Apparent color and Raman vibrational modes of the unconventional superconductor Bi_2Sr_2CaCu_2O_8+δ exfoliated flakes
arxiv(2024)
摘要
Studying and controlling the properties of individual exfoliated materials is
one of the first steps towards the fabrication of complex van der Waals
systems. However, prolonged exposure to ambient conditions can affect the
properties of very thin exfoliated materials altering their physical
properties. For this reason, it is imperative to employ versatile
characterization strategies compatible with reduced ambient exposure times. In
this work, we demonstrate that optical microscopy and Raman spectroscopy are
quick and non-invasive techniques to study flakes of the high-temperature
superconductor Bi_2Sr_2CaCu_2O_8+δ (BSCCO-2212). The apparent
color of BSCCO-2212 exfoliated flakes on SiO_2/Si has been studied allowing a
rough and fast identification of the number of layers. Moreover, we find that
thin flakes have a refractive index of around 1.7 in the visible range and 0.5
for the absorption coefficient near the maximum at 550 nm. We determine the
optimal combination of illumination wavelength and substrate properties for the
identification of different numbers of unit cells of BSCCO-2212. In addition,
we report the hardening of the characteristic Raman modes at 116 cm^-1 and
460 cm^-1 as flake thickness decreases, possibly due to strain in the BiO
and CuO_2 planes, respectively. Moreover, the evolution of the Raman modes
establishes a second approach to determine the thickness of BSCCO-2212 thin
flakes. As BSCCO-2212 is a challenging material to be due to its sensitivity to
ambient conditions deriving in an insulating state, the present work provides a
guide for the fabrication and characterization of complex van der Waals systems
paving the way for studying heterostructures based on unconventional
superconductors in the 2D limit.
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