Question 6 of 6: Sketches of Electronic, Dielectric, Magnetic and Superconducting Property Dependences

Question 6: Sketches of Electronic, Dielectric, Magnetic and Superconducting Property Dependences (40 marks, mandatory)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

This mandatory question surveys nine characteristic property-vs-temperature (or property-vs-field) dependences across electronic, dielectric, magnetic and superconducting behaviour. Each sub-part below traces the dependence back to a single competing-energy argument: thermal energy (k_BT) working against a band gap, an activation energy, an ordering energy, or an applied bias, pushing the material between two regimes.

(a) Diode output current under a sinusoidal drive

The ideal diode law I = I_s[exp(qV/k_BT) − 1] means forward current rises steeply (and non-linearly) with V once V > 0, while for V < 0 the current saturates at the small, nearly constant reverse leakage −I_s. The output I(t) is therefore not a scaled copy of the sinusoid V(t): it looks like a half-wave-rectified, sharply peaked waveform in phase with the positive lobes of V(t) and is essentially flat and near-zero during the negative lobes.

(b) Dielectric constant vs. temperature for ferroelectrics

Below Tc the material is ferroelectric (a spontaneous, switchable polarization exists); above Tc thermal agitation destroys the long-range dipole ordering and the material becomes simply paraelectric. The dielectric constant is maximum exactly at the transition, where the polarization becomes most susceptible to a small applied field.

(c) Critical field vs. temperature for superconductors

At T = 0 the material can sustain up to Hc(0) before flux penetrates and resistance returns; at T = Tc even H = 0 is enough to destroy superconductivity. NbTi (low-field superconducting magnets, e.g. MRI) and Nb₃Sn (high-field magnets) are two low-T_c superconductors in routine practical use; YBa₂Cu₃O₇ (YBCO) is a widely used high-T_c example.

(d) Conductivity vs. temperature for metals

Increasing temperature increases the phonon population, which scatters conduction electrons more frequently and shortens their mean free path and mobility μ; since carrier density n is essentially fixed in a metal, σ = nqμ falls with T (roughly as 1/T at moderate-to-high temperature).

(e) Conductivity vs. temperature for extrinsic semiconductors

At low T, thermal energy is still ionizing the dopant atoms and σ rises with T (freeze-out region). Once essentially all dopants are ionized, carrier density is fixed by the doping level and σ is nearly flat, drifting only slowly downward from mobility loss (extrinsic/exhaustion plateau). At high T, thermally generated intrinsic electron-hole pairs eventually outnumber the fixed extrinsic carriers and σ rises steeply again (intrinsic region) — a three-region shape that has no counterpart in a metal's simple monotonic decrease.

(f) Conductivity vs. temperature for ionically bonded materials

Conduction in an ionic solid proceeds by thermally activated ion (or vacancy) hopping through the lattice, σ = σ₀exp(−E_a/k_BT), so more thermal energy makes ion migration easier and σ increases with T — the opposite temperature trend from a metal, because the charge carriers and the conduction mechanism are entirely different (ions hopping between lattice sites, not free electrons being scattered).

(g) Remanent magnetization vs. temperature for ferromagnets

At absolute zero the magnetic moments are fully aligned by the exchange interaction; increasing temperature progressively randomizes their orientation (a Brillouin-type curve: nearly flat at low T, falling steeply as Tc is approached), and above Tc thermal agitation overcomes the exchange coupling entirely, leaving the material paramagnetic.

(h) Flux density vs. µ₀H for the four magnetic material classes

Diamagnetic materials (e.g. copper, bismuth, silver) have a small negative susceptibility, so B sits just below the µ₀H reference line. Paramagnetic materials (e.g. aluminum, chromium, manganese) have a small positive susceptibility, so B sits just above the line. Ferrimagnetic materials (e.g. magnetite Fe₃O₄, nickel-zinc and manganese-zinc ferrites) and ferromagnetic materials (e.g. iron, cobalt, nickel) both respond strongly and non-linearly, rising quickly and saturating at a material-specific flux density — ferromagnets typically reach a higher saturation than ferrimagnets of comparable composition because all of their sublattice moments (rather than partially opposing sublattices) add constructively.

(i) Hysteresis loop for ferromagnets

Sweeping H from positive saturation to negative saturation and back traces a closed loop rather than retracing the same curve, because domain-wall motion lags the applied field. Fe-Si (soft, low H_c, used in transformer cores where a narrow loop minimizes hysteresis loss) and Alnico or Nd-Fe-B (hard, high H_c, used in permanent magnets where a wide loop resists demagnetization) illustrate the soft/hard extremes of ferromagnetic behaviour.