SQUID
SQUID is an acronym for English s uperconducting qu antum i nterference d evice (dt. Superconducting quantum interference device ). A SQUID is a sensor for the very precise measurement of extremely small changes in the magnetic field. Based on the theoretical work of Brian D. Josephson , the experimental realization was successfully implemented in 1964 in the Ford Research Labs by Robert Jaklevic, John J. Lambe, James Mercereau and Arnold Silver.
construction
A SQUID consists of a superconducting ring that is interrupted at one point ( rf -SQUID, sometimes also called ac -SQUID) or two points ( dc -SQUID) by a normally conducting or electrically insulating material. This interruption must be so thin that the superconducting electron pairs (the Cooper pairs ) can tunnel through this gap . Such tunnel contacts are called Josephson contacts .
functionality
The functionality of a SQUID is based on the effect of flux quantization in superconducting rings and the Josephson effect . For quantum mechanical reasons, only a magnetic flux can flow through a superconducting ring, the size of which is an integral multiple of the elementary magnetic flux quantum Φ 0 = 2.07 × 10 −15 Vs. If the external magnetic field changes, an electrical circulating current is excited in the ring, which is exactly large enough to increase or decrease the magnetic flux in the superconducting ring to the nearest multiple of the flux quantum. This magnetic field-dependent change in the current is difficult to detect in a simple superconducting ring, which is why the Josephson effect is used. In the superconducting ring (in the case of the dc-SQUID) two Josephson contacts are inserted, which divides the ring into two parts. Now the two ring parts are contacted and a direct current is passed through the SQUID. This causes a measurable electrical voltage to drop across the SQUID . This depends on the externally applied direct current, but also on the compensation currents that flow in the superconducting ring due to the flux quantization.
If the external magnetic field changes, the current in the ring changes and with it the voltage at the dc-SQUID. The flux-voltage characteristic of the SQUID is periodic (approximately sinusoidal ) and the period is exactly one magnetic flux quantum.
The functionality of an rf-SQUID is based on the same effects, only that the bias current is not a direct current, but an alternating current in the frequency range of a few 10 megahertz. This is not applied directly to the SQUID, but is coupled inductively via a coil. It is also read out via this coil.
production
Most SQUIDs today are manufactured using thin-film technology ( sputtering or laser ablation ).
Different materials are used in the production of SQUIDs, which become superconducting at different temperatures. With conventional SQUIDs, classic superconductors made of metals or metal compounds with transition temperatures of up to 40 K (−233.15 ° C) are used. A very common material for conventional SQUIDs is niobium , which has a transition temperature of 9.5 K (−263.65 ° C) and, for cooling it to operating temperature, usually liquid helium with a temperature of approx. 4 K (−269.15 ° C) is used.
Another group are SQUIDs, which are made of high-temperature superconductors : they consist of ceramic metal oxides, which have transition temperatures of up to approx. 140 K (−133.15 ° C). The use of high-temperature superconductors means that complex and cost-intensive cooling using liquid helium can be dispensed with; instead, liquid nitrogen , which is easier to obtain , 77 K (−196.15 ° C) is used.
Although operating costs can be saved through the use of high-temperature superconductors, it should be noted that the crystalline material results in a complex, error-prone and correspondingly expensive production process.
SQUIDs made from high-temperature superconductors have a significantly higher 1 / f noise compared to conventional SQUIDs due to internal effects . In recent years, however, significant advances have been made in this area through targeted changes in the manufacturing process.
business
Due to the periodicity of the flux-voltage characteristic, a SQUID cannot measure absolute values of magnetic field strengths, only changes in field strength . If you want to measure flux changes that are greater than a flux quantum, electronics must be connected downstream of the SQUID, which, via an induction coil, compensates for the respective flux change in the SQUID ring and thus operates it at a fixed operating point . Such electronics are called a flow control loop .
Due to omnipresent magnetic background fields (for example the earth's magnetic field , but also interference from power lines and electrical devices in the vicinity), a SQUID is constantly exposed to strong interference. In order to suppress this to some extent, the measurement can either be carried out in a magnetically shielded environment or, for example, two SQUIDs can be coupled close to each other and opposite one another (SQUID gradiometer) in order to only perceive fields that originate in the immediate vicinity of the SQUID.
Applications
SQUIDs allow the highly accurate measurement of the magnetic flux. SQUID susceptometers are used to measure the magnetic properties of matter.
In medicine , SQUIDs are used to measure the magnetic fields generated by currents in the human body, e.g. B. brain waves ( magnetoencephalography [MEG]) or heart waves ( magnetocardiography [MKG]). They are also used to detect nuclear magnetic resonances in weak magnetic fields, which opens up a further field of application in medicine, namely the creation of magnetic resonance tomographies .
In geology and archeology , SQUIDs are used to determine very fine changes in the earth's magnetic field on the surface. This makes it possible to discover subterranean structures (geological layers, ore deposits or structures of building remains) that cannot be determined using other methods. SQUIDs are also used for non-destructive material testing. Scanning SQUID microscopes and SQUID-detected eddy current testing methods should be mentioned in particular . In addition, SQUIDs are used as highly sensitive measuring amplifiers ( SQUID amplifiers ).
Many cryo-detectors are currently using SQUIDs as the basis of their readout electronics. An example of this is the CRESST experiment to search for WIMPs (a possible constituent of dark matter).
In recent times there are research projects with the aim of using rf-SQUIDS as qubits for quantum computers .
Others
The electrostatic analogue of the SQUID is the single electron transistor (SET).
Individual evidence
- ↑ US Patent US3363200 A (filed 1964 / granted 1967)
- ^ Ann Johnson: How Ford invented the SQUID . IEEE Spectrum, No. 11.14, Posted 27 Oct 2014 (English)
- ^ Robin Cantor, Frank Ludwig: SQUID Fabrication Technology . In: J. Clarke, AI Braginski (Ed.): The SQUID Handbook , Volume Vol. I: Fundamentals and Technology of SQUIDs and SQUID Systems. Wiley-VCH, Weinheim 2004, ISBN 3-527-40229-2 , pp. 93-118.
literature
- John Clarke, Alex I. Braginski: The SQUID Handbook, Vol.1: Fundamentals and Technology of SQUIDs and SQUID Systems, Wiley-VCH, 2004, ISBN 3527402292
- John Clarke, Alex I. Braginski: The SQUID Handbook, Vol.2: Applications, Wiley-VCH, 2006, ISBN 3527404082
- Werner Buckel, Reinhold Kleiner: Superconductivity Basics and Application, Wiley-VCH, 2004 (6th edition), ISBN 3527403485
