Superconductivity, the phenomenon in which a material conducts electrons without resistance, is the tantalizing theoretical mystery of condensed matter physics attracting some of the greatest minds in the field. Now, some incredible news has come out of the Max Planck Institute in Germany that will add even more debate and wonder surrounding the origins of superconductivity in materials near room temperature. Researchers there have found that sulfur hydride (H2S) superconducts at an astounding 190 K, only about 100 K away from the holy grail of the field – room temperature superconductivity (around 290-300 K)!
This finding, if replicated, could completely change how researchers are currently searching for high-temperature superconductors. Discovering a material that can superconduct at room temperature would transform how our society transports and stores energy forever. Before we get into the details of this new finding, let’s first set the stage to better understand why this finding could be so important.
For some time, experts have believed that they understand how certain classes of superconducting materials work, known as conventional superconductors. After superconductivity was first discovered in 1911, theorists scrambled to describe some model explaining the surprising zero-resistance phenomenon found at low temperatures. It took a long time. Four decades later, Bardeen, Cooper, and Schrieffer proposed a mechanism, now known as BCS theory, in which vibrations of the atomic lattice (red atoms below) work coherently and bring pairs of electrons (green ‘-‘ below) close together.
Electrons and nuclei are attracted to one another, so an electron will pull all the surrounding nuclei closer to it. This, in turn, creates a region of higher positive charge density that attracts another electrons. These two electrons, known as Cooper pairs, have highly correlated motion and can be viewed as one object, a boson in physics-speak. In a given material, there are many, many of these Cooper pairs that highly overlap such that they all form what is known in physics as a condensate. Below a critical temperature (Tc) that changes for different materials, the material undergoes a phase change in which the condensate is bound together strongly enough that lattice vibrations from surrounding atoms don’t have enough energy to break it up. All these electrons in the condensate move collectively with zero resistance! At high enough temperatures, nuclei begin to vibrate with much more energy and eventually can disrupt the condensate, destroying superconductivity and restoring the material to a state with finite resistance.
Since this mechanism has been understood, researchers have tried to take advantage by designing materials with the essential ingredient: lighter elements that have higher frequency lattice vibrations (the high frequency vibrations help bring the Cooper pairs together). Some materials have been found this way, but the critical temperature has always remained low, with MgB2 holding the record at Tc = 39 K. Many physicists believed that something about the BCS theory prohibited supeconductivity above about 30 K. Back in the 60’s, another theoretical physicist, Ashcroft, predicted that hydrogen-based materials should have great potential as BCS superconductors due to the element’s light weight, but nothing ever came from these conjectures.
In the 1980’s, a new type of superconductivity, which we still don’t completely understand, was found in ceramic materials that often contain copper and oxygen (known as cuprates), that set the new record for superconductivity around 130 K!
These sexy new materials have attracted much of the research effort in past years, from both theorists and experimentalists trying to understand the novel mechanism that appears to be different than Cooper pair formation. Meanwhile, BCS theory has taken a back seat…until now!
The researchers at Max Planck revisited the ideas of Ashcroft from 50 years ago and looked into hydrogen-based materials that exhibit the high-frequency lattice vibrations thought to be so important. Choosing H2S possibly because of its complex phase diagram (or possibly by trial error/chance), they squeezed the substance to high pressures, a method that is known to assist in creating superconducting phase transitions. The way experimentalists do this is quite cool – they use an anvil cell in which two diamonds squeeze the substance from both sides to incredibly high pressures.
Amazingly, the researchers found a superconducting state for H2S at Tc = 190 K by applying a pressure of P = 150 GPa! GPa is a gigapascal. To put this in perspective, the pressure you feel from all the mass in an atmospheric column above you is about 0.0001 GPa, so this is a pressure of 1.5 million atmospheres pushing down on you. So, yes, this finding isn’t exactly useful for everyday applications that you would want to work at lower pressures. But the mere existence of this high critical temperature for a conventional BCS superconductor is monumental, surpassing previous Tc records by 160 K, after many thought BCS theory was basically useless after 30K! This will motivate many, many researchers to pursue conventional superconductivity again, likely in hydrogen-based molecules, and the more minds being applied, the faster progress will be made.
These results definitely need to be replicated, but the authors included a thorough analysis of all the classic measurements used to show BCS-type superconductivity. First, they measured resistance as a function of the material’s temperature at increasing pressures, shown below on the left:You can see the resistance (y-axis) decreases as more pressure (each line signifies a different pressure) is applied in the anvil cell, and a critical point in temperature occurs when the resistance dramatically drops to zero. This is a first clear sign of superconductivity. Second, shown above on the right, the researchers compared sulfur hydride with sulfur deuteride, in which hydrogen is replaced with deuterium, a heavier hydrogen isotope that contains a neutron. BCS theory predicts that lighter atoms lead to higher frequency lattice vibrations and a higher critical temperature, which is exactly what the authors find above. The sulfur deuteride (red line) has a Tc = 90 K, about half that of H2S (green line). This is the first test to show that conventional Cooper pair formation is probably the type of superconductivity at play in sulfur hydride.
The final test for BCS superconductivity uses magnetic fields. BCS theory predicts the so-called Meissner effect, stating that conventional superconductors should expel all magnetic fields. Conversely, strong applied magnetic fields can destroy existing superconductivity, which the authors test and report in the results shown below.As seen here, increasing magnetic field strength (each curve represents a different strength measured in Teslas), leads to lower and lower critical temperatures, measured along the x-axis. This is an indication that the magnetic field is destroying the superconducting state and another strong piece of evidence that this is conventional superconductivity.
The next step for the authors of this paper will be to examine other materials made of hydrogen and carbon instead of sulfur – fullerenes, graphane, and aromatic hydrocarbons. A key improvement on the current findings will be to find a way other than pressure to increase the superconducting critical temperature. Doping of these materials is one possible route and theoretical work providing options for suitable dopants will be important complements to ongoing experimental work. It’s always exciting for the scientific community when a finding like this opens up so many new avenues of research!
P.S. I’d like to thank my colleague, Robert Hembree, for letting me know about this article!
A. P. Drozdov, M. I. Eremets, & I. A. Troyan (2014). Conventional superconductivity at 190 K at high pressures arXiv arXiv: 1412.0460v1