Graphene Sublattice Symmetry and Isospin Determined by Circular Dichroism in Angle-Resolved Photoemission Spectroscopy
ABSTRACT The Dirac-like electronic structure of graphene originates from the equivalence of the two basis atoms in the honeycomb lattice. We show that the characteristic parameters of the initial state wave function (sublattice symmetry and isospin) can be determined using angle-resolved photoemission spectroscopy (ARPES) with circularly polarized synchrotron radiation. At a photon energy of hν = 52 eV, transition matrix element effects can be neglected allowing us to determine sublattice symmetry and isospin with high accuracy using a simple theoretical model.
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ABSTRACT: Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollow-core waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution. C ircularly-polarized radiation in the extreme ultraviolet (EUV) and soft X-ray spectral regions has proven to be extremely useful for investigating chirality-sensitive light–matter inter-actions. It enables studies of chiral molecules using photoelectron circular dichroism 1 , ultrafast molecular decay dynamics 2 , the direct measurement of quantum phases (for example, Berry's phase and pseudo-spin) in graphene and topological insulators 3,4Nature Photonics 12/2014; 9(2). DOI:10.1038/NPHOTON.2014.293 · 29.96 Impact Factor
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ABSTRACT: We have investigated graphene using circularly polarized light via angle-resolved photoemission spectroscopy. We observe that photoelectron intensity rotates around a constant energy contour towards the opposite direction upon changing the chirality of light. Interestingly, the circular dichroism is found to be asymmetric with respect to the Dirac energy, which is not explained by the Berry phase effect (Liu et al 2011 Phys. Rev. Lett. 107 166803). We also report that the energy spectra taken using the light with different chiralities show a finite separation from each other. We discuss possible origins of the unusual circular dichroism observed in graphene.Journal of Physics Condensed Matter 07/2014; 26(33):335501. DOI:10.1088/0953-8984/26/33/335501 · 2.22 Impact Factor
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ABSTRACT: We have investigated the effect of imperfect circular polarization on the angle-resolved photoemission spectroscopy signal, using graphene as a prototypical system that can be understood within tight-binding formalism. We found that perfect left- and right-circularly polarized lights give the same photoelectron intensity distribution around a constant energy contour of the graphene π band. On the other hand, upon breaking the purity of the polarization, photoelectron intensity starts to show circular dichroism, which is enhanced with further increasing the imperfection. Our results predict the existence of an additional factor for the circular dichroism observed in the photoemission signal from graphene and hence suggest the importance of experimental conditions to understand circular dichroism observed via photoemission spectroscopy.Journal of Electron Spectroscopy and Related Phenomena 11/2014; 198. DOI:10.1016/j.elspec.2014.10.011 · 1.55 Impact Factor