to extract weak signals from thermal noise. The
voltage drop from the sample is pre-amplified by a factor of 200-2000 and digitalized[19].
The output current is calculated from direct voltage to current conversion with a resistor
100 [Ohm] which is put in series with the AC generator. This signal is also amplified and
digitalized. The I-V data are sampled during 1 sec and saved into data files. Sample
temperature is controlled by heating the variable temperature insert (VTI) see Figure 19.
Temperature of the sample was stable during measurements for low fields it deviated from
the set value about 1-5 [mK] from set value. At very strong fields the temperature of sample
showed field dependence. During T=1.8 [K] measurement of MR (parallel) the temperature
shifted maximum of 40 [mK] from the set value. Measurements are controlled and recorded
by LabVIEW all calculated quantities was configured in LabVIEW with the help of Taras
Golod.
4 Results and discussion
The measurements were preformed on a commercial SIS junction. The junction is a square
overlap Nb/Al-AlOx/Nb junction with a different geometry than the fabricated sample. The
electrodes on bottom and top layer have different thickness 150 [nm] and 50 [nm] respectively,
more information about the junction can be found in Ref [6]. The IV’s are measured at AC
frequency of 1 [Hz] and MR data measured with 100 [µA] bias current with a frequency 23
[Hz]4
. All the IV-characteristic can be found in Appendix B
4.1 Critical temperature and field alignment
Before the sample is loaded into the cryostat we measure the resistance at room temperature
across the electrodes to check if there are any shorts. The measured resistance is 360, 300
and 280 [Ohm] (smallest to largest junction). Critical temperature is measured at H=0 [T]
by slowly ramping the temperature below Tc.
4
We use larger frequency because strong magnetic fields create small frequency noise

4.2 Magnetoresistance 20
2 4 6 8 10 12
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
-120 -110 -100 -90 -80 -70 -60
55
60
65
70
75
Resistance vs angle
Rac[Ohm]
T [K]
Tc= 8.7 [K]
Rac=76 [Ohm]
Critical temperature
angle [deg]
90.6 [deg]
H=3 [T]
Figure 20: In the left figure it is seen that the top and bottom electrode have slightly different
Tc due to the different thickness of the films. Right figure minimum in MR(angle) correspond
to field being parallel to surface
In Figure 20 we see that the resistance does not go to zero at Tc as expected, because re-
sistance is measured across the junction therefore we get junction contribution to resistance.
The true angle between the sample and the field is calculated from the components of the
resistance of the Hall probes behind the sample holder. Similar alignment was preformed
for perpendicular field showed a minimum in MR at −2 [deg] at H = 1.9 [T].
4.2 Magnetoresistance
The resistance is still measured across the junction similar to Tc measurement, for the same
reason R = 0 below Hc1. Hc3 is extracted by numerical differentiation dR
dH (H), in normal
state this quantity expected to be close to zero. However we always measured a slight MR in
normal state. From the dR
dH as a function of field an estimation of the error in Hc3 is made.
In Figure 21 a) (green curve) one can see that it is not trivial to find the third critical field
just by looking at MR data. Hc3 is somewhere in the tail marked by the black ellipse. For
temperatures near Tc it was not possible to extract the third critical field since the electrodes
turned into normal state and is not able to carry the super current. At these temperatures
it is a sharp step like transition between Hc2 and the normal state. The signature of surface
superconductivity in R vs H which is the smooth tail between Hc2 and Hc3 is not observed.
For comparison we did a series of measurements of field perpendicular at T=1.8 [K] and 6.5
[K] ≤ T ≤ 8.5 [K].

