Superconductor-Insulator-Superconductor
Encyclopedia
The superconducting tunnel junction (STJ) — also known as a superconductor–insulator–superconductor tunnel junction (SIS) — is an electronic
device consisting of two superconductors separated by a very thin layer of insulating material. Current passes through the junction via the process of quantum tunneling. The STJ is a type of Josephson junction. These devices have a wide range of applications, including high-sensitivity detectors
of electromagnetic radiation
, superconducting quantum interference devices (SQUIDs), high speed digital circuit elements, and superconducting quantum computing
circuits.
, first predicted by Brian David Josephson
in 1962. For this prediction, Josephson received the Nobel prize in physics
in 1973. The second is the quasiparticle
current, which, in the limit of zero temperature, arises when the energy from the bias voltage
exceeds twice the value of superconducting energy gap Δ. At finite temperature, a small quasiparticle tunneling current — called the subgap current — is present even for voltages less than twice the energy gap due to the thermal promotion of quasiparticles above the gap.
If the STJ is irradiated with photons of frequency
, the dc current-voltage curve will exhibit both Shapiro steps and steps due to photon-assisted tunneling. Shapiro steps are constant-voltage steps in the supercurrent at voltages equal to 
, where 
is Planck's constant, 
is the electron
charge, and
is an integer
. Photon-assisted tunneling is seen as steps in the quasiparticle current at voltages equal to
relative to the gap voltage.
. The deposition is performed inside a vacuum chamber
. Oxygen
gas is then introduced into the chamber, resulting in the formation of an insulating layer of aluminum oxide (
) with a typical thickness of several nanometers. After the vacuum is restored, an overlapping layer of superconducting metal is deposited, completing the STJ. To create a well-defined overlap region, a procedure known as the Dolan bridge technique is commonly used. This technique uses a suspended bridge of resist
with a double-angle deposition to define the junction.
Aluminum is widely used for making superconducting tunnel junctions because of its unique ability to form a very thin (2-3 nm) insulating oxide layer with no defects that short-circuit the insulating layer. The superconducting critical temperature of aluminum is approximately 1.2 kelvin
(K). For many applications, it is convenient to have a device that is superconducting at a higher temperature, in particular at a temperature above the boiling point
of liquid helium
, which is 4.2 K at atmospheric pressure. One approach to achieving this is to use niobium
, which has a superconducting critical temperature in bulk form of 9.3 K. Niobium, however, does not form an oxide that is suitable for making tunnel junctions. To form an insulating oxide, the first layer of niobium can be coated with a very thin layer (approximately 5 nm) of aluminum, which is then oxidized to form a high quality aluminum oxide tunnel barrier before the final layer of niobium is deposited. The thin aluminum layer is proximitized by the thicker niobium, and the resulting device has a superconducting critical temperature above 4.2 K. Early work used lead
-lead oxide-lead tunnel junctions. Lead
has a superconducting critical temperature of 7.2 K in bulk form, but lead oxide tends to develop defects (sometimes called pinhole defects) that short-circuit the tunnel barrier when the device is thermally cycled between cryogenic temperatures and room temperature, and as result lead is no longer widely used to make STJs.
receivers in the 100 GHz to 1000 GHz frequency range, and hence are used for radio astronomy
at these frequencies. In this application, the STJ is dc biased
at a voltage just below the gap voltage (|V| = 2Δ/e). A high frequency signal from an astronomical object of interest is focused onto the STJ, along with a local oscillator
source. Photons absorbed by the STJ allow quasiparticles to tunnel via the process of photon-assisted tunneling. This photon-assisted tunneling changes the current-voltage curve, creating a nonlinearity that produces an output at the difference frequency of the astronomical signal and the local oscillator. This output is a frequency down-converted version of the astronomical signal. These receivers are so sensitive that an accurate description of the device performance must take into account the effects of quantum noise
.
detection, STJs can also be used as direct detectors. In this application, the STJ is biased with a dc voltage less than the gap voltage. A photon
absorbed in the superconductor breaks Cooper pairs and creates quasiparticles. The quasiparticles tunnel across the junction in the direction of the applied voltage, and the resulting tunneling current is proportional to the photon energy. STJ devices have been employed as single-photon detectors for photon frequencies ranging from X-rays to the infrared
.
is based on a superconducting loop containing Josephson junctions. SQUIDs are the world's most sensitive magnetometer
s, capable of measuring a single magnetic flux quantum
.
