Instrumentation & Measurement Magazine 23-2 - 6

beyond dc. PJVS have been proven effective in many applications such as traveling standards for international
comparisons, in a watt balance, in quantum impedance and
power standards. They provide dc and ac peak voltages in excess of 10 V, up to several kHz. PJVS systems are commercially
available [21], [22].
AC Josephson Voltage Standard (ACJVS) consist of a
single array driven by a frequency modulated pulsed radio frequency (RF) signal with typical clock frequency 15 GHz. JAWS
can generate spectrally pure ac signals up to 2 V and 1 MHz. It
has been shown that the use of short pulses instead of a sinusoidal RF signal allows modulating the signal effectively over
a wide range of frequencies, keeping junctions on a quantized
step [4]. The output voltage is then exactly determined by the
knowledge of the pulse repetition rate.
The LTS Programmable Josephson ac Voltage Standards
and pulse-driven ac Josephson Voltage Standard are replacing thermal voltage converters used to reproduce the ac volt
in terms of dc volt, especially at low frequencies (<10 kHz)
[23]-[25] where the determination of metrological parameters
of thermal converters is troublesome [26]. However, the thermal converters will still play an important role in ac voltage
metrology, at least in the next decade, in particular above 100
kHz [27], [28].

Cryocooling Methods
The LTS JJs used in quantum voltage standards operate around
4 K, i.e., the temperature of boiling 4He. In general, two cooling
methods are used to achieve such low temperatures: passive
or active. In the former, the JJ is immersed in a bath cryostat in
the form of a Dewar container filled with liquid helium. Unfortunately, high cost and short supply of helium make the
operation of passive-cooled LTS quantum voltage standards
expensive. The active method uses cryocoolers [29]. Lately,
extensive work was performed to model and optimize the operation of the LTS cryocoolers [30], [31]. However, operation
at 4 K requires expensive and energy consuming compressors
(typically a few kW), and requires mastery of difficult cryogenic techniques. A completely different landscape would be
provided by HTS arrays operating at 77 K, the liquid nitrogen
boiling temperature. Cryocoolers operating at 77 K are much
smaller, and consume less energy (about 100 W), so it would be
possible to include a quantum standard for ac and dc into e.g.,
a commercial voltmeter. This argument alone is sufficient to
show the enormous impact from the development of HTS Josephson arrays.

Application of High-temperature
Superconductor Josephson Junctions
Non-hysteretic junctions used in modern voltage standards [4]
are naturally available in high-temperature superconductor
technology. Series arrays of shunted YBCO bicrystal Josephson junctions were used to generate dc voltages at elevated
temperatures [32]. Quantum voltages of an array of YBCO
bicrystal junctions were calibrated against a PJVS of the Physikalisch-Technische Bundesanstalt [33]. The coincidence of
6

quantum voltages of the array of HTS junctions at 64 K and of
the array of niobium junctions at 4.2 K was measured with an
uncertainty of 1.7 parts in 108. This result has shown that arrays of HTS junctions operate properly and can be used for
high-precision measurements. At the same time, these junctions have large critical currents, and first current steps, mA, at
temperatures up to 80 K, and provide stability against noise as
well as a large output current.
For large scale application of HTS JJs in cryoelectronics, both the development of the deposition technology of
thin HTS epitaxial films and the technology to fabricate JJs
were necessary. An important step in the development of basic and applied research was the development of deposition
techniques to grow epitaxial films of the most used high-temperature superconductor YBCO. This technological success,
as well as the subsequent demonstration of very low microwave losses in YBCO thin films, initiated the development of
thin-film passive microwave devices that are currently used in
nuclear magnetic resonance or magnetic resonance imaging
(receiving coils), in mobile communication systems (resonators and filters) and other passive microwave systems [7]. For
the fabrication of a JJ, it is necessary to create a weak link between two superconducting electrodes. As the material for
the weak link, dielectrics, normal metals, or superconductors with a lower critical temperature compared to electrodes
can be used. The length of this connection, L, is limited by several coherence lengths ξ, in the ideal case L ≤ 3ξ [34]. Unlike
low-temperature superconductors, HTS are characterized by
strong anisotropy of superconducting properties. Mainly used
YBCO single crystal films with orthorhombic crystallographic
structure have a coherence length ξYBCO equal to 1.5 nm in a
plane parallel to the substrate. This coherence length is much
shorter as compared with ξNb value for Nb which is equal to 40
nm. A very small coherence length ξYBCO is a most important
limitation in the development of JJs technology. The traditional
methods for creating weak links, which are widespread in the
manufacture of JJs from LTS, such as niobium, are practically
difficult to implement. In particular, the HTS SIS junctions
with hysteretic IVC were not demonstrated up to now. Additional restrictions on technology are imposed by the need to
create arrays of JJs with a small spread of Ic and Rn. Moreover,
the performance parameters Ic, Rn and fc should be adjusted for
the specific applications at elevated temperatures, e.g., higher
than 55K, where a small scale cryocooler can be used.
Different routes to make reliable HTS JJ have been
explored, among them the most successful one, at least for metrology applications, is the "bicrystal" technology described
below. Alternative techniques are still under development,
as the promising He FIB one presented thereafter to make
nanojunctions.

Application of Bicrystal Josephson Junctions
For growing single-crystal YBCO films, substrates with the
parameters of the crystal lattices close to the film crystal
parameters are required. YBCO single crystals have an orthorhombic lattice with the parameters: a = 3.82 Å, b = 3.88 Å,

IEEE Instrumentation & Measurement Magazine

April 2020

Instrumentation & Measurement Magazine 23-2

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 23-2