Pranjal Kumar Gogoi

Tunable optical absorption and interactions in graphene via oxygen plasma

We report significant changes of optical conductivity in single layer graphene induced by mild oxygen plasma exposure, and explore the interplay between carrier doping, disorder, and many-body interactions from their signatures in the absorption spectrum. The first distinctive effect is the reduction of the excitonic binding energy that can be extracted from the renormalized saddle point resonance at 4.64 eV. Secondly, the real part of the frequency-dependent conductivity is nearly completely suppressed below an exposure-dependent threshold in the near infrared range. The clear step-like suppression follows the Pauli blocking behaviour expected for doped monolayer graphene. The nearly zero residual conductivity at frequencies below 2Ef can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controlably dope graphene without introducing the strong physical and chemical changes that are common in other approaches to oxidized graphene, allowing a controllable manipulation of the optical properties of graphene.

Observation of room-temperature high-energy resonant excitonic effects in graphene

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3 eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers N≈75, thus suggesting a poor screening in graphene. The optical conductivity (σ1) of a resonant exciton scales linearly with the number of graphene layers (up to at least 8 layers), implying the quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4 eV, which is a mixture of interband transitions from π to π∗ at the M point and a π plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for N⩽ 28 the σ1 is dominated by the resonant exciton, while for N> 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite.

Optical conductivity study of screening of many-body effects in graphene interfaces

Theoretical studies have shown that electron-electron (e-e) and electron-hole (e-h) interactions play important roles in many observed quantum properties of graphene making this an ideal system to study many-body effects. In this report we show that spectroscopic ellipsometry can enable us to measure this interactions quantitatively. We present spectroscopic data in two extreme systems of graphene on quartz (GOQ), an insulator, and graphene on copper (GOC), a metal which show that for GOQ, both e-e and e-h interactions dominate while for GOC e-h interactions are screened. The data further enables the estimation of the strength of the many-body interaction through the effective fine-structure constant, α*g. The α*g for GOQ indicates a strong correlation with an almost energy-independent value of about 1.37. In contrast, the α*g value of GOC is photon energy dependent and is almost two orders of magnitude lower at low energies indicating very weak correlation.

Optical conductivity renormalization of graphene onSrTiO3due to resonant excitonic effects mediated by Ti3dorbitals

We present evidence of a drastic renormalization of the optical conductivity of graphene on SrTiO

Temperature-dependent dielectric function of bulkSrTiO3: Urbach tail, band edges, and excitonic effects

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Optical and electronic structure of quasi-freestanding multilayer graphene on the carbon face of SiC

resulting in almost full transparency in the ultraviolet region. These findings are attributed to resonant excitonic effects further supported by \emph{ab initio} Bethe-Salpeter equation and density functional theory calculations. The (

Modification of Vapor Phase Concentrations in MoS2 Growth Using a NiO Foam Barrier

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Effect of Oxygen Plasma on the Optical Properties of Monolayer Graphene

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Temperature-dependent and anisotropic optical response of layered Pr0.5Ca1.5MnO4 probed by spectroscopic ellipsometry

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Anomalous excitons and screenings unveiling strong electronic correlations inSrTi1−xNbxO3 (0≤x≤0.005)

*)-orbitals of graphene and Ti-3\textit{d}