utilizes STJ-based circuits, including charge qubit
s, flux qubit
s and phase qubit
s.
or RSFQ fast logic circuits.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
device consisting of two superconductors separated by a very thin layer of insulating material. Current passes through the junction via the process of quantum tunneling. The STJ is a type of Josephson junction. These devices have a wide range of applications, including high-sensitivity detectors
Cryogenic particle detectors
Cryogenic particle detectors operate at very low temperature, typically only a few degrees above absolute zero. These sensors interact with an energetic elementary particle and deliver a signal which can be related to the type of particle and the nature of the interaction...
of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
, superconducting quantum interference devices (SQUIDs), high speed digital circuit elements, and superconducting quantum computing
Superconducting quantum computing
Superconducting quantum computing is a promising implementation of quantum information that involves nanofabricated superconducting electrodes coupled through Josephson junctions...
circuits.
Quantum tunneling
All currents flowing through the STJ pass through the insulating layer via the process of quantum tunneling. There are two components to the tunneling current. The first is from the tunneling of Cooper pairs. This supercurrent is described by the ac and dc Josephson relationsJosephson effect
The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...
, first predicted by Brian David Josephson
Brian David Josephson
Brian David Josephson, FRS is a Welsh physicist. He became a Nobel Prize laureate in 1973 for the prediction of the eponymous Josephson effect....
in 1962. For this prediction, Josephson received the Nobel prize in physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
in 1973. The second is the quasiparticle
Quasiparticle
In physics, quasiparticles are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space...
current, which, in the limit of zero temperature, arises when the energy from the bias voltage
exceeds twice the value of superconducting energy gap Δ. At finite temperature, a small quasiparticle tunneling current — called the subgap current — is present even for voltages less than twice the energy gap due to the thermal promotion of quasiparticles above the gap.If the STJ is irradiated with photons of frequency
, the dc current-voltage curve will exhibit both Shapiro steps and steps due to photon-assisted tunneling. Shapiro steps are constant-voltage steps in the supercurrent at voltages equal to , where is Planck's constant, is the electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
charge, and
is an integerInteger
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
. Photon-assisted tunneling is seen as steps in the quasiparticle current at voltages equal to
relative to the gap voltage.Device fabrication
The device is typically fabricated by first depositing a thin film of a superconducting metal such as aluminum on an insulating substrate such as siliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
. The deposition is performed inside a vacuum chamber
Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space...
. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
gas is then introduced into the chamber, resulting in the formation of an insulating layer of aluminum oxide (
) with a typical thickness of several nanometers. After the vacuum is restored, an overlapping layer of superconducting metal is deposited, completing the STJ. To create a well-defined overlap region, a procedure known as the Dolan bridge technique is commonly used. This technique uses a suspended bridge of resistResist
In semiconductor fabrication, a resist is a thin layer used to transfer a circuit pattern to the semiconductor substrate which it is deposited upon. A resist can be patterned via lithography to form a micrometer-scale, temporary mask that protects selected areas of the underlying substrate during...
with a double-angle deposition to define the junction.
Aluminum is widely used for making superconducting tunnel junctions because of its unique ability to form a very thin (2-3 nm) insulating oxide layer with no defects that short-circuit the insulating layer. The superconducting critical temperature of aluminum is approximately 1.2 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(K). For many applications, it is convenient to have a device that is superconducting at a higher temperature, in particular at a temperature above the boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of liquid helium
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
, which is 4.2 K at atmospheric pressure. One approach to achieving this is to use niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
, which has a superconducting critical temperature in bulk form of 9.3 K. Niobium, however, does not form an oxide that is suitable for making tunnel junctions. To form an insulating oxide, the first layer of niobium can be coated with a very thin layer (approximately 5 nm) of aluminum, which is then oxidized to form a high quality aluminum oxide tunnel barrier before the final layer of niobium is deposited. The thin aluminum layer is proximitized by the thicker niobium, and the resulting device has a superconducting critical temperature above 4.2 K. Early work used lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
-lead oxide-lead tunnel junctions. Lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
has a superconducting critical temperature of 7.2 K in bulk form, but lead oxide tends to develop defects (sometimes called pinhole defects) that short-circuit the tunnel barrier when the device is thermally cycled between cryogenic temperatures and room temperature, and as result lead is no longer widely used to make STJs.