4.3 Tunneling spectroscopy 21
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
10
20
30
40
50
60
70
80
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
10
20
30
40
50
60
70
80
T [K]=
1.8
2.5
3.5
4.5
5.5
6.5
7
7.5
8
8.5
Rac[Ohm]
Magnetoresistance
Hc3
a)
b)
T [K]=
1.8
6.5
7
7.5
8
8.5
Rac[Ohm]
H [T]
Figure 21: a) Field parallel to junction plane b) field perpendicular. Closer investigation of the
curve T=7.5 [K] in parallel alignment observation of a small increase of resistance ∼0.2 [Ohm]
(!) near transition from normal to superconducting is made, a similar peak is observed at T=8
[K].
In Figure 21 we see a decrease in resistance (negative MR) at low fields. This decrease
can be understood by looking at the IV-curves in Appendix B. The tunneling conductance
increases as a function of field below Vp. Therefore we see as the decrease of resistance as a
function of field.
4.3 Tunneling spectroscopy
Tunneling spectroscopy is the method used to extracting the energy gap from the IV curves
as a function of field. At the same time it is possible to extract the tunneling conductance
at the voltage peak Vp. This is done by numerical differentiation, the algorithm that is used
takes n points and calculates the slope between the points, the slope of the fit is assumed
to be the derivative at the middle (n/2+1) point5
. In the calculations I used n=20, using
more points gives a smoother curve but one looses features from the IV’s. This was done
for all IV’s and different fields in Appendix B. Below is an example tunneling conductance
σ(V ) where we see the BSC singularity at E=eVp.
5
Source: Macro in Kaleidagraph 4.03 made by A. Yurgens 1995

-4 -2 0 2 4
0.01
0.1
1
10dI/dV
Voltage [mV]
dI/dVp
Tunneling conductance (Semi-log)
T=2.5 [K]
H=2 [mT]
Figure 22: The energy gap eVp=2∆ is extracted from the conductance peak in semi-log scale
and the tunneling conductance at Vp is extracted in normal scale. Rn = 1
σn
is extracted from
the horizontal line after the minima above Vp.
We use two different methods of extracting Hc2 from dI
dV curves the methods are explained
in detail in Ref [6]. The first method is based on the fact that the tunneling resistance (dV
dI )
becomes Ohmic at Hc2. Therefore we use the scaling of dV
dI ( H
Hc2
) 1
Rn
, extraction of Hc2 is
done by extrapolation to unity see Figure 23 and 25. This method gives an approximation
of Hc2 since the relation is not completely linear[6]. The second method uses the universal
linear scaling of eVp( H
Hc2
)[6] the relation is linear in the whole temperature range see the
theoretical curves in Figure 24. For this method we first extract the 2∆(H = 0[T]) from the
energy gap vs field data through linear extrapolation to H=0 [T], these values are presented
in Figure 27. eVp is normalized by the extracted value 2∆(H = 0[T]). With a linear fit we
extrapolate the curve to 1/2 to obtain Hc2 [6]. Interestingly the voltage peak does not go to
zero at H=Hc2, but stops half way at eV∼ ∆(H = 0[T]). Why this happens is beyond the
scope of this thesis more information can be found in Ref [6]. The uncertainty is obtained
from the linear fit in all measurements we have neglected the uncertainty in magnetic field.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
Perpendicular
H
H c2
dV/dIp/Rn
1.8 [K]
6.5 [K]
7 [K]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
H
H c2
dV/dIp/Rn
7.5 [K]
8 [K]
8.5 [K]
Figure 23: Scaling of tunneling resistance (H⊥), dashed lines are theoretical curves: black line
is for T=1.96 [K] and the blue is for T=7.3 [K].
In this ﬁeld orientation we can compare our experimental data with numerical calcula-
tions obtained from Ref [6]. Figure 23 shows that at about T ≥ 7.5[K] the data starts to
deviate from the theoretical curves. At these temperatures it was not easy to extract Rn
because the electrodes was turned into resistive state.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.5
0.6
0.7
0.8
0.9
1
H
H c2
eVp/2∆(H=0)
Perpendicular
1.8 [K]
6.5 [K]
7 [K]
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.5
0.6
0.7
0.8
0.9
1
H
H c2
eVp/2∆(H=0)
7.5 [K]
8 [K]
8.5 [K]
Figure 24: Scaling of voltage peak (H⊥), dashed lines are theoretical curves: black line is for
T=1.96 [K] and the blue is for T=7.3 [K]

