Another discovery in superconductivity was made in 1933 by Germany’s
Meissner and Oschenfeld, who found that superconductors not only lacked resistance but demonstrated a self-repulsion effect in which a magnetic field inside the
superconductor was expelled outward. Now called the Meissner effect, it is recognized alongside the absence of resistance as one of the most basic characteristics
of superconductivity.
The first to successfully ascertain the cause of superconductivity were the U.S.
scientists Bardeen, Cooper, and Schrieffer. Their theory, which was announced in
1957, came to be known as the BCS Theory after their initials. It was a somewhat
complex theory, holding that lattice vibration occurred as electrons of opposite spin
joined into “Cooper pairs,” and that while electrons always moved with the same
energy at opposing speeds at the relatively macro-level scale, they flowed within the
lattice with a general lack of resistance as it vibrated. The three were awarded the
1972 Nobel Prize in Physics. While lattice vibration in a material typically produces
resistance to electrical current, in a superconducting state it produces Cooper pairs,
and electrons in this state slip out of the atom’s lattice as though surfing waves.
B. Types of Superconductors
Superconductors come in two main types, known as Type I and Type II. Type I is
the kind of superconducting material initially discovered by Kamerlingh Onnes; the
most representative example may be mercury. Because these materials have a
relatively low threshold, they pose numerous difficulties for application. Type II is
divided in turn into low-temperature and high-temperature superconductors.
Because Type II materials have a relative higher threshold than Type I materials, it
was the discovery of Type II superconductors that truly paved the way for superconductor applications.
The biggest difference between Type I and II concerns the intermediate stage
when undergoing a phase transition in a superconducting state. Type I superconductors have this intermediate stage. It exists momentarily, so that if any part of the
superconductor undergoes a phase transition, the rest of it immediately does so as
well. Type II superconductors, in contrast, have a mixed state in which superconductor and normal conductor coexist. The mixed state is physically stable and can
continue as long as the threshold is not exceeded (Schnyder et al. 2012).
Low-temperature superconductors—so called because of their low threshold
temperature—become superconductors like Type I superconductors in the presence
of liquid helium. Low-temperature superconductors have a high critical current
density, but a relatively low critical field or temperature. High-temperature superconductors use liquid nitrogen as a refrigerant, which makes them far more economical than low-temperature superconductors.
C. High-Temperature Superconductors
The key issue in superconductor application is temperature. More than 1000 types
of superconducting materials have been discovered, including metals, organic
8.3 Bio Materials
255
Meissner and Oschenfeld, who found that superconductors not only lacked resistance but demonstrated a self-repulsion effect in which a magnetic field inside the
superconductor was expelled outward. Now called the Meissner effect, it is recognized alongside the absence of resistance as one of the most basic characteristics
of superconductivity.
The first to successfully ascertain the cause of superconductivity were the U.S.
scientists Bardeen, Cooper, and Schrieffer. Their theory, which was announced in
1957, came to be known as the BCS Theory after their initials. It was a somewhat
complex theory, holding that lattice vibration occurred as electrons of opposite spin
joined into “Cooper pairs,” and that while electrons always moved with the same
energy at opposing speeds at the relatively macro-level scale, they flowed within the
lattice with a general lack of resistance as it vibrated. The three were awarded the
1972 Nobel Prize in Physics. While lattice vibration in a material typically produces
resistance to electrical current, in a superconducting state it produces Cooper pairs,
and electrons in this state slip out of the atom’s lattice as though surfing waves.
B. Types of Superconductors
Superconductors come in two main types, known as Type I and Type II. Type I is
the kind of superconducting material initially discovered by Kamerlingh Onnes; the
most representative example may be mercury. Because these materials have a
relatively low threshold, they pose numerous difficulties for application. Type II is
divided in turn into low-temperature and high-temperature superconductors.
Because Type II materials have a relative higher threshold than Type I materials, it
was the discovery of Type II superconductors that truly paved the way for superconductor applications.
The biggest difference between Type I and II concerns the intermediate stage
when undergoing a phase transition in a superconducting state. Type I superconductors have this intermediate stage. It exists momentarily, so that if any part of the
superconductor undergoes a phase transition, the rest of it immediately does so as
well. Type II superconductors, in contrast, have a mixed state in which superconductor and normal conductor coexist. The mixed state is physically stable and can
continue as long as the threshold is not exceeded (Schnyder et al. 2012).
Low-temperature superconductors—so called because of their low threshold
temperature—become superconductors like Type I superconductors in the presence
of liquid helium. Low-temperature superconductors have a high critical current
density, but a relatively low critical field or temperature. High-temperature superconductors use liquid nitrogen as a refrigerant, which makes them far more economical than low-temperature superconductors.
C. High-Temperature Superconductors
The key issue in superconductor application is temperature. More than 1000 types
of superconducting materials have been discovered, including metals, organic
8.3 Bio Materials
255