EL EFECTO MEISSNER Y LA LEVITACIÓN MAGNÉTICA
La evolución de nuestra comprensión del estado superconductor de los elementos puros se llevó una buena parte de la mitad de este centenario. Durante los primeros 22 años, se fueron encontrando nuevos elementos, compuestos y aleaciones superconductoras; sin embargo, se creía que todo conductor perfecto podría ser considerado superconductor. Fue entonces, cuando W. Meissner y R. Ochsenfeld, en 1933, observaron el diamagnetismo perfecto de estos materiales. El campo magnético dentro del material superconductor no podía existir: efecto Meissner - Ochsenfeld. En el terreno de la teoría, cabe destacar que los avances se desataron con el descubrimiento de esta propiedad. En un principio, se elaboraron teorías fenomenológicas desarrolladas a partir de consideraciones clásicas: Ecuación de London y Teoría de Ginzburg - Landau. La teoría que desarrollaron los hermanos London, está basada en una visión microscópica clásica del efecto Meissner, tomando en cuenta la termodinámica y el electromagnetismo (ecuaciones de Maxwell). Fritz y Heinz, en 1935, asumieron que en el superconductor se presentaban dos fluidos uno de electrones normales y otro de electrones superconductores que podían moverse libremente en el material. Con ello llegaron a una expresión sencilla para lo que se denomina longitud de penetración del campo magnético. El éxito de esta ecuación se reflejó en su aplicación a varios materiales. Por lo que se refiere a la teoría elaborada por V. Ginzburg y L. Landau, en 1950, podemos decir que es fenomenológica y está basada en el efecto del diamagnetismo perfecto, pero su enfoque es macroscópico y aborda el comportamiento magnético del material cerca de la temperatura crítica. Este desarrollo teórico concluyó con dos magníficas ecuaciones: una que relacionan la longitud de penetración de London con la diferencia entre la temperatura del material y la temperatura crítica; y otra, la que define una nueva característica de los materiales, la llamada longitud de coherencia de los electrones. La combinación de ambas expresiones proporciona el parámetro de Ginzburg - Landau que determina dos tipos de superconductividad, a partir de su valor. Sobre estos dos tipos de superconductor hablaremos más adelante. No fue sino hasta el año 1957 cuando J. Bardeen, L. Cooper y R. Schrieffer desarrollaron una teoría microscópica de la superconductividad, conocida ahora, en su honor, como teoría BCS. Esta teoría está basada en la propuesta de una atracción entre los electrones presentes en la estructura cristalina de los metales, formándose un par de Cooper y dando origen a la aparición de una brecha en la densidad de estados electrónicos. Esta teoría fue todo un éxito y llevó a la entrega del premio Nobel de Física, en 1989, a sus autores. Diversas técnicas han permitido confirmar que la superconductividad se manifiesta a través de los pares de Cooper como portadores con carga -2e (e - carga del electrón): efecto Josephson, por ejemplo. Además, I. Giaever comprobó experimentalmente mediante una ingeniosa técnica, la existencia de la brecha energética, denominada Gap en la densidad de estados. Este experimento de tunelaje electrónico de 1959, permite visualizar prácticamente la brecha (ver figura 3). Giaever compartió el premio Nobel de Física con Esaki y Josephson, en 1973, por esta importante contribución al entendimiento del estado superconductor.\cite{lopez2011}
Superconductivity
Superconductivity, complete disappearance of electrical
resistance in various solids when they are cooled below a characteristic
temperature. This temperature, called the
transition temperature, varies for different materials but generally is below 20
K (−253 °C).
The use of superconductors in magnets is limited by the fact that strong
magnetic fields above a certain critical value, depending upon the material, cause a superconductor to revert to its normal, or nonsuperconducting, state, even though the material is kept well below the transition temperature.
Suggested uses for superconducting materials include medical magnetic-imaging devices, magnetic energy-storage systems, motors, generators, transformers, computer parts, and very sensitive devices for measuring magnetic fields, voltages, or currents. The main advantages of devices made from superconductors are low power
dissipation, high-speed operation, and high sensitivity.
Superconductivity was discovered in 1911 by the Dutch physicist
Heike Kamerlingh Onnes; he was awarded the Nobel Prize for Physics in 1913 for his low-temperature research. Kamerlingh Onnes found that the electrical
resistivity of a mercury wire disappears suddenly when it is cooled below a temperature of about 4 K (−269 °C);
absolute zero is 0 K, the temperature at which all matter loses its disorder. He soon discovered that a superconducting material can be returned to the normal (i.e., nonsuperconducting) state either by passing a sufficiently large current through it or by applying a sufficiently strong magnetic field to it.
