Tokyo Institute of Technology research: Insights into the stages of high-temperature superconductivity

(Tokyo, 20 May 2014) Researchers at Tokyo Institute of Technology uncover the complexities of quantum phase fluctuations during the superconductor-insulator transition in high-temperature superconductors.

The superconductor-insulator transition (SIT) in high-temperature copper-oxide (‘cuprate’) superconductors is commonly triggered by the application of a magnetic field. However, due to the complexities of superconductivity, many questions are still to be answered about the exact process which underpins the SIT and the associated quantum phases the material undergoes.

Scientists had thought that high-temperature superconductors had a single quantum critical point at which the material switches from a superconductor to an insulator when a particular strength of magnetic field was applied. Now, an international team of researchers from the USA and Japan, including Takao Sasagawa at Tokyo Institute of Technology, have uncovered a two-stage transition in lanthanum-strontium-copper-oxide high-temperature superconductors (LSCOs), leading to the first complex phase diagram of the behavior of LSCOs.

“The delicate interplay of thermal fluctuations, quantum fluctuations and disorder leads to a complex H-T [magnetic field-temperature] phase diagram of vortex matter,” the authors state in their paper published in Nature Physics.

The researchers measured electrical resistivity of the material in magnetic fields up to 18 T at various temperatures down to 0.09 K, revealing the complete picture of the SIT. They deliberately used a variety of LSCOs that had been created using different techniques, so as to separate out the effects of sample preparation from more general superconductive behavior.

Sasagawa’s team discovered that the LSCOs showed a two-stage magnetic-field-induced transition at T = 0 K before they become insulators. Firstly, the material forms a superconducting vortex lattice state known as ‘Bragg glass’. In this phase, the material shows zero resistivity at finite temperature. After a first critical point is reached it passes into a disordered superconducting phase, or ‘Vortex glass’, wherein the arrangement of vortices becomes amorphous. In this phase, zero resistivity is only realized at absolute zero. After a second critical point is reached, superconductivity is lost and the LSCOs become insulating.

The researchers conclude; “Our results provide important insight into the interplay of vortex line physics and quantum criticality in high-temperature superconductors, bridging the gap between their behavior in the high-T ‘classical’ region and the less-explored low-T ‘quantum’ region.”