LECTURE 4

CONTENTS

SUPERCONDUCTORS

APPLICATIONS OF SUPERCONDUCTORS

DETAILED STUDY ON PHOTONIC

MATERIALS(LED AND LCD)

SUPERCONDUCTORIntroduction

Father of Super conductivity is H.Kamerlingh Onnes’ Duch Physist (1911).

In his experiments on the properties of metals in general and on the electrical conductivity (thereby resistivity) of Mercury (Hg).

He observed that, when pure mercury is cooled, its resistivity vanished abruptly at 4.2 K. Above this temperature, the resistivity is immensurable, while below this temperature the resistivity is very small that it is essentially zero. ( is in the order of 105 ohm cm).i.e., at 4.2 K, Hg is converted into a superconductor.

This phenomenon of loosing resistivity absolutely when cooled to a sufficiently, low temperature is called `super conductivity’.

Transition Temperature (or) Critical Temperature

The temperature at which the transition of a normal conductor into a superconductor occurs is called as the `Transition temperature or critical temperature [Tc].

Above Tc- the substance is in the normal state, but Below Tc-The substance is in the super conducting state. For semiconductors -Tc varies from, 0.3K to 1.25K For metals -Tc varies from 0.35K to 9.22K and For alloys -Tc varies from 18.1K to 22.65K.

Resistance of mercury vs temperature

High Temperature Superconductors (HTS)High-temperature superconductors (abbreviated high Tc) are

a family of superconducting materials containing copper-oxide planes as a common structural feature. For this reason, the term is often used interchangeably with cuprate superconductors.

This feature allows some materials to support superconductivity at temperatures above the boiling point of liquid nitrogen (77 K) . Indeed, they offer the highest transition temperatures of all superconductors. The ability to use relatively inexpensive and easily handled liquid nitrogen as a coolant has increased the range of practical applications of superconductivity.

Notations Chemical formula Tc (K)

1 2 3 YBa2 Cu3 O7 90

Tl-1212 Tl Ba2 CaCu2 O7 80

Some examples of HTS

Characteristics of HTSC

Superconductors are characterized by a material-dependent magnetic field H, above which the superconducting state disappears.

The critical field is a function of temperature. All the HTS materials are type II superconductors .When the applied field H < Hc1, the material is in the superconducting Meissner state whereas in the mixed state, the magnetic field penetrates partly into the material in the form of vortices.

Type II superconducting materials have usually higher critical fields than type I superconductors which makes them suitable for many advanced applications.

(a) (b)

Critical magnetic field as a function of temperature for (a) type I superconductors and (b) type II superconductors.

Most of the HTS materials are layered cuprates, i.e., they consist of CuO2 planes separated by layers of other elements or oxides. Because of the layered structure, HTS materials exhibit strong anisotropy: the values of the superconducting parameters are different in different directions. In addition, charge transport is mainly confined to the CuO2 planes.

IMPORTANT FEATURES HTS• They have high Tc.• They have PEROVSKITE crystal structure.• They are direction dependent• They are reactive, brittle and cannot be easily formed (or) joined.

HTS Material - YBCO HTS materials usually have complicated crystal structures. The compounds of HTS almost consists more than three different chemical elements and the materials with the highest Tc have seven elements in the crystal lattice. Ex: YBa2Cu3O7-d (YBCO). YBCO has numerous advantages compared to other ceramic superconductors

• This is only known stable four-element compound with a Tc

above 77 K. Includes neither toxic elements nor volatile compounds Easy to make single-phase YBCO. Less anisotropic than other HTS materials, carries higher current densities at higher magnetic fields.

Structure of a single unit cell of YBCO

The critical temperature of YBCO is approximately 90 K and the critical magnetic field can be as high as 300 T. For thin-film applications, critical current density (Jc) is an important parameter and, in the case of YBCO, it is typically Jc > 1 MA/cm².

The dimensions of a single unit cell of YBCO are a = 3.82 Å, b = 3.89 Å, and c = 11.68 Å. The lattice is composed of so-called perovskite layers (ACuO3) where A is a rare-earth or alkaline-earth element (e.g., Y or Ba in YBCO). The term 7-d in the chemical formula implies a slight deficiency of oxygen. If d=0, the lattice is in the orthorhombic phase whereas in the case of d=1, the material has a tetragonal structure. Only the orthorhombic configuration is superconducting but it is stable only at temperatures below 500°C.

