Superconductors — Zero Resistance and the Meissner Effect
Below a critical temperature some materials conduct with exactly zero resistance and expel magnetic fields. How it works, from mercury at 4.2 K to high-Tc…
Quick answer
A superconductor cooled below its critical temperature Tc conducts electricity with literally zero resistance and expels magnetic fields from its interior — the Meissner effect. The first was mercury at 4.2 K, discovered by Kamerlingh Onnes in 1911.
Unit: kelvin (K) for critical temperature
Example: Start a current in a superconducting ring and it flows for years with no battery. In copper the same current would die in a fraction of a second — zero resistance means truly zero energy loss.
What you will learn
- Define superconductivity and the critical temperature Tc
- Explain zero resistance via persistent currents
- Describe the Meissner effect and why it differs from perfect conductivity
- Distinguish Type I and Type II superconductors
- Summarize BCS theory's Cooper-pair idea
The 1911 discovery
Heike Kamerlingh Onnes had just learned to liquefy helium, reaching temperatures near absolute zero, and he used the new cold to test mercury's electrical resistance. He expected it to level off. Instead, at 4.2 K, the resistance abruptly vanished — not small, but zero, below anything his instruments could detect.
He had found superconductivity: below a material-specific critical temperature , electrical resistance is exactly zero. The discovery won him the 1913 Nobel Prize in Physics.
Zero resistance, exactly
Zero does not mean 'very small' — it means a current started in a superconducting ring keeps flowing with no power source, decaying so slowly that experiments have watched persistent currents for years without measurable loss. Ordinary copper would kill the same current in milliseconds.
There are limits: push the current past a critical current density, apply too strong a magnetic field, or warm above , and superconductivity quenches back to normal. Superconductors live inside a three-dimensional boundary of temperature, field, and current.
The Meissner effect: expelling magnetism
In 1933 Meissner and Ochsenfeld found the second miracle: a superconductor doesn't just conduct perfectly — it actively expels magnetic fields from its interior, becoming a perfect diamagnet. Drop a magnet above a cooled superconductor and it levitates, pinned in midair.
This is not what a merely perfect conductor would do. A perfect conductor traps whatever field was present when it became perfect; a superconductor expels the field even if applied after cooling. The Meissner effect proves superconductivity is a distinct thermodynamic state, not just infinite conductivity.
Perfect conductivity would freeze magnetic field lines in place. The Meissner effect pushes them out. That difference is why superconductivity needed a whole new theory, not just 'really good metal'.
Type I vs Type II
Type I superconductors (pure mercury, lead, aluminum) expel fields completely — until a modest critical field destroys superconductivity all at once. Their critical fields are tiny, useless for magnets.
Type II superconductors (niobium alloys, cuprates) allow magnetic field through in microscopic quantized filaments while staying superconducting around them — the mixed state — surviving fields thousands of times stronger. Every practical superconducting magnet, from MRI machines to the Large Hadron Collider, is Type II.
High-temperature superconductors
For 75 years every superconductor needed liquid helium (below ~20 K). Then in 1986 Bednorz and Müller found superconductivity in copper-oxide ceramics near 35 K — work that won the 1987 Nobel Prize — and within a year YBCO (yttrium barium copper oxide) reached K.
That crossed a magic line: 93 K is above liquid nitrogen's boiling point (77 K), so cooling got enormously cheaper. But 'high-temperature' still means −180 °C, and after decades of research the pairing mechanism in cuprates is still debated — the honest frontier of the field.
BCS theory: Cooper pairs
In 1957 Bardeen, Cooper, and Schrieffer explained conventional superconductors: at low temperatures, lattice vibrations give electrons a slight effective attraction, and electrons pair up into Cooper pairs.
Pairs behave nothing like single electrons — they condense into a single quantum state that flows without scattering off the lattice. No scattering means no resistance. Break the pairs (with heat, field, or current) and normal resistance returns. BCS predicts values beautifully for conventional materials — and fails for the cuprates, which is why they remain mysterious.
Where superconductors work today
Superconducting magnets are the quiet workhorses of big science and medicine: every MRI scanner uses niobium-titanium coils bathed in liquid helium, the Large Hadron Collider steers protons with more than a thousand superconducting dipole magnets, maglev trains float on superconducting repulsion, and pilot power cables carry city-scale current with no resistive loss.
The dream — a room-temperature superconductor for lossless grids and pocket MRIs — stays unfulfilled. Verified claims would transform civilization; unverified ones surface every few years and collapse under scrutiny.
Variables and units
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Critical temperature | K | Below this, the material superconducts | |
| Critical magnetic field | T | Field strength that destroys superconductivity | |
| Critical current density | A/m² | Maximum current before quenching | |
| Cooper pair | — | Bound electron pair flowing without scattering |
Common mistakes
Only below Tc. Warm mercury even slightly above 4.2 K is an ordinary resistive metal — the transition is sharp, not gradual.
A perfect conductor would trap existing field lines; a superconductor expels them even when the field is applied after cooling. Different physics, different state of matter.
The record cuprates work near 93–135 K — still around −180 °C. 'High' is relative to liquid helium, not to your kitchen.
Real-world applications
MRI scanners image bodies with niobium-titanium superconducting magnets.
The Large Hadron Collider steers particle beams with more than a thousand superconducting dipole magnets.
Maglev trains levitate on superconducting magnetic repulsion.
Pilot superconducting cables carry grid-scale power without resistive losses.
Frequently asked questions
What is the critical temperature?
The temperature below which a material becomes superconducting — 4.2 K for mercury, 9.2 K for niobium, ~93 K for YBCO. Each material has its own Tc, and crossing it changes the resistance from normal to exactly zero, sharply.
Why do superconductors need to be so cold?
Heat shakes the crystal lattice and breaks Cooper pairs apart. Only when thermal energy drops below the pair binding energy can pairs survive and condense into the resistance-free state. Higher-Tc materials have stronger pairing — the search is really a search for stronger glue.
What is a Cooper pair?
Two electrons bound weakly together by lattice vibrations (phonons): one electron distorts the lattice, and the second is attracted to that distortion. The pair has opposite momenta and spins, and huge numbers of pairs lock into one quantum state that flows without scattering.
Will we ever get room-temperature superconductors?
Unknown. It would revolutionize power grids, transport, and medicine — but every claimed room-temperature superconductor so far has failed independent verification. Treat new announcements with excitement and skepticism in equal measure until reproduced.