By: Fakharyar Khan
Above a conductor’s critical temperature, it’s resistance increases linearly with temperature and becomes nearly exponential as the resistance approaches zero. This makes sense since temperature influences the motion of the electrons in the conductor and resists the current and so the resistance could never be zero. However, when some materials reach a critical temperature, they become superconductors and achieve zero resistance and this causes it to release a strong magnetic field that is known as the Meissner effect.
Due to the unusually strong magnetic field that they exert , superconductors have many uses in public transportation, electric generators, and particle accelerators. Since they have zero resistance, they are 100% energy efficient as none of the electrical energy is converted into heat. However, superconductors can only be achieved at temperatures close to absolute zero and the goal of Professor J.J Hamlin and his team’s experiment is to explore how applying and decreasing pressure of superconductors, namely Barium, at low temperatures can create metastable structures that can exist in relatively higher temperatures.
The researchers take an interest in the phase Barium VI because it has a maximum transition temperature, the temperature at which a superconductor reverts back to a conductor, of 8K and exists at pressures from 12-30 GPa which are both substantially higher than those of the other phases of Barium. Also, BaVI has an orthohombric structure, a crystal structure where the axes are unequal and perpendicular to each other, which is much simpler than BaV and easier to do calculations with. Barium VI can be created by applying constant pressure at room temperature and then cooling it down to below 100K. This happens because when Barium-II reaches temperatures below 100K, there isn’t energy for it to transition into Ba-IV but the applied pressure changes the structure of Ba-II to form a semi-permanent structure called Ba-VI.
The researchers used a membrane driven diamond anvil cell to exert pressure onto pieces of Barium of size 100 um X100 um X 10 um. They then used a Van der Pauw method to measure the resistance of the conductor as this would tell them when the piece of Barium reaches superconductivity. They found that the optimal conditions for which Barium VI will form is when it is given a pressure of 8GPa at room temperature and cooled down to below 100K.
They now wanted to explore the superconductive properties of Barium VI. They used a primary coil system and connected it to a transformer to measure AC magnetic susceptibility. This introduced a sinusoidal varying magnetic field by an AC wire which they then used the transformer to measure the magnetic susceptibility which is a measure of how much a conductor has become magnetized.
The figure below is a graph of temperature versus magnetic susceptibility at different pressures for Barium. The rough drops in the first half of the graph are when the phase transitions of Barium occur. They concluded that during the Ba-IV -> Ba V transition, the graph of transition temperature vs pressure is a smooth function of order 2. They also found that the transition of Ba-VI -> BaV at above 30 GPa wasn’t immediate and took a significant amount of time which could be useful.
The researchers believe that these states are created by the interactions between charge density waves and the formation of pseudo gaps in the crystal structure. A charge density wave is created through the interference patterns of the wave functions of several electrons of opposite momenta. Pseudo gaps are formed when an electron is repelled from its Cooper pair leaving an energy gap in the lattice which can be intercepted by electrons travelling parallel to the bond. As atoms tend towards a state of least energy these two paths compete for electrons which is attributed to an increase in the transition temperature and the formation of a new structure.
I believe that their experiment was very rigorous and left little room for error. However, as they have readily acknowledged, in calculating the resistivity of the superconductor, they left the thickness of the piece of barium constant throughout the experiment when the piece would have become compressed and the thickness would increase. They estimated the error bound to be within a factor of 2 which I believe is significant considering the type of conclusions made for that part of the experiment. The thickness should be accounted for even if there is a possibility that it is insignificant. They also mentioned in the introduction of a previous experiment where a researcher had released pressure at below 50K and measured the transition temperature to be 7K at 27 GPa which is the researchers believe was the formation of Ba-VI. I think that they should have a trial where they test the effect of decreasing instead of increasing pressure while decreasing the temperature and see how much that affects the transition temperature.
Sources Cited
arXiv:1707.05860 [cond-mat.supr-con]