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arXiv:1707.03362 · Physical review. B./Physical review. B

Revisiting quasiparticle scattering interference in high-temperature superconductors: The problem of narrow peaks

Miguel Antonio Sulangi, Milan P. Allan, Jan Zaanen

Published Oct 9, 2017• 15 citations
arXivHTMLPDF

Abstract

We revisit the interpretation of quasiparticle scattering interference in cuprate high-${T}_{c}$ superconductors. This phenomenon has been very successful in reconstructing the dispersions of $d$-wave Bogoliubov excitations, but the successful identification and interpretation of quasiparticle interference (QPI) in scanning tunneling spectroscopy (STS) experiments rely on theoretical results obtained for the case of isolated impurities. We introduce a highly flexible technique to simulate STS measurements by computing the local density of states using real-space Green's functions defined on two-dimensional lattices with as many as 100 000 sites. We focus on the following question: to what extent can the experimental results be reproduced when various forms of distributed disorder are present? We consider randomly distributed pointlike impurities, smooth ``Coulombic'' disorder, and disorder arising from random on-site energies and superconducting gaps. We find an apparent paradox: the QPI peaks in the Fourier-transformed local density of states appear to be sharper and better defined in experiment than those seen in our simulations. We arrive at a no-go result for smooth-potential disorder since this does not reproduce the QPI peaks associated with large-momentum scattering. An ensemble of pointlike impurities gets closest to experiment, but this goes hand in hand with impurity cores that are not seen in experiment. We also study the effects of possible measurement artifacts (the ``fork mechanism''), which turn out to be of relatively minor consequence. It appears that a more microscopic model of the tunneling process needs to be incorporated in order to account for the sharpness of the experimentally obtained QPI peaks.

DOI
10.1103/physrevb.96.134507

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We revisit the interpretation of quasiparticle scattering interference in cuprate high-Tc{T}_{c}Tc​ superconductors. This phenomenon has been very successful in reconstructing the dispersions of ddd-wave Bogoliubov excitations, but the successful identification and interpretation of quasiparticle interference (QPI) in scanning tunneling spectroscopy (STS) experiments rely on theoretical results obtained for the case of isolated impurities. We introduce a highly flexible technique to simulate STS measurements by computing the local density of states using real-space Green's functions defined on two-dimensional lattices with as many as 100 000 sites. We focus on the following question: to what extent can the experimental results be reproduced when various forms of distributed disorder are present? We consider randomly distributed pointlike impurities, smooth Coulombic'' disorder, and disorder arising from random on-site energies and superconducting gaps. We find an apparent paradox: the QPI peaks in the Fourier-transformed local density of states appear to be sharper and better defined in experiment than those seen in our simulations. We arrive at a no-go result for smooth-potential disorder since this does not reproduce the QPI peaks associated with large-momentum scattering. An ensemble of pointlike impurities gets closest to experiment, but this goes hand in hand with impurity cores that are not seen in experiment. We also study the effects of possible measurement artifacts (the fork mechanism''), which turn out to be of relatively minor consequence. It appears that a more microscopic model of the tunneling process needs to be incorporated in order to account for the sharpness of the experimentally obtained QPI peaks.
We explore the effects of various kinds of random disorder on the quasiparticle density of states of two-dimensional ddd-wave superconductors using an exact real-space method, incorporating realistic details known about the cuprates. Random on-site energy and pointlike unitary impurity models are found to give rise to a vanishing DOS at the Fermi energy for narrow distributions and low concentrations, respectively, and lead to a finite, but suppressed, DOS at unrealistically large levels of disorder. Smooth disorder arising from impurities located away from the copper-oxide planes meanwhile gives rise to a finite DOS at realistic impurity concentrations. For the case of smooth disorder whose average potential is zero, a resonance is found at zero energy for the quasiparticle DOS at large impurity concentrations. We discuss the implications of these results on the computed low-temperature specific heat, the behavior of which we find is strongly affected by the amount of disorder present in the system. We also compute the localization length as a function of disorder strength for various types of disorder and find that intermediate- and high-energy states are quasiextended for low disorder, and that states near the Fermi energy are strongly localized and have a localization length that exhibits an unusual dependence on the amount of disorder. We comment on the origin of disorder in the cuprates and provide constraints on these based on known results from scanning tunneling spectroscopy and specific heat experiments.
In this paper, we study the quasiparticle scattering interference phenomenon in the presence of a single impurity within the renormalized Hubbard model. By calculating the energy and momentum dependence of the Fourier-transformed local density of states in the full Brillouin zone, we can qualitatively describe the main features of the quasiparticle scattering interference phenomenon in cuprate superconductors using a single point-like impurity. In particular, we show that with increasing energy, the position of the peak along the nodal ([0, 0] → [ π, π ]) direction moves steadily to a large momentum region, while the position of the peak along the antinodal ([0, 0] → [ π , 0]) direction moves toward the center of the Brillouin zone.
We study the local density of states around potential scatterers in d-wave superconductors, and show that quantum interference between impurity states is not negligible for experimentally relevant impurity concentrations. The two-impurity model is used as a paradigm to understand these effects analytically and in interpreting numerical solutions of the Bogoliubov--de Gennes equations on fully disordered systems. We focus primarily on the globally particle-hole symmetric model which has been the subject of considerable controversy, and give evidence that a zero-energy delta function exists in the density of states. The anomalous spectral weight at zero energy is seen to arise from resonant impurity states belonging to a particular sublattice, exactly as in the two-impurity version of this model. We discuss the implications of these findings for realistic models of the cuprates. In particular, we show how apparently isolated impurity states can be observed in STM experiments, despite long-range interference, due to averaging over a finite energy range within the impurity band.
We illustrate the importance of many-body effects in the Fourier-transformed local density of states (FT-LDOS) of ddd-wave superconductors from a model of electrons coupled to an Einstein mode with energy ${\ensuremath{\Omega}}_{0}$. For bias energies significantly larger than ${\ensuremath{\Omega}}_{0}$, the quasiparticles have short lifetimes due to this coupling and the FT-LDOS is featureless if the electron-impurity scattering is treated within the Born approximation. In this regime, it is important to include boson exchange for the electron-impurity scattering, which provides a ``step down'' in energy for the electrons and allows for long lifetimes. This many-body effect produces qualitatively different results, namely the presence of peaks in the FT-LDOS, which are mirrors of the quasiparticle interference peaks which occur at bias energies smaller than $\ensuremath{\sim}{\ensuremath{\Omega}}_{0}$. The experimental observation of these quasiparticle mirages would be an important step forward in elucidating the role of many-body effects in FT-LDOS measurements.
Quasi-particle interference (QPI) measurements have provided a powerful tool for determining the momentum dependence of the gap of unconventional superconductors. Here we examine the possibility of using such measurements to probe the frequency and momentum dependence of the electron self-energy. For illustration, we calculate the QPI response function for a cuprate-like Fermi surface with an electron self-energy from a random phase approximation. Then we try to re-extract the self-energy from the dispersion of the peaks in the QPI response function using different approaches. We show that in principle it is possible to extract the self-energy from the QPI response for certain nested momentum directions. We discuss some of the limitations that one faces.
The low-energy quasiparticle states of a disordered d-wave superconductor are investigated theoretically. A class of such states, formed via tunneling between the Andreev bound states that are localized around extended impurities (and result from scattering between pair-potential lobes that differ in sign), is identified. Its (divergent) contribution to the total density of states is determined by taking advantage of connections with certain one-dimensional random tight-binding models. The states under discussion should be distinguished from those associated with nodes in the pair potential.