Atomic-Scale Quasi-Particle Scattering Resonances in Bi ₂ Sr ₂ CaCu ₂ O _8+δ

Abstract

Low-temperature scanning tunneling spectroscopy of the high transition temperature (high-Tc) cuprate Bi2Sr2CaCu2O8+delta reveals the existence of large numbers of identical regions with diameters of about 3 nanometers that have a relatively high density of low-energy quasi-particle states. Their spatial and spectroscopic characteristics are consistent with theories of strong quasi-particle scattering from atomic-scale impurities in a d-wave superconductor. These characteristics include breaking of local particle-hole symmetry, a diameter near twice the superconducting coherence length, and an inverse square dependence of their local density-of-states on distance from the scattering center. In addition to the validation of d-wave quasi-particle scattering theories, these observations identify a source for the anomalously high levels of low-energy quasi-particles in Bi2Sr2CaCu2O8+delta at low temperatures.

Distinguishing models for the pseudo-gap in cuprate superconductors by probing the spatial distribution of impurity states

“…As was shown by Pan et al [4] a nonmagnetic impurity as Zn locally destroys the superconductivity, as indicated by the absence of a hole-like resonance. Nevertheless a particle-like resonance is observed, due to the depletion of states near the Fermi level, which we attribute to the pseudo-gap.…”

“…As various different theoretical proposals for the pseudo-gap have been put forward the need of experimental probes that are able to distinguish between them is of great interest. Here we argue that scanning tunneling microscopy (STM) experiments in underdoped HTSC, that serve as a local probe of impurity states [4,5], can be used in order to differentiate between at least two classes of proposals.…”

“…Unfortunately, until now experiments on resonant impurity states have only been conducted on optimally doped samples of BSCCO [4,5], where no pseudo gap is present. Since understanding the pseudo-gap is considered the key for the theory of HTSC, a corresponding STM experiment on underdoped samples with a very low T c and a huge pseudo-gap is in demand.…”

“…As various different theoretical proposals for the pseudo-gap have been put forward the need of experimental probes that are able to distinguish between them is of great interest. Here we argue that scanning tunneling microscopy (STM) experiments in underdoped HTSC, that serve as a local probe of impurity states [4,5], can be used in order to differentiate between at least two classes of proposals.In one class of models the pseudo-gap is ascribed to a precursor pairing amplitude whose phase coherence is destroyed above T c [6,7]. Since such a state exhibits off-diagonal short-range order, particle and hole states get mixed and consequently an induced impurity state is composed of both a particle and a hole-like component.…”

Quasiparticle echoes in scanning tunneling microscopy

“…In the case of a superconductor, the method can potentially distinguish the nature of the scattering by a particular impurity. PACS numbers: 74.55.+v,72.10.Fk,73.20.At,74.72.ah Scanning Tunneling Microscopy (STM), which measures the "local density of states" (LDOS) as a function of position and energy set by the bias voltage, has opened the door to imaging the sub-nanoscale topography and electronic structure of materials, including normal metals [1] and especially cuprate superconductors [2,3,4,5,6,7,8,9].The dispersion relations of (Landau or Bogoliubov) quasiparticles may be extracted from STM data on normal metals [10,11] and superconductors [13], via the inverse method called Fourier transform scanning tunneling spectroscopy (FT-STS) [10,13], or directly in real space [11]. This technique is based on the fact that impurities produce spatial modulations of the LDOS in their vicinity -standing waves in the electronic structure that generalize the Friedel oscillations found in metals at the Fermi energy.…”