196 research outputs found

    Electron-boson glue function derived from electronic Raman scattering

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    Raman scattering cross sections depend on photon polarization. In the cuprates nodal and antinodal directions are weighted more strongly in B2gB_{2g} and B1gB_{1g} symmetry, respectively. On the other hand in angle-resolved photoemission spectroscopy (ARPES), electronic properties are measured along well-defined directions in momentum space rather than their weighted averages. In contrast, the optical conductivity involves a momentum average over the entire Brillouin zone. Newly measured Raman response data on high-quality Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} single crystals up to high energies have been inverted using a modified maximum entropy inversion technique to extract from B1gB_{1g} and B2gB_{2g} Raman data corresponding electron-boson spectral densities (glue) are compared to the results obtained with known ARPES and optical inversions. We find that the B2gB_{2g} spectrum agrees qualitatively with nodal direction ARPES while the B1gB_{1g} looks more like the optical spectrum. A large peak around 30−40 30 - 40\,meV in B1gB_{1g}, much less prominent in B2gB_{2g}, is taken as support for the importance of (π,π)(\pi,\pi) scattering at this frequency.Comment: 7 pages, 3 figure

    Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates

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    We consider the lightly doped cuprates Y0.97_{0.97}Ca0.03_{0.03}BaCuO6.05_{6.05} and La2−x_{2-x}Srx_xCuO4_4 (with x=0.02x=0.02,0.04), where the presence of a fluctuating nematic state has often been proposed as a precursor of the stripe (or, more generically, charge-density wave) phase, which sets in at higher doping. We phenomenologically assume a quantum critical character for the longitudinal and transverse nematic, and for the charge-ordering fluctuations, and investigate the effects of these fluctuations in Raman spectra. We find that the longitudinal nematic fluctuations peaked at zero transferred momentum account well for the anomalous Raman absorption observed in these systems in the B2gB_{2g} channel, while the absence of such effect in the B1gB_{1g} channel may be due to the overall suppression of Raman response at low frequencies, associated with the pseudogap. While in Y0.97_{0.97}Ca0.03_{0.03}BaCuO6.05_{6.05} the low-frequency lineshape is fully accounted by longitudinal nematic collective modes alone, in La2−x_{2-x}Srx_xCuO4_4 also charge-ordering modes with finite characteristic wavevector are needed to reproduce the shoulders observed in the Raman response. This different involvement of the nearly critical modes in the two materials suggests a different evolution of the nematic state at very low doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201

    Pinpointing Gap Minima in Ba(Fe0.94_{0.94}Co0.06)2_{0.06})_{2}As2_2 \textit{via} Band Structure Calculations and Electronic Raman Scattering

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    A detailed knowledge of the gap structure for the Fe-pnictide superconductors is still rather rudimentary, with several conflicting reports of either nodes, deep gap minima, or fully isotropic gaps on the Fermi surface sheets, both in the kx−kyk_{x}-k_{y} plane and along the c-axis. In this paper we present considerations for electronic Raman scattering which can help clarify the gap structure and topology using different light scattering geometries. Using density functional calculations for the Raman vertices, it is shown that the location of the gap minima may occur on loops stretching over a portion of the c-axis in Ba(Fe0.94_{0.94}Co0.06)2_{0.06})_{2}As2_2.Comment: 4+ pages, three figure

    Band and momentum dependent electron dynamics in superconducting Ba(Fe1−xCox)2As2{\rm Ba(Fe_{1-x}Co_{x})_2As_2} as seen via electronic Raman scattering

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    We present details of carrier properties in high quality Ba(Fe1−xCox)2As2{\rm Ba(Fe_{1-x}Co_{x})_2As_2} single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition highlighting the importance of complex band-dependent interactions. The presence of low energy spectral weight deep in the superconducting state suggests a gap with accidental nodes which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band and momentum dependent carrier dynamics indicate that the iron arsenides may have several competing superconducting ground states.Comment: 5 pages, 4 figure

    A balancing act: Evidence for a strong subdominant d-wave pairing channel in Ba0.6K0.4Fe2As2{\rm Ba_{0.6}K_{0.4}Fe_2As_2}

