Effect of unitary impurities on non-STM types of tunneling in high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>superconductors

Zhu, Jian‐Xin; Ting, C. S.; Hu, Chia-Ren

doi:10.1103/physrevb.62.6027

Cited by 53 publications

(66 citation statements)

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“…The attribution of the maximum at the center of the STM pattern to an effect of hybridization of impurity wavefunctions with the states between the CuO 2 and the surface bears some similarity to an explanation which has been discussed previously in the framework of phenomenological tight binding models 14,15 . In this approach, the presence of the BiO and SrO layers serve to act as a "fork" or "filter" which forces the electron to tunnel with equal probability from the Bi site above the impurity to the Cu sites neighboring the impurity; thus the measured LDOS at the surface is found to reflect a linear combination of the impurity-induced wavefunctions on the nearest-neighbor Cu sites in the CuO 2 layer below.…”

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confidence: 91%

Ab initiocalculation of impurity effects in copper oxide materials

2005

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We describe a method for calculating, within density functional theory, the electronic structure associated with typical defects which substitute for Cu in the CuO 2 planes of high-T c superconducting materials. The focus is primarily on Bi 2 Sr 2 CaCu 2 O 8 , the material on which most STM measurements of impurity resonances in the superconducting state have been performed. The magnitudes of the effective potentials found for Zn, Ni and vacancies on the in-plane Cu sites in this host material are remarkably consistent with phenomenological fits of potential scattering models to STM resonance energies. The effective potential ranges are quite short, of order 1 Å with weak long range tails, in contrast to some current models of extended potentials which attempt to fit STM data.For the case of Zn and Cu vacancies, the effective potentials are strongly repulsive, and states on the impurity site near the Fermi level are simply removed. The local density of states (LDOS) just above the impurity is nevertheless found to be a maximum in the case of Zn and a local minimum in case of the vacancy, in agreement with experiment. The Zn and Cu vacancy patterns are explained as due to the long-range tails of the effective impurity potential at the sample surface. The case of Ni is richer due to the Ni atom's strong hybridization with states near the Fermi level; in particular, the short range part of the potential is attractive, and the LDOS is found to vary rapidly with distance from the surface and from the impurity site. We propose that the current controversy surrounding the observed STM patterns can be resolved by properly accounting for the effective impurity potentials and wave-functions near the cuprate surface. Other aspects of the impurity states for all three species are discussed.2

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confidence: 91%

Ab initiocalculation of impurity effects in copper oxide materials

2005

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“…The on-site potential scattering was taken to be either zero (Zhang et al, 2001) or very weak (Polkovnikov, 2002;Polkovnikov et al, 2001) so that the resonance peak is located very close to the Fermi energy. Zhu and Ting (Zhu and Ting, 2001b) took into account the quasiparticle scattering from a geometrical hole, where electrons are allowed to hop onto the four neighbors of the impurity site, and obtained a double-peak structure around the Fermi energy. They further (Zhu and Ting, 2001c) considered the potential scattering term to be in the unitary limit (U → ∞), and found that the Kondo resonance effect is weaved into that from the strong potential scattering to determine the low energy quasiparticle states.…”

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Impurity-induced states in conventional and unconventional superconductors

2006

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We review recent developments in our understanding of how impurities influence the electronic states in the bulk of superconductors. Our focus is on the quasi-localized states in the vicinity of impurity sites in conventional and unconventional superconductors and our goal is to provide a unified framework for their description. The non-magnetic impurity resonances in unconventional superconductors are directly related to the Yu-Shiba-Rusinov states around magnetic impurities in conventional s-wave systems. We review the physics behind these states, including quantum phase transition between screened and unscreened impurity, and emphasize recent work on d-wave superconductors. The bound states are most spectacularly seen in scanning tunneling spectroscopy measurements on high-Tc cuprates, which we describe in detail. We also discuss very recent progress on the states coupled to impurity sites which have their own dynamics, and impurity resonances in the presence of an order competing with superconductivity. Last part of the review is devoted to influence of local deviations of the impurity concentration from its average value on the density of states in s-wave superconductors. We review how these fluctuations affect the density of states and show that s-wave superconductors are, strictly speaking, gapless in the presence of an arbitrarily small concentration of magnetic impurities.

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“…The discrepancies between experiments and the potential scattering scenario have led to the suggestion of various filter functions motivated by the fact that the measurements are performed on the topmost BiO layer whereas the nonmagnetic impurity substitutes a Cu atom in the CuO 2 plane two layers below the top BiO layer [8,9,10]. The significance of the filter functions remains controversial.…”

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Andreev States near Short-Ranged Pairing Potential Impurities

et al. 2006

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We study Andreev states near atomic scale modulations in the pairing potential in both s-and d-wave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi2Sr2CaCu2O8+x (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.PACS numbers: 74.45.+c, 74.62.Dh Motivated by the experimental ability to determine the LDOS with high resolution in both energy and real space using scanning tunneling microscopy (STM), there has recently been a large interest in the perturbations caused by impurities in superconductors. This is because impurities disturb the underlying superconducting state and hence constitute a natural probe of the state in which they are embedded [1]. For magnetic adatoms on the surface of conventional s-wave superconductors such experiments were performed by Yazdani et al [2]. In the superconducting state of BSCCO, STM measurements have provided detailed information about the electronic structure near Ni and Zn impurities [3,4,5]. Near Zn it was found that each impurity generates a sharp conductance peak close to the Fermi level (ω B ∼ −1.5meV). By fixing the bias voltage between the tip and the sample at ω B , the spatial structure of this state can be mapped out: it is strongly localized near the Zn atom with maximum intensity on the impurity site and second largest intensity on the next-nearest neighbor sites.Although there exists a large number of theoretical treatments dealing with the resonant states generated by nonmagnetic impurities in d-wave superconductors, no consensus has been reached on their relevance to experiments [1]. Within the most straightforward scenario where Zn is modelled as a delta-function potential scatterer, low-energy resonant states are generated in the unitary limit where the potential is large compared to all other energy scales of the problem [6]. For realistic bands with particle-hole asymmetry, a particular value of the impurity potential V must be chosen to tune the resonance energy ω B to be close to the Fermi level. Thus, the potential scattering model can reproduce the energetics of the resonant state. However, as is well-known, it produces a low-energy spatial LDOS pattern with a severely suppressed amplitude on the impurity site, in contrast to experimental results on Zn [5]. Physically, this is clear since a large on-site potential V penalizes any substantial amplitude of the impurity-state wavefunction on the impurity site. In fact, within this model the largest intensity is found on the nearest neighbor site to the impurity.These properties of the delta-function potential scatterer are shown in Fig.1. The discrepanc...

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Effect of unitary impurities on non-STM types of tunneling in high-Tcsuperconductors

Cited by 53 publications

References 60 publications

Ab initiocalculation of impurity effects in copper oxide materials

Ab initiocalculation of impurity effects in copper oxide materials

Impurity-induced states in conventional and unconventional superconductors

Andreev States near Short-Ranged Pairing Potential Impurities

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