Papers by Pranjal Kumar Gogoi
Theoretical studies have shown that electron-electron (e-e) and electron-hole (e-h) interactions ... more 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.
Physical Review B, 2015
Electron-electron (e-e) and electron-hole (e-h) interactions are often associated with many exoti... more Electron-electron (e-e) and electron-hole (e-h) interactions are often associated with many exotic phenomena in correlated electron systems. Here, we report an observation of anomalous excitons at 3.75 , 4.67 and 6.11 eV at 4.2 K in bulk -SrTiO3. Fully supported by ab initio GW Bethe-Salpeter equation calculations, these excitons are due to surprisingly strong e-h and e-e interactions with different characters: 4.67 and 6.11 eV are resonant excitons and 3.75 eV is a bound Wannier-like exciton with an unexpectedly higher level of delocalization. Measurements and calculations on SrTi1−xNbxO3 for 0.0001≤x ≤0.005 further show that energy and spectral-weight of the excitonic peaks vary as a function of electron doping (x ) and temperature, which are attributed to screening effects. Our results show the importance of e-h and e-e interactions yielding to anomalous excitons and thus bring out a new fundamental perspective in SrTiO3.
EPL (Europhysics Letters)
Theoretical studies have shown that electron-electron (e-e) and electron-hole (e-h) interactions ... more 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, $\alpha_{g}^{*}$. The $\alpha_{g}^{*}$ for GOQ indicates a strong correlation with an almost energy independent value of about 1.37. In contrast, $\alpha_{g}^{*}$ value of GOC is photon energy dependent, is almost two orders of magnitude lower at low energies indicating very weak corre...
Thin Solid Films, 2008
Organic solar cells with copper pthalocyanine (CuPC) and fullerene (C60) blends with efficiency a... more Organic solar cells with copper pthalocyanine (CuPC) and fullerene (C60) blends with efficiency around 5% have already been demonstrated. However, the data of reliable optical constants of these organic materials and their blends in thin films are not readily available. In this paper, we discuss thin films of CuPC and C60 fabricated using multiple evaporation cells at a base pressure b 10 − 8 torr are studied using spectroscopic ellipsometry. Ellipsometric data were obtained in the spectral range of 0.75 eV to 4.5 eV for determining the optical constants. We model CuPC and C60 using Tauc-Lorentz dispersion relation. The blend at the composition ratio of 1:1 were modelled by Maxwell-Garnett and Bruggeman effective medium approximations. In addition, a new approach is taken to model the blend by adding separately all the oscillators for each energy transition for both the composites and were found to fit the raw data better than the Maxwell-Garnett and Bruggeman effective medium approximations. The obtained refractive index data by this approach is then presented. We find that the Q-band and B-band of CuPC and the broad absorption of C60 around 3.56 eV still maintain their optical identity in the blend.
Physical Review B, 2015
We present evidence of a drastic renormalization of the optical conductivity of graphene on SrTiO... more We present evidence of a drastic renormalization of the optical conductivity of graphene on SrTiO3 resulting in almost full transparency in the ultraviolet region. These findings are attributed to resonant excitonic effects further supported by ab initio Bethe-Salpeter equation and density functional theory calculations. The (π,π*)-orbitals of graphene and Ti-3d t2g orbitals of SrTiO3 are strongly hybridized and the interactions of electron-hole states residing in those orbitals play dominant role in the graphene optical conductivity. These interactions are present much below the optical band gap of bulk SrTiO3. These results open a possibility of manipulating interaction strengths in graphene via d -orbitals which could be crucial for optical applications.
EPL (Europhysics Letters), 2014
ABSTRACT We report the optical and electronic properties of multilayer graphene films grown epita... more ABSTRACT We report the optical and electronic properties of multilayer graphene films grown epitaxially on the carbon face (C-face) of 4H-SiC probed using spectroscopic ellipsometry (SE) and angle-resolved photoemission spectroscopy (ARPES). The optical conductivity in the energy range from 1.0 to 5.3 eV extracted from SE reveals two important features: the presence of universal conductivity at the near-infrared region and asymmetric resonant excitons at 4.5 eV. Furthermore, ARPES shows the presence of independent linear electronic dispersion. These features resemble quasi-freestanding properties of multilayer graphene grown on the C-face of SiC.
