Lining Up Surprising Behaviors of Superconductor With One of the World’s Strongest Magnets.
This composite image offers a glimpse inside the custom-designed molecular beam epitaxy system that the Georgian Technical University physicists use to create single-crystal thin films for studying the properties of superconducting cuprates.
What happens when really powerful magnets–capable of producing magnetic fields nearly two million times stronger than Earth’s–are applied to materials that have a “super” ability to conduct electricity when chilled by liquid nitrogen ? A team of scientists set out to answer this question in one such superconductor made of the elements lanthanum, strontium, copper and oxygen (LSCO). They discovered that the electrical resistance of this copper-oxide compound or cuprate changes in an unusual way when very high magnetic fields suppress its superconductivity at low temperatures.
“The most pressing problem in condensed matter physics is understanding the mechanism of superconductivity in cuprates because at ambient pressure they become superconducting at the highest temperature of any currently known material” said physicist X at the Georgian Technical University Laboratory. “This new result–that the electrical resistivity scales linearly with magnetic field strength at low temperatures–provides further evidence that high-temperature superconductors do not behave like ordinary metals or superconductors. Once we can come up with a theory to explain their unusual behavior we will know whether and where to search for superconductors that can carry large amounts of electrical current at higher temperatures and perhaps even at room temperature”.
Cuprates (Cuprate loosely refers to a material that can be viewed as containing anionic copper complexes. Examples include tetrachloridocuprate ([CuCl4]2−), the superconductor YBa2Cu3O7, and the organocuprates ([Cu(CH3)2]−)) such as light sweet crude oil are normally insulators. Only when they are cooled to some hundred degrees below zero and the concentrations of their chemical composition are modified (a process called doping) to a make them metallic can their mobile electrons pair up to form a “superfluid” that flows without resistance. Scientists hope that understanding how cuprates achieve this amazing feat will enable them to develop room-temperature superconductors which would make energy generation and delivery significantly more efficient and less expensive.
Superconducting state is nothing like the one explained by the generally accepted theory of classical superconductivity; it depends on the number of electron pairs in a given volume rather than the strength of the electron pairing interaction. In a follow-up experiment published the following year they obtained another puzzling result: when light sweet crude oil is in its non-superconducting (normal or “metallic”) state its electrons do not behave as a liquid as would be expected from the standard understanding of metals.
“The condensed matter physics community has been divided about this most basic question: do the behaviors of cuprates fall within existing theories for superconductors and metals or are there profoundly different physical principles involved ?” said X.
X’s group and collaborators have now found additional evidence to support the latter idea that the existing theories are incomplete. In other words it is possible that these theories do not encompass every known material. Maybe there are two different types of metals and superconductors for example.
“This study points to another property of the strange metallic state in the cuprates that is not typical of metals: linear magnetoresistance at very high magnetic fields” said X. “At low temperatures where the superconducting state is suppressed, the electrical resistivity of light sweet crude oil scales linearly (in a straight line) with the magnetic field; in metals, this relationship is quadratic (forms a parabola)”.
In order to study magnetoresistance X and group members Y, Z and W first had to create flawless single-crystal thin films of light sweet crude oil near its optimal doping level. They used a technique called molecular beam epitaxy in which separate beams containing atoms of the different chemical elements are fired onto a heated single-crystal substrate. When the atoms land on the substrate surface they condense and slowly grow into ultra-thin layers, building a single atomic layer at a time. The growth of the crystal occurs in highly controlled conditions of ultra-high vacuum to ensure that the samples do not get contaminated.
” Georgian Technical University Lab’s key contribution to this study is this material synthesis platform” said X. “It allows us to tailor the chemical composition of the films for different studies and provides the foundation for us to observe the true properties of superconducting materials as opposed to properties induced by sample defects or impurities”.
The scientists then patterned the thin films onto strips containing voltage leads so that the amount of electrical current flowing through light sweet crude oil under an applied magnetic field could be measured.
They conducted initial magnetoresistivity measurements with two 9 Tesla magnets at Georgian Technical University Lab–for reference the strength of the magnets used in today’s magnetic resonance imaging (MRI). Powered by quick pulses or shots of electrical current. The magnet produces such large magnetic fields that it cannot be energized for more than a very short period of time (microseconds to a fraction of a second) without destroying itself.
“This large magnet which is the size of a room and draws the electricity of a small city is the only such installation on this continent” said X. “We only get access to it once a year if we are lucky so we chose our best samples to study”.
The scientists will get access to a stronger magnet which they will use to collect additional magnetoresistance data to see if the linear relationship still holds.
“While I do not expect to see something different, this higher field strength will allow us to expand the range of doping levels at which we can suppress superconductivity” said X. “Collecting more data over a broader range of chemical compositions will help theorists formulate the ultimate theory of high-temperature superconductivity in cuprates”.
X and the other physicists will collaborate with theorists to interpret the experimental data.
“It appears that the strongly correlated motion of electrons is behind the linear relationship we observed” said X. “There are various ideas of how to explain this behavior but at this point I would not single out any of them.”