Copper oxides, or cuprates, exhibit high-temperature superconductivity when electrons and holes (vacant spaces left by electrons) are introduced into their crystal structure through doping. In the low-doped state, a pseudogap-a partial gap in the electronic structure-emerges, which is considered a possible factor in superconductivity.
Previous studies have identified a long-range charge density wave (CDW) order in low-doped cuprates that disrupts the crystal symmetry of the copper oxide (CuO2) plane. CDW is a wave-like pattern of electrons that affects conductivity. This symmetry breaking is significant because superconductivity often arises near such symmetry-broken states.
In the bismuth-based cuprate superconductor Bi2Sr2-xLaxCuO6+d (Bi2201), strong magnetic fields can induce long-range CDW order. Despite extensive research, the exact role of these phenomena in cuprate superconductivity is still uncertain.
A new study led by Associate Professor Shinji Kawasaki from Okayama University's Department of Physics investigated the origin of high-temperature superconductivity in the pseudogap state of cuprates using a novel approach.
Prof. Kawasaki stated, "In this study, we have discovered the existence of a long-range CDW order in the optimally doped Bi2201, induced by tensile-compressive strain applied by a novel piezo-driven uniaxial strain cell, which deliberately breaks crystal symmetry of the CuO2 plane." The findings were published in Nature Communications on June 14, 2024. The research team included Ms. Nao Tsukuda and Professor Guo-qing Zheng from Okayama University, and Dr. Chengtian Lin from Max-Planck-Institut fur Festkorperforschung, Germany.
The researchers used nuclear magnetic resonance (NMR) to observe changes in the electronic structure of the optimally doped Bi2201 superconductor under uniaxial compressive and tensile strains. Results showed that when strain exceeded 0.15%, the short-range CDW order transformed into a long-range CDW order.
Additionally, increasing strain suppressed superconductivity while enhancing CDW order, indicating that superconductivity and long-range CDW can coexist. These results suggest that a hidden long-range CDW order exists in the pseudogap state of cuprates, becoming apparent under strain.
"This finding challenges the conventional belief that magnetism is the primary driver in copper oxides and provides valuable insights for constructing theoretical models of superconductivity," commented Prof. Kawasaki.
Highlighting the study's potential applications, he added, "The findings of this study hold immense promise for elucidating the underlying mechanisms of high-temperature superconductivity, paving the way for the development of more practical superconducting materials. High temperature superconductors hold great potential for lossless power transmission and storage, contributing significantly to energy conservation and the pursuit of carbon neutrality. Furthermore, the application of superconductors in MRI technology has the potential to reduce costs and make advanced medical imaging more accessible."
This study represents a significant step towards understanding high-temperature superconductivity, emphasizing the value of uniaxial strain as a tool for exploring superconductivity in similar materials.
Research Report:Strain-induced long-range charge-density wave order in the optimally doped Bi2Sr2-xLaxCuO6+d superconductor
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