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Anisotropy of graphite optical conductivity  [PDF]
L. A. Falkovsky
Physics , 2010, DOI: 10.1134/S0021364010170169
Abstract: The graphite conductivity is evaluated for frequencies between 0.1 eV, the energy of the order of the electron-hole overlap, and 1.5 eV, the electron nearest hopping energy. The in-plane conductivity per single atomic sheet is close to the universal graphene conductivity $e^2/4\hbar$ and, however, contains a singularity conditioned by peculiarities of the electron dispersion. The conductivity is less in the $c-$direction by the factor of the order of 0.01 governed by electron hopping in this direction.
Anisotropy of the optical conductivity of high Tc cuprates  [PDF]
D. van der Marel
Physics , 1999, DOI: 10.1103/PhysRevB.60.R765
Abstract: The optical conductivity along and perpendicular to the planes is calculated assuming strong k-dependence of the scattering rate and the c-axis hopping parameter. Closed analytical expressions for the optical conductivy along these directions are obtained, and are shown to be integrable at low and high frequencies. A large and qualitatively different frequency dependence for both polarizations follows directly from the model. The expression for the in-plane conductivity has an effective scattering rate proportional to frequency, and can be easily generalized to provide a simple analytical expression, which may replace the Drude formula in the case of non-Fermi liquids.
Many-body large polaron optical conductivity in SrTi$_{1-x}$Nb$_x$O$_3$  [PDF]
J. T. Devreese,S. N. Klimin,J. L. M. van Mechelen,D. van der Marel
Physics , 2010, DOI: 10.1103/PhysRevB.81.125119
Abstract: Recent experimental data on the optical conductivity of niobium doped SrTiO$_{3}$ are interpreted in terms of a gas of large polarons with effective coupling constant $\alpha_{eff}\approx2$. The {theoretical approach takes into account} many-body effects, the electron-phonon interaction with multiple LO-phonon branches, and the degeneracy and the anisotropy of the Ti t$_{2g}$ conduction band. {Based on the Fr\"{o}hlich interaction, the many-body large-polaron theory} provides an interpretation for the essential characteristics, except -- interestingly -- for the unexpectedly large intensity of a peak at $\sim130$ meV, of the observed optical conductivity spectra of SrTi$_{1-x}$Nb$_{x}$O$_{3}$ \textit{without} any adjustment of material parameters.
Anisotropy and temperature dependence of the optical conductivity in LaMnO_3  [PDF]
K. H. Ahn,A. J. Millis
Physics , 1999,
Abstract: A tight binding parameterization of the band structure, along with a mean field treatment of Hund, electron-electron, and electron-lattice couplings, is used to obtain the full optical conductivity tensor of LaMnO_3 as a function of temperature. We predict striking changes with temperature in the functional form and magnitude of the optical absorption. Comparison of our results to data will determine the Hund, electron-lattice, and electron-electron interactions.
Origin of In-Plane Anisotropy in Optical Conductivity for Antiferromagnetic Metallic Phase of Iron Pnictides  [PDF]
K. Sugimoto,E. Kaneshita,T. Tohyama
Physics , 2010, DOI: 10.1143/JPSJ.80.033706
Abstract: We examine the optical conductivity in antiferromagnetic (AFM) iron pnictides by mean-field calculation in a five-band Hubbard model. The calculated spectra are well consistent with the in-plane anisotropy observed in the measurements, where the optical conductivity along the direction with the AFM alignment of neighboring spins is larger than that along the ferromagnetic (FM) direction in the low-energy region; however, that along the FM direction becomes larger in the higher-energy region. The difference between the two directions is explained by taking account of orbital characters in both occupied and unoccupied states as well as of the nature of Dirac-type linear dispersions near the Fermi level.
