1,721,016 research outputs found
Induced charge in a Frohlich polaron: Sum rule and spatial extent
No full textWithin the path-integral formalism, we derive exact expressions for correlation functions measuring the lattice charge induced by an electron and associated polarization in the Frohlich polaron problem. We prove that a sum rule for the total induced charge, already obtained with approximate approaches, is indeed exact. As a consequence the total induced charge is shown rigorously to be temperature independent. In addition, we perform path integral Monte Carlo calculations of the correlation functions and compare them to variational results based on the Feynman method. As the temperature increases the polaron radius decreases. On the other hand, at high temperatures the electron motion is not hindered by the lattice. These apparently contradictory results are discussed
Stripes in cuprate superconductors: Excitations and dynamic dichotomy
No full textWe present a short account of the present experimental situation of stripes in cuprates followed by a review of our present understanding of their ground state and excited state properties. Collective modes, the dynamical structure factor, and the optical conductivity of stripes are computed using the time-dependent Gutzwiller approximation applied to realistic one band and three band Hubbard models, and are found to be in excellent agreement with experiment. On the other hand, experiments like angle-resolved photoemission and scanning tunneling microscopy show the coexistence of stripes at high energies with Fermi liquid quasiparticles at low energies. We show that a phenomenological model going beyond mean-field can reconcile this dynamic dichotomy. (C) 2012 Elsevier B. V. All rights reserved
Nematic phase without Heisenberg physics in FeAs planes
No full textWe use Monte Carlo simulations and analytical arguments to analyze a frustrated Ising model with nearest-neighbor antiferromagnetic coupling J(1) and next-nearest-neighbor coupling J(2). The model is inspired on the physics of pnictide superconductors and to some extent we argue that it can be more representative of this physics than the Heisenberg counterpart. Parameters are chosen such that the ground state is a columnar or striped state, as observed experimentally, but is close to the transition to the simple Neel ordered antiferromagnetic state R = J(2)/vertical bar J(1 vertical bar) greater than or similar to 0.5. We find that a nematic phase is induced close to R = 0.5 by finite-size effects and argue that this explains experiments in imperfect samples that find a more robust nematic state as the quality of the sample decreases [A. Jesche et al., Phys. Rev. B 81, 134525 ( 2010)]. Including the effect of a weak coupling with the lattice, we find that a structural transition occurs associated with a nematic phase, with a magnetic transition occurring at a lower temperature. These two transitions merge into a single structural and magnetic transition with a stronger first-order character for larger spin-lattice couplings. These two situations are in agreement with the different phenomenologies found in different families of pnictides
Checkerboard and stripe inhomogeneities in cuprates
No full textWe systematically investigate charge-ordering phases by means of a restricted and unrestricted Gutzwiller approximation to the single-band Hubbard model with nearest- (t) and next-nearest- (t(')) neighbor hopping. When parallel to t(')/t parallel to is small, as appropriate for La2-xSrxCuO4, stripes are found, whereas in compounds with larger parallel to t(')/t parallel to (such as Ca2-xNaxCuO2Cl2 and Bi2Sr2CaCu2O8+delta) checkerboard structures are favored. In contrast to the linear doping dependence found for stripes the charge periodicity of checkerboard textures is locked to 4 unit cells over a wide doping range. In addition we find that checkerboard structures are favored at surfaces
Dynamical charge and spin density wave scattering in cuprate superconductors
No full textWe show that a variety of spectral features of high-Tc cuprates can be understood on the basis of the coupling of charge carriers to some kind of dynamical order that we exemplify in terms of fluctuating charge and spin density waves. Two theoretical models that capture different aspects of such a dynamical scattering are investigated. The first approach leaves the ground state in the disordered phase but couples the electrons to bosonic degrees of freedom, corresponding to the quasi-singular scattering associated with closeness to an ordered phase. The second, more phenomological approach starts from the construction of a frequency-dependent order parameter that vanishes for small energies. Both theories capture scanning tunneling microscopy and angle-resolved photoemission experiments that suggest the protection of quasi-particles close to the Fermi energy but the manifestation of long-range order at higher frequencies
Influence of correlations on transitive electron-phonon couplings in cuprate superconductors
No full textWe investigate a model for the CuO2 plane of high-T-c superconductors where the charge carriers are coupled to A(1g) and B-1g symmetric out-of plane vibrations of the oxygen atoms in the presence of local Hubbard correlations. The coupling is implemented via a modulation of the hopping integral and we calculate the renormalization of vertex and pairing scattering functions based on the time-dependent Gutziller approximation. Contrary to local electron-phonon couplings we find that the transitive coupling can even be enhanced by correlations for certain momenta and symmetries of the vibrations. While this effect may be important for certain properties, we find that, with regard to superconductivity, electron-electron correlations still generically lead to a suppression of the pairing correlations. Our results allow for an estimate of correlation effects on the electron-phonon induced pair scattering from weak electron-electron interactions up to the Mott regime. For onsite repulsions relevant to cuprate superconductors our calculations reveal a significant contribution of B-1g phonons to d-wave superconductivity
Dynamics of Electronic Inhomogeneities in Cuprates
No full textA phenomenological model is constructed which can reconcile the simultaneous existence of low-energy Bogoljubov quasiparticles and high-energy electronic order as observed by scanning tunneling microscopy (STM) experiments in cuprates. The theory can account for the high-energy contrast reversal in the STM spectra between positive and negative bias as characteristic for charge-density wave scattering. On the other hand the low-energy Fourier-transform of the local density of states is an even function of bias which provides an alternative to the formation of pair-density waves
Spin excitations of ferronematic order in underdoped cuprate superconductors
No full textHigh-temperature superconductors exhibit a characteristic hourglass-shaped spectrum of magnetic fluctuations which most likely contribute to the pairing glue in the cuprates. Recent neutron scattering experiments in strongly underdoped compounds have revealed a significant low energy anisotropy of these fluctuations which we explain by a model in which topological defects of the antiferromagnet clump to producing domain wall segments with ferronematic order. This state does not invoke global charge order but breaks C 4 rotational and inversion symmetry. The incommensurability of the low doping charge-disordered state is in good agreement with experiment and interpolates smoothly with the incommensurability of the stripe phase at higher doping. Within linear spin-wave theory the dynamic structure factor is in very good agreement with inelastic neutron scattering data and can account for the observed energy dependent anisotropy. © 2014 Macmillan Publishers Limited
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