Layer-cake 2-D superconductivity: Developing clean 2-D superconductivity in a bulk van der Waals superlattice

Materials science has had a profound historical impact on humanity since the advent of the Iron and Bronze ages. Presently, materials scientists are intrigued by a class of materials known as quantum materials, whose electronic or magnetic behavior cannot be explained by classical physics. Discoveries in the field of quantum materials are followed by a surge of research to uncover new physics or quantum information in science. In a new report now published on Science, A. Devarakonda and a team of scientists in physics at the Massachusetts Institute of Technology, Harvard University and the Riken Center for Emergent Matter Science in the U.S. and Japan reported the synthesis of a highly interesting novel quantum material.

The construct may allow physicists to study obscure quantum effects that have hitherto remained unknown. In this study, the team developed a bulk superlattice containing the transition metal dichalcogenide (TMD) superconductor 2H-niobium disulfide (2H-NbS_2, 2H phase) to generate enhanced two-dimensional (2-D), high-electronic quality and clean-limit inorganic superconductivity.

Superconductivity

Superconductivity can hypothetically allow high-speed applications without power loss and contribute to the development of concepts such as levitating express trains. Researchers can partly realize such applications at present using materials that superconduct at high enough temperatures and use 2-D materials to simplify problems, while highlighting the physics behind superconductivity. Early experimental works with granular aluminum (Al) and amorphous bismuth (Bi) films showed 2-D superconductivity by precisely controlling the superconducting layer thickness, which were later used in ground-breaking studies. These include the Berezinskii-Kosterlitz-Thouless (BKT) transition; an early example describing the topological transition, for which physicist received the Nobel prize in physics in 2016.

In parallel, scientists have also studied anisotropic bulk superconductivity to understand the superconducting state in the context of 2-D superconductivity, which include transition metal dichalcogenides (TMDs), i.e. atomically thin semiconductors of the type MX₂ where M is a transition metal and X is a chalcogen atom (group 16 elements in the periodic table). Recent advancements in materials engineering have also shown the possibility to exfoliate van der Waals (vdW) layered materials to allow atomically thin 2-D superconductors to be readily accessible. However, such flakes of exfoliation can degrade, reducing the sample quality. Devarakonda et al. therefore used high quality 2H-NbS₂ (2H-niobium disulfide) monolayers in this work with a clean-limit 2-D superconductor exhibiting a BKT (Berezinskii-Kosterlitz-Thouless) transition. They then further synthesized a single-crystal material; Ba₆Nb₁₁S₂₈ using high-quality H-NbS₂ monolayers and Ba₃NbS₅ block layers, within which the TMD layers were strongly decoupled.

Developing and characterizing the layer-cake

Devarakonda et al. therefore made the resulting material (Ba₆Nb₁₁S₂₈) as pure as possible to study the pure physics of 2-D superconductivity. The discovery of clean 2-D superconductivity in Ba₆Nb₁₁S₂₈ will open the door to better understand 2-D superconductivity associated with quantum phenomena. The material contained alternating layers of the 2-D superconductor NbS₂ and an electronically uninteresting spacer layer Ba₃NbS₅ – much like a layer cake with a thin layer of chocolate (i.e. NbS₂) between thicker layers of cake (i.e. the spacer layer). The layering protected the NbS₂ layer from cracking or air/moisture exposure to allow much cleaner 2-D superconductivity. The team used high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to investigate the resulting structure. The material displayed a clean-limit 2-D superconductor exhibiting a BKT transition at T_BKT = 0.82 K and prominent 2-D Shubnikov-de Haas (SdH) quantum oscillations; a macroscopic manifestation of the inherent quantum nature of matter.

Properties of the new quantum material

Using magnetotransport measurements, Devarakonda et al. showed the cleanliness of the material and further evidence for the 2-D electronic architecture. The results were qualitatively different from the starting material 2H-NbS₂, which maintained warped and elliptical Fermi surfaces in its electronic structure. Although quantum oscillations had not yet been reported in 2H-NbS₂, the team noted the onset of Shubnikov-de Haas (SdH) quantum oscillations in Ba₆Nb₁₁S₂₈ within magnetic fields between 2 and 3 Tesla to indicate quantum mobilities. The scientists analyzed the quantum oscillations and low-field magnetoresistance of Ba₆Nb₁₁S₂₈, which placed the materials in the clean limit of superconductivity.

They also recorded the current/voltage characteristics of the material across the superconducting transition. In addition to the observed BKT transition, the work showed the appearance of a clean, 2-D superconducting state with enhanced stability, which Devarakonda et al. credited to the high purity of the NbS₂ layers in Ba₆Nb₁₁S₂₈. Since the bulk Ba₆Nb₁₁S₂₈ material already displayed 2-D physics, the research team originally proposed the now known process of inserting the spacer layers instead of fabricating exfoliated nanodevices detailed in previous work. The team also noted the scope to functionalize the spacer layer by introducing magnetic constituents. In this way, the large electronic mean free path (average distance traveled by a moving particle) of Ba₆Nb₁₁S₂₈ allowed clean-limit super-conductivity to potentially realize unconventional phases as predicted in monolayer superconductors.

Impact of the new quantum material

The inherent beauty of the material remains in the natural growth of the heterostructure, which much like a layer-cake naturally separated during the process of synthesis. This allowed the synthetic process to be much less labor-intensive compared to the manual addition of each layer. The ease of synthesis may allow different types of layered materials to be developed where the 2-D layers are naturally protected by their environment. The technique can yield different types of quantum materials aside from superconductors, including topological insulators suited for quantum computing. The new discovery allows a simpler, alternative approach to the existing process of exfoliated nanodevice fabrication. A. Devarakonda and colleagues envision extending this strategy to other materials beyond the Ba₆Nb₁₁S₂₈ detailed here.

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