Why don't room temperature superconductors exist?

Question

It seems like high-temperature and low-temperature superconductors are not too rare. But, why don't any superconductors work at room temperature? No theories seem to predict their occurrence, but none seem to contradict their existence.

Answer 1

The honest answer is we don't know

As DrMoishe Pippik points out in their answer we have a good theoretical explanation of superconductivity only for one class of superconductors. And that Nobel Prize winning BCS theory explains why that class of superconductors has a problem with high temperatures because higher levels of vibration in the lattice of the structure interferes with the mechanism of superconductivity.

But chemists have discovered superconductivity in other classes of compound where that mechanism can't be the explanation.

The original (BCS) superconductors were mostly metals or alloys. The first "high temperature" superconductors (if temperatures of -200°C can be thought of as "high") were ceramic materials. Some more recent discoveries are different classes of compounds.

The problem with many of the recent compounds is that theory has not caught up with experiment. There are theories but none are yet proved. We know from some observations that the mechanisms cannot be the BCS mechanism observed for the original superconductors. But we can't even be sure that the mechanisms for high temperature superconductivity are the same in all the different classes of new superconductors.

So, at least until theory catches up with experiment, we just don't have a convincing explanation for why room temperature superconductivity has not yet been observed. But experimental chemistry is fun and we might find some room temperature superconductors, before we understand how they work.

Answer 2

(My answer is a bit off-topic to the theoretical "why" answered well by others, but I believe the particular wording of OP's post is evocative of a relatively common misunderstanding of the peculiar terminology, which I'll address here if not for OP then for others who may need it.)

It seems like high-temperature and low-temperature superconductors are not too rare. But, why don't any superconductors work at room temperature?

I think it is important to know that "low-temperature" and "high-temperature" are jargon-specific terms in this context and don't relate to "room-temperature" the way one would expect in normal everyday language.

Finding a room-temperature superconductor isn't a matter of finding something between high-temperature and low-temperature examples, it's actually a matter of discovering a "higher-than-high-temperature" superconductor!

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The "low-temperature" and "high-temperature" naming convention is an unfortunate byproduct of superconductivity's scientific history.

Superconductors were first discovered in 1911 when mercury was cooled below 4.15K and then later other pure metals like tin and lead were found to be superconductive at similarly cold temperatures (3.7K and 7.2K respectively). Various other pure metal superconductors were discovered thereafter, but still they were all superconducting only in these very cold single-digit-Kelvin temperature ranges. Additionally, superconductors that weren't pure-metals, but rather compounds and alloys, were discovered over the following decades, but even among the record-breaking standouts the temperature range for superconductivity was below ~25K.

Then, everything changed with the discovery of a 35.1K cuprate superconductor in 1986. This was a huge leap compared to the previous 23.2K record-holding compound of niobium-germanium, but what's more, is that by 1987 other scientists had discovered a similar cuprate compound with a mind-boggling 93K superconducting temperature. This was especially impressive as it was the first material discovered to superconduct above the 77K boiling point of liquid-nitrogen!

It is around this time that the unfortunate naming of "low-temperature" versus "high-temperature" superconductors cropped up. To superconductivity scientists whose specialty was performing cryogenic experiments in the <30K temperature range, this new class of superconductors operating above 77K was absolutely sweltering! So, as unfortunate as it is, a naming convention of "low-temperature superconductor" (for the oldschool <30K stuff) and "high-temperature superconductor" (for the >30K new stuff) ended up becoming the widespread shorthand jargon in the scientific community.

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All that is to say that scientists are still doing the same work as ever of trying to discover the record-holding highest-temperature superconductor.

The late-1910s record-breaking superconductors worked above the liquid-helium boiling point of 4.2K.

The late-1980s brought the record above the liquid-nitrogen boiling point of 77K.

The record holders from the 1990s brought things closer to ~150K.*
*(Which is better than 77K by a good margin, but still significantly colder than say the 194K of dry-ice).

But, same as ever, getting a record-holder to work somewhere in the ballpark of ice-bath (273K) to water-bath (283K) temperatures, or ideally even higher, would be extremely important for practical applications.

Answer 3

For classical superconductivity, the Bardeen–Cooper–Schrieffer (BCS) theory states electrons travel in Cooper pairs. Since, at the quantum level, it is hard to distinguish one electron from the other, it forms a condensate. When one electron runs into an imperfection, it simply tunnels around it, with its mate. "Because the pairing increases this energy barrier, kicks from oscillating atoms in the conductor (which are small at sufficiently low temperatures) are not enough to affect the condensate as a whole."

That said, there are higher temperature superconductors, and some do work near room temperature (well, perhaps in Hôtel de Glace, with the caveat at high pressure. For example, $\ce{LaH10}$, at 170 GPa and -23°C.

Answer 4

Different materials have different limitations. Sometimes spins of charge-carriers are not paired strongly enough, sometimes all the paired electrons form localized covalent bonds.

To form a stable superconducting condensate, a material needs multiple simultaneously present structural features:

1. Self-doping ability: If no mixed-valence element was present, and an usual dopant was added, the resulting irregularity would create weak links in the condensate.(The idea of adding some usual dopant in an ordered structure is experimentally unfeasible).
2. Polarizable insulating units, which interact with the condensate to suppress the amplitudes of thermal fluctuations of condensate density.
3. Covalent bonds to prevent the whole material from expanding (because the condensate itself is essentially non-bonding).
4. Effectively reduced dimensionality over the places with the highest condensate density to create a locally antiferromagnetic-like pairing of spins.
5. High density of delocalized (relative to the lattice) electrons (the condensate): Low density would allow thermalization.

All of the above needs to fit into a single material. Keeping in mind, that each atom has only a few valence electrons, and that using larger molecules as building blocks is complicated by the previously mentioned sensitivity to possible structural irregularity, we must admit the highest-temperature-superconductor synthesis is going to be a multi-step process.

It gets even worse, because the transition temperature is sensitively dependent on the carrier concentration, which is hard to predict in the first place.

Answer 5

Entropy.

At room temperature and atmospheric pressure the structure of any material is too noisy. Spins flipping every now and then, atoms vibrating. The current high temperature superconductor works at 93 K (−180.2 °C) because it has a strong crystalline structure.

Furthermore we know that putting some material under an enormous pressure allows superconductivity at room temperature.