For many years it was believed that, except for the fact that they had no electrical resistance (i.e., that they had
infinite electrical conductivity), superconductors had the same properties as normal materials. This belief was shattered in 1933 by the discovery that a superconductor is highly
diamagnetic; that is, it is strongly repelled by and tends to expel a magnetic field. This phenomenon, which is very strong in superconductors, is called the
Meissner effect for one of the two men who discovered it. Its discovery made it possible to formulate, in 1934, a theory of the electromagnetic properties of superconductors that predicted the existence of an electromagnetic penetration depth, which was first confirmed experimentally in 1939. In 1950 it was clearly shown for the first time that a theory of superconductivity must take into account the fact that free electrons in a crystal are influenced by the vibrations of atoms that define the
crystal structure, called the lattice vibrations. In 1953, in an analysis of the thermal
conductivity of superconductors, it was recognized that the distribution of energies of the free electrons in a superconductor is not uniform but has a separation called the
energy gap.
\cite{ginsberg2013}
Superconductor Definition, Types, and Uses
A superconductor is an element or metallic alloy which, when cooled below a certain threshold temperature, the material dramatically loses all electrical resistance. In principle, superconductors can allow
electrical current to flow without any energy loss (although, in practice, an ideal superconductor is very hard to produce). This type of current is called a supercurrent.
The threshold temperature below which a material transitions into a superconductor state is designated as Tc, which stands for critical temperature.
Not all materials turn into superconductors, and the materials that do each have their own value of Tc.
TYPES OF SUPERCONDUCTORS
- Type I superconductors act as conductors at room temperature, but when cooled below Tc, the molecular motion within the material reduces enough that the flow of current can move unimpeded.
- Type 2 superconductors are not particularly good conductors at room temperature, the transition to a superconductor state is more gradual than Type 1 superconductors. The mechanism and physical basis for this change in state is not, at present, fully understood. Type 2 superconductors are typically metallic compounds and alloys.
DISCOVERY OF THE SUPERCONDUCTOR
Superconductivity was first discovered in 1911 when mercury was cooled to approximately 4 degrees Kelvin by Dutch physicist Heike Kamerlingh Onnes, which earned him the 1913 Nobel Prize in physics. In the years since, this field has greatly expanded and many other forms of superconductors have been discovered, including Type 2 superconductors in the 1930s.
The basic theory of superconductivity, BCS Theory, earned the scientists—John Bardeen, Leon Cooper, and John Schrieffer—the 1972 Nobel Prize in physics. A portion of the 1973 Nobel Prize in physics went to Brian Josephson, also for work with superconductivity.
In January 1986, Karl Muller and Johannes Bednorz made a discovery that revolutionized how scientists thought of superconductors.
Prior to this point, the understanding was that superconductivity manifested only when cooled to near
absolute zero, but using an oxide of barium, lanthanum, and copper, they found that it became a superconductor at approximately 40 degrees Kelvin. This initiated a race to discover materials that functioned as superconductors at much higher temperatures.
In the decades since, the highest temperatures that had been reached were about 133 degrees Kelvin (though you could get up to 164 degrees Kelvin if you applied a high pressure). In August 2015, a
paper published in the journal Nature reported the discovery of superconductivity at a temperature of 203 degrees Kelvin when under high pressure.
APPLICATIONS OF SUPERCONDUCTORS
Superconductors are used in a variety of applications, but most notably within the structure of the Large Hadron Collider. The tunnels that contain the beams of charged particles are surrounded by tubes containing powerful superconductors. The supercurrents that flow through the superconductors generate an intense magnetic field, through
electromagnetic induction, that can be used to accelerate and direct the team as desired.
In addition, superconductors exhibit the
Meissner effect in which they cancel all magnetic flux inside the material, becoming perfectly diamagnetic (discovered in 1933).
In this case, the magnetic field lines actually travel around the cooled superconductor. It is this property of superconductors which is frequently used in magnetic levitation experiments, such as the quantum locking seen in quantum levitation. In other words, if
Back to the Future style hoverboards ever become a reality. In a less mundane application, superconductors play a role in modern advancements in
magnetic levitation trains, which provide a powerful possibility for high-speed public transport that is based on electricity (which can be generated using renewable energy) in contrast to non-renewable current options like airplanes, cars, and coal-powered trains.
\cite{jones2017}
Superconductors and Superconducting Materials Information
Superconductors and superconducting materials are metals, ceramics, organic materials, or heavily doped semiconductors that conduct electricity without resistance.