Figure 24 presents data points using the second method and all lines coincide over a
large temperature range.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0
0.2
0.4
0.6
0.8
1
Parallel
H
H c2
dV/dIp/Rn
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0
0.2
0.4
0.6
0.8
1
H
H c2
dV/dIp/Rn
1.8 [K]
2.5 [K]
3.5 [K]
4.5 [K]
5.5 [K]
6.5 [K]
7 [K]
7.5 [K]
8 [K]
8.5 [K]
Figure 25: Scaling of tunneling resistance for all temperatures (H )
For this ﬁeld orientation no theoretical curves are available. For low temperatures all the
data points coincide except the data from T=4.5 [K], these data points was measured with
the wrong frequency. The dI
dV curves was not easy to analyze for this temperature. At T=7.5
[K] and after we see that the data points starts to deviate similar to the perpendicular ﬁeld
orientation.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0.6
0.7
0.8
0.9
1
Parallel
H
H c2
eVp/2∆(H=0)
1.8 [K]
2.5 [K]
3.5 [K]
5.5 [K]
0.1 0.2 0.3 0.4 0.5 0.6
0.75
0.8
0.85
0.9
0.95
1
H
H c2
eVp/2∆(H=0)
6.5 [K]
7 [K]
7.5 [K]
8 [K]
8.5 [K]
Figure 26: Scaling of voltage peak for all temperatures (H )
0 1 2 3 4 5 6 7 8
0
0.5
1
1.5
2
2.5
Eg(H = 0[T ]) vs Temperature
T [K]
Eg=2∆
e[mV]
H||
H⊥
Figure 27: Temperature dependence of the energy gap
In Figure 27 we show that the extracted Eg(H = 0) is independent of ﬁeld direction.
Which is what we expect since for clean superconductors (Nb). The energy gap according
to BCS-theory is ∆ =
2 vf
πξ [7], where vf is the Fermi velocity. The quantity is determined
by constants speciﬁc to materials.

1 2 3 4 5 6 7 8 9
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Parallel Field
T [K]
H[T]
Tunnel data (dI/dV) Hc2
Tunnel data (Gap) Hc2
MR data Hc3
Figure 28: Extracted critical fields for field parallel to sample, for T=4.5 [K] it was not possible
to extract the Hc2 since it was measured in wrong frequency.
1 2 3 4 5 6 7 8 9
0
0.5
1
1.5
2
2.5
3
Perpendicular field
T [K]
H[T]
Tunnel data (dI/dV) Hc2
Tunnel data (Gap) Hc2
MR data Hc3
Figure 29: Extracted critical fields for field perpendicular to sample
In the Figures 28 and 29 above we see that Hc2 decrease significantly upon reaching Tc.
The figure also show that Hc3 decrease linearly as a function of temperature. We can also
compare the temperature dependence of the upper critical fields from our measurements
with what we saw in theory see Figure 7. Where it is clear that the two upper critical
fields is starting to become indistinguishable near T=7 [K]. The extracted critical fields are

