Which defines a superconductor?

Resistance falls abruptly to zero below Tc and magnetic flux is expelled

Resistance falls gradually to a small residual value near 0 K

It is any metal with very high conductivity at room temperature

It has zero resistance only above Tc

Superconductivity exists only while which quantities are below their critical values?

Voltage, frequency and pressure

Resistance, capacitance and inductance

How do the two types respond as the field rises?

Type-I stays fully diamagnetic up to Hc then turns normal suddenly; type-II has a mixed state between Hc1 and Hc2

Type-I has a mixed state between Hc1 and Hc2; type-II turns normal suddenly at Hc

Both turn normal suddenly, but type-II at a higher field

Type-I has two critical fields and type-II has one

Why is type-II called a hard superconductor?

Vortices are pinned at defects, giving irreversible magnetization, hysteresis and high Jc, and it survives very high fields

It is made of pure soft metals like Pb and Hg

It has a lower critical field than type-I

It shows completely reversible magnetization

Asked 2/28
2068 Shrawan · 8 marks
2068 Baisakh · 3+5 marks

Superconductor types I and II

Short answer

A superconductor has zero resistance and perfect diamagnetism below Tc. Type-I: single critical field Hc, complete flux expulsion up to Hc then abrupt transition (e.g., Pb). Type-II: two critical fields Hc1, Hc2; mixed vortex state between them; high Hc2 allows high current, used in magnets (e.g., NbTi, YBCO).

What is superconductor? Differentiate between Type-I and Type-II superconductor.

Superconductor

A superconductor is a material whose electrical resistance falls abruptly to zero when it is cooled below a certain critical temperature $T_c$ , and which expels magnetic flux from its interior (Meissner effect). Example: mercury becomes superconducting at 4.2 K (discovered by Kamerlingh Onnes, 1911); YBa $_2$ Cu $_3$ O $_7$ at about 92 K.

Superconductivity exists only while temperature, magnetic field and current density are all below their critical values ( $T_c$ , $H_c$ , $J_c$ ). It is explained by BCS theory: electrons bind into Cooper pairs through lattice vibrations and move without scattering.

  R
    |        ___----
    |   ___--
    |  |   normal metal
    |  |
    |__|__________ T
       Tc

Type-I vs type-II superconductors

Type-I: perfect diamagnet up to $H_c$ , then suddenly normal.
Type-II: perfect diamagnet up to $H_{c1}$ , mixed (vortex) state between $H_{c1}$ and $H_{c2}$ , normal above $H_{c2}$ .

 -M                         -M
  |    /|                    |   /\
  |   / |                    |  /  \
  |  /  |                    | /    \_
  | /   |                    |/       \__
  |/    |____ H              +----|------|--- H
  0    Hc                    0   Hc1    Hc2
   Type-I                     Type-II

Point	Type-I (soft)	Type-II (hard)
Critical field	Single $H_c$	Two: $H_{c1}$ and $H_{c2}$
Meissner effect	Complete up to $H_c$	Complete only up to $H_{c1}$
Transition	Sharp, normal at $H_c$	Gradual; mixed (vortex) state between $H_{c1}$ and $H_{c2}$
Value of field	Low, about 0.01 to 0.2 T	$H_{c2}$ very high, up to 20 to 100 T
Current capacity	Low $J_c$	High $J_c$ (with flux pinning)
Magnetization	Reversible	Irreversible, hysteresis
Materials	Pure metals: Pb, Hg, Sn, Al, In	Alloys, compounds: NbTi, Nb $_3$ Sn, V $_3$ Ga, YBCO
Uses	Limited (research, SQUID parts)	Magnets for MRI, accelerators, maglev

✓ Answer checked by Claude Opus 5.5 (high effort)

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