Measurements of optical
activity can be readily performed in transparent
matter by means of a rotation of transmitted light polarization. In
the case of opaque bulk materials, such measurements cannot be performed,
making it difficult to assess possible chiral properties. In this
work, we present full angular polarization dependencies of the Raman
modes of bulk 1T-TaS
2
, which has recently been suggested
to have chiral properties after pulsed laser excitation. We found
that a mechanical rotation of the sample does not alter polarization-resolved
Raman spectra, which can only be explained by introducing an antisymmetric
Raman tensor, frequently used to describe Raman optical activity (ROA).
Raman spectra obtained under circularly polarized excitation demonstrate
that 1T-TaS
2
indeed shows ROA, providing strong evidence
that 1T-TaS
2
is chiral under the used conditions of laser
excitation. Our results suggest that ROA may be used as a universal
tool to study chiral properties of quantum materials.
We present results of µ-Raman and µ-photoluminescence study of few-layer WS2 flakes that have been locally thinned down by a focused laser beam. The Raman spectroscopy measurements prove that the investigated flake was locally thinned down to a monolayer. Interestingly, µ-photoluminescence experiments allowed us to observe huge intensity fluctuations at the boundary of laser-thinned region. Similar effects were found at the edges of a WS2 bilayer flake, which has not been subjected to laser-thinning. The origin of the observed time evolution of the photoluminescence response is discussed in terms of potential fluctuations resulting from light-induced changes of the charge state of defects.
The influence of intrinsic defects of 1T-TaS2 on charge density waves (CDWs) is studied using scanning tunneling microscopy and spectroscopy (STM, STS), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT). We identify several types of structural defects and find that most have a local character limited to a single CDW site, with a single exception which effectively behaves as a dopant, leading to band-bending and affecting multiple neighboring sites. While only one type of defect can be observed by STM topographic imaging, all defects are easily resolved in STS mapping. Our results indicate modulation of the Mott band gap commensurate with the CDW and breaking of the three-fold symmetry of electronic states. DFT calculations (with included Coulomb interactions) are used to investigate the electronic structure, focusing on both sulfur vacancy and oxygen-sulfur substitution. The sulfur vacancy system, characterized with a metallic behavior, is identified as the origin of one of the experimentally observed defects. Additionally, the effect of oxidation of 1T-TaS2 depends on the substitution site, leading to the heterogeneity of electronic properties.
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