4.4 Angular-dependent magnetoresistance 27
smaller for perpendicular field orientation, in this particular alignment vortices are easily
formed since the field penetrates a larger area. And we know that vortices generates super
currents see Figure 6. Eventually superconducting state is destroyed by the high current
density generated by vortices.
1 2 3 4 5 6 7 8
1
1.2
1.4
1.6
1.8
2
Parallel Field
H3
Hc2
dI/dV
Eg
Theory
1 2 3 4 5 6 7 8
1
1.2
1.4
1.6
1.8
2
Perpendicular Field
T [K]
H3
Hc2
dI/dV
Eg
Theory
Figure 30: The temperature dependence of Hc3
Hc2
, error propagation is made to determine the
error in the ratio.
Both methods seems to estimate Hc2 quite accurately at low temperatures. However at
higher temperatures the method using the tunneling resistance method underestimates Hc2.
This is due to the fact that the method using the tunneling resistance is very sensitive to the
choice of normal tunneling resistance Rn at high temperatures. In the previous experiment[6]
it is reported that the Ohmic Rn is T independent. In this case it was not always trivial
to find Rn the data showed that Rn had slight field and temperature dependence. In fact
we wanted to eliminate this effect with the junction geometry of the fabricated sample to
establish a true four point probe which eliminates the parasitic electrode resistance. The
second method where we used the energy gap there is no room for bias since we do not
need know Rn to extract Hc2. Here we scale the Vp with the extracted gap value at zero
field from the experimental data. In Figure 30 we see the decrease in ratio as a function
of temperature, already at T=7 [K] the ratio is very close to 1 in parallel field orientation.
Since we were not able to measure junction properties at higher temperatures we were not
able to locate the tricritical point.
4.4 Angular-dependent magnetoresistance
Finally we wanted to explore the possibility to detect surface superconductivity by measuring
the resistance of a peace of electrode far away from the junction. Where there we do not
get junction contribution to resistance. By rotating the sample with respect to the field, we
vary the components of the field H and H⊥. Bulk superconductivity is 3D, as long as the
components do not exceed Hc2 bulk superconductivity prevails. Surface superconductivity
however is 2D it requires that the field is parallel to the surface. In the region Hc2 < H < Hc3
it is expected a cusp-like angle dependence similar to magenta curve in the left figure 31.
While bulk superconductivity would look like the black curve in the same figure.

-120 -100 -80 -60 -40 -20 0
0.00
0.05
0.10
0.15
0.20
0.25
-98 -96 -94 -92 -90 -88 -86 -84
0.00
0.05
0.10
0.15
0.20
0.25
H [T] =
2
2.5
2.75
2.8
2.85
2.9
3
Rac[Ohm]
angle [deg]
T=1.8 [K]
I [uA]=
100
200
400
1000
angle [deg]
Current dependence
H = 2.8 [T]
T = 1.8 [K]
Resistance vs angle (H)
Figure 31: Measuremenets where we show angle dependence of resistance left figure T=1.8 [K].
In the right figure we try to suppress surface superconductivity by increasing the current.
It is not easy to draw conclusions from the top right figure, where we study the current
dependence, since the measured resistance depends on the current we are sending through
the electrode. The current also generates a field which at I=100[µA] H=2.8 [T] and at
I=1[mA] field is 2.85 [T]. However we see a current dependence and this indicates that we
are suppressing superconducting state on the surface of the sample.

-120 -100 -80 -60 -40 -20 0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
-120 -100 -80 -60 -40 -20 0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
-120 -100 -80 -60 -40 -20 0 20
0.00
0.05
0.10
0.15
0.20
0.25
0.30
-120 -100 -80 -60 -40 -20 0 20
0.00
0.05
0.10
0.15
0.20
0.25
0.30
H [T]=
1.3
1.4
1.45
1.5
1.6
1.8
Rac[Ohm]
a)
H [T]=
0.8
0.93
1
1.3
b)
H [T]=
0.6
0.7
0.75
0.8
1
Rac[Ohm]
angle [deg]
c)
Resistance vs angle (H)
H [T]=
0.5
0.52
0.53
0.535
0.6
angle [deg]
d)
Figure 32: Angle dependence for a) T=5.5 [K] b) T=6.5 [K] c) T=7 [K] and d) T=7.5 [K]
The most clear trace of surface superconductivity is from the lowest measured tempera-
ture T=1.8 [K] the magenta and cyan curve. Seen as a sharp cusp in R(θ). The resistance is
not completely zero because the surface superconducting layer only carries a small portion
of the current the rest is sent through the normal bulk.

