Superconductor material

1. Superconductor material Supervision: dr.issamrashid. Done by :hendzaid.

2. What is a Superconductor?? Superconductor:An element inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature called critical temperature (Tc). Resistance is undesirable because it produces losses in the energy flowing through the material.

3. The history of superconductor: • 1) In 1911 superconductivity was first observed in mercury by physicist Heike Kamerlingh OnnesWhen he cooled it to the temperature of liquid helium, 4 degrees Kelvin ( -269C), its resistance suddenly disappeared. in 1913, he won a Nobel Prize. • 2) In 1933Walther Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. A magnet moving by a conductor induces currents in the conductor. This phenomenon is known as "Meissner effect" (The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material.) Meissner Onnes

4. 3) In 1935, Fritz and Heinz London showed that the Meissner effect was a consequence of the minimization of the electromagnetic free energy carried by superconducting current. 4) In 1957 ,John Bardeen, Leon Cooper, and John Schrieffer put a theory ( became know as the BCS theory ) and they own a Nobel Prize in 1972. The mathematically-complex BCS theory explained superconductivity at temperatures close to absolute zero for elements and simple alloys. 5) In 1962 Brian D. Josephson predicted that electrical current would flow between 2 superconducting materials - even when they are separated by a non-superconductor or insulator. Nobel Prize in 1973.

5. Types of superconductors: Type I: • Suddenly loss it magnetization. • Exhibit Meissner Effect. • it has One HC = 0.1 tesla • No mixed state • Soft superconductor • Eg.s – Pb, Sn, Hg • B=0 • R=0

6. Type II: • Gradual loss of magnetization • Does not exhibit complete Meissner Effect • Two HCs – HC1 & HC2 (≈30 tesla) • Mixed state present • Hard superconductor • Eg.s – NbSn, Nb-Ti • Type II: can then bear much higher magnetic fields. • B‡0 • R=0 • B=0 R=0

7. Vortices (for type 2) Under a small magnetic field (with a weak magnet or a magnet far from the sample), type2 react like type I superconductors and completely expel the magnetic field. But when the magnetic field is stronger, they prefer to adopt a compromise situation and allow some of the magnetic field to penetrate along “vortices”. These vortices are like tubes that go through the sample from side to side and in which the material is no longer superconducting since the magnetic field can penetrate it. The material then becomes a sieve. In order to enable this magnetic field to go through the vortex, the material develops superconducting currents circulating around this pillar in a spiral motion justifying the name, “vortex”.

8. Resistance In a superconductor below a temperature called the “critical temperature”, the electric resistance very suddenly falls to zero,the material conducts current perfectly. This is incomprehensible because the flaws and vibrations of the atoms should cause resistance in the material when the electrons flow through it. However, in a superconductor, the electric resistance is equal to zero although the flaws and vibrations still exist.!!!! Explanation : the electrons form a quantum collective state that is not sensitive to collisions anymore. The electrons are not slowed, and the electric resistance has disappeared.

9. Levitation • there are two different forces that can make a magnet levitate: • Meissner effect :expels the magnet . • vortex pinning: maintains the magnet. •  when the material becomes superconducting, the magnet begins to levitate.  From where does this strange effect come? • In a superconductor, the electrons merge into one collective quantum wave called the condensate. • The magnetic field acts to “bend” this wave. But the superconducting wave is rigid and unbendable, otherwise it might break. The superconductor “tries” to protect itself from the magnetic field by expelling it. This is called the • Meissner effect.

10. Meissner effect. a b c a) The black arrows represent the external magnetic field applied to the superconducting sample.b) Currents appear on the surface of the superconductor (in red). These currents create a magnetic field (the external magnetic field is not represented).c) The total magnetic field is the sum of the applied magnetic field and the magnetic field that appears as a result of the reaction. In the superconductor, as a whole, the magnetic field is equal to zero.

11. The superconducting state is defined by three very important factors: • critical temperature (Tc) It corresponds to the temperature below which the material becomes superconducting. • critical magnetic field (Hc )It corresponds to the maximum magnetic field that can be applied to a superconductor. Above the critical field Hc, the material becomes a normal metal again. For instance, in mercury, this field is 0.04 Tesla. For some superconductors, we have to define two critical fields,Hc1 and Hc2, because of the presence of vortices. For example, in a  cuprate, Bc1 is about 0.01 Tesla, but Bc2 can be as high as 60 • teslas.