Radio astronomy
STJs are the most sensitive heterodyneHeterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
receivers in the 100 GHz to 1000 GHz frequency range, and hence are used for radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
at these frequencies. In this application, the STJ is dc biased
DC bias
When describing a periodic function in the frequency domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform...
at a voltage just below the gap voltage (|V| = 2Δ/e). A high frequency signal from an astronomical object of interest is focused onto the STJ, along with a local oscillator
Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
source. Photons absorbed by the STJ allow quasiparticles to tunnel via the process of photon-assisted tunneling. This photon-assisted tunneling changes the current-voltage curve, creating a nonlinearity that produces an output at the difference frequency of the astronomical signal and the local oscillator. This output is a frequency down-converted version of the astronomical signal. These receivers are so sensitive that an accurate description of the device performance must take into account the effects of quantum noise
Quantum noise
Quantum noise is uncertainty of some physical quantity due to its quantum origin.In the case of number of particles , the quantum noise is also called shot noise. Most optical communications use amplitude modulation...
.
Single-photon detection
In addition to heterodyneHeterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
detection, STJs can also be used as direct detectors. In this application, the STJ is biased with a dc voltage less than the gap voltage. A photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
absorbed in the superconductor breaks Cooper pairs and creates quasiparticles. The quasiparticles tunnel across the junction in the direction of the applied voltage, and the resulting tunneling current is proportional to the photon energy. STJ devices have been employed as single-photon detectors for photon frequencies ranging from X-rays to the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
.
SQUIDs
The superconducting quantum interference device or SQUIDSQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
is based on a superconducting loop containing Josephson junctions. SQUIDs are the world's most sensitive magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
s, capable of measuring a single magnetic flux quantum
Magnetic flux quantum
The magnetic flux quantum Φ0 is the quantum of magnetic flux passing through a superconductor. The phenomenon of flux quantization was discovered B. S. Deaver and W. M. Fairbank and, independently, by R. Doll and M. Nabauer, in 1961...
.
Quantum computing
Superconducting quantum computingSuperconducting quantum computing
Superconducting quantum computing is a promising implementation of quantum information that involves nanofabricated superconducting electrodes coupled through Josephson junctions...
utilizes STJ-based circuits, including charge qubit
Charge qubit
In quantum computing, a charge qubit is a superconducting qubit whose basis states are charge states . A charge qubit is formed by a tiny superconducting island coupled by a Josephson junction to a superconducting reservoir...
s, flux qubit
Flux qubit
In quantum computing, flux qubits are micrometer sized loops of superconducting metal interrupted by a number of Josephson junctions. The junction parameters are engineered during fabrication so that a persistent current will flow continuously when an external flux is applied...
s and phase qubit
Phase qubit
The phase qubit is a superconducting device based on the superconductor-insulator-superconductor Josephson junction, designed to operate as a quantum bit, or qubit. The phase qubit is closely related, yet distinct from, the flux qubit and the charge qubit, which are also quantum bits implemented...
s.
RSFQ
The STJ is the primary active element in rapid single flux quantumRapid single flux quantum
In electronics, rapid single flux quantum is a digital electronics technology that relies on quantum effects in superconducting devices, namely Josephson junctions, to process digital signals. Josephson junctions are the active elements for RSFQ electronics, like transistors are the active...
or RSFQ fast logic circuits.
Josephson voltage standard
Shapiro steps arise from Cooper pairs synchronizing with an external source of electromagnetic radiation and can be used to convert a frequency to a dc voltage. Because frequency can be measured with very high precision, this effect is used as the basis of the Josephson voltage standard, which defines the international standard volt.See also
- SuperconductivitySuperconductivitySuperconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
- Josephson effectJosephson effectThe Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...
- Quantum tunneling
- Superconducting quantum interference device (SQUID)
- Superconducting quantum computingSuperconducting quantum computingSuperconducting quantum computing is a promising implementation of quantum information that involves nanofabricated superconducting electrodes coupled through Josephson junctions...
- Rapid single flux quantumRapid single flux quantumIn electronics, rapid single flux quantum is a digital electronics technology that relies on quantum effects in superconducting devices, namely Josephson junctions, to process digital signals. Josephson junctions are the active elements for RSFQ electronics, like transistors are the active...
(RSFQ) - Cryogenic particle detectorsCryogenic particle detectorsCryogenic particle detectors operate at very low temperature, typically only a few degrees above absolute zero. These sensors interact with an energetic elementary particle and deliver a signal which can be related to the type of particle and the nature of the interaction...