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**Introduction:**

Solid state physics is a study of crystalline solids, through methods like quantum mechanics, electromagnetism, crystallography, and metallurgy. It is largest branch of condensed matter physics. Solid state physics studies how large-scale properties of solid materials result from atomic-scale properties. Therefore, solid state physics forms theoretical basis of material science. It has direct applications, for instance in the technology of transistors and semiconductors. Solid materials can be separated into two distinct groups on the basis of their behaviour under influence of the external electric field.

Conductors - those in which there are electrons that are free to move in presence of field. Examples of these are metals, carbon, etc.

ii. Insulators or dielectrics - those in which electrons are strongly bound to atoms or molecules composing material and can't be detached by application of the electric field to the materials. Examples are sulphur, porcelain, mica, etc.

*Dielectric Properties:*

The dielectric is a non-conducting material, like glass, rubber, or waxed paper. The following are some of the properties of dielectric:

i) When the dielectric material is inserted between plates of capacitor, the capacitance increases by factor K, known as dielectric constant of dielectric

C = KC_{0}

Where C_{0} is capacitance in absence of dielectric

ii) the dielectric constant K has no unit and it is characteristic of given material

iii) If ε_{o} is permittivity of free space and ε is permittivity of dielectric material, therefore

K = ε/ε_{0 }

iv) With introduction of dielectric materials, energy density becomes

u = 1/2εE^{2}

v) Potential difference V between plates when the dielectric material is inserted, given that charge remains unchanged, is provided as:

V = V_{0}/K

Where V_{o} is potential difference when dielectric is not inserted. Equation defines that potential difference V between plates decreases by the factor of K.

vi) If potential difference is kept unchanged when dielectric material is inserted, then we have,

Q = KQ_{0}

Therefore, charge on the capacitor with fixed potential difference between its plates is increased by factor K

vii) If electric field between parallel plate capacitor is E_{o} and it is E after material with dielectric constant K is inserted into space between plates, then

E = E_{0}/K

The electric field within dielectric is reduced by factor equal to dielectric constant

viii) Net field E within dielectric is provided as

E = E_{0} - E_{ind}

E_{ind} = E_{0}(1 - 1/K)

ix) The induced electric field in dielectric is related to induced charge density σ_{ind} through relationship E_{ind} = σ_{ind}/ε_{0}

x) σ_{ind} = σ(1 - 1/K) and Q_{ind} = Q(1 - 1/K)

As K is always greater than 1, these expressions show that E_{ind} > E_{o}, σ_{ind} > σ and Q_{ind} > Q.

*Local Electric Field:*

It must be noted that:

- every material is composed of very large number of atoms/molecules
- Atom comprises of positively charged nucleus and negatively charged electrons
- System comprising of two equal and opposite charges q, separated by the certain distance d, is electric dipole
- When the atom or molecule of dielectric is placed in electric field, positive and negative charges feel opposite forces and are separated forming a dipole.

Assume the charges -q and +q are placed, respectively, at d/2 and -d/2 from origin, as shown below. Magnitude of potential because of this system at a point X is provided by

Φ(r) = q/4πε_{0}(1/r_{1} - 1/r_{2})

Where r_{1}, r_{2} are distances of X from + q and - q respectively.

If θ is angle between axis of charges and position vector of X, we can write

1/r1 ≈ 1/r + dcosθ/2r^{2} and 1/r^{2} ≈ 1/r - dcosθ/2r^{2}

After solving:

Φ(r) = (q/4πε0)(dcosθ/r^{2})

Dipole moment, p, is vector along axis of dipole pointing in direction - q to + q and having magnitude qd. In terms of dipole moment, Equation becomes:

Φ(r) = pcosθ/4πε_{0}r^{2} = p‾.r‾/4πε_{0}|r|^{3}

Φ(r) = -P‾/4πε_{0}r^{2} . ∇(1/r) = -p‾.∇Φ_{0}

Φ_{0} is potential of unit charge.

Field of electric dipole may also be stated in following way,

E‾ = 1/4πε_{0}[(3(p‾.r‾)r‾ - r^{2}p‾)/r^{5}]

Considering cubic crystal dielectric and suppose electric field of magnitude E_{o} is applied parallel to one of axes of crystal. Field will push positively charged nucleus of atoms of dielectric slightly in direction of field and negatively charged electrons in opposite direction. We say that atom is polarized under influence of applied external field and charges are called polarization charges. The polarization P is stated as dipole moment p per unit volume, averaged over volume of the cell. Total dipole moment is stated as

P‾ = Σq_{n}r_{n}‾

Let us imagine that small sphere is cut out of specimen around reference point; then E_{1} is field of polarization charges on inside of cavity left by sphere, and E_{2} is field of atoms within cavity. Polarization charges on outer surface of specimen produce depolarization field, E_{3}. Depolarization field is opposite to P, that is, field E_{3} is known as depolarization field, as within body it tends to oppose applied field E_{o}. The dielectric obtains polarization because of applied electric field E_{o}. This polarization is result of redistribution of charges inside dielectric.

The local electric field E_{loc} at any atom of dielectric may be expressed as

E_{loc} = E_{0} + E_{1} + E_{2} + E_{3}

Contribution E_{1} + E_{2} + E_{3} is total effect at one atom of dipole moments of all the other atoms in system (in CGS units):

E_{1} + E_{2} + E_{3} = (3(p‾.r‾)r‾ - r^{2}p)/r^{5 }and in SI unit E_{1} + E_{2} + E_{3} = 1/4πε_{0}[(3(p‾.r‾)r‾ - r^{2}p)/r^{5}]

If P_{x}, P_{y}, P_{z} are the components of the polarisation P referred to the principal axes of an ellipsoid, then the components of the depolarization field are written

E_{3x} = -N_{x}P_{x}/ε_{0}; E_{3y} = -N_{y}P_{y}/ε_{0}; E_{3Z} = -N_{z}P_{z}/ε_{0}

*Dielectric Constant and Polarizability:*

Consider dielectric behaviour of molecules that have permanent dipole moment. Dielectric constant ε has been expressed for isotropic or cubic medium relative to vacuum as:

ε = D/E = 1 + P/ε_{0}E = 1+χ

Where χ is electric susceptibility.

Susceptibility (in SI unit) is related to dielectric constant by

χ = P/ε_{0}E = ε - 1

If field is not too large, strength of induced dipole moment in the atom i is proportional to local electric field acting on dielectric. That is

P_{i} ∝ E_{loc} = P_{i} = α_{i}E^{i}_{loc}

Where α_{i} is Polarizability and p_{i} is dipole moment of atom i.

Electric susceptibility can now be expressed as:

χ = P/E = Σ_{i}E^{i}_{loc}N_{i}α_{i}/E_{loc} - (4π/3)P

Using Lorentz relation,

P/E = ε - 1/4π

Therefore (in CGS) ΣN_{i}α_{i} = 3/4π((ε - 1)/(ε + 2))

This equation is called as Clausius-Mossotti equation, defines relation between dielectric constant and atomic polarizabilities

*Dipole Relaxation and Dielectric Losses:*

Dipole relaxation time is time interval characterizing restoration of disturbed system to equilibrium configuration; the relaxation frequency is stated as reciprocal of relaxation time. Orientational contribution to dielectric constant is the major cause to difference between low frequency dielectric constant and high frequency dielectric constant. Orientational relaxation frequencies are strongly dependent on temperature and frequency as:

α = α_{0}/(1 + iωτ)

Where τ is called as Debye relaxation time, α_{0} is static orientational Polarizability and ω is angular frequency.

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