The new interfacial superconductor has new properties that raise new fundamental questions and could be useful for quantum information processing or quantum detection.
Interfaces in solids form the basis of much of modern technology. For example, the transistors found in all of our electronic devices work by controlling the electrons at the interfaces of semiconductors. More generally, the interface between any two materials can have unique properties that are radically different from those found in either material separately, paving the way for new discoveries.
Like semiconductors, superconducting materials have many important implications for technology, from magnets for MRIs to accelerating electrical connections or perhaps quantum technology. The vast majority of superconducting materials and devices are in 3D, which gives them properties well understood by scientists.
One of the fundamental questions of superconducting materials concerns the transition temperature – the extremely cold temperature at which a material becomes superconducting. All materials superconducting at regular pressures become superconductive at temperatures well below the coldest day outside.
Now, researchers at the US Department of Energy’s Argonne National Laboratory have discovered a new way to generate 2D superconductivity at a material interface at a relatively high transition temperature – albeit still cold. This interfacial superconductor has new properties that raise new fundamental questions and could be useful for quantum information processing or quantum detection.
In the study, Argonne postdoctoral researcher Changjiang Liu and his colleagues, working in a team led by Argonne materials scientist Anand Bhattacharya, found that a new 2D superconductor forms at the interface of an oxide insulator called KTaO.3 (KTO). Their results were published online in the journal Science on February 12.
In 2004, scientists observed a thin layer of conducting electrons between two other oxide insulators, LaAlO3 (LAO) and SrTiO3 (STO). It was later shown that this material, called 2D electron gas (2DEG) can even become superconducting – allowing electricity to be transported without dissipating energy. Importantly, superconductivity could be turned on and off using electric fields, just like in a transistor.
However, to achieve such a superconducting state, the sample had to be cooled to around 0.2 K – a temperature close to absolute zero (- 273.15 ° C), requiring a specialized device known as a dilution refrigerator. . Even with such low transition temperatures (TVS), the LAO / STO interface has been extensively studied in the context of superconductivity, spintronics and magnetism.
In the new research, the team found that in KTO, interfacial superconductivity could emerge at much higher temperatures. To obtain the superconducting interface, Liu, graduate student Xi Yan and colleagues developed thin films of europium oxide (EuO) or LAO on KTO using state-of-the-art thin film growth facilities at Argonne. .
“This new oxide interface makes the application of 2D superconducting devices more feasible,” Liu said. “With its transition temperature an order of magnitude higher of 2.2 K, this material will not need a dilution refrigerator to be superconducting. Its unique properties raise many interesting questions.”
A strange superconductor
Surprisingly, this new interfacial superconductivity shows a strong dependence on the orientation of the crystal facet where the electron gas is formed.
Adding to the mystery, the measurements suggest the formation of band-shaped superconductivity in low-doping samples where streams of superconducting regions are separated by normal, non-superconducting regions. This type of spontaneous scratch formation is also called nematicity and is commonly found in liquid crystal materials used for displays.
“Electronic realizations of nematicity are rare and of great fundamental interest. It turns out that the overcoat of EuO is magnetic, and the role of this magnetism in achieving the nematic state in KTO remains an open question. “said Bhattacharya.
In their scientific article, the authors also discuss the reasons why electron gas is formed. Using atomic resolution transmission electron microscopes, Jianguo Wen at the Center for Nanoscale Materials in Argonne, as well as the group of Professor Jian-Min Zuo at the University of Illinois at Urbana-Champaign, showed that the defects formed during the growth of the overlay can play a central role. role.
In particular, they found evidence of oxygen vacancies and substitution defects, where potassium atoms are replaced with europium or lanthanum ions – all of which add electrons to the interface and turn it into a conductor. 2D. Using advanced photon source (APS) ultra-bright x-rays, Yan, along with Argon scientists Hua Zhou and Dillon Fong, probed the KTO interfaces buried under the superimposed layer and observed the spectroscopic signatures of these extra electrons. near the interface.
“The interface-sensitive radiographic toolkits available at APS allow us to reveal the structural basis of 2DEG formation and the unusual dependence of crystal facets on 2D superconductivity. A more detailed understanding is underway, ”Zhou said.
Beyond the description of the 2DEG formation mechanism, these results indicate the way to improve the quality of the interfacial electron gas by controlling the synthesis conditions. Since superconductivity occurs for both the EuO and LAO oxide superpositions which have been tried so far, many other possibilities remain to be explored.