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Solids with weak and strong electron correlations  [PDF]
Prof. Dr. Peter Fulde
Physics , 1998,
Abstract: A number of methods are discussed which may serve for a treatment of electron correlations in solids. When the electron correlations are relatively weak like in semiconductors or a number of ionic crystals one may start from a self-consistent field calculation and include correlations by quantum chemical methods. An incremental computational scheme enables us to obtain results of high quality for the ground state of those systems. A number of examples demonstrates that explicitely. Solids with strongly correlated electrons require the use of model Hamiltonians. With their help one can tackle the problem of determining spectral densities for those systems. The projection technique is a useful tool here. In strongly correlated $f$ electron systems electron or holes can crystallize with quite different physical consequences as in the case of a Wigner crystal or Verwey transition. Finally, different routes to heavy-fermion behavior are discussed, another hallmark of strongly correlated electrons.
Correlations in optical phonon spectra of complex solids  [PDF]
G. Fagas,Vladimir I. Fal'ko,C. J. Lambert,Yuval Gefen
Physics , 1999, DOI: 10.1103/PhysRevB.61.9851
Abstract: Spectral correlations in the optical phonon spectrum of a solid with a complex unit cell are analysed using the Wigner-Dyson statistical approach. Despite the fact that all force constants are real, we find that the statistics are predominantly of the GUE type depending on the location within the Brillouin zone of a crystal and the unit cell symmetry. Analytic and numerical results for the crossover from GOE to GUE statistics are presented.
Optical signatures of electron correlations in the cuprates  [PDF]
D. van der Marel
Physics , 2003,
Abstract: The f-sum rule is introduced and its applications to electronic and vibrational modes are discussed. A related integral over the intra-band part of sigma(omega) which is also valid for correlated electrons, becomes just the kinetic energy if the only hopping os between nearest-neighbor sites. A summary is given of additional sum-rule expressions for the optical conductivity and the dielectric function, including expressions for the first and second moment of the optical conductivity, and a relation between the Coulomb correlation energy and the energy loss function. It is shown from various examples, that the optical spectra of high Tc materials along the c-axis and in the ab-plane direction can be used to study the kinetic energy change due to the appearance of superconductivity. The results show, that the pairing mechanism is highly unconventional, and mostly associated with a lowering of kinetic energy parallel to the planes when pairs are formed. Keywords: Optical conductivity, spectral weight, sum rules, reflectivity, dielectric function, inelastic scattering, energy loss function, inelastic electron scattering, Josephson plasmon, multi-layers, interlayer tunneling, transverse optical plasmon, specific heat, pair-correlation, kinetic energy, correlation energy, internal energy.
Universal probes for antiferromagnetic correlations and entropy in cold fermions on optical lattices  [PDF]
E. V. Gorelik,D. Rost,T. Paiva,R. Scalettar,A. Klümper,N. Blümer
Physics , 2012, DOI: 10.1103/PhysRevA.85.061602
Abstract: We determine antiferromagnetic (AF) signatures in the half-filled Hubbard model at strong coupling on a cubic lattice and in lower dimensions. Upon cooling, the transition from the charge-excitation regime to the AF Heisenberg regime is signaled by a universal minimum of the double occupancy at entropy s=S/(N k_B)=s*=ln(2) per particle and a linear increase of the next-nearest neighbor (NNN) spin correlation function for s
A Coherent Nonlinear Optical Signal Induced by Electron Correlations  [PDF]
Shaul Mukamel,Rafal Oszwaldowski,Lijun Yang
Physics , 2007, DOI: 10.1063/1.2820379
Abstract: The correlated behavior of electrons determines the structure and optical properties of molecules, semiconductor and other systems. Valuable information on these correlations is provided by measuring the response to femtosecond laser pulses, which probe the very short time period during which the excited particles remain correlated. The interpretation of four-wave-mixing techniques, commonly used to study the energy levels and dynamics of many-electron systems, is complicated by many competing effects and overlapping resonances. Here we propose a coherent optical technique, specifically designed to provide a background-free probe for electronic correlations in many-electron systems. The proposed signal pulse is generated only when the electrons are correlated, which gives rise to an extraordinary sensitivity. The peak pattern in two-dimensional plots, obtained by displaying the signal vs. two frequencies conjugated to two pulse delays, provides a direct visualization and specific signatures of the many-electron wavefunctions.