Advanced Materials Research, 2014
ABSTRACT The significant alteration of absorption (A) of monolayer graphene under mild oxygen pla... more ABSTRACT The significant alteration of absorption (A) of monolayer graphene under mild oxygen plasma exposure has been observed. The first important effect is the reduction of the excitonic resonance peak at 4.64 eV. Secondly, in the near infrared range, A is gradually suppressed below an exposure-dependent threshold in sense that A << A0. Quantity A0 (given by πα and α is the fine structure constant) denotes constant absorption and relates to universal optical conductivity σ0. The suppression of A0 can be thought as the weakening of electron-hole interaction as displayed by the reduction of the excitonic resonance peak at 4.64 eV. The weakening of this interaction is due to the disorder introduced by the oxygen plasma exposure.
Physical Review B, 2014
We report significant changes of optical conductivity ( ) in single layer graphene induced by mil... more 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, 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 below ω~2E F can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controllably 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.
Physical Review B, 2013
We study the temperature dependence as well as anisotropy of optical conductivity (σ 1 ) in the p... more We study the temperature dependence as well as anisotropy of optical conductivity (σ 1 ) in the pseudocubic single crystal Pr 0.5 Ca 1.5 MnO 4 using spectrocopic ellipsometry. Three transition temperatures are observed and can be linked to charge-orbital (T CO/OO ∼320 K), two-dimensional-antiferromagnetic (2D-AFM) (∼200 K), and three-dimensional AFM (T N ∼125 K) orderings. Below T CO/OO , σ 1 shows a charge-ordering peak (∼0.8 eV) with a significant blue shift as the temperature decreases. Calculations based on a model that incorporates a static Jahn-Teller distortion and assumes the existence of a local charge imbalance between two different sublattices support this assignment and explain the blue shift. This view is further supported by the partial spectral weight analysis showing the onset of optical anisotropy at T CO/OO in the charge-ordering region (0.5-2.5 eV). Interestingly, in the charge-transfer region (2.5-4 eV), the spectral weight shows anomalies around the T 2D-AFM that we attribute to the role of oxygen-p orbitals in stabilizing the CE-type magnetic ordering. Our result shows the importance of spin, charge, orbital, and lattice degrees of freedom in this layered manganite.
Physical Review B, 2011
Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry... more Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3eV 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 resonant exciton scales linearly with number of graphene layer (up to at least 8 layers) implying quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4eV, 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.
Nature Communications, 2014
In condensed matter physics the quasi two-dimensional electron gas at the interface of two differ... more In condensed matter physics the quasi two-dimensional electron gas at the interface of two different insulators, polar LaAlO3 on non-polar SrTiO3 (LaAlO3/SrTiO3) is a spectacular and surprising observation. This phenomenon is LaAlO3 film thickness-dependent and may be explained by the polarization catastrophe model, in which a charge transfer of 0.5efrom the LaAlO3 film into the LaAlO3/SrTiO3 interface is expected. Here we show that in conducting samples (≥4 unit cells of LaAlO3) there is indeed a ~0.5etransfer from LaAlO3 into the LaAlO3/SrTiO3 interface by studying the optical conductivity in a broad energy range (0.5-35 eV). Surprisingly, in insulating samples (≤3 unit cells of LaAlO3) a redistribution of charges within the polar LaAlO3 sub-layers (from AlO2 to LaO) as large as ~0.5eis observed, with no charge transfer into the interface.
EPL (Europhysics Letters), 2012
Theoretical studies have shown that electron-electron (e-e) and electron-hole (e-h) interactions ... more 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, α * g value of GOC is photon energy dependent, is almost two orders of magnitude lower at low energies indicating very weak correlation. PACS numbers: 73.22.Pr, 78.67.Wj, 07.60.Fs
Bulletin of the American …, 2010
Almost all discussion of the optical absorption, the interlayer interaction and relationship betw... more Almost all discussion of the optical absorption, the interlayer interaction and relationship between them in multilayer graphene is almost conclusively decided~ taking into account only the charge dynamic in states at K point in the hexagonal Brillouin zone while other states are ...
Uploads
Papers by Pranjal Kumar Gogoi