30
5 Conclusion
During this project I have learned nanoscale fabrication of Josephson junction using ad-
vanced nanofabrication techniques such as 3D FIB sculpturing, lithography and deposition
of thin films. I also learned measuring techniques in low temperatures and high magnetic
fields. I studied three different fabricated junctions and a commercial sample. The fab-
ricated junctions did not have a proper insulating layer. After analyzing the Fraunhofer
modulations we could observe traces of the junction. This revealed that we had shorts be-
tween the Nb electrodes. We found two plausible explanations to this: The bottom Nb layer
was unevenly sputtered, the tension between the top and bottom layer destroyed the oxide
layer creating shorts between the electrodes. Second possible explanation is that the top
Nb layer might have been sputtered too hard onto the oxide layer which destroyed it. The
focus of the work was investigation of high magnetic field properties of the Nb/Al-AlOx/Nb
tunnel junction. Preformed both magnetic transformed measurement and tunneling spec-
troscopy studies. Which provides direct information about the temperature dependence and
magnetic field dependence of the energy gap and thus allows to unambiguously determine
the upper critical fields of the electrode. At fields above the upper critical field we observe an
extended region of surface superconductivity which ended up at the third critical field Hc3.
We study the temperature dependence of both Hc2 and Hc3 for different field orientations
and then we observed that the ratio Hc3
Hc2
is decreasing approaching Tc. Which is consistent
with the existence of critical point for disappearance of surface super superconductivity.

31
Acknowledgements
I would like to express my deepest gratitude to the Experimental Condensed Matter Physics
Group for making me feel as a part of the team and for always being helpful. Thanks to
Andrey Boris and Thorsten Jacobs for all the help with sample characterization of the fabri-
cated samples. Thank you Donato Campanini and Zhu Diao for helping me with OriginLab
and for being supportive and helpful to me. Holger Motzkau thank you for explaining how
the cryostat works and for answering my questions conserning the measurements. Adrian
Iovan thank you for the help with the FIB sculpturing when Taras was on leave I learned a
lot from that session. A very special thanks to Vladimir Krasnov for always being supportive
and leading me in the right direction whenever I was lost. Thank you for your patience and
for always having your oﬃce open for me. You have been an amazing teacher. The biggest
thanks of all goes to Taras Golod you are truly one of the most generous persons I have
ever meet. Thank you for spending over a month (sometimes until very late at night) with
us in the cleanroom and teaching me everything I know about micro- and nanofabrication
techniques. You were also there during the measurements sharing tips and tricks about
measurement techniques and LabVIEW for this I am very grateful.

32
A Appendix
Power supply: RF=Radio frequency. DC=Direct curren. [sccm]=Standard cubic centimeter
1. Substrate cleaning
RF substrate = 25 [W]
Ar pressure 5 [mTorr] @ 25 [sccm]
Time: 5 min
2. Nb pre cleaning
DC = 250 [W]
Ar preassure 5 [mTorr] @ 25 [sccm]
Time: 5 min
3. Nb deposition
DC = 250 [W]
Time: 31 min 11 sec
4. Al pre cleaning
same as 2.
5. Al deposition
DC = 150 [W]
Time: 17 min 40 sec
6. Oxidation
O2 preasssure 20 [mTorr] @ 10 [sccm]
Time: 2 Hrs
7. Nb pre cleaning
same as 2 without RF.
8. Nb deposition
Same as 3. except without RF.
Time: 26 min 32 sec

33
B Appendix
0.0 0.2 0.4 0.6
0.00
0.02
0.04
0.06
0.08
0.10
0.0 0.5 1.0 1.5 2.0
0.00
0.05
0.10
0.15
0.20
1 2
0.0
0.1
0.2
0.3
0.0 0.5 1.0 1.5 2.0 2.5
0.0
0.1
0.2
0.3
0.4
0 1 2 3
0.0
0.1
0.2
0.3
0.4
0 1 2 3 4
0.0
0.1
0.2
0.3
0.4
I[mA]
H [T]=
0.005
0.01
0.02
0.03
T=8.5 [K]
H [T]=
0.003
0.008
0.03
0.05
0.08
0.11
0.13
T= 8[K]
I[mA]
H [T]=
0.003
0.01
0.02
0.04
0.07
0.09
0.11
0.16
0.23
T=7.5 [K] T=7 [K]
H [T]=
0.003
0.02
0.03
0.05
0.07
0.10
0.13
0.16
0.19
0.24
0.31
0.37
Voltage [mV]
I[mA]
Voltage [mV]
H [T]=
0.003
0.02
0.045
0.067
0.1
0.12
0.15
0.19
0.24
0.31
0.39
0.43
0.5
T=6.5 [K]
H [T]=
0.01
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.9
1.1
1.31
1.41
T=1.8 [K]
IV Characteristic (Perpendicular Field)
Figure 33: IV measurements perpendicular ﬁeld