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    We present an analysis of the Raman spectra of optimally doped Ba0.6K0.4Fe2As2{\rm Ba_{0.6}K_{0.4}Fe_2As_2} based on LDA band structure calculations and the subsequent estimation of effective Raman vertices. Experimentally a narrow, emergent mode appears in the B1gB_{1g} (dx2−y2d_{x^2-y^2}) Raman spectra only below TcT_c, well into the superconducting state and at an energy below twice the energy gap on the electron Fermi surface sheets. The Raman spectra can be reproduced quantitatively with estimates for the magnitude and momentum space structure of the s+−_{+-} pairing gap on different Fermi surface sheets, as well as the identification of the emergent sharp feature as a Bardasis-Schrieffer exciton, formed as a Cooper pair bound state in a subdominant dx2−y2d_{x^2-y^2} channel. The binding energy of the exciton relative to the gap edge shows that the coupling strength in this subdominant dx2−y2d_{x^2-y^2} channel is as strong as 60% of that in the dominant s+−s_{+-} channel. This result suggests that dx2−y2d_{x^2-y^2} may be the dominant pairing symmetry in Fe-based sperconductors which lack central hole bands.Comment: 10 pages, 6 Figure

    Raman-Scattering Detection of Nearly Degenerate ss-Wave and dd-Wave Pairing Channels in Iron-Based Ba0.6_{0.6}K0.4_{0.4}Fe2_2As2_2 and Rb0.8_{0.8}Fe1.6_{1.6}Se2_2 Superconductors

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    We show that electronic Raman scattering affords a window into the essential properties of the pairing potential Vk,k′V_{\mathbf{k},\mathbf{k^{\prime}}} of iron-based superconductors. In Ba0.6_{0.6}K0.4_{0.4}Fe2_2As2_2 we observe band dependent energy gaps along with excitonic Bardasis-Schrieffer modes characterizing, respectively, the dominant and subdominant pairing channel. The dx2−y2d_{x^2-y^2} symmetry of all excitons allows us to identify the subdominant channel to originate from the interaction between the electron bands. Consequently, the dominant channel driving superconductivity results from the interaction between the electron and hole bands and has the full lattice symmetry. The results in Rb0.8_{0.8}Fe1.6_{1.6}Se2_2 along with earlier ones in Ba(Fe0.939_{0.939}Co0.061_{0.061})2_2As2_2 highlight the influence of the Fermi surface topology on the pairing interactions.Comment: 5 pages, 4 figure

    Electron interactions and charge ordering in La2−x_{2-x}Srx_xCuO4_4

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    We present results of inelastic light scattering experiments on single-crystalline La2−x_{2-x}Srx_{x}CuO4_4 in the doping range 0.00≤x=p≤0.300.00 \le x=p \le 0.30 and Tl2_2Ba2_2CuO6+δ_{6+\delta} at p=0.20p=0.20 and p=0.24p=0.24. The main emphasis is placed on the response of electronic excitations in the antiferromagnetic phase, in the pseudogap range, in the superconducting state, and in the essentially normal metallic state at x≥0.26x \ge 0.26, where no superconductivity could be observed. In most of the cases we compare B1g_{1g} and B2g_{2g} spectra which project out electronic properties close to (π,0)(\pi,0) and (π/2,π/2)(\pi/2, \pi/2), respectively. In the channel of electron-hole excitations we find universal behavior in B2g_{2g} symmetry as long as the material exhibits superconductivity at low temperature. In contrast, there is a strong doping dependence in B1g_{1g} symmetry: (i) In the doping range 0.20≤p≤0.250.20 \le p \le 0.25 we observe rapid changes of shape and temperature dependence of the spectra. (ii) In La2−x_{2-x}Srx_{x}CuO4_4 new structures appear for x<0.13x < 0.13 which are superposed on the electron-hole continuum. The temperature dependence as well as model calculations support an interpretation in terms of charge-ordering fluctuations. For x≤0.05x \le 0.05 the response from fluctuations disappears at B1g_{1g} and appears at B2g_{2g} symmetry in full agreement with the orientation change of stripes found by neutron scattering. While, with a grain of salt, the particle-hole continuum is universal for all cuprates the response from fluctuating charge order in the range 0.05≤p<0.160.05 \le p < 0.16 is so far found only in La2−x_{2-x}Srx_{x}CuO4_4. We conclude that La2−x_{2-x}Srx_{x}CuO4_4 is close to static charge order and, for this reason, may have a suppressed TcT_c.Comment: 17 pages, 15 figure

    Raman scattering evidence for a cascade-like evolution of the charge-density-wave collective amplitude mode

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    The two-dimensional rare-earth tri-tellurides undergo a unidirectional charge-density-wave (CDW) transition at high temperature and, for the heaviest members of the series, a bidirectional one at low temperature. Raman scattering experiments as a function of temperature on DyTe3_3 and on LaTe3_3 at 6 GPa provide a clear-cut evidence for the emergence of the respective collective CDW amplitude excitations. In the unidirectional CDW phase, we surprisingly discover that the amplitude mode develops as a succession of two mean-field, BCS-like transitions in different temperature ranges
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