APPLICATIONS OF SUPERCONDUCTORS

1. Superconducting Transmission Lines Since 10% to 15% of generated electricity is

dissipated in resistive losses in transmission lines, the prospect of zero loss superconducting transmission lines is appealing.

Current experiments with power applications of high-temperature superconductors focus on uses of BSCCO in tape forms and YBCO in thin film forms. Current densities above 10,000 amperes per square centimeter are considered necessary for practical power applications, and this threshold has been exceeded in several configurations.

2. Superconducting Motors and Generators Superconducting motors and generators could be

made with a weight of about one tenth that of conventional devices for the same output. This is the appeal of making such devices for specialized applications. Motors and generators are already very efficient, so there is not the power savings associated with superconducting magnets. It may be possible to build very large capacity generators for power plants where structural strength considerations place limits on conventional generators.

3. Superconducting Magnetic Energy StorageSuperconducting magnetic energy storage (SMES)

stores electricity for long periods of time in superconductive coils. SMES will be used by electrical utilities some day.

4.ComputersIf computers used superconducting parts they

would be much more faster than the computers today. They would much smaller because no space for heat would be required. Computers of today need a great deal of space for cooling.Computers are being developed today that use Josephson junctions. The Josepson effect states that electrons are able to flow across an insulating barrier placed between two superconducting materials. Josephson junctions have a thin layer of insulating materials squeezed between superconductive material. Josephson junctions require little power to operate, thus creating less heat.

5. Josephson Devices

Devices based upon the characteristics of a Josephson junction are valuable in high speed circuits. Josephson junctions can be designed to switch in times of a few picoseconds. Their low power dissipation makes them useful in high-density computer circuits where resistive heating limits the applicability of conventional switches.

6. SQUID Magnetometer

The superconducting quantum interference device (SQUID) consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. The device may be configured as a magnetometer to detect incredibly small magnetic fields -- small enough to measure the magnetic fields in living organisms. Squids have been used to measure the magnetic fields in mouse brains to test whether there might be enough magnetism to attribute their navigational ability to an internal compass.

Threshold for SQUID: 10-14 T

Magnetic field of heart: 10-10 T

Magnetic field of brain: 10-13 T

The great sensitivity of the SQUID devices is associated with measuring changes in magnetic field associated with one flux quantum

7. Superconductors in NMR Imaging Superconducting magnets find application in magnetic

resonance imaging (MRI) of the human body. Besides requiring strong magnetic fields on the order of a Tesla, magnetic resonance imaging requires extremely uniform fields across the subject and extreme stability over time. Maintaining the magnet coils in the superconducting state helps to achieve parts-per-million spacial uniformity over a space large enough to hold a person, and ppm/hour stability with time.

8. Fault-Current Limiters High fault-currents caused by lightning strikes are a

troublesome and expensive nuisance in electric power grids. One of the near-term applications for high temperature superconductors may be the construction of fault-current limiters which operate at 77K. The need is to reduce the fault current to a fraction of its peak value in less than a cycle (1/60 sec).

9. Magnetically Levitated Trains Perhaps the most famous and fascinating

superconducting invention is magnetically levitated trains, or "maglev" trains. Maglev trains have no wheels and friction. The trains float silently on a magnetic field due to diamagnetic behaviour.

Photonic materials – Light Emitting DiodesLED (Light Emitting Diode) is a semiconductor p-n junction

diode which converts electrical energy to light energy under forward biasing.Principle

The diode is forward biased. Due to forward bias, the majority carriers from ‘n’ and ‘p’ regions cross the junction and become minority carriers in the other junction (i.e.) Electrons, which are majority carriers in ‘n’ region cross the junction and go to ‘p’ region and become minority carriers in p-region

Similarly, holes which are majority carries in ‘p’ region cross the junction and go to ‘n’ region and become minority carriers in ‘n’ region and this phenomenon is called minority carrier injection.

Radiative recombination

Now if the biasing voltage is further increased, these excess minority carriers diffuse away from the junction and they directly recombine with the majority carriers. (i.e.) the electrons, which are excess minority carriers in p-region recombine with the holes which are the majority carriers in ‘p’ region and emit light. Similarly, the holes which are excess minority carriers in ‘n’ region recombine with the electrons which are majority carriers in ‘n’ region and emit light.

