One of the great challenges for solid-state physicists is to find materials that superconduct at room temperature. Conventional superconductors have never seemed promising candidates for this accolade— the highest superconducting temperature that these materials have reached is 39 Kelvin for magnesium diboride.

In 1986, physicists discovered that some ceramic materials could superconduct at much higher temperatures. These rely in an entirely different way to conventional superconductors. Nobody is quite sure how but the current record-holder—a ceramic material made of mercury, barium, calcium, copper and oxygen—superconducts at 133 kelvin (-138 degrees Celsius). Even higher temperatures are possible (164 Kelvin) at high pressure.

Today, Mikhail Eremets and a couple of pals from the Max Planck Institute for Chemistry in Mainz, Germany, say they have smashed this record. These guys have measured sulphur hydride superconducting at a temperature of 190 Kelvin (-83 degrees Celsius). There is a caveat, of course. The material has to be squeezed at pressures greater than 150 gigapascals — that’s about half the pressure at the centre of the Earth.

But here’s the thing: sulphur hydride is not one of these new ceramic high-temperature superconductors. On the contrary, it appears to be a conventional superconductor described by a theory that has been well understood for over 50 years.

If the discovery is confirmed, sulphur hydride is likely to become one of the most closely studied materials on the planet, not least because a better understanding of its properties could open the floodgates to the discovery of other materials that superconduct at even higher temperatures.

First, some background. The theory that describes conventional superconductivity was proposed in 1957 by John Bardeen, Leon Cooper and John Schrieffer and consequently given the name BCS theory. Their idea is that when certain metals are cooled to low temperature, the lattice becomes rigid enough to allow the coherent movement of vibrations called phonons.

Since the lattice is positively charged, these vibrations can resonate with pairs of negatively charged electrons called Cooper pairs. These also form when the temperature drops below some critical threshold.

It turns out that a phonon and a Cooper pair can together travel through the lattice with little or no resistance. This is conventional superconductivity.

However, as the temperature increases, the lattice begins to vibrate more strongly and at a critical temperature, the Cooper pairs break up and the superconductivity stops. The measurement of this change at a critical temperature is one of the standard tests for superconductivity along with the measurement of zero resistance.

BCS theory predicts that superconducting materials also expel any magnetic field, the so-called Meissner effect. This is another key test of superconductivity. The theory goes on to suggest that the material can expel fields only up to a certain strength. So there is also a critical field strength above which the material becomes an ordinary conductor again.

The BCS theory has another consequence. Because the vibrations within the lattice depend on the mass of the atoms, the critical superconducting temperature depends on this mass. So when an atom is replaced by a heavier or lighter isotope, the superconducting temperature should change accordingly.

Physicists have found all of these effects in conventional superconductors and they are now used as important tests of whether or not superconductivity has been observed.

Many physicists think that BCS theory must somehow forbid superconductivity at temperatures above 30 or 40 Kelvin. And the observational evidence, until now, has certainly supported this view. Nobody has found a conventional superconductor that operates above these temperatures.

But in fact, there is nothing in the theory to prevent conventional superconductivity at higher temperatures. Indeed, theoretical physicists have had some fun predicting just how hot these materials can become and still superconduct.

Back in 1968, the British physicist Neil Ashcroft predicted that hydrogen could superconduct at high temperatures and pressures. His idea was that because hydrogen is so light, it should form a lattice that is able to vibrate at very high frequencies and therefore superconduct at high temperatures.

Since then, other physicists have shown that hydrogen ought to be up to do this even at room temperature (albeit at pressures greater than those at the centre of the Earth).

Ashcroft has gone even further. In 2004, he predicted that metallic alloys of hydrogen known as hydrides should also superconduct at high temperatures and suggested that this should be possible at pressures that are achievable with the current generation of diamond anvil cells.

It is this prediction that has the closest resemblance to the work of Eremet and co. To carry out the experiment, these guys poured liquid sulphur hydride at a temperature of around 200 Kelvin into a diamond anvil cell equipped with electrodes capable of measuring conductivity and resistance. They then varied the pressure and temperature inside the cell to see how the resistivity changed.

The results make for fascinating reading. They first showed that the resistivity of sulphur hydride indeed drops by several orders of magnitude at low temperatures. That’s not really surprising.

However, they also found that the critical temperature at which superconductivity occurs increased as they increased the pressure. To their surprise, at the pressure of 150 gigapascals, the critical temperature rose to 190 Kelvin. By this, they mean they measured a phase change at this temperature in which the conductivity changed by several orders of magnitude. “We found superconductivity with Tc≈190 K in a H2S sample pressurized to P>150 GPa at T>220 K,” they say

That is an extraordinary result. It smashes the record for a conventional superconductor by 150 kelvin and beats the record for a ceramic high-temperature superconductor under pressure by more than 20 kelvin.

Of course, the phase change and zero resistance that they measured are just two of the crucial tests necessary to confirm superconductivity. However, the team also replaced the hydrogen in the experiments with its heavier isotope, deuterium. In this case, they found that the critical temperature drops accordingly, just as BCS theory predicts.

They also subjected the sample to a powerful magnetic field of up to 7 Tesla in the hope of destroying the superconductivity. In this way they were able to lower the critical temperature, as expected, but were not able to destroy the superconductivity. They say the data suggests that fields of up to 70 Tesla would be required for this.

All this amounts to important evidence that superconductivity was indeed taking place. “We proved occurrence of superconductivity by the drop of the resistivity at least 50 times lower than the copper resistivity, the decrease of Tc with magnetic field, and the strong isotope shift of Tc in D2S which evidences a major role of phonons in the superconductivity,” say Eremet and co.

However, it is fair to say that this result raises some important questions. The first is that theoretical studies of sulphur hydride predict that it should superconduct at temperatures of up to 80 Kelvin. Eremet and co certainly seem to have confirmed this, an extraordinary result by itself.

But the additional jump in critical temperature to 190 Kelvin is unexpected and unexplained. A good theoretical treatment that explains this jump will be required before other physicists accept it as a genuine phenomenon of BCS theory.

Physicists might also want a clear demonstration of the Meissner effect to confirm the presence of superconductivity. An alternative demonstration might involve spinning the superconducting sample to create a magnetic field, an effect known as the London moment.

The history of superconductivity is littered with false claims of high-temperature events that have subsequently turned out to be ambiguous and unreproducible. Indeed, physicists have a name for these observations— unidentified superconducting objects or USOs.

So the most important additional evidence must come in the form of the reproduction of this result in another lab. Clearly, Eremet and co have put their work on the arXiv for precisely this reason. The pedigree of the team and their institution — the Max Planck Institute for Chemistry— should certainly have piqued the interest of many colleagues.

The experiment is certainly within the capability of many other labs around the world so if there is going to be confirmation, we should hear about it soon. If not, the work may join the many other USOs that have plagued this field over the years.

Confirmation will be big news though. If sulphur hydride superconducts at high temperature, many other hydrogen-bearing materials ought to as well, possibly at room temperature. Eremet and co say it may be worth taking a closer look at fullerenes, aromatic hydrocarbons and graphanes. And they raise the tantalising prospect that superconductivity in these substances may come about, not by increasing pressure, but by doping instead.

A field worth watching!

Ref: arxiv.org/abs/1412.0460 : Conventional Superconductivity at 190 K at High Pressures