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Electronic Raman scattering in correlated materials: exact treatment of nonresonant, mixed, and resonant scattering with dynamical mean field theory

We solve for the electronic Raman scattering response functions on an infinite-dimensional hypercubic lattice employing dynamical mean field theory. This contribution extends previous work on the nonresonant response to include the mixed and resonant contributions. We focus our attention on the spinless Falicov-Kimball model, where the problem can be solved exactly, and the system can be tuned to go through a Mott-Hubbard-like metal-insulator transition. Resonant effects vary in different scattering geometries, corresponding to the symmetries of the charge excitations scattered by the light. We do find that the Raman response is large near the double resonance, where the transfered frequency is close to the incident photon frequency. We also find a joint resonance of both the charge-transfer peak and the low-energy peak when the incident photon frequency is on the order of the interaction strength. In general, the resonance effects can create order of magnitude (or more) enhancements of features in the nonresonant response, especially when the incident photon frequency is somewhat larger than the frequency of the nonresonant feature. Finally, we find that the resonant effects also exhibit isosbestic behavior, even in the A1g and B2g sectors, and it is most prominent when the incident photon frequency is on the order of the interaction energy.Comment: (20 pages, 13 figures

Evolution of Superconductivity in Electron-Doped Cuprates: Magneto-Raman Spectroscopy

The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. The SC pairing strength is found to be consistent with a weak-coupling regime except in the under-doped region where we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers divide into coherent quasi-particles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (\pi/2, \pi/2) regions of the Brillouin zone (BZ). We find that only coherent QPs contribute to the superfluid density in the B_{2g} channel. The persistence of SC coherence peaks in the B_{2g} channel for all dopings implies that superconductivity is mainly governed by interactions between the hole-like coherent QPs in the vicinity of (\pi/2, \pi/2) regions of the BZ. We establish that superconductivity in the electron-doped cuprates occurs primarily due to pairing and condensation of hole-like carriers. We have also studied the excitations across the SC gap by Raman spectroscopy as a function of temperature (T) and magnetic field (H) for several different cerium dopings (x). Effective upper critical field lines H*_{c2}(T, x) at which the superfluid stiffness vanishes and H^{2\Delta}_{c2}(T, x) at which the SC gap amplitude is suppressed by field have been determined; H^{2\Delta}_{c2}(T, x) is larger than H*_{c2}(T, x) for all doping concentrations. The difference between the two quantities suggests the presence of phase fluctuations that increase for x< 0.15. It is found that the magnetic field suppresses the magnitude of the SC gap linearly at surprisingly small fields.Comment: 13 pages, 8 figures; submitted to Phys. Rev.

Observation of Leggett's collective mode in a multi-band MgB2 superconductor

We report observation of Leggett's collective mode in a multi-band MgB2 superconductor with T_c=39K arising from the fluctuations in the relative phase between two superconducting condensates. The novel mode is observed by Raman spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed mode frequency is consistent with theoretical considerations based on the first principle computations.Comment: Accepted for PR

Stripe orders in the extended Hubbard model

We study stripe orders of charge and spin density waves in the extended Hubbard model with the nearest-neighbor Coulomb repulsion V within the mean field approximation. We obtain V vs. T(temperature) phase diagram for the on-site Coulomb interaction U/t=8.0 and the filling n=0.8, here t is a nearest-neighbor transfer energy. Our result shows that the diagonal stripe spin density wave state (SDW) is stable for small V, but for large V the most stable state changes to a charge density wave-antiferromagnetic (CDW-AF) state. Especially we find at low temperature and for a certain range of value of V, a vertical stripe CDW-AF state becomes stable.Comment: LaTeX 9 pages, 17 figures, uses jpsj.st

Constant effective mass across the phase diagram of high-Tc_{c} cuprates

We investigate the hole dynamics in two prototypical high temperature superconducting systems: La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}$O$_{y}$ using a combination of DC transport and infrared spectroscopy. By exploring the effective spectral weight obtained with optics in conjunction with DC Hall results we find that the transition to the Mott insulating state in these systems is of the "vanishing carrier number" type since we observe no substantial enhancement of the mass as one proceeds to undoped phases. Further, the effective mass remains constant across the entire underdoped regime of the phase diagram. We discuss the implications of these results for the understanding of both transport phenomena and pairing mechanism in high-T$_{c}$ systems.Comment: 5 pages, 2 figure

Nonmonotonic d_{x^2-y^2} Superconducting Order Parameter in Nd_{2-x}Ce_xCuO_4

Low energy polarized electronic Raman scattering of the electron doped superconductor Nd_1.85Ce_0.15CuO_4 (T_c=22 K) has revealed a nonmonotonic d_{x^2-y^2} superconducting order parameter. It has a maximum gap of 4.4 k_BT_c at Fermi surface intersections with antiferromagnetic Brillouin zone (the ``hot spots'') and a smaller gap of 3.3 k_BT_c at fermionic Brillouin zone boundaries. The gap enhancement in the vicinity of the ``hot spots'' emphasizes role of antiferromagnetic fluctuations and similarity in the origin of superconductivity for electron- and hole-doped cuprates.Comment: 4 pages, 4 figure