Thus radiative recombination events lead to photon emission. The number of radiative recombination is proportional to the carrier injection rate and hence to the total current flowing through the device as given by

1exp0 kT

eVII

where I0 - the saturation current ; V- the forward bias voltage; k - the Boltzmann constant ; -varies from 1 and 2 depending on the semiconductor and temperature.

The optical photon emitted due to radiative recombination has the energy very close to the bandgap energy Eg and frequency of the emitted photon is given by

gEhc

where - the photon wavelength; h - Plancks constant; c - the velocity of light in vacuum.

LED ConstructionAn LED must be constructed such that the light emitted by

the radiative recombination events can escape the structure.

Sketches of LEDs

LEDs can be designed as either surface or edge emitters. Surface emitting LEDs can be made such that the bottom edge reflects light back towards the top surface to enhance the output intensity. The main advantage of edge emitter LEDs is the emitted radiation is relatively direct. Hence edge emitter LEDs have a higher efficiency in coupling to an optical fibre.

Although the internal quantum efficiency of LEDs is 100%, the external efficiencies are much lower. The main reason is that most of the emitted light radiation strikes the material interface at greater than critical angle and hence trapped with in the device. The internal critical angle at the semiconductor – air boundary is given by

1

2

n

nsin c

Where n1 is the refractive index of air = 1.0 n2 is the refractive index of the semiconductorFor group III semiconductor n2 = 3.5

Therefore c = 16°

Critical angle

Therefore all rays of light striking the surface at an angle exceeding 16° suffer total internal reflection and as a result most of the emitted light is reflected back inside the semiconductor crystal.

Two methods used to reduce reflection losses in LEDs

Hence to improve the external efficiency losses caused bulk absorption has to be minimized and the surface transmission has to be increased. One method to achieve this is to give the semiconductor a dome structure. Hemi spherical domes made from plastics are effective in increasing the external efficiency by a factor 2 or 3. There will be some losses at the plastic/ air interface but these are easily minimized by molding the plastic into an approximately hemispherical shape.

Materials

The choice of the materials for an LED is decided by the spectral requirements for a particular application. The most commonly used materials for LEDs are GaP, GaAs and their related ternary compound Ga Asx P1-x

The bandgap radiation of GaP, GaAs and GaAsP. GaP which gives a peak at 560 nm is very close to the wavelength of maximum eye response.

This makes GaP one of the most useful of all visible semiconductor light sources since in addition to green light both red and other colours can be produced by appropriate dopants.

Wavelength response of LED materials

Material DopantBandgap(eV)

Wavelength ( Nm)

Quantum efficiency

( %)

GaPGaPGaP

GaAsAlGa

NZn0NP

As

2.881.802.251.881.84

430690550660675

0.60.20.10.20.2

Photonic materials – Liquid Crystal DisplayLiquid crystals are organic compounds that flow like a liquid

while maintaining a long range orderliness of a solid.The molecules of liquid crystal compound are in the form of long cigar shaped rods.

Liquid CrystalTypes Based on the orientation of these rods–like polar molecules, the liquid crystals are classified into three basic types. They are smectic, nematic and chloesteric.(i) Smectic

The Smectic phase consists of flat layers of cigar shaped molecules with their long axes oriented perpendicular to the plane of the layer. The molecules within each layer remain oriented within each layer and do not move between layers. This most ordered smectic mesophase structural model.

(ii) NematicThe nematic phase also has molecules with their long

axes parallel to each other, but they are separated into layers. In the nematic mesophase, while the molecules maintain their orientation, the individual molecules can move freely up and down.

The nematic liquid crystal molecule consists of two benzene rings linked with a central group. A typical example is 4-methoxybenzenylidene-4-butylanaline (MBBA). The nematic liquid exhibits crystalline property over the temperature range 20°C to 47°C.

(iii) ChloestericChloesteric mesophase can be defined as a special type of

nematic in which the thin layers of mostly parallel molecules have their longitudinal axes twisted (rotated) in adjacent layers at a definite angle. This is the most ordered phase. Each layer is basically nematic.