12. Occurrence of Superconductivity

13. critical current density (Jc): It corresponds to the maximum current that can go through a superconductor. If the current exceeds this value, the material becomes a normal metal again and starts resisting and heating like any other metal. More often, physicists use the term “current density” (jc), that is the current divided by the cross section of the electric wire.  For instance, in a cuprate, the current density is more than 10 000 amperes per cm2. 4) The penetration length λ: When a magnetic field is applied to a superconductor, it is expelled and can only penetrate a short depth of the surface of the sample, measured as the penetration length. It is usually very small, from 10 to 100 nanometres.

14. 5) The gap: The superconductor is characterized by its wave function formed from the Cooper pairs. This wave function has an energy called “gap”: it corresponds to the minimum energy needed to break one of the Cooper pairs.

16. BCS theory (by Bardeen, Cooper, Schrieffer) • They put theoretical model (that has since been called “BCS”, after their initials), they proposed the following explanation: electrons form a collective quantum state made up of pairs of electrons of opposite spin and momentum. This remarkable state, called a pair condensate, explained all known superconducting properties and made possible the prediction of new ones. This BCS theory has since been proven by numerous experiments in metals and alloys. But it cannot apply straightforward in the case of some new superconductors such as cuprites or pnictides, • and scientists are still working • on finding • new explanations for these • materials.

17. Cooper pair:  “Pauli exclusion principle”, only allows the existence of such a condensate if the waves which compose it are carried by particles called bosons. Unfortunately, electrons. are fermions, not bosons. !!!!! The solution:  in order to form a condensate, electrons must form pairs first. Indeed, a pair of electrons can form a boson. In conventional superconductors, the creation of electron pairs and the formation of the condensate happen instantaneously. The Cooper pairs will move at the same speed, the electrons of the pairs still moving at opposite speeds, following the Pauli principle.

18. Josephson effect: • When two superconductors are separated by a very thin insulating layer, quite unexpectedly, a continuous electric current appears, the value of which is linked to he characteristics of the superconductors. • This effect was predicted in 1962 by Brian Josephson. • Where does such an effect come from? • When a material becomes superconducting, the electrons form Cooper pairs and condensate in the shape of a unique collective quantum wave. If the electric insulator separating the two superconductors is very thin (only a few nanometers), then the wave can somehow spill out of the superconductor, which enables the electron pairs to go through the insulator thanks to a quantum effect called tunneling effect.

19. London equation This proposed equations are consistent with the Meissner effect and can be used with Maxwell's equations to predict how the magnetic field and surface current vary with distance from the surface of a superconductor.In order to account for the Meissner effect, the London brothers proposed that in a superconductor, Maxwell's equations is replaced by equ. 1 & 2 : V=RI = - = = 0 B = constant. = - B ….. equ. 1 … equ.2

20. Ampere law says ∇×B = µ0 • Put this equ. in equ.1 we will have : • ∇(∇×B) = ∇(∇.B)- B= 0- B= - = - B • =- B , where λ= = B ….equ.3 • Equ.3 is correct when B=0 (for superconductor). • The sol.of equ.3 is

21. Cold Mercury is the metal in which superconductivity was first observed. It had to be cooled to -269°C, really close to absolute zero. The best superconductor we have yet discovered becomes superconducting below -135°C. The physics of superconductors is hence closely linked to the study of very low temperatures accessible with specific cooling techniques, using cryogenic liquids such as liquid helium or nitrogen, and even more sophisticated tools.  In order to reach very low temperatures – a branch of physics called “cryogenics” – we use machines that pump the heat out of an object in order to cool it.  liquid oxygen (90.2 K) liquid nitrogen (77.4K) liquid hydrogen (20.3 K) liquid helium (4.2 K)

22. “New superconductors” : • refer to a wide variety of materials • often artificially synthesized in laboratory • cannot be described with the “classical” BCS theory of superconductivity. • The origin of their superconductivity is probably not the same as that of lead or mercury, especially as far as the process of two electrons forming a Cooper pair is concerned. • still struggle to understand how they work. • the list of these new superconductors will get longer and longer as years go by.

24. Examples of New superconductors : • Cuprates(King of the superconductors): • discovered in 1986 by Georg Bednorz and Alex Müller. • Today, they are the materials that become superconducting at the highest temperatures: -135°C or 138 K. • These materials become superconducting at the temperature of liquid nitrogen. • Consists of copper and oxygen layer. • Pnictides(The youngest one): • discovered In 2008, by the team of the Japanese. • iron-based compounds at temperatures almost as “high” as cuprats • we find it in semimetals composed of iron and arsenic. • After cuprates, they are the “hottest” superconductors with • a maximum critical temperature of (-217°C).

25. Nano-superconductors: Nanophysics corresponds to physics at the scale of the nanometer, a billionth of a meter, or a hundred thousandth of the width of a hair. It corresponds to the size of about ten atoms. Observing and manipulating superconductivity at the scale of the nanometer is one of the issues of modern research, not only to understand superconductivity better, but also to make non-superconducting materials superconducting, or to invent new electronic components with very strange properties.

26. Applications of Nano-superconductors: • Graphene and superconductivity a graphene sheet sprinkled with superconducting nanoparticles (made of indium). The scale bar goes from millimeters to nanometers.  • Nanosquid • This nanoSQUID is made of a superconducting loop (of niobium) interrupted by two non superconducting (normal) wires, made of gold. At low temperature a super current runs through the gold wires, and the interference between the two branches is modulated by a magnetic field.  • Nanotubes and superconductivity • a carbon nanotubeconnected to two superconducting electrodes, with nearby a third electrode which controls the parity of electrons in the nanotubes and modulates the super current through the tube. 

27. Applications of superconductivity • Electrical applications(Zero resistance, zero loss) • power cables: • smaller cables with more current. •  cables have to be cooled to become superconducting. However, cable prototypes made of superconducting cuprates cooled with liquid nitrogen have been built on small distances. • SMES (Energy storage): • SMES = Superconducting Magnetic Energy Storage. Power cable made of superconductors;

28. The energy is stored via an electric current sent in a coil made of a superconducting wire. • Once the coil is short circuited (closed), the current stays forever since there is no loss and produces a magnetic field as in MRI coils. Energy storage • Medical applications: • MRI(Magnetic Resonance Imagery) • used everyday to diagnose tumors, sclerosis and edemas. • use the small magnets inside the nuclei of the human body atoms to visualize what surrounds them (brain, muscles…). • the nuclei magnets called “spin” must first be lined up thanks to a magnetic field in which the patient is put.  • To produce the field, a strong electric current must flow through a coil of thousands of wire loops, called turns. •  once the magnetic field has been created, the coil can be closed. The current (and the magnetic field) keeps flowing since there is no resistance.

29. NMR(Nuclear Magnetic Resonance) • to understand what characterizes the electrons surrounding these nuclei. • In chemistry, this enables to determine the structure of complicated molecules. • In physics, NMR enables to measure the static and dynamic magnetic properties of the electrons in matter. • It is used to study superconductors, for instance. MRI NRI

30. TRAINS • MAGLEV( “Magnetic Levitation trains”) • levitate thanks to the use of a magnetic field. • A moving magnetic field produces inducted currents that will produce a second magnetic field interacting with the first one. • It is this force that lifts the Maglev. Maglev, the fastest train in the world, uses superconductors.

31. SQUID (The super-detector) • SQUID(“superconducting quantum interference device”) • is an electronic system that uses a superconducting ring in which one or two small insulating layers have been inserted (see figure). • This device based onthe Josephson effect in the superconductor-insulator-superconductor sandwich • and on the flux quantization in the ring makes it ultra-sensitive to any magnetic field. • Squids are hence the most efficient systems to measure magnetic fields with great accuracy, even the weaker ones. A SQUID circuit made of superconductors

32. How does a squid work? • An entirely superconducting ring only allows certain magnetic flux values in its centre : the magnetic field (penetration of the field in the ring) is quantized and must be an integer multiple of the flux quantum f0. In the squid, the situation is subtler because of the junctions. Indeed, each junction allows a small electric current to go through by Josephson effect, and that current is also very sensitive to the magnetic field in the ring.

33. Accelerators: • The electric field that accelerates the particles is produced by radio-frequency (RF) resonant cavities • whereas the magnetic field that guides and focuses them is produced by electromagnets. • Superconductivity gives access to stronger fields and reduces the energy loss in RF cavities: it enables to build more powerful accelerators that are cheaper to use. •  Thus, the large hadron collider (LHC) of the CERN in uses thousands superconducting magnets spread on , producing a magnetic field four times higher than classical electromagnets, with an electric intake ten times smaller. 

34. Thank you Any question please??