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Optical characterization of BSCCO, a high-temperature superconducting 2D material


bandgap opening

Colour scale showing the apparent colour vs. thickness of exfoliated flakes of BSCCO. Credit: Ignacio Figueruelo.

  • IMDEA Nanociencia researchers present a systematic study of the optical properties of a high-temperature superconducting 2D material.
  • The study provides a new scale that relates the colour of the thin layers of this material to their nanometre thickness.
  • The researchers demonstrate that these are fast and non-invasive techniques for studying air-sensitive materials, which oxidize or change their composition when exposed to ambient conditions.
  • The results are a guide for the fabrication of layered, van der Waals-type devices that include 2D superconductors.

Madrid, May 7th, 2024. Two-dimensional (2D) materials are of great interest because of their properties, which are very different from those of their bulk counterparts. These materials, which belong to the family of quantum materials, can present superconductivity, topological phases or correlated electronic states, all of which are exotic properties of great interest that make 2D materials an ideal playground to study quantum phenomena. 2D materials can be prepared by exfoliation, a low-cost technique in which researchers extract fine flakes by taping to a thicker piece of material. The characterization of 2D materials prepared using this exfoliation technique is complex. In some cases it is difficult to figure out their thickness or their number of monatomic layers without altering the material. In addition, exfoliated flakes have very small lateral dimensions, which makes them difficult to manipulate, rendering their characterization costly in terms of time.

The 'Transport in Quantum Materials' Group of IMDEA Nanociencia, led by Dr. Mariela Menghini, has carried out a systematic study of a 2D material with a high potential to fabricate complex structures incorporating superconducting materials. This is the BSCCO compound, a complex oxide that exhibits 'high temperature' superconductivity. Commonly, superconducting materials must be cooled near 0 K, the absolute zero temperature, to observe their superconductivity. However, this BSCCO material exhibits superconductivity at a temperature as high as 90 K, making it an attractive candidate for making superconducting devices that operate at liquid nitrogen temperature, and therefore consume less energy.

materialBSCCO is a quaternary compound, made up of 4 elements – bismuth, strontium, calcium and copper – plus oxygen; It is, therefore, a complex oxide, and whose chemical formula is written Bi2Sr2CaCu2O8+d. In typical superconducting materials, such as lead or mercury, the mechanism of superconductivity is well understood. However, in BSCCO the mechanism of superconductivity is still unknown. In order to study the properties of a 2D material in detail, it must be well characterized: what side dimension, how many layers, and how thick they are. Currently, there are techniques to measure the thickness of 2D materials; the problem with BSCCO lies in its sensitivity to environmental conditions. Handling BSCCO in the air causes it to lose its superconducting properties.

The work led by Mariela Menghini and Ignacio Figueruelo, published in the journal 2D Materials, proposes to use a property visible to the naked eye –using an optical microscope- to characterize BSCCO flakes. The researchers use the apparent colour of the flakes in the 2D material to obtain their thickness. In general, the colour of bulky materials has to do with their light absorption spectrum. But in nanometre-thick materials, other factors such as light interference play a crucial role in their apparent colour. That is why the group has studied and developed a colour scale, a rainbow, that directly relates the apparent colour of the BSCCO to its thickness.

The study provides a new scale that relates the colour of the thin layers of this material to their nanometre thickness

In addition to the colour scale, the group has managed to obtain the microscopic refractive index of the BSCCO, which was not known until now, and is a crucial parameter when implementing it in optical systems. This complete optical characterization has allowed the researchers to create a chart from which the wavelength (colour) for observing individual layers of BSSCO with optimal contrast can be obtained, according to the thickness of the silicon substrate that supports this 2D material.

The characterization of the BSCCO material has also included the study of its Raman spectrum. The vibration modes of the crystal structure, evidenced in the Raman spectrum, have been "hardened" for the thinner flakes. The authors justify this fact by alluding to internal stresses in the exfoliated material on the substrate, and also to deformations caused by exfoliating it.

Combining different 2D materials with vertical stacking opens up a plethora of exciting opportunities. These heterostructures are held together by weak van der Waals forces and allow for more combinations than any conventional method of growth. Combining the best of different materials to create the ultimate device is an attractive idea, even if it is a considerable challenge to implement. With this study, the IMDEA Nanociencia researchers go a step further, facilitating the characterization of materials by their colour, and constituting a guide for the fabrication of 'van der Waals' layered materials that include 2D superconductors.

The work is a collaboration between researchers from IMDEA Nanociencia, the Complutense University of Madrid, the Institute of Ceramics and Glass (CSIC) and the Atomic Center of Bariloche (Argentina), and has been partially funded by the Severo Ochoa Seal of Excellence awarded to IMDEA Nanociencia (2017 and 2021).


Ignacio Figueruelo-Campanero,  and  2D Mater. 11 025032 (2024). DOI: 10.1088/2053-1583/ad349e




Dr. Mariela Menghini
Group webpage: Transport in Quantum Materials Group

Oficina de Divulgación y Comunicación en IMDEA Nanociencia
divulgacion.nanociencia [at]imdea.org
Twitter: @imdea_nano
Facebook: @imdeananociencia
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Fuente: IMDEA Nanociencia.