Strong electron correlations in FeSb2: An optical investigation and comparison with RuSb2  [PDF]
A. Herzog,M. Marutzky,J. Sichelschmidt,F. Steglich,S. Kimura,S. Johnsen,B. B. Iversen
Physics , 2010, DOI: 10.1103/PhysRevB.82.245205
Abstract: We report investigations of the optical properties of the narrow gap semiconductor FeSb2 in comparison with the structural homolog RuSb2. In the infrared region the latter shows insulating behavior in whole investigated temperature range (10 - 300 K) whereas the optical reflectivity of FeSb2 shows typical semiconductor behavior upon decreasing the temperature. The conduction electron contribution to the reflectivity is suppressed and the opening of a direct and an indirect charge excitation gap in the far-infrared energy region is observed. Those gap openings are characterized by a redistribution of spectral weight of the optical conductivity in an energy region much larger than the gap energies indicating that strong electron-electron correlations are involved in the formation of the charge gap. Calculations of the optical conductivity from the band structure also provided evidence for the presence of strong electronic correlations. Analyzing the spectra with a fundamental absorption across the gap of parabolic bands yields a direct gap at 130 meV and two indirect gaps at 6 and 31 meV. The strong reduction in the free-carrier concentration at low energies and low temperatures is also reflected in a change in the asymmetry of the phonon absorption which indicates a change in the phonon-conduction-electron interaction.
Nanodiamond Landmarks for Subcellular Multimodal Optical and Electron Imaging  [PDF]
Mark A. Zurbuchen,Michael P. Lake,Sirus A. Kohan,Belinda Leung,Louis-S. Bouchard
Physics , 2015, DOI: 10.1038/srep02668
Abstract: There is a growing need for biolabels that can be used in both optical and electron microscopies, are non-cytotoxic, and do not photobleach. Such biolabels could enable targeted nanoscale imaging of sub-cellular structures, and help to establish correlations between conjugation-delivered biomolecules and function. Here we demonstrate a subcellular multi-modal imaging methodology that enables localization of inert particulate probes, consisting of nanodiamonds having fluorescent nitrogen-vacancy centers. These are functionalized to target specific structures, and are observable by both optical and electron microscopies. Nanodiamonds targeted to the nuclear pore complex are rapidly localized in electron-microscopy diffraction mode to enable "zooming-in" to regions of interest for detailed structural investigations. Optical microscopies reveal nanodiamonds for in-vitro tracking or uptake-confirmation. The approach is general, works down to the single nanodiamond level, and can leverage the unique capabilities of nanodiamonds, such as biocompatibility, sensitive magnetometry, and gene and drug delivery.
Investigation on Dynamic Calibration for an Optical-Fiber Solids Concentration Probe in Gas-Solid Two-Phase Flows  [PDF]
Guiling Xu,Cai Liang,Xiaoping Chen,Daoyin Liu,Pan Xu,Liu Shen,Changsui Zhao
Sensors , 2013, DOI: 10.3390/s130709201
Abstract: This paper presents a review and analysis of the research that has been carried out on dynamic calibration for optical-fiber solids concentration probes. An introduction to the optical-fiber solids concentration probe was given. Different calibration methods of optical-fiber solids concentration probes reported in the literature were reviewed. In addition, a reflection-type optical-fiber solids concentration probe was uniquely calibrated at nearly full range of the solids concentration from 0 to packed bed concentration. The effects of particle properties (particle size, sphericity and color) on the calibration results were comprehensively investigated. The results show that the output voltage has a tendency to increase with the decreasing particle size, and the effect of particle color on calibration result is more predominant than that of sphericity.
Signatures of electron-electron correlations in the optical spectra of $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ ($M$=NH$_4$ and Rb)  [PDF]
N. Drichko,M. Dressel,A. Greco,J. Merino,J. Schlueter
Physics , 2005, DOI: 10.1007/BF02679519
Abstract: We interpret the optical spectra of $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (M=NH$_4$ and Rb) in terms of a 1/4 filled metallic system close to charge ordering and show that in the conductivity spectra of these compounds a fraction of the spectral weight is shifted from the Drude-peak to higher frequencies due to strong electronic correlations. Analyzing the temperature dependence of the electronic parameters, we distinguish between different aspects of the influence of electronic correlations on optical properties. We conclude, that the correlation effects are slightly weaker in the NH$_4$ compound compared to the Rb one.
Electron and orbital correlations in Ca_{2-x}Sr_{x}RuO_{4} probed by optical spectroscopy  [PDF]
J. S. Lee,Y. S. Lee,T. W. Noh,S. -J. Oh,Jaejun Yu,S. Nakatsuji,H. Fukazawa,Y. Maeno
Physics , 2002, DOI: 10.1103/PhysRevLett.89.257402
Abstract: The doping and temperature dependent optical conductivity spectra of the quasi-two-dimensional Ca_{2-x}Sr_xRuO_4 (0.0=
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