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HIDDEN QUANTUM INTERFERENCE AND ACHIRAL SYMMETRY BREAKING REVEALED BY NONLINEAR OPTICAL HARMONIC GENERATION SPECTROSCOPY
Subedi, Sujan Babu
Subedi, Sujan Babu
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2024-04
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Physics
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http://dx.doi.org/10.34944/dspace/10210
Abstract
Rotational anisotropy second harmonic generation measurements conducted with incoming photon energies range of 1.1 - 1.8 eV revealed the presence of strong magnetic dipole (MD) transitions that we assign to the trivalent lanthanide ions. Extracting the spectra of the MD transition susceptibility tensor, we observe an asymmetric resonance at $\sim$ 1.5 eV, consistent with a Fano lineshape. Comparison of our data with a band structure obtained from density function theory revealed the hybridization of an unoccupied band deriving from unassigned orbitals in LaAlSi and CeAlSi, the unoccupied $^1D_2$ state of PrAlSi, and occupied $^4I_{\frac{9}{2}}$ state in NdAlSi. Moreover,polarimetry measurement was conducted at normal incidence in the paramagnetic phase of LnAlSi revealed a novel, nonlinear form of electromagnetically induced chirality (EIC) that derives
from resonant MD transitions. This form of nonlinear magnetoelectricity is expressed as $P_i = \chi^{eem}_{ijk}E_jH_k$, which causes the emission of elliptically polarized SHG.
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In a separate study, we used the second and third harmonic nonlinear harmonic generation spectroscopy to study \ce{1T-TiSe2} below its charge density wave phase transition temperature at 200 K. The lack of a second harmonic signal below and above the transition temperature indicated that the low-temperature symmetry-breaking phase is achiral. Further study using rotational anisotropy nonlinear third harmonic generation revealed that \ce{1T-TiSe2} experiences a non-trivial phase transition at $\sim$ 180 K within the charge density phase that comprises two separate domain types that randomly rearrange upon thermal cycling. Taken together, our data support the onset of orbital ordering at 180 K without the emergence of an electronically chiral state.\\\par
Finally, we describe THz emission spectroscopy to study the circular photogalvanic effect (CPGE ) and linear photogalvanic effect (LPGE) spectra of the structurally chiral Weyl semimetal PdGa. We reveal that CPGE and LPGE spectra produce signals of opposite signs for the two PdGa enantiomers, as predicted by theory. Measurement of the spectra deriving from the material's bulk reveals a peak in the photogalvanic spectra at 0.68 eV, which is due to optical transitions between the parallel bands near the Weyl nodes dispersing from $\Gamma$ to R and similar transitions between M and R. Surface sensitive measurement are consistent with helicoidally dispersing states of opposite helicity from the two different chirality samples.
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