Instrumentation & Measurement Magazine 23-2 - 9

condensate has a d-wave symmetry, as opposed to most of
conventional superconductors. As a consequence, any crystallographic defect acts as a "depairing center," i.e., a place where
superconducting properties are weakened. An increasing density of defects lowers the superconducting critical temperature
Tc and, beyond the threshold, makes the material metallic,
even when insulating at large dose, because the crystalline
structure of the material is too altered to keep its metallicity.
It is therefore possible to fabricate a JJ, which is a weak link
between two superconducting reservoirs separated by a nonsuperconducting material, by locally disordering the high Tc
material on a scale which is on the order of a few tens of nanometers, to keep phase coherence across the junction.
Ion irradiation has been used to create such a local disorder. High-energy ions (typically 100 keV oxygen) are sent onto
a high Tc (very often YBCO) channel made as a thin film (50
to 150 nm thick) which is protected by a resistor, except in its
central part, where a tiny aperture (20 to 40 nm) lets the ions
penetrate the sample and locally increase the defect density.
For a proper dose in the range of a few 1013 oxygen ions/cm2,
Tc is depressed and the Josephson coupling is observed below
a temperature TJ, which depends on the irradiation conditions. Such a JJ displays nice characteristics [41], which have
been used in different applications such as dc SQUIDs [42],
high frequency (400 GHz) mixers [43] or RF magnetometers
[44]. This technique used to create high Tc JJ is interesting for
applications which require a large number of JJs and rather
complex design. However, the Josephson characteristic voltage IcRn is moderate (in the range of a few 100 μV) which is
too low for some applications, and the spread in parameters
(in the 10% range for Ic and Rn) is a bit large for very demanding applications.
Recently, S. A. Cybart and his collaborators at University
of California San-Diego developed a parent technique [45],
where the local ion irradiation is performed directly on the thin
film in a Zeiss-Orion ion microscope. This advanced Focused
Ion Beam (FIB) system allows the direct patterning of JJ by introducing the defects with a sub-nanometer 30 keV helium
beam. Because helium is light, ion straggling in the YBCO thin
film is reduced, and the resolution of the defect density profile
is much better than with the high energy oxygen irradiation
technique. Since the ion energy is rather low, and therefore the
range of the ions in matter is limited, thinner YBCO films must
be used with typical thickness of 50 nm or lower. Cybart and
co-workers convincingly demonstrated that high-Tc JJ can be
fabricated with this technique [45], in which TJ, Ic and Rn can be
adjusted with the He dose.
In the first series of experiments, they made two types of
JJ, depending on the ion dose. For low dose, typically 2·1016
ions/cm2, the irradiated part is essentially a normal metal (N)
above TJ, with a decreasing resistance as the temperature is
lowered. The JJ is therefore of SNS type, with a critical current
Ic rising up quadratically below TJ as expected, and I-V characteristics very close to the conventional Resistively Shunted
Junction (RSJ) model. For higher dose, beyond 6·1016 ions/cm2,
the disordered material shows an insulating (I) behavior with
April 2020	

an increasing resistance at low temperature. This SIS type JJ
has a linearly increasing Ic at low temperature and does not
display hysteresis because of its very small capacitance. In
this tunnel junction, the dynamical resistance dV/dI displays
a gap like feature which is reminiscent of the gap opening in
the density of states of a superconductor below Tc. A temperature dependence following the Barden-Cooper-Schrieffer
(BCS) theory is reported, with a zero-temperature gap value
of around 30 mV.
Recent experiments performed by the ESPCI Paris team
confirm that the He FIB technique produces interesting and
versatile high-Tc JJ [46]. In these experiments, standard highenergy ion irradiation technique is used to fabricate a 4 μm
wide YBCO channel in a 50 nm thick YBCO film. The JJ is then
created by means of a 0.7 nm diameter He ion beam at 30 kV
scanned across the channel. Fig. 4 shows the resistance as a
function of temperature of a junction made with a dose of 400
ions/nm introduced in a single scan. Below the critical temperature Tc of the reservoirs, a resistance plateau develops until
Josephson coupling takes place at TJ. Below this temperature,
the I-V characteristics of the JJ follows closely the standard RSJ
model (inset in Fig. 4).
One advantage of these JJs is the wide range of parameters
accessible by changing the ion dose, as compared to high energy ion irradiated ones. For a 4 μm wide channel, Ic can vary
from 1 μA to more than 1 mA, Rn from a few hundred mΩ to
10 Ω, and the IcRn product can reach almost 1 mV at low temperature [46].
Another decisive advantage of these He FIB-made JJs is the
possibility of placing several junctions at nanometric distances
to each other, to make compact 1D arrays with very good characteristics. Experiments conducted by L. Kasaei et al. on the
superconductor MgB2 showed that JJ arrays made by He FIB
have the lowest spread in critical current (∼3.5 %) ever recorded
in this material [47]. Correlatively, they obtained flat giant Shapiro steps under microwave irradiation at 12 GHz for 60 JJ in

Fig. 4. Resistance versus temperature curve of a JJ irradiated with 400 ions/
nm. The critical temperature of the reservoirs Tc and the Josephson coupling
temperature TJ are shown. Insert: Current-Voltage characteristic of the JJ
measured at T = 60 K.

IEEE Instrumentation & Measurement Magazine	9



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