LIQUID CRYSTAL ORIENTATIONIn LCD’s two preffered orientations of LC molecules are

used (i). Homeotrophic[With long axis of the molecules parallel to the glass plates and electrodes] and (ii). Homogeneous[With long axis of the molecules perpandicular to the glass plates and electrodes]

LC molecular orientations

The dielectric layer near the electrodes has preferred orientation which in turn aligns LC molecules. The top and bottom dielectric layers are by 90° with respect to one another. Therefore the direction of the crystal is rotated 90° with respect to the bottom of the liquid crystal. The liquid crystal thus acts like a set of polarisers whose optic axes are parallel to each other in the presence of electric field and in crossed position in the absence of electric field.

LC molecular orientations

The orientation of the LC molecules parallel to the glass plates is achieved by the deposition of a layer of dielectric over the transparent electrodes.

The dielectric layer near the electrodes has preferred orientation which in turn aligns LC molecules. The top and bottom dielectric layers are by 90° with respect to one another. Therefore the direction of the crystal is rotated 90° with respect to the bottom of the liquid crystal. The liquid crystal thus acts like a set of polarisers whose optic axes are parallel to each other in the presence of electric field and in crossed position in the absence of electric field.

Effect of electric fieldThe fundamental property of LCs that makes them useful as

display device is that they are sensitive to an external electric field.

The nematic liquid crystal finds its applications increasingly in electro-optic devices since their molecules can be aligned by electric and magnetic fields to produce sufficient change in their optical properties. Liquid crystal molecules rotate as a result of external electric field. The behaviour of initially ordered liquid crystal material due to increase in electric field.

Behaviour of LC molecules in an electric field

Let ‘E’ be the electric field applied in a direction perpendicular to the liquid crystal / solid interface. Also let ‘Ec’ be the electric field strength at which LC molecules change from homogeneous order to a homeotropic type.

When E<Ec the ordering existsFig-a[Above]. If E<Ec the liquid crystal molecules away from the electrode begins align along the field direction as in Fig-b[Above]. When E>>Ec then most of the molecules align along the field direction as in Fig-c[Above].

LCD cell construction

The most important structure of twisted nematic mesophase liquid crystal cell. It consists of a thin layer of LC material between two glass plates that are fused together. The thickness of the LC is 10 to 25 m. The two glass plates have transparent electrodes on their inside faces made of conducting material indium tin oxide.

LCD operation

The working of LC display device is shown in above fig.

The cell is assembled so that LC molecules undergo 90° twist from the top plate to the bottom plate. The cell is sandwiched between two polarisers with their polarisation direction is parallel to the LC direction of each plate.

The incident unpolarised light on the cell is polarised linearly as indicated and undergoes 90° rotation as it passes through the LC before exiting the bottom of the polariser. In this mode of operation, frequently used in LCDs, the cell is transmissive, in the absence of electric field.

When voltage is applied to the electrodes the LC molecules will align with field. Now the incident light do not undergo rotation in polarization direction due to liquid crystal and therefore absorbed by the exit polarizer. Thus the twisted nematic liquid crystal cell is opaque in driven state and transmissive in non driven state.

The LC display device can be operated either in transmission or reflection mode. In the reflector mode, a reflector is placed below the bottom of the polarizer. With no field, therefore, the device reflects the incident light and appears bright. When the field is applied, the direction of polarisation of light travelling across the cell is not rotated and cannot pass through the second polarizer and the device will appear dark.Limitation of twisted nematic displays

(i) Viewing angle is restricted to 45°(ii) Use of polarisers reduces the maximum amount of light

that can be reflected.

Super twisted nematic displays

They are basically TN displays only but they have a twist of 90° to 270° from top to bottom plates, Super twisted nematic displays have greater image contrast and wider range of viewing angles. High resolution displays are developed using STN.

S.No.LED LCD

Demerits Merits

1 Cost is high compared to LCD Cost is very low.

2. Not suitable for large area display Suitable for large areas display

3. High consumption Power (milliwatts) Low power consumption (microwatts)

Merits Demerits

4. Operating temperature is 0° to 70°C. Operating temperature is 10°C to 47°C.

5. Response time is in nano seconds (10-9 sec)

Response time is in microseconds (10-6 sec)

6. Intensity of light can be controlled Intensity of light cannot be controlled.

7. Different colour displays are available at low cost. Colour displays will not be available at low cost.

Comparison between LED and LCD

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