34
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.00
0.02
0.04
0.06
0.08
0.10
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.00
0.05
0.10
0.15
0.20
0.25
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.00
0.05
0.10
0.15
0.20
0.25
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
T=8.5 [K]
I[mA]
H [T] =
0.006
0.024
0.043
0.064
T=8 [K]
H [T]=
0.006
0.09
0.12
0.15
0.20
T=7.5 [K]
I[mA]
H [T]=
0.009
0.09
0.12
0.16
0.21
0.27
0.34
0.45
H [T] =
0.002
0.07
0.11
0.15
0.20
0.25
0.30
0.40
0.50
0.55
T=5.5 [K]
T=6.5 [K]
T=7 [K]
I[mA]
Voltage [mV]
H [T] =
0.007
0.08
0.12
0.15
0.25
0.30
0.35
0.41
0.47
0.53
0.64
IV Characteristic (Parallel Field)
Voltage [mV]
H [T] =
0.008
0.11
0.16
0.21
0.43
0.55
0.69
0.78
0.95
1.1
Figure 34: IV measurements parallel ﬁeld
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0
0.1
0.2
0.3
0.4
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0
0.1
0.2
0.3
0.4
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0
0.1
0.2
0.3
0.4
0.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.0
0.1
0.2
0.3
0.4
0.5
T=1.8 [K]T=2.5 [K]
T=3.5 [K]T=4.5 [K]
I[mA]
H [T] =
0.007
0.09
0.15
0.33
0.43
0.55
0.69
0.85
1.06
1.25
1.37
H [T] =
0.0035
0.10
0.15
0.30
0.46
0.71
0.90
1.12
1.32
1.72
IV Characteristic (Parallel Field)
I[mA]
Voltage [mV]
H [T]=
0.002
0.05
0.11
0.15
0.31
0.51
0.71
0.90
1.02
1.22
1.42
1.62
1.94
Voltage [mV]
H [T] =
0.008
0.06
0.072
0.10
0.19
0.40
0.50
0.60
0.91
1.12
1.32
1.72
2.1
Figure 35: IV characteristic at 4.5 [K] was measured with wrong frequency, it was not possible
to extract the energy gap. However it was possible to extract the conductance.

35
C Errata
The deﬁnition of Josephson junctions at the end of introduction is misplaced this paragraph
should be moved to row 14 page 3.
Figure 15 a) is not the actual sample.

REFERENCES 36
References
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1976.
[4] S. O. Katterwe, Th. Jacobs, A. Maljuk, and V. M. Krasnov. Low anisotropy of the upper
critical eld in a strongly anisotropic layered cuprate: Evidence for paramagnetically
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[5] C. W. Leung, C. Bell, G. Burnell, and M. G. Blamire. In situ fabrication of a cross-
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[10] Michael Tinkham. Introduction to superconductivity. Dover Publications, 2004.
[11] http://encyclopedia2.thefreedictionary.com/Superconductivity.
[12] James F. Annett. Superconductivity, Superfluids, and Condensates. Oxford University
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[13] S. R. Park, S. M. Choi, D. C. Dender, J. W. Lynn, and X. S. Ling. Fate of the peak
effect in a type-ii superconductor: Multicriticality in the bragg-glass transition. Phys.
Rev. Lett., 91:167003, Oct 2003.
[14] Pradip Das, C. V. Tomy, S. S. Banerjee, H. Takeya, S. Ramakrishnan, and A. K.
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Dec 2008.
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[18] Taras Golod. Mesoscopic phenomena in hybrid superconductor/ferromagnet structures.
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