Tuning Optical Conductivity of Large-Scale CVD Graphene by Strain Engineering  [PDF]
Guang-Xin Ni,Hong-Zhi Yang,Wei Ji,Seung-Jae Baeck,Chee-Tat Toh,Jong-Hyun Ahn,Vitor M. Pereira,Barbaros ?zyilmaz
Physics , 2013, DOI: 10.1002/adma.201304156
Abstract: Strain engineering has been recently recognized as an effective way to tailor the electrical properties of graphene. In the optical domain, effects such as strain-induced anisotropic absorption add an appealing functionality to graphene, opening the prospect for atomically thin optical elements. Indeed, graphene is currently one of the notable players in the intense drive towards bendable, thin, and portable electronic displays, where its intrinsically metallic, optically transparent, and mechanically robust nature are major advantages. Given that the intrinsic transparency of a graphene monolayer is 97.7 %, any small, reproducible, controllable, and potentially reversible modulation of transparency can have a significant impact for graphene as a viable transparent conducting electrode. Even more so, if the degree of modulation is polarization dependent. Here we show that the transparency in the visible range of graphene pre-strained on a Polyethylene terephthalate (PET) substrate exhibits a periodic modulation (0.1 %) as a function of polarization direction, which we interpret as strain-induced optical anisotropy. The degree of anisotropy is varied by reversible external manipulation of the level of pre-strain. The magnitude of strain is monitored independently by optical absorption and Raman spectroscopy, and the experimental observations are consistent with the theoretically expected modification of the optical conductivity of graphene arising from the strain-induced changes in the electronic dispersion of graphene. The strain sensitivity of the optical response of graphene demonstrated in this study can be potentially utilized towards novel ultra-thin optical devices and strain sensing applications.
Optical Conductivity Anisotropy in the Undoped Three-Orbital Hubbard Model for the Pnictides  [PDF]
Xiaotian Zhang,Elbio Dagotto
Physics , 2011, DOI: 10.1103/PhysRevB.84.132505
Abstract: The resistivity anisotropy unveiled in the study of detwinned single crystals of the undoped 122 pnictides is here studied using the two-dimensional three-orbital Hubbard model in the mean-field approximation. Calculating the Drude weight in the x and y directions at zero temperature for a Q=(\pi,0) magnetically ordered state, the conductance along the antiferromagnetic direction is shown to be larger than along the ferromagnetic direction. This effect is caused by the suppression of the d_{yz} orbital at the Fermi surface, but additional insight based on the momentum dependence of the transitions induced by the current operator is provided. It is shown that the effective suppression of the inter-orbital hopping d_{xy} and d_{yz} along the y direction is the main cause of the anisotropy.
Optical conductivity and superconductivity in LaSb$_2$  [PDF]
J. F. DiTusa,V. Guritanu,S. Guo,D. P. Young,P. W. Adams,R. G. Goodrich,J. Y. Chan,D. van der Marel
Physics , 2010, DOI: 10.1088/1742-6596/273/1/012151
Abstract: We have measured the resistivity, optical conductivity, and magnetic susceptibility of LaSb$_2$ to search for clues as to the cause of the extraordinarily large linear magnetoresistance and to explore the properties of the superconducting state. We find no evidence in the optical conductivity for the formation of a charge density wave state above 20 K despite the highly layered crystal structure. In addition, only small changes to the optical reflectivity with magnetic field are observed indicating that the MR is due to scattering rate, not charge density, variations with field. Although a superconducting ground state was previously reported below a critical temperature of 0.4 K, we observe, at ambient pressure, a fragile superconducting transition with an onset at 2.5 K. In crystalline samples, we find a high degree of variability with a minority of samples displaying a full Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The application of pressure stabilizes the superconducting transition and reduces the anisotropy of the superconducting phase.
Optical conductivity and surface plasmon modes of U(1) spin liquid states with large spinon Fermi surfaces  [PDF]
Yuan-Fei Ma,Tai-Kai Ng
Physics , 2014, DOI: 10.1103/PhysRevB.91.075106
Abstract: In this paper we study the optical properties of $U(1)$ spin liquids with large spinon Fermi surfaces based on a simple formula for the bulk optical conductivity obtained through the Ioffe-Larkin composition rule. We show that the optical conductivity of $U(1)$ spin liquids at energies above the charge gap has a unique feature that distinguishes them from ordinary insulators. In particular we show the existence of a long-life surface plasmon mode propagating along the interface between a linear medium and the spin liquid at frequencies above the charge gap, which can be detected by the widely used Kretschmann-Raether three-layer configuration.
Conductivity of Strongly Coupled Striped Superconductor  [PDF]
Jimmy A. Hutasoit,George Siopsis,Jason Therrien
Physics , 2012, DOI: 10.1007/JHEP01(2014)132
Abstract: We study the conductivity of a strongly coupled striped superconductor using gauge/gravity duality (holography). The study is done analytically, in the large modulation regime. We show that the optical conductivity is inhomogeneous but isotropic at low temperatures. Near but below the critical temperature, we calculate the conductivity analytically at small frequency \omega, and find it to be both inhomogeneous and anisotropic. The anisotropy is imaginary and scales like 1/\omega. We also calculate analytically the speed of the second sound and the thermodynamic susceptibility.
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