Phase transitions occur when a substance changes from one solid, liquid, or gaseous state to another, such as ice melt or vapor condensation. During these phase transitions there is a point where the system can simultaneously display properties of both matter states. A similar effect occurs when normal metals go into superconductors – properties vary and properties that are expected to belong to one state are transposed into the other.
Scientists at Harvard have developed a bismuth-based two-dimensional superconductor just one nanometer thick. By studying fluctuations in this ultra-thin material as it transitioned into superconductivity, scientists gained insights into the processes that control superconductivity more generally. Because superconducting materials can be used in virtually any technology in which electricity is used because of their improved ability to conduct zero-resistance electrical currents.
Harvard scientists used the new technology to experimentally confirm a 23-year-old theory of superconductors developed by scientist Valerii Vinokur from the Argonne National Laboratory of the US Department of Energy (DOE).
A phenomenon of interest to scientists is the complete reversal of the well-studied Hall effect in the transition from materials to superconductors. When a normal, non-superconductive material conducts an applied current and is exposed to a magnetic field, a voltage is induced across the material. In this normal Hall effect, the voltage will be in a certain direction depending on the orientation of the field and the current.
Interestingly, the sign of Hall voltage reverses when materials become superconductors. The "positive" end of the material becomes "negative". This is a well-known phenomenon. While the Hall effect has long been an important tool for scientists to study the types of electronic properties that make a material a good superconductor, the cause of this reverse Hall effect has been puzzling for scientists for decades, especially in terms of highly sensitive materials , Temperature superconductor for which the effect is stronger.
In 1996, the theorist Vinokur, an Argonne Distinguished Fellow, and his colleagues presented a comprehensive description of this effect (and more) in high-temperature superconductors. The theory took into account all the driving forces and contained so many variables that it seemed unrealistic to test them experimentally.
"We thought we had really solved these problems," Vinokur said, "but the formulas felt useless at the time because they contained many parameters that were difficult to compare to the experiments that existed at the time . "
Scientists knew that the reverse Hall effect results from magnetic vortices that emerge in the magnet in the superconducting material field. Vortexes are singularity points in the fluid of superconducting electrons – Cooper pairs – that flow around the Cooper pairs, creating circulating superconducting microcurrents that introduce new physical features of the Hall effect in the material.
Normally Distribution of Electrons The material causes the Hall voltage, but in superconductors, vortices move under the applied current, creating electronic pressure differences that are mathematically similar to those that hold a plane in flight. These pressure differences change the course of the applied current, just as the wings of an aircraft change the course of the passing air, raising the aircraft. The vortex movement distributes electrons differently and changes the direction of the Hall voltage to the opposite of the usual purely electronic Hall voltage.
The theory of 1996 quantitatively described the effects of these vortices, which had only been understood qualitatively. The theory was tested and validated with a novel material that took five years to develop by Harvard scientists.
The thin bismuth-based material is practically only one atomic layer thick and thus essentially two-dimensional. It is one of the few of its kind, a thin-film high-temperature superconductor; The production of the material alone is a technological breakthrough in superconductor science.
"By reducing the size from three to two, the variability of material properties becomes much clearer and easier to study," said Philip Kim, a senior scientist at the Harvard Group. "We have created an extreme form of material that allowed us to quantitatively examine the theory of 1996."
A prediction of the theory was that the anomalous Reverse Hall effect could exist outside of the temperatures at which the material is a superconductor. This study provided a quantitative description of the effect that perfectly matched the theoretical predictions.
"Before we were sure what role swirls play in reverse Hall effect, we could not reliably use it as a gauge," Vinokur said. "Now that we know we're right, we can use the theory to study other variations in the transition phase, ultimately leading to a better understanding of the superconductors."
Although the material in this study is two-dimensional, scientists believe that Theory applies to all superconductors. Future research will include deeper investigations of the materials – the behavior of the vertebrae is even used in mathematical research.
Eddies are examples of topological objects or objects with unique geometric properties. They are currently a popular subject in mathematics because they shape and deform and change the properties of a material. The theories of 1996 used topology to describe the behavior of vortices, and topological properties of matter could bring a lot of new physics.
"Sometimes you discover something new and exotic," Vinokur said about the research, "but sometimes you confirm that you understand the behavior of everyday life right in front of you."
A Treatise with the The title "Sign reversing Hall effect in atomic thin high temperature superconductors" was published on June 21 in Physical Review Letters .
A peculiar ground state phase for 2-D superconductors
Frank Zhao et al., Sign-Reversing Hall effect in atomically thin high-temperature Bi2.1Sr1.9CaCu2.0O8 + δ superconductors, Physical Review Letters (2019). DOI: 10.1103 / PhysRevLett.122.247001
Confirmation of the old theory leads to a new breakthrough in superconductive science (2019, June 28)
retrieved on June 28, 2019
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