8/12/2015 1 S.No.Module Lectur e No. PPT Slide No. 1 properties of superconductors. L7-83-11 2 Types of superconductors L9-1012-28 3. DC & AC Josephson. - презентация

1 8/12/ S.No.Module Lectur e No. PPT Slide No. 1 properties of superconductors. L Types of superconductors L DC & AC Josephson effect L ApplicationsL UNIT INDEX

2 8/12/ APPLIED PHYSICS CODE : 07A1BS05 CODE : 07A1BS05 I B.TECH I B.TECH CSE, IT, ECE & EEE CSE, IT, ECE & EEE UNIT-5: CHAPTER-2 UNIT-5: CHAPTER-2 NO. OF SLIDES :37 NO. OF SLIDES :37

3 8/12/ SUPERCONDUCTIVITY. Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by almost zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect). Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by almost zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect). materials temperatureselectrical resistance magnetic fieldMeissner effect materials temperatureselectrical resistance magnetic fieldMeissner effect Lecture-7

4 8/12/ Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity does not occur in noble metals like gold and silver, nor in most ferromagnetic metals. Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity does not occur in noble metals like gold and silver, nor in most ferromagnetic metals.tinaluminiumalloyssemiconductors noble metalsgoldtinaluminiumalloyssemiconductors noble metalsgold

5 Tc O Resistivity TEMP(K) Impure Pure Resistance of superconducter suddenly drops to zero

6 8/12/ Critical temperature The temperature at which the transition from normal state to superconducting state takes place on cooling in the absence of magnetic field is called the critical temperature or the transition temperature The temperature at which the transition from normal state to superconducting state takes place on cooling in the absence of magnetic field is called the critical temperature or the transition temperature

7 8/12/ A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (the Meissner effect). This current effectively forms an electromagnet that repels the magnet. A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (the Meissner effect). This current effectively forms an electromagnet that repels the magnet.magnet high-temperature superconductor liquid nitrogen Meissner effectmagnet high-temperature superconductor liquid nitrogen Meissner effect

8 8/12/ Persistent current Lecture-8 Persistent current The electrical current in a superconducter,in superconducting state remains for a long time. The electrical current in a superconducter,in superconducting state remains for a long time. This current remains for very long period without attenuation. This current remains for very long period without attenuation. The time taken by the super current to reduce 1/e times of its initial value is more than The time taken by the super current to reduce 1/e times of its initial value is more than 1,00,000 years. 1,00,000 years. This current is called called persistent current. This current is called called persistent current.

9 8/12/ Effect of magnetic field. By applying magnetic field of sufficient strength, superconductivity of material can be destroyed. By applying magnetic field of sufficient strength, superconductivity of material can be destroyed. The minimum magnetic field strength required to destroy superconductivity of substance,below Tc is called critical magnetic field (Hc) at that temperature. The minimum magnetic field strength required to destroy superconductivity of substance,below Tc is called critical magnetic field (Hc) at that temperature. Hc = Ho [1-(T/Tc)2]. Hc = Ho [1-(T/Tc)2].

10 8/12/ Meissner effect. T > Tc T

11 8/12/ Meissner Effect High Tc Superconductor and High Energy Permanent Magnet Magnets in repulsive mode for levitation Levitation Experiments

12 8/12/ Types of Superconductors. Depending on the way of transition from superconducting state to normal state by the application of magnetic field, superconductors are classified into Depending on the way of transition from superconducting state to normal state by the application of magnetic field, superconductors are classified into Type-I superconductors and Type-I superconductors and Type-II superconductors. Type-II superconductors. Lecture-1Lecture-1 Lecture-9

13 8/12/ TYPE-I SUPERCONDUCTORS Superconductors exhibiting complete Meissner effect (perfect diamagnetism) are called Type-Superconductors. Superconductors exhibiting complete Meissner effect (perfect diamagnetism) are called Type-I Superconductors. Superconductors. They are also known as soft Superconductors.

14 8/12/ Transition between normal and superconducting states is sharp and well defined. Transition between normal and superconducting states is sharp and well defined. There is only one value of critical magnetic field H c. There is only one value of critical magnetic field H c.

15 8/12/ Critical temperatures are low. Hence these are not commercially useful but are useful to understand the exciting phenomenon of superconductivity. Critical temperatures are low. Hence these are not commercially useful but are useful to understand the exciting phenomenon of superconductivity. Type-Superconductors are mostly of pure specimens. Type-I Superconductors are mostly of pure specimens. Examples: Pure specimens of Al, Zn, Hg and Sn.

16 SUPER CONDUCTING STATE NORMAL STATE. Hc M O TYPE-I SUPERCONDUCTERS RELATION BETWEEN MAGNETIZATION AND APPLIED MAGNETIC FIELD FOR TYPE-I SUPER CONDUCTERS.

17 8/12/ TYPE-II SUPERCONDUCTERS They are developed from alloys, compounds, ceramics, transition metals etc. They are developed from alloys, compounds, ceramics, transition metals etc. For any Type2 material, two critical values of applied magnetic field Hc 1 and Hc 2 can be identified. In between, there is a thermodynamic critical magnetic field H c corresponding to that of type1 materials. For any Type2 material, two critical values of applied magnetic field Hc 1 and Hc 2 can be identified. In between, there is a thermodynamic critical magnetic field H c corresponding to that of type1 materials.

18 8/12/ The material behaves as a perfect superconductor in the range 0H c2 the material returns to normal state. When H>H c2 the material returns to normal state. Nb and Zr are some examples of this type. Nb and Zr are some examples of this type.

19 SUPERCONDUCTIONG STATE. MIXED STATE (OR) VORTEX STATE NORMAL STATE. Hc1Hc2 O M H TYPE-II SUPERCONDUCTERS. Variation of Magnetization with applied magnetic field for Type –II superconducters.

20 8/12/ Super electrons According to London brothers, a superconductor is composed of two distinct type of electrons, i.e., normal electrons and super electrons. super electrons experience no scattering. According to London brothers, a superconductor is composed of two distinct type of electrons, i.e., normal electrons and super electrons. super electrons experience no scattering.

21 8/12/ Penetration depth According to London equations, the magnetic flux does not drop to zero suddenly at the surface of Type-I superconductors, but decreases exponentially. The depth from the surface at which the magnetic flux density falls to 1/e of its initial value at the surface is called penetration depth. According to London equations, the magnetic flux does not drop to zero suddenly at the surface of Type-I superconductors, but decreases exponentially. The depth from the surface at which the magnetic flux density falls to 1/e of its initial value at the surface is called penetration depth. Lecture-10

22 8/12/ BCS theory According to BCS theory, superelectrons are responsible for the superconductivity. They exist as Cooper pairs. They form a bound single system. Their motions are correlated. According to BCS theory, superelectrons are responsible for the superconductivity. They exist as Cooper pairs. They form a bound single system. Their motions are correlated.

23 Metal Insulater V I Quantum Tunneling Lecture-1Lecture-1

24 Metal Super conducter Insulater V I Quantum Tunneling Vc

25 Ef Insulater E E1 METAL Super Conducter Available States. QUANTUM TUNNELING Lecture-1Lecture-1

26 8/12/

27 8/12/ Cooper Pairs

28 8/12/ Flux quantization The magnetic flux enclosed by a ring is quantized. This concept is known as flux quantization. The magnetic flux enclosed by a ring is quantized. This concept is known as flux quantization.

29 8/12/ Josephson effect When a thin insulating layer is sandwiched between a metal and a superconductor or two superconductors, electrons can tunnel through the junction. Their wave functions on both sides are highly correlated. This is known as Josephson effect. When a thin insulating layer is sandwiched between a metal and a superconductor or two superconductors, electrons can tunnel through the junction. Their wave functions on both sides are highly correlated. This is known as Josephson effect. Lecture-11

30 8/12/ d.c. Josephson effect A d.c. current flows across the junction of two superconductors separated by a thin insulating layer in the absence of any external electric or magnetic field. A d.c. current flows across the junction of two superconductors separated by a thin insulating layer in the absence of any external electric or magnetic field.

31 8/12/ a.c.Josephson effect When d.c. voltage applied across the junction of the two superconductors separated by a thin insulating layer then microwaves are emitted. When d.c. voltage applied across the junction of the two superconductors separated by a thin insulating layer then microwaves are emitted.

32 8/12/ Applications of Josephson effect Josephson effect is used to generate microwaves with frequency W = 2eVo/ħ Josephson effect is used to generate microwaves with frequency W = 2eVo/ħ A.C. Josephson effect is used to define standard volt A.C. Josephson effect is used to define standard volt Lecture-12

33 8/12/ Applications of Josephson effect A.C. Josephson effect is used to measure very low temperatures based on the variation of frequency of the emitted radiation with temperature A.C. Josephson effect is used to measure very low temperatures based on the variation of frequency of the emitted radiation with temperature A Josephson junction is used for switching of signals from one circuit to another. The switching time is of the order of 1ps and hence very useful in high speed computers. A Josephson junction is used for switching of signals from one circuit to another. The switching time is of the order of 1ps and hence very useful in high speed computers.

34 8/12/ Applications of Superconductors 1.It is a basis of new generation of energy saving power system. Superconducting generators are smaller in size and less in weight compare with conventional generators. These generators consume very low energy, hence more energy will be saved. 2.All electric power companies are looking forward to the superconducting transmission system that would save most of the energy now being last Lecture-13

35 8/12/ from conventional power lines in the form of useless radiation and heat..3.In japan, Superconducting magnets have been used to levitate an experimental train above its track and can drive it at a great speed of 500 Km/h with minimum expenditure of energy. A similar magnetic propulsion system may be used to launch satellites into orbits directly from the earth without the use of rockets.

36 8/12/ High efficiency ore-separating machines are built using Super-conducting magnets, which are also used to separate tumour cells from healthy cell by High Gradient Magnetic separation method. 5.Superconducting materials can be used as a memory or storage device in computers, since the current in it can flow without any change in its value with time.

37 8/12/ Using Superconducting elements one can build up an extremely fast and large-scale computer in a compact size. The power consumed by this computer will be less than 0.5 watt. 7. The Josephon devices are used to produce microwaves, which are made up of superconductors. 7. The Josephon devices are used to produce microwaves